Compound motor characteristics basically depends on the fact whether the motor is cumulatively compound or differential compound. All the characteristics of the compound motor are the combination of the shunt and series characteristic.
Cumulative compound motor is capable of developing large amount of torque at low speeds just like series motor. However it is not having a disadvantages of series motor even at light or no load. The shunt field winding produces the definite flux and series flux helps the shunt field flux to increase the total flux level.
So cumulative compound motor can run at reasonable speed and will not run with dangerously high speed like series motor, on light or no load condition.
In differential compound motor, as two fluxes oppose each other, the resultant flux decreases as load increases, thus the machine runs at a higher speed with increase in the load. This property is dangerous as on full load, the motor may try to run with dangerously high speed. So differential compound motor is generally not used in practice.
The various characteristics of both the types of compound motors cumulative and the differential are shown in the Fig. 1(a), (b) and (c).
|Fig. 1 characteristics of d.c. compound motor
The exact shape of these characteristics depends on the relative contribution of series and shunt field windings. If the shunt field winding is more dominant then the characteristics take the shape of the shunt motor characteristics. While if the series field winding is more dominant then the characteristics take the shape of the series characteristics.