Thursday, January 29, 2015

Engr. Aneel Kumar

THREE PHASE TRANSFORMER WINDING CONNECTIONS

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A three-phase transformer bank can be easily created by using three single-phase transformers. The two sides of these three transformers can be either connected in a wye or a delta configuration, thus allowing four possible types of connections. These are:

• WYE WYE:

With the wye-wye (Y-Y) connection, the secondary side is in phase with the primary circuit, and the ratio of primary to secondary voltage is the same as the ratio of turns in each of the phases. A possible connection is shown in Figure 1. Power distribution circuits supplied from a wye-wye bank often create series disturbances in communication circuits (e.g., telephone interference) in their immediate vicinity. One of the advantages of this connection is that when a system is changed from a delta to a four-wire wye to increase system capacity, existing transformers can be used.
Figure 1 Y-Y transformer with 0° phase shift between the primary and the secondary sides. 

• WYE-DELTA:

In the Y-Δ connection, there is a 30° phase angle shift between the primary and secondary sides. The phase angle difference can be made either lagging or leading, depending on the external connections of the transformer bank.

The case with the primary side lagging is shown in Figure 2, and the case with the primary side leading is shown in Figure 3. The transformation ratio is times the ratio of turns in each of the phases.
Figure 2 Y-Δ transformer with the primary side lagging the secondary side by 30°.
Figure 3 Y-Δ transformer with the primary side leading the secondary side by 30°.

• DELTA-WYE:

With the Δ-Y connection, the neutral of the secondary wye can be grounded and single-phase loads connected across the phase and the neutral conductor.

Three-phase loads are connected across the phases. The phasor relationship between the primary and the secondary sides is shown in Figure 4. The transformation ratio is 1/√3 times the ratio of turns in each of the phases.
Figure 4 Δ-Y transformer with the primary side leading the secondary side by 30°.

• DELTA-DELTA:

The Δ-Δ connection does not cause a phase shift between the primary and the secondary sides. The phasor relationship of this transformer is shown in Figure 5. The transformation ratio is equal to the ratio of the turns in each of the phases. There is no problem from third-harmonic over voltage or telephone interference because such disturbances get trapped in the delta and do not pass into the lines.
Figure 5 Δ-Δ transformer with 0° phase shift between the primary and the secondary sides.
Although these four configurations are the most common ones used, other arrangements are possible, including:

• OPEN-DELTA:

An advantage of the Δ-Δ connection is that if one of the single- phase transformers becomes damaged or is removed for maintenance, the remaining two can be operated in a so-called open-delta connection. Because the currents in each of the two remaining transformers are the same as the line current, each transformer carries times the current it was carrying in the closed-delta connection. The open-delta bank continues to deliver three-phase currents and voltages in their correct phase relationship. To keep the transformers from being overloaded, however, it is necessary to reduce the line currents by approximately 1/√3 .

• SCOTT OR T-CONNECTION:

The Scott or T-connection is used when a two-phase (or a transformed three-phase) supply is needed from a three-phase system. In general, the T-connection is used for deriving a three-phase transformation, and the Scott connection is mainly used for obtaining a two-phase output. The two connections are similar in basic design. Either connection requires two specially wound single-phase transformers. The main transformer has a 50 percent tap on the primary winding, whereas the other transformer, called the teaser transformer, has an 86.6 percent tap. The main transformer is connected between two primary lines, whereas the teaser transformer is connected from the center tap of the main transformer to the third primary line. The secondary sides of the transformers provide two-phase service. A T-connection is shown in Figure 6.
Figure 6 The T connection for a three-phase to two-phase transformation.
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Friday, January 23, 2015

Engr. Aneel Kumar

POWER FACTOR

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WHAT IS POWER FACTOR

Power factor (PF) is simply the relationship between the active and reactive power on an electricity distribution network and a measure of whether the system’s voltage and current are ‘in phase’. Take, for example, a frothy latte. The coffee body is the ‘active power’ that you can use to do work. The froth on the top is ‘reactive power’; some is useful, but too much is simply a waste the same as the foam you leave behind in your glass.
  • If a network is 100% efficient (i-e if no reactive power is present) its power factor (PF) is 1 or unity. This is the ideal for power transmission, but is practically impossible to attain. Variation in power factor is caused by different types of electrical devices connected to the grid that consume or generate reactive power. Unless this variation is corrected, higher currents are drawn from the grid, leading to grid instability, higher costs and reduced transmission capacity
  • A poor PF results in additional costs for the electricity supplier
  • These costs are passed on to the customer as a ‘reactive power charge’ or ‘exceeded capacity charge’

THE CAUSES OF POOR POWER FACTOR

The causes of poor PF include inductive loads on equipment such as AC motors, arc welders, furnaces, fluorescent lighting and air conditioning. The more inductive loads there are on the network, the greater the possibility there is of a poor PF.

THE BENEFITS OF POWER FACTOR CORRECTION

The following are the main benefits of PFC
  • Eliminating expensive utility penalties for a poor power factor
  • Improved energy efficiency: reduced system currents and kW losses
  • Security of supply: reduction in peak currents prevents fuse failure and loss of supply
  • Release additional capacity: to take advantage of the full current capacity available in existing transformers, switch-gear and supply cables
  • Increase system load without the need to invest in additional infrastructure
  • Environmentally friendly: reduced kWh losses mean that less power needs to be generated, so less CO2 is produced
  • Increased infrastructure service life: since the amount of heat generated within cables, switch-gear, transformers and other equipment is reduced.
Power factor correction equipment is not only fast and cost-effective to install but it starts paying back on your investment immediately, with typical payback times from 12 to 24 months.
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Thursday, January 22, 2015

Engr. Aneel Kumar

CONSTRUCTION AND OPERATION OF A SYNCHRONOUS MOTOR

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The stator winding of a synchronous motor is similar to that of a 3 phase induction motor. The rotor consists of salient poles excited by dc field windings like that of inward-projecting poles of a dc motor. The rotor field windings are energized by direct current passed through slip rings from an external source or from a dc generator, mounted on the same rotor shaft.

When the stator winding is energized from a 3 phase supply, a revolving magnetic field, the speed of which is given by Ns=120f/P is produced. This speed is called synchronous speed. To enable the synchronous motor to run at the above mentioned synchronous speed the rotor field winding is energized and at the same time brought near to the synchronous speed, by some other means. The rotor poles, which are always equal to that of the stator poles, are pulled to synchronous speed and the two set of poles lock with each other and the rotor starts rotating at synchronous speed. Thus, to run a synchronous motor, the rotor has to be brought near to synchronous speed first by some means, say by some external prime mover. This is a big disadvantage of this motor. However, a synchronous motor is made self-starting by providing a squirrel cage winding (like that of an induction motor) along with the dc field winding on the rotor. In such a case when three phase supply is applied across the stator windings the rotor starts rotating as an induction motor and when it reaches near its final speed (near synchronous speed), dc field winding is energized and the rotor thus pulls into synchronism with the revolving field and continues to run at synchronous speed. At synchronous speed there will be no current in the squirrel cage winding since at synchronous speed slip is zero. The squirrel cage winding therefore is designed only for short duty services. During the starting period the dc field winding has to be kept shorted through a discharge resistance. This is done so as to avoid building up of an extremely high voltage in the winding. If field is left open circuited a high voltage will develop in the open field winding as it has large number of turns and the relative speed of stator flux to the windings of the poles is high during starting. But this induced high voltage will gradually decrease as the motor will be picking up speed. The induced emf in the field winding is kept to a safe value by shorting the winding. This would limit the demagnetizing effect on the main flux otherwise caused due to current flowing in the dc field winding as a result of induced emf in it. This demagnetizing effect, if allowed to happen will reduce the starting torque of the motor. If in some special applications a higher starting torque is required the field winding can be left open circuited, but should be sectionalized, to have reduced voltage induced across the separated portions.
From the above, it is seen that the primary purpose of the squirrel cage in this motor, is for starting the motor. As mentioned earlier this winding is designed for low thermal capacity. If, however, the motor picks up speed too slowly under some loading condition, it will run as induction motor for extended period of time and as a result the squirrel cage winding may get over heated and get damaged. To overcome this problem a certain protection must be provided which should disconnect the motor from the supply in the event of its failure to get synchronized properly within a certain prescribed time. A timing relay is used for this purpose to open the control circuit if the motor fails to synchronize within the set time.

Synchronous motors, like the induction motors, can be started by applying line voltage, reduced voltage, or using part winding controllers depending upon the kind of load, frequency of starting, and power service restrictions. The starters for the motor can be manual, semiautomatic, or fully-automatic using a polarizing frequency relay.

From the above it follows that synchronous motor control has two basic functions:

(i) To start the motor as an induction motor (the motor can be started by any schemes such as across the line, auto-transformer, primary resistor or any other method);

(ii) To bring the motor up to synchronous speed by exciting the dc field. Different types of synchronous motor starters are discussed as follows.
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Engr. Aneel Kumar

TYPES OF SINGLE PHASE MOTORS

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Single phase motors are manufactured in fractional kilowatt range to be operated on single phase supply and for use in numerous applications like ceiling fans, refrigerators, food mixers, hair driers, portable drills, vacuum cleaners, washing machines, sewing machines, electric shavers, office machinery etc. Single phase motors are manufactured in different types to meet the requirements of various applications. Single phase motors are classified on the basis of their construction and starting methods employed. The main types of single phase motors are:
(a) Induction motors
(b) Synchronous motors
(c) Commutator motors

The various types of motors under each class are shown as under:
Repulsion, repulsion induction and reluctance start motors are not used these days, they have been largely replaced by split phase motors with special capacitors which can be designed to perform equally well as repulsion types. In addition they offer such advantages as lower cost and trouble free service.

Shaded pole motors are extremely popular motors used in low-starting torque applications. Split phase motors are widely used and are designed in several ways to develop different values of starting torque.

Universal series motor is another very popular type of motor which can operate on both ac and dc supply. They generally run at high speed and employ special design features to reduce commutation and armature reaction difficulties on ac supply.

Synchronous motors such as reluctance motor and hysteresis motor operate at synchronous speed for all values of load. They are manufactured in very small ratings. Practically all single phase motors are designed for line voltage starting and take inrush currents that may be little more than the rated values in some types and six or more times as much in others. Like polyphase motors they are also frequently jogged, plugged, reversed, dynamically braked and plug reversed. When the above mentioned operations are done, contactors of larger ratings than normal are used, as their frequent operations cause overheating and excessive wear of contacts. Control circuits of single phase motors are however simple as few contactors and relays are used.
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Engr. Aneel Kumar

REVERSING DIRECTION OF ROTATION OF UNIVERSAL MOTOR

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The direction of rotation of a universal motor can be changed by either: (i) Reversing the field connection with respect to those of armature; or (ii) By using two field windings wound on the core in opposite directions so that the one connected in series with armature gives clockwise rotation, while the other in series with the armature gives counterclockwise rotation.

The second method, i.e, the two field method is used in applications such as motor operated rheostats and servo systems. This method has somewhat simpler connections than the first method.
For simple applications like portable drills etc. manual switches are frequently used for reversing the direction of rotation of the motor. Figure 1(a and b) shows how a DPDT (Double Pole Double Throw) switch and a three position switch may be used for reversing the direction of rotation of single field and double field type of motors respectively.
Figure 1 Reversing of a universal motor (a) Armature reversing method using a reversing switch (b) Two-field method using a three-position switch
In Figure 1 (a), when the DPDT switch is in the position shown, the switch blade bridges the switch terminals A – A1 and B – B1, current through series field and armature flows in the direction as indicated by arrows. When direction of rotation of the motor is to be reversed the switch is thrown to position 2.

Now, terminal A gets connected to terminal A2 and terminal B gets connected to B2.  Current through the armature reverses while direction of current through the series field remains unchanged. This leads to reversal of direction of rotation of the motor. As shown in Figure 1(b) the direction of rotation of the motor is reversed by moving the selector switch in position 1 or 2. In position 1, the FORW-field runs the motor in one direction and in position 2, REV-field runs the motors in the reverse direction. Power and control circuit for a double field universal motor has been shown in Figure 2.
Figure 2 Power and control circuit for reversing of a two-field universal motor
When the FOR-push button is pressed, the forward contactor F get energized and its contacts F1 and F2 close to energies the armature and the FORW-field winding. Similarly when the reverse contactor is energized the REV-field winding and the armature get energized by receiving supply through contacts R1 and R2. In the control circuit, push button interlocking, as also auxiliary contact interlocking have been provided to avoid simultaneous energization of contactors F and R.
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Engr. Aneel Kumar

STANDARD SPLIT PHASE MOTORS

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The stator of a standard split phase motor has two windings viz., a main winding and an auxiliary winding. These two windings have different ratios of resistance to inductive reactance. The windings are connected in parallel across the single phase ac supply. The line current is thus split into two parts; one part flowing through the main winding and the other part flowing through the auxiliary windings. Because of different ratio of resistance to inductive reactance of these two windings current flowing through them will have a time phase difference of 30 electrical degrees or more. In some motors, both the windings are energized continuously, while in most of them, the auxiliary winding is used only during the starting period along with the main winding to develop the required starting torque. When the motor reaches final speed the auxiliary winding is disconnected from the supply. The auxiliary windings can be disconnected by using a centrifugal switch in series with it. During starting, the centrifugal switch remains closed. When the motor reaches normal speed the centrifugal switch opens and disconnects the auxiliary winding. This has been shown in Figure 1(a).
Another method of disconnecting the auxiliary winding when the motor has picked up speed is by using an electromagnetic relay as has been shown in Figure 1(b).
Figure: 1 Use of (a) Centrifugal switch (b) Electro-magnetic relay for connecting an auxiliary winding in parallel with the main winding during starting
During starting the relay picks up due to high starting current and connects the auxiliary winding in the circuit. When the motor reaches normal speed, current drops to normal value and the electromagnetic relay is dropped and therefore the auxiliary winding gets disconnected from supply.

Reversal of direction of rotation of a split phase motor is obtained by interchanging the auxiliary winding terminal connections with respect to the main winding. The motor must be brought to rest or the centrifugal switch must be closed before reversal is attempted. The basic circuit for reversal of small split phase motor using a DPDT (Double Pole Double Throw) switch and contactors has been shown in Figure 2(a and b).
Figure: 2 Reversing of split phase motor
Speed control of standard split phase motors is achieved with difficulty. The usual method is to use two or more windings designed to produce different number of poles. The complications arise as both the main and the auxiliary windings have to be arranged for pole changing. The capacitor type split phase motor which will now be discussed have better possibility of speed adjustments.
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Engr. Aneel Kumar

CAPACITOR TYPE SPLIT PHASE MOTORS

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Capacitor type split phase motors are generally similar to the standard split phase construction except for the addition of capacitor and a slightly modified switching arrangement for the two value capacitor type motors. The three types of capacitor split phase motors have been shown in Figure.
Figure: Three types of capacitor type split phase motors

1) CAPACITOR START SPLIT PHASE MOTORS

These motors are used where high starting torque is required. To accomplish this, capacitors of large values are to be used. Electrolytic capacitors designed for short duty service are available and used in these motors. The motor should come upto its speed quickly and disconnect the capacitor from the windings, otherwise the capacitor will get damaged. The centrifugal switch or the relay that function in auxiliary winding should be very reliable.

2) PERMANENT SPLIT CAPACITOR MOTORS

These motors are mainly used for low starting torque loads where they are generally shaft mounted. Their particular fields of applications are in air moving equipment such as fans, blowers, oil burners etc. where quiet operation is required.

3) TWO VALUE CAPACITOR MOTORS

This type of capacitor motors has the advantage that they develop extremely high locked rotor torque with one value of capacitance in the auxiliary winding circuit and gives quiet running performance of the motor with another capacitor. Electrolytic capacitors are used for starting. Their values are 10 to 15 times as much as the value of the running capacitor.
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Engr. Aneel Kumar

STARTERS FOR CAPACITOR TYPE SPLIT PHASE MOTORS

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We know that direction of a split phase motor can be reversed if the connection of one of the windings is reversed with respect to the other. In an automatic starter this can be achieved by using two contactors i.e., one for forward direction of rotation and the other for reverse direction of rotation. The connections for control circuit and the power circuit diagrams have been shown in Figure. The circuit does not need any further explanation as it is a simple forward reverse starter scheme.
Figure: Forward/reverse starter for a permanent split capacitor motor
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Engr. Aneel Kumar

STARTER FOR TWO VALUE CAPACITOR TYPE SPLIT PHASE MOTORS

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This type of split phase motors have two capacitors of different values in the auxiliary winding. The motors develop a high starting torque with one capacitor of a higher value and gives a quite running performance with the other capacitor of comparatively lower value. The circuit for forward / reverse operation of a two value capacitor motor has been shown in Figure.
Figure Forward/reverse starter for a two value capacitor motor
In the circuit shown an electrolytic capacitor has been used for starting purpose and an oil filled capacitor has been used for continuous running operation. During starting both the capacitors are in the circuit. When the motor picks up speed the electrolytic capacitor gets disconnected due to opening of a relay contact. The circuit operation is as follows:
When the START-push button is pressed the M contactor is energized which in turn energizes both the main and the auxiliary windings (with both the capacitors in the auxiliary winding circuit) due to closing of contacts M2 and M3. A thermal timer TR is also energized along with the contactor.

This thermal timer consists of a heater coil wound on a bimetallic strip. After a pre-set delay the bimetallic strips bends and closes contact TR1. Closing of contact TR1 causes energization of relay R which then gets hold through its own contact R2. At the same time relay R disconnects the electrolytic capacitor from the auxiliary winding circuit due to opening of its normally closed (NC) contact R3. The thermal timer gets de-energized as soon as relay R is energized due to opening of its contact R1.
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Engr. Aneel Kumar

STARTER FOR A TWO VALUE CAPACITOR MOTOR USING A CURRENT RELAY AND AN AUTO TRANSFORMER

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This type of starter uses a current relay in series with the main winding and an auto-transformer in the auxiliary winding to increase the effective value of the capacitors in the auxiliary winding circuit. The circuit has been shown in Figure.
In this circuit, when contactor M is energized, a high inrush current flows through the main winding which energizes relay R. Its contact R1 closes and contact R2 opens. Due to closing of contact R1, the auto transformer winding comes in circuit and the effective value of capacitor is increased. The current relay is designed to pick up at 3 times the full load current and to drop at twice the full-load value. The motor therefore develops high starting torque during starting due to high effective capacitance. When the motor picks up speed, the current drops and the relay R drops out. Due to opening of contact R1 and closing of contact R2, the auto-transformer gets open circuited. The auxiliary winding then gets connected through the capacitor only. The relay R can pick up again, if load on the motor causes current to rise to its pick up value and thus re-inserts the auto-transformer. Thus, this increased capacitance permits the motor to develop more torque and prevents stalling at heavier loads.
Figure: Starter for a two value capacitor motor using an overcurrent relay and an auto-transformer
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Engr. Aneel Kumar

DYNAMIC BREAKING OF SPLIT PHASE MOTORS

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The principle of dynamic breaking discussed earlier in connection with poly phase induction motors, also applies well to split phase motors. The procedure, as already explained, is to disconnect the stator windings from ac supply while the motor is running and instead, connect dc supply across the windings. Full wave rectifier is used to obtain dc supply from the available ac source. Precaution must be taken to avoid, simultaneous energization of the stator windings from both ac as well as dc supply. This is taken care of by providing electrical and/or mechanical interlocking of the contactors. A rheostat is provided in the dc circuit to control the time required for the motor to stop. Less is the value of rehostatic value of resistance, more quickly will the rotational energy get converted to electrical energy and get dissipated as heat and hence less will be the time required for the motor to stop. The control diagram for dynamic breaking of a split phase motor is shown in Figure.
Figure: Control circuit for dynamic breaking of a split phase induction motor
When the STOP-push button is pressed, contactor M gets de-energized and closes its contact M2. Contactor DB gets energized through the back contact of the STOP-push button, closed contact TR2 and contact M2. Timer TR also gets energized along with contactor DB. The instantaneous contact of relay TR, i.e, TR1 closes and thus both contactor DB and relay TR remain energized. Contactor DB energizes a bridge rectifier which feeds dc current into the stator winding of the motor. Current to be fed is adjustable through a rheostat, R. Due to feeding of dc current the motor slows down and comes to a quick stop. After a pre-set delay which would match with the time taken by the motor to stop, the timer TR operates and opens its delayed contact TR2 and thus de-energizes contactor DB and gets reset. The time allowed to stop the motor, as explained earlier, can be varied by adjusting the rheostat R.
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Engr. Aneel Kumar

PLUG REVERSING OF CAPACITOR START MOTORS

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The capacitor start motors suffer from the disadvantage that they are not easily reversible due to the centrifugal switch connected in the auxiliary winding. The motor cannot be instantaneously reversed by simple control as the auxiliary winding remains disconnected till the motor comes near to zero speed. However, by proper design of the control circuit the motor can be made instantly reversible. This is accomplished by using an electromagnetic relay along with a special two contact centrifugal switch as shown in Figure.

The circuit shown in Figure is for a small hoist using a capacitor start motor. The upper and lower limits of travel are controlled by two limit switches viz. LSU and LSD.
When UP-push button is pressed, contactor U gets energized. Its contacts U1 and U2 energies the main winding. Closing of its contact U3 causes relay R to get energized through the centrifugal switch contacts A—B. After the energization of relay R auxiliary winding gets energized through capacitor C, the centrifugal switch, and the closed contact R2 of relay R.
Figure: Control circuit for a small hoist using a capacitor start
When the motor reaches near normal speed, the centrifugal switch opens and the auxiliary winding gets connected to a high resistance R. Under this condition the auxiliary winding is effectively an open circuit and therefore the developed torque is only by the main winding (recall that this is what is required during normal operation of the motor). When the hoist will reach the upper limit the limit switch LSU will be actuated, U contactor will get de-energized, and relay R also drops out due to opening of contact U3. If, to achieve instant reversal, the DOWN push button is pressed, contactor D would pick-up energizing the main and also the auxiliary winding through normally closed (NC) contact R1 (as relay R will not pick-up because of centrifugal switch being open). The centrifugal switch would close when speed becomes zero, the relay R would then get energized. The auxiliary winding would also get energized through the centrifugal switch contact B. In the downward direction when motor again reaches its final speed the centrifugal switch would open thus connecting the auxiliary winding to the high resistance R.
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Engr. Aneel Kumar

SPEED CONTROL OF SPLIT PHASE MOTORS

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Speed control of split phase motors (without capacitors) is obtained by either pole changing method or by using special winding arrangements. These methods will, however, increase the size of such motors and make them expensive.

To obtain two speed operation of a split phase motor, the motor will have two main windings and two auxiliary windings, each set wound for different number of poles. However centrifugal switch is set to open at the lower of the two speeds. In case of fast speed operation, therefore, the rotor has to accelerate to full speed on the main winding only. No problem would arise on this account if the load on motor is moderate.

Figure shows the control circuit for a two speed motor with two main and two auxiliary windings. The control circuit permits starting of the motor at slow or fast speed depending upon the setting of the selector switch. During running condition, transfer can be made from one speed to the other speed.
Figure: Control circuit for a two speed split phase induction motor having two main and two auxiliary windings
For the selector switch setting for slow speed, as shown in Figure, the motor can be started by pressing the SLOW-push button. A relay CRS first gets energized through contact S3 and then gets hold through its contact CRS1. Contact CRS2 also closes and makes the circuit ready for fast operation. Whenever a changeover is required for fast operation, the FAST-push button is to be pressed which would first release the slow contactor S and then energies the fast contactor F. A relay CRF would then get energized and get hold through its own contact. Contact CRF2 of this relay closes allowing transfer from fast speed to slow speed operation when required. Thus, it follows from the above that the motor can be started for a particular operation, fast or slow, depending upon the setting of the selector switch and then change over to the other operation by pressing the corresponding push button.
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Engr. Aneel Kumar

SPEED CONTROL OF PERMANENT SPLIT CAPACITOR MOTORS

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The speed of a permanently split capacitor motor can be adjusted by connecting it to a variable voltage source such as an auto-transformer. The limitation of this method is that the starting torque developed is very low, especially when the motor is started on low speed. Another limitation is that speed is sensitive to voltage changes on low speed connections. Further speed varies considerably for different loading conditions of the motor.

The motor can be started in three different ways using an auto-transformer as shown in Figure 1. In Figure 1 (a) the voltage across the main winding and the auxiliary winding are varied simultaneously resulting in low starting torque for low speed operation. This shortcoming, however, can be overcome by first starting the motor on high speed and then stepping it down to low speed as shown in Figure 1 (a).
Good starting torque at all speeds can be obtained if the auxiliary winding is connected across the mains supply and voltage adjustment is provided only for the main winding as shown in Figure 1 (b).

In the third method of connections shown in Figure 1 (c), the two windings are connected to a tapped auto transformer so that any voltage change across the main winding is accompanied by an inverse change across the auxiliary winding. This method with a properly designed motor can be applied to an installation whose speed should not be sensitive to variations of normal line voltage.
Figure 1 Voltage control method for speed adjustment of split capacitor motors
In another design of split motor a special winding arrangement is made to eliminate the requirement of an auto transformer. The motor in this case, in addition to the main and the auxiliary windings, is provided with a so called intermediate winding. The intermediate winding so provided is in space phase with the main winding. The main and the intermediate windings occupy the same slots, the later placed directly over the former. They need not have the same number of turns. The wire of the intermediate winding is invariably smaller in size than the main winding. Two different connections for this type of motor design are shown in Figure 2 (a and b).
Figure 2 Speed control of split capacitor motor with intermediate winding
The connection scheme shown in Figure 2 (a) is generally used for 110 volt motors. The main and the intermediate winding are connected in series across the auxiliary winding and the capacitor. The terminal of SPDT (Single Pole Double Throw) switch H and L are wired to the intermediate winding as shown in Figure 2(a).

Connections shown in Figure 2 (b) are used for 230 voltage motors.
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Engr. Aneel Kumar

SPEED CONTROL OF UNIVERSAL MOTOR

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There are various methods of controlling the speed of a universal motor. A wide range of speed control is possible by inserting a rheostat in the line circuit which causes variable voltage to appear across the motor terminals resulting in reduced motor speed. Another method of speed control, not very commonly used is by brush shifting mechanism. The speed of the motor increases when the brushes are moved backward relative to the direction of rotation. However, only a limited range of speed control is possible by this method. This is because when the brushes are moved further from the magnetic neutral, commutation worsens.

Another speed control method makes use of a tapped field winding. Universal motors are always bipolar. The number of turns on the two poles need not always be the same as the air gap flux is created by series combination of mmfs of the two pole windings.
As shown in Figure 1 the field winding having larger number of turns is tapped at three points thus making possible a total of four operating speeds. For a given value of load, minimum speed will be obtained when the entire winding is used (this gives maximum mmf and flux).
Figure: 1 Tapped field winding speed adjustment method for a universal motor
Maximum speed will result when the selector switch is on point H at which minimum flux is obtained. This method offers possibility of tapping the field winding at appropriate points to permit the motor to run at the same speed on direct current and also on alternating current for a particular input. This, however, does not mean that speeds will be the same on ac and dc supplies for some other values of input currents.

Another popular method of speed control is the governor controlled speed adjusting method. In this method of speed adjustment of a series motor, a governor consisting of an assembly of a spring loaded contacts is mounted on the shaft of the motor. The current enters the governor contacts through carbon brush and slip-ring arrangement. When the motor is running, governor contacts open and close very rapidly depending upon the natural resonant frequency of the moving contacts. For a given spring tension setting, the contacts vibrate at a certain rate. Figure 2 shows how a governor is connected to a series motor for non-reversing as well as for two field reversing service.
Figure: 2 Governor controlled speed adjustment method for a series motor
For a given spring tension setting, contacts vibrate at a certain rate. When the speed rises above the particular value set by the spring tension, the contacts remain open for a relatively longer period of time, than the time the contacts take for closing. This keeps a line resistance R in the circuit a little longer and acts to reduce the speed. The reverse is true when speed falls below the adjusted value. Now the closing time of contact is longer than its opening time and hence the resistance remains in circuit for smaller duration of time and as a consequence the motor speed tends to rise.

In this type of speed adjustment the motor does not run at a constant speed but runs in a narrow range above and below the set speed. A wide range of speed adjustment is thus possible by adjusting the spring tension of the contacts. A small capacitor is used across the contacts to prevent excessive arcing.
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Engr. Aneel Kumar

UNIVERSAL SERIES MOTORS

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The construction and principle of working of a universal series motor are similar to a dc series motor. To enable the motor to work satisfactorily on ac supply also, some modifications are required in its construction. The important modification required are:

(i) Field structure should be completely laminated to avoid losses due to eddy currents:

(ii) To combat effects of armature reaction and resulting poor commutation the armature is to be designed to have lower voltage gradient between adjacent commutator segments than in an equivalent dc motor;
(iii) Poor commutation with ac (due to emf induced by the alternating main field flux in a coil undergoing commutation) is improved by using distributed field windings and compensating field windings that are placed in a slotted stator core. When a universal motor is used with ac supply the armature reactance drop exerts a speed lowering effect with increased loading. At the same time at increased loading the effective flux per ac ampere is less than that produced per dc ampere. This condition tends the motor to run faster on ac supply. Out of the above two factors i.e., of armature reactance drop, and effective flux, the factor which pre-dominates determines the speed of the motor on ac supply.
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Engr. Aneel Kumar

TYPES OF DC MOTORS AND THEIR APPLICATIONS

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DC motors are classified into three types depending on the way their field windings are excited. Field winding connections for the three types of dc motors have been shown in Figure. Brief description of different types of dc motors are given as follows:

SHUNT MOTOR

In this type of motor, the field winding is connected in parallel with armature as shown in Figure (a). There are as many number of field coils as there are poles. When connected to supply, constant voltage appears across the field windings (as they are connected in parallel with armature). The field current is therefore constant and is independent of the load current.

Shunt field winding usually are designed to have large number of turns of fine wire. Its resistance, therefore, is high enough to limit the shunt field current to about 1 to 4 percent of the rated motor current.

A shunt motor operates at nearly constant speed over its normal load range. It has a definite stable no-load speed. The motor is adaptable to large speed variations. The disadvantage of the motor is that it has low starting torque and over-load torque capability.

SERIES MOTOR

A series motor receives its excitation from a winding which is connected in series with the armature and carries load current. As the series field has to carry high load current, it is made of a thick wire and a few turns. As the resistance is low, the voltage drop across the series winding is small.

This motor has excellent starting and over-load torque characteristics. The disadvantages are that the motor attains dangerously high speed at no-load. Speed adjustment of the motor is somewhat difficult.

COMPOUND MOTOR

In compound motors excitation results from combined action of both shunt field winding and series field winding. Figure (c) shows the winding connections with the series field of the compound motor carrying the armature current (the long-shunt connection). In the short-shunt connection, which is sometimes used, the shunt field is directly connected in parallel with the armature, in which case, the series field current is the same as the line current. Excitation of a compound motor is a combination of series and shunt excitation. The motor, therefore, has mixed characteristic between that of a series motor and a shunt motor. This motor behaves somewhat better than a shunt motor from the point of view of starting and overload torque; and has definite stable no-load speed like a shunt motor. Speed of this motor is adjustable as easily as that of a shunt motor. It’s speed, however, tends to change as much as 25 percent between full-load and no-load due to the effect of series winding.

A brief description of some special field windings used in modern motors for corrective influence upon the operation of the motor under load is given as follows.

These field windings are called corrective fields. Their purpose is to reduce the effects of armature reaction such as poor commutation, instability at high speeds, and commutator flashover under conditions of suddenly applied overloads. Inter pole windings are most widely used corrective field windings. Inter pole windings are connected permanently in series with the armature circuit. This field maintains the magnetic neutral axis in the same position under all load conditions and thereby permits the motor to commutate well i.e., without sparking at the brushes. Stabilizing field winding is used only in shunt motors that are made to operate at high speeds by shunt-field weakening. This a series field winding placed directly over the shunt winding whose moderate flux tends to prevent run away operation or instability that may result from the demagnetizing effect of armature reaction.

Compensating winding is placed in slots or holes in the main pole faces. This winding is also connected in series with the armature circuit. This winding creates a magnetic field that tends to offset the armature reaction which acts to distort the flux-density distribution under the pole faces. If this flux distortion is left uncorrected, it would increase the probability of flashover between brushes under conditions of suddenly applied overloads.
Figure: Connections of field windings for different types of dc motors
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Engr. Aneel Kumar

SEMI AUTOMATIC AND AUTOMATIC CONTROL OF MODERN MACHINERY

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Control of a machine can be semi-automatic or fully automatic. There are probably more machines operated by semi-automatic control than by manual or fully automatic controls. Consider, for example, an over-head tank which supplies drinking water to a factory.

If we provide a manual switch near the pump motor and depute an operator to switch it ON when water level falls, then this is classified as manual control. Here, the operator has to go to the pump site to fill the tank. For the same pump if a magnetic starter is provided near the pump motor and for its starting, a switch is provided near foreman’s desk it may be classified as a semi-automatic control. A lamp indication or a bell can also be provided near the desk to indicate if the tank is full. The foreman can switch ON the pump from his desk without going to the pump site. Over-flow can also be avoided by switching OFF the pump when the lamp glows or the bell rings. If a float switch is provided in the tank to switch ON the pump motor when water level falls below a certain lower limit, and switch it OFF when water level rises beyond a certain upper limit, then the control becomes fully automatic. The cost of installation of an automatic control system will be higher than the other two types of controls. However, an automatic control arrangement relieves the operator from the task of keeping an eye on the water level and operate the pump. Also there is no danger of over-flow from the tank. Thus it is seen that the basic difference in manual, semi-automatic and fully automatic control lies in the flexibility it provides to the system being controlled.
The study of control circuits involves study of the construction and principle of operation of various control components and learning the art of designing control circuits for various functions of machines. In this text, we have first discussed the various control components and then control schemes for ac and dc motors.

Modern machines have large number of operations requiring extensive control circuits consisting of large number of relays. Thus the control panel occupies a lot of space and control circuit design also becomes tedious.

Static control is used for such machines as the control design is easy with static control devices. The static devices used for design of control circuits are the digital logic gates. With much advancement in the field of computers this static control is also becoming obsolete as more and more machines are now being controlled by programmable controllers. Inspite of all these developments as far as single motor control or a machine having few operations is concerned, the magnetic control using contactor and relay will continue to be in use because it is the simplest and cheapest method of control for such applications.
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Engr. Aneel Kumar

ADVANTAGES OF MAGNETIC CONTROL OF MACHINES

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Having discussed the starting and stopping of a motor by using control devices like push buttons, contactors and over-load relays, we are in a position to discuss the advantages of magnetic control over the manual control. The various advantages are listed as follows:

1) Magnetic control permits installation of power contacts close to motor whereas the actuating control device i.e., a push button switch could be located away from the motor in a position most convenient to the operator.

2) Magnetic control provides safety to the operator as remote operation described above minimizes the danger to the operator of coming into accidental contact with live parts or being exposed to power arc and flashes at the main contacts.
3) The most important advantage of magnetic control is the elimination of dependence on operators’ skill for control of motor performance. Current and torque peaks could be limited thus resulting in less wear and less maintenance.

4) Magnetic control also makes interlocking (to be discussed later) between various operations of a multi motor drive easy. The various operations can be performed in the desired sequence automatically.

5) With the demand for more production in industry, it became necessary to atomize the machinery to meet the challenge. Today in our industrial plants most of the machines are automatic. Once the machine is started most of the operations are carried out automatically.
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Engr. Aneel Kumar

DISADVANTAGES OF MANUAL CONTROL OF MACHINES

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When electric motors were first introduced, simple manual switches were used to start and stop the motor. The only protective device used was the fuse. Progress was subsequently made along the lines of improving the reliability, flexibility and make-break performance of the manual switches. In those days one large motor was used to drive a line shaft through a belt pulley arrangement. Individual machines were then connected to the line shaft through belt and pulley arrangements. This system of driving a number of individual loads from a common line shaft and manual switching of motors had many disadvantages as listed below:
1) Starting, stopping and speed control of motor had to be performed by hand every time.

2) The operator had to move a manual switching device from one position to another.

3) Switching of large motors required great physical effort.

4) Operator had to remain continuously alert to watch indicators so as to adjust motor performance according to drive requirements.

5) Sequence operations of number of motors could not be accomplished in common line shaft arrangement.

6) The varied needs of individual machines like frequent starts and stops, periodic reversal of direction of rotation, high-starting torque requirement, constant speed, variable speed, etc., could not be accomplished in common line shaft arrangement.
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Wednesday, January 21, 2015

Engr. Aneel Kumar

SYNCHRONOUS MOTOR STARTING BY USING DAMPER (AMORTISSEUR) WINDING

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Most of the large synchronous motors are provided with damper windings, in order to nullify the oscillations of the rotor whenever the synchronous machine is subjected to a periodically varying load. Damper windings are special bars laid into slots cut in the pole face of a synchronous machine and then shorted out on each end by a large shorting ring, similar to the squirrel cage rotor bars.
When the stator of such a synchronous machine is connected to the 3-Phase AC supply, the machine starts as a 3-Phase induction machine due to the presence of the damper bars, just like a squirrel cage induction motor. Just as in the case of a 3-Phase squirrel cage induction motor, the applied voltage must be suitably reduced so as to limit the starting current to the safe rated value. Once the motor picks up to a speed near about its synchronous speed, the DC supply to its field winding is connected and the synchronous motor pulls into step i.e. it continues to operate as a Synchronous motor running at its synchronous speed.
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Engr. Aneel Kumar

SYNCHRONOUS MOTOR STARTING WITH AN EXTERNAL MOTOR

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The second method of starting a synchronous motor is to attach an external starting motor (pony motor) to it and bring the synchronous machine to near about its rated speed (but not exactly equal to it, as the synchronization process may fail to indicate the point of closure of the main switch connecting the synchronous machine to the supply system) with the pony motor. Then the output of the synchronous machine can be synchronized or paralleled with its power supply system as a generator, and the pony motor can be detached from the shaft of the machine or the supply to the pony motor can be disconnected. Once the pony motor is turned OFF, the shaft of the machine slows down, the speed of the rotor magnetic field BR falls behind Bnet, momentarily and the synchronous machine continues to operate as a motor. As soon as it begins to operates as a motor the synchronous motor can be loaded in the usual manner just like any motor.
This whole procedure is not as cumbersome as it sounds, since many synchronous motors are parts of motor-generator sets, and the synchronous machine in the motor-generator set may be started with the other machine serving as the starting motor. Moreover, the starting motor is required to overcome only the mechanical inertia of the synchronous machine without any mechanical load (load is attached only after the synchronous machine is paralleled to the power supply system). Since only the motor’s inertia must be overcome, the starting motor can have a much smaller rating than the synchronous motor it is going to start. Generally most of the large synchronous motors have brushless excitation systems mounted on their shafts. It is then possible to use these exciters as the starting motors. For many medium-size to large synchronous motors, an external starting motor or starting by using the exciter may be the only possible solution, because the power systems they are tied to may not be able to handle the starting currents needed to use the damper (amortisseur) winding approach.
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Engr. Aneel Kumar

SYNCHRONOUS MOTOR STARTING BY REDUCING THE SUPPLY FREQUENCY

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If the rotating magnetic field of the stator in a synchronous motor rotates at a low enough speed, there will be no problem for the rotor to accelerate and to lock in with the stator’s magnetic field. The speed of the stator magnetic field can then be increased to its rated operating speed by gradually increasing the supply frequency f up to its normal 50- or 60-Hz value.

This approach to starting of synchronous motors makes a lot of sense, but there is a big problem: Where from can we get the variable frequency supply? The usual power supply systems generally regulate the frequency to be 50 or 60 Hz as the case may be. However, variable-frequency voltage source can be obtained from a dedicated generator only in the olden days and such a situation was obviously impractical except for very unusual or special drive applications.
But the present day solid state power converters offer an easy solution to this. We now have the rectifier- inverter and cyclo-converters, which can be used to convert a constant frequency AC supply to a variable frequency AC supply. With the development of such modern solid-state variable-frequency drive packages, it is thus possible to continuously control the frequency of the supply connected to the synchronous motor all the way from a fraction of a hertz up to and even above the normal rated frequency. If such a variable-frequency drive unit is included in a motor-control circuit to achieve speed control, then starting the synchronous motor is very easy-simply adjust the frequency to a very low value for starting, and then raise it up to the desired operating frequency for normal running.

When a synchronous motor is operated at a speed lower than the rated speed, its internal generated voltage (usually called the counter EMF) EA = Kϕω will be smaller than normal.

As such the terminal voltage applied to the motor must be reduced proportionally with the frequency in order to keep the stator current within the rated value. Generally, the voltage in any variable-frequency power supply varies roughly linearly with the output frequency.
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Engr. Aneel Kumar

METHODS OF STARTING SYNCHRONOUS MOTOR

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Basically there are three methods that are used to start a synchronous motor:

1) To reduce the speed of the rotating magnetic field of the stator to a low enough value that the rotor can easily accelerate and lock in with it during one half-cycle of the rotating magnetic field’s rotation. This is done by reducing the frequency of the applied electric power. This method is usually followed in the case of inverter-fed synchronous motor operating under variable speed drive applications.

2) To use an external prime mover to accelerate the rotor of synchronous motor near to its synchronous speed and then supply the rotor as well as stator. Of course care should be taken to ensure that the direction of rotation of the rotor as well as that of the rotating magnetic field of the stator are the same. This method is usually followed in the laboratory- the synchronous machine is started as a generator and is then connected to the supply mains by following the synchronization or paralleling procedure. Then the power supply to the prime mover is disconnected so that the synchronous machine will continue to operate as a motor.
3) To use damper windings or amortisseurs windings if these are provided in the ma- chine. The damper windings or amortisseurs windings are provided in most of the large synchronous motors in order to nullify the oscillations of the rotor whenever the synchronous machine is subjected to a periodically varying load.
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Engr. Aneel Kumar

PROPERTIES OF TRANSFORMER OIL

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Even though the basic functions of the oil used in transformers are (a) heat conduction and (b) electrical insulation, there are many other properties which make a particular oil eminently suitable. Organic oils of vegetative or animal origin are good insulators but tend to decompose giving rise to acidic by-products which attack the paper or cloth insulation around the conductors.

Mineral oils are suitable from the point of electrical properties but tend to form sludge. The properties that are required to be looked into before selecting an oil for transformer application are as follows:

INSULTING PROPERTY: This is a very important property. However most of the oils naturally fulfill this. Therefore deterioration in insulating property due to moisture or contamination may be more relevant.

VISCOSITY: It is important as it determines the rate of flow of the fluid. Highly viscous fluids need much bigger clearances for adequate heat removal.

PURITY: The oil must not contain impurities which are corrosive. Sulphur or its compounds as impurities cause formation of sludge and also attack metal parts.

SLUDGE FORMATION: Thickening of oil into a semisolid form is called a sludge. Sludge formation properties have to be considered while choosing the oil as the oil slowly forms semi-solid hydrocarbons. These impede flows and due to the acidic nature, corrode metal parts. Heat in the presence of oxygen is seen to accelerate sludge formation. If the hot oil is prevented from coming into contact with atmospheric air sludge formation can be greatly reduced.

ACIDITY: Oxidized oil normally produces CO2 and acids. The cellulose which is in the paper insulation contains good amount of moisture. These form corrosive vapors. A good breather can reduce the problems due to the formation of acids.

FLASH POINT AND FIRE POINT: Flash point of an oil is the temperature at which the oil ignites spontaneously. This must be as high as possible (not less than 160◦C from the point of safety). Fire point is the temperature at which the oil flashes and continuously burns. This must be very high for the chosen oil (not less than 200◦C).

Inhibited oils and synthetic oils are therefore used in the transformers. Inhibited oils contain additives which slow down the deterioration of properties under heat and moisture and hence the degradation of oil. Synthetic transformer oil like chlorinated di-phenyl has excellent properties like chemical stability, non-oxidizing, good dielectric strength, moisture repellent, reduced risk due fire and explosion.

It is therefore necessary to check the quality of the oil periodically and take corrective steps to avoid major break downs in the transformer.

There are several other structural and insulating parts in a large transformer. These are considered to be outside the scope here.
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