The Compound Motor

Author: E.E. Kimberly

The compound motor is simply a shunt motor with a series winding added to the main poles. This series winding is similar to that in the compound generator. When the load current flowing through the series winding produces an mmf having the same direction as the mmf of the shunt-field winding, the motor is called a cumulative compound motor. When the two windings tend to produce fluxes in opposite directions, the motor is called a differential compound motor.

When a cumulative compound motor is loaded, its main field is stronger than when the motor is not loaded because of the added excitation of the series winding. Equation (11-3) applies to the compound motor as well as to the shunt motor. However, because of its greater field strength, the compound motor does not need so much speed to generate the counter emf Eg as does the shunt motor, and it therefore runs at a somewhat lower speed and has greater speed regulation when all other conditions are the same.

The cumulative compound motor has somewhat higher starting torque than a shunt motor of the same rating. In some applications, such as punch-press service, shears, and other apparatus having large fly-wheels, it is essential that the motor speed decrease more than could be accomplished with a shunt motor when the heavy part of the load occurs, in order that part of the fly-wheel energy may be delivered up to smooth out the fluctuations of power from the power line. The cumulative compound motor is well adapted to applications of this kind. A typical compound-motor characteristic is shown in Fig. 11-7. Curve (a) is for a shunt motor, and curve (6) is for the same motor after a series winding has been added.

In the differential compound motor, the series-field mmf is opposite in direction to the shunt-field mmf, and so the resultant main flux is less than it would be if the series field were not used. Inasmuch as the field is weakened by increase in load, the speed rises with increase in load. This is an unstable condition at loads greater than about one-third of the rated load (the limit depends on the relative strengths of the shunt-field and series-field mmf s), and the motor may reach an unsafe uncontrollable speed or "run away" very quickly. If the series field is exceptionally strong, the motor may even stop and reverse its direction before running away. There are extremely few practical applications for the differential compound motor.

When the connections of a compound motor must be made the first time without the benefit of a connection diagram, a simple test should be made to assure proper connection of the series field. With the shunt field fully excited, the motor should be starteD₁ slowly. If the motor over-speeds or otherwise performs erratically, it should be stopped and the series-field connections should be reversed. Even if the motor reaches full speed uneventfully, it cannot yet be assumed tnat tne series-field connections are correct. A slight load should be applied. If the speed decreases, the series-field connections are correct. If the speed increases when the motor is loaded, the series field is differentially connected and must be reversed.

Fig. 11-7. Shunt-Motor and Compound-Motor Characteristics

Another method, which is possibly more convenient, consists of starting the motor slowly and noting the direction of rotation; and then again starting it with the shunt field opened. If the direction of rotation is the same in both trial starts, then the compounding is cumulative.