Typical compound motors designed for industrial application obtain approximately 50% of their mmf from the series field wen operating at rated load.
There are two types of compound motors connection,
If the connection to the series and shunt winding is in such a way that their respective mmfs are additive is called cumulative compound motor.
If the series field is reversed with respect to the shunt field, its mmf will subtract from the shunt field mmf, causing the net flux to decrease with increasing load, resulting in excessive speed, which is differential compound motor.
Stabilized - shunt motor
Compound motors, whose series field are designed to provide just enough mmf to nullify the equivalent demagnetizing mmf of armature reaction and provide a very slight speed droop, are called stabilized – shunt motors. The series field winding of such machines generally have one – half to one and half turns / pole and depending on the application, provide approximately 3 to 10 percent of the total field mmf at rated load. The speed of stabilized – shunt motors is fairly constant, with only a slight droop in speed with increasing load. Stabilized – shunt motors are used in applications that require a fairly constant speed and a moderate starting torque.
Reversing the direction of rotation of compound or stabilized – shunt motors is accomplished by reversing the armature branch or reversing both the series field and the shunt field.