A DC Compound Motor is a type of DC motor that combines the features of both shunt motor and series motor. It has both series field winding (connected in series with the armature) and shunt field winding (connected in parallel with the armature). Because of this combination, the DC compound motor exhibits characteristics that lie in between shunt and series motors.
Types of DC Compound Motors
- Cumulatively Compounded Motor: In this type, the series field winding assists the shunt field winding. Hence, the flux is strengthened, providing high starting torque.
- Differentially Compounded Motor: In this type, the series field opposes the shunt field winding. The flux is weakened, and the motor becomes unstable, so it is rarely used in practice.
Characteristics of DC Compound Motor
1. Torque vs. Armature Current (T-I Characteristic)
The torque-current characteristic of a cumulatively compounded motor lies between that of a shunt and a series motor. It provides better starting torque than a shunt motor but less than a pure series motor.
- High starting torque due to series field winding.
- Torque increases nearly linearly with armature current at low currents.
- At high currents, the torque saturates because of magnetic saturation.
2. Speed vs. Armature Current (N-I Characteristic)
The speed-current characteristic also lies between shunt and series motors:
- At no-load, the motor runs at a speed close to that of a shunt motor.
- As load increases, speed drops slightly because of the effect of series winding.
- Unlike a series motor, it does not dangerously increase in speed at no load, which makes it safer to operate.
3. Torque vs. Speed (T-N Characteristic)
The torque-speed characteristic of a DC compound motor is a compromise between the shunt and series motors:
- At high torque, the motor runs at lower speed, similar to a series motor.
- At moderate loads, it behaves almost like a shunt motor with nearly constant speed.
- This combination makes the motor suitable for applications requiring both high starting torque and constant speed.
Applications of DC Compound Motor
- Cranes and Hoists: Because of high starting torque.
- Elevators: Where load handling and constant speed are important.
- Rolling Mills: Heavy starting load with speed regulation.
- Press Machines: Requires sudden torque demand.
- Conveyors: Where variable load is applied.
Advantages of DC Compound Motor
- Combines the good features of both series and shunt motors.
- Provides better speed regulation than a series motor.
- Higher starting torque compared to a shunt motor.
- Safe from “runaway” condition at no-load.
Disadvantages of DC Compound Motor
- More expensive than shunt or series motor due to extra windings.
- Efficiency is slightly reduced due to copper losses in both windings.
- Differentially compounded motors are unstable and rarely used.
Conclusion
DC Compound Motors offer the perfect balance of starting torque and speed regulation. With characteristics that lie between shunt and series motors, they are widely used in industrial applications like elevators, cranes, and rolling mills. Their ability to provide high torque at starting and maintain nearly constant speed under varying load makes them one of the most versatile types of DC motors.
Important Point
A compound motor has both series field and shunt field. The shunt field is always stronger than the series field.
Compound motors are of two types:
- Cumulative-compound motors in which series field aids the shunt field.
- Differential-compound motors in which series field opposes the shunt field.
FAQs on DC Compound Motor Characteristics
- What are the types of DC Compound Motors?
There are two types: Cumulatively Compounded and Differentially Compounded. - Which type of compound motor is used in industries?
Cumulatively compounded motors are widely used, while differential compound motors are rarely used. - Why is DC compound motor preferred?
Because it provides both high starting torque and good speed regulation. - What is the difference between cumulative and differential compound motor?
In cumulative type, the series field aids the shunt field, while in differential type, it opposes the shunt field.