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.
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Figure: Connections of field windings for different types of
dc motors
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