5 . 1 . 3 Compound Motor
5 . 1 . 3 Compound Motor
5 . 1 . 3 Compound Motor In compound motors excitation results from combined action of both shunt field winding and series field winding. Fig. 5 . 1(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 . Interpole windings are most widely used corrective field windings . Interpole 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 . Stabilising 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 demagnetising 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 .