ENGINEERING ARTICLES

Solid-state Controls systems differ from off-line designs in the following ways: 1. Solid-state Controls offers an on-line, double conversion UPS system. Therefore, the client’s critical load is being powered continuously from the inverter. The battery is always being floated by the fully-rated rectifier/charger and is always connected to the inverter input. As result, all components used in the Solid-state Controls design are fully rated to carry 120% of the load on a continuous basis over a 20-year life. 2. Solid-state Controls’ Ferro resonant design has a built in capacity to supply nonlinear (crest factor) loads of up to 3:1 without oversizing. 3. Solid-state Controls’ on-line UPS design is provided with a make-before-break static switch to aid in downstream fault clearing and to protect against possible system failures. 4. Solid-state Controls’ on-line UPS system is a double conversion type, converts AC to DC and then reconverts DC back to AC. It is also insensit...

The following is an outline of some of the major problems associated with off-line (stand by) UPS designs. INPUT FREQUENCY/ VOLTAGE PASS THROUGH The off-line unit is designed to pass through the input line voltage and frequency to the load. (Note: the power passed through is non-conditioned utility power.) While this may be fine for office environments, it is not acceptable for industrial settings with periodic voltage and frequency deviations. Due to the design of off-line systems, some of the deviations will be passed directly to the loads, causing loads to drop and/or loss of data. Off-line UPS suppliers could tighten input parameters so these levels of voltage and frequency are not passed through. However, this would require the systems’ batteries to assume the load more frequently. BATTERY PICKUP If the input voltage and frequency deviate outside of acceptable limits, the systems’ batteries will automatically assume the supply of the charger/inverter. While this m...

The D-STATCOM is basically one of the custom power devices. It is nothing but a STATCOM but used at the Distribution level. The D-STATCOM is a voltage or current source inverter based custom power device connected in shunt with the power system. It is connected near the load at the distribution systems. The key component of the D-STATCOM is a power VSC that is based on high power electronics technologies. Basically, the D-STATCOM system is comprised of three main parts: a VSC, a set of coupling reactors and a controller. The basic principle of a D-STATCOM installed in a power system is the generation of a controllable ac voltage source by a voltage source converter (VSC) connected to a dc capacitor (energy storage device). The ac voltage source, in general, appears behind a transformer leakage reactance. The active and reactive power transfer between the power system and the D-STATCOM is caused by the voltage difference across this reactance. The D-STATCOM is connected in shunt w...

Principle: An electrical generator is a machine which converts mechanical energy into electrical energy. The energy conversion is based on the principle of the production of dynamically induced emf, where a conductor cuts magnetic flux, dynamically induced emf is produced in it according to Faraday’s Laws of electromagnetic Induction. This emf causes a current to flow if the conductor circuit is closed. Hence, two basic essential parts of an electrical generator are (i) a magnetic field and (ii) a conductor or conductors which can so move as to cut the flux. The following figure shows a single-turn rectangular copper coil rotating about its own axis in a magnetic field provided by either permanent magnets or electromagnets. The two ends of the coil are joined to two slip-rings ‘a’ and ‘b’ which are insulated from each other and from the central shaft. Two collecting brushes (of carbon or copper) press against the slip-rings. Their function is to collect the current induced in the coi...

(1) By using a starting motor. This motor is directly coupled to the motor. It may be an induction motor which can run on a synchronous speed closer to the synchronous speed of the main motor. (2) Starting as an induction motor. This is the most usual method in which the motor is provided with a special damper winding on rotor poles. The stator is switched on to supply either directly or by star delta/reduced voltage starting. When the rotor reaches more than 95% of the synchronous speed, the dc circuit breaker for field excitation is switched on and the field current is gradually increased. The rotor pulls into synchronism (A) Pull-in torque. It is the maximum constant load torque under which the motor will pull into synchronism at the rated rotor supply voltage and rated frequency, when the rated field current is applied (B) Nominal pull in torque. It is the value of pull in torque at 95 percent of, the synchronous speed with the rated voltage and frequency applied to th...

1. Shunt generators with field regulators are used for ordinary lighting and power supply purposes. They are also used for charging batteries because their terminal voltages are almost constant or can be kept constant. 2. Series generators are not used for power supply because of their rising characteristics. However, their rising characteristic makes them suitable for being used as boosters in certain types of distribution systems particularly in railway service. 3. Compound generators: The cumulatively-compound generator is the most widely used dc generator because its external characteristic can be adjusted for compensating the voltage drop in the line resistance. Hence, such generators are used for motor driving which require dc supply at constant voltage, for lamp loads and for heavy power service such as electric railways. The differential-compound generator has an external characteristic similar to that of a shunt generator but with large demagnetization armature react...

Following are the three most important characteristics or curves of a dc generator: 1. No-load saturation Characteristic (E 0 /I f ): It is also known as Magnetic Characteristic or Open-circuit Characteristic (O.C.C.). It shows the relation between the no-load generated MMF in armature, E 0  and the field or exciting current I f  at a given fixed speed. It is just the magnetization curve for the material of the electromagnets. Its shape is practically the same for all generators whether separately-excited or self-excited. 2. Internal or Total Characteristic (E/I a ): It gives the relation between the MMF E actually induces in the armature (after allowing for the demagnetizing effect of armature reaction) and the armature current I a . This characteristic is of interest mainly to the designer. 3. External Characteristic (V/I): It is also referred to as performance characteristic or sometimes voltage-regulating curve. It gives relatio...