DC Compound motor

Everything You Need to Know About DC Compound Motor

Definition

Compound DC motor is a compound of both series and shunt motor. It is consist of a Series field coil, which is connected in series with the armature field winding, and a shunt field coil winding, which is connected in parallel with the armature field winding. Presence of both the field windings i.e. series field winding and shunt field winding gave this motor the name of compound DC motor.

Types of DC Compound Motor

There are two major types of compound DC motors depending upon the connections of shunt field winding with the armature winding and series winding. Those two types are:

  1. Long shunt
  2. Short shunt

In the case of long shunt compound DC motor, the shunt field winding is connected in parallel with both armature coil winding and the series field winding. On the other hand, in the case of short shunt compound DC motor, a shunt resistor is only in parallel with the armature field coil. Figure- 2 represents the schematic comparison of both types.

One more classification of DC compound motors can be made depending upon the connection polarities of armature coil, series field coil, and shunt coil.

DC Compound Motor

Figure- 2

Therefore, a compound DC motor can be further subdivided into two types, known as:

  1. Cumulative Compound Motor
  2. Differential Compound Motor

In cumulative compound motor, the flux produced by shunt coil winding is in line with the flux produced by the series field winding. Therefore, both the fluxes add up to provide more flux. In contrast to the cumulative compound motor, in the case of a differential compound motor, the flux produced in shunt coil is in opposite direction to the flux produced in series coil winding, resulting in total flux equal to the difference of both the fluxes.

Characteristics

As DC compound motor is a compound of both series and shunt DC motors. Therefore, it utilizes positives from both types of motors resulting in a motor suitable for all kinds of applications. Shunt DC motor has the property of good speed regulation while the series DC motor has a high starting torque. Compound DC motor combine both these properties to provide good speed regulation for different loads with high starting torque suitable for different application.

In the case of a cumulative compound motor, both shunt and series fields will act simultaneously and assist each other. This results in more flux for any given current. More flux will mean more torque and reduced speed as compared to the shunt DC motor.

A compound motor can also be run at no load, and it will not be destroyed due to very high speed as the case present in the series motor. The compound motor has a specific no-load speed.

On the other hand, in the case of a differential compound motor, both fields oppose each other, resulting in reduced flux for any given value of current fewer flux results in less torque and more speed as compared to shunt DC motor. Differential type compound DC motor is never practically used, as it is unstable for all possible applications. Its speed increases with the increase in the load, leading to unitability.

Speed control Methods

Speed of compound DC motor can be controlled by changing the applied voltage, resistance of armature, or by changing the flux. Depending on these options, speed control can be classified into two categories named:

  1. Armature control method
  2. Field control method

The speed of the motor is directly dependent on the applied voltage across the armature points. By decreasing the voltage, the speed of the motor can be reduced or by increasing the voltage, speed will also increase accordingly.

Another type of armature control-based method of speed control is the use of rheostat in series with the armature winding. The addition of this extra resistance in series will reduce the current flow and some parts of the voltage applied across armature will drop across the resistance, resulting in less voltage across the armature. Change in rheostat resistance will directly change the voltage, resulting in a direct effect on the speed of the DC compound motor.

In this method of using a rheostat for speed control, a lot of power is wasted due to the extra voltage drop across the resistor. Therefore, this method is not much used and discouraged due to the non-economical solution of speed control.

The field control method is based on controlling the flux produced by field winding. Speed of motor is inversely proportional to the field flux, Hence reduction in field winding flux will result in an increase of speed of the motor. The flux of field winding can be reduced by placing a rheostat in series with the field winding. This rheostat will limit the current flow through the field winding resulting in less flux, which will directly affect the speed of the motor. Therefore, by varying the field winding flux, the speed of the compound motor can easily be controlled.

Advantages and Disadvantages of DC Compound Motor

Advantages

The compound motor is compound of series and shunt motor resulting in a motor capable of taking positives out of both types of motors. For example, a shunt motor operates at a constant speed, so it is not suitable for the applications with variable speed requirements. However, the compound motor has very good speed control due to the presence of a series field.  Similarly, the series motor provides good speed control but cannot be used for the applications where the load is varied and initially set to zero. As a series DC motor can destroy itself due to high speed at no load. On the other hand, a compound DC motor can be used even at no load. It will run at some speed within the limits. Therefore, compound DC motor can be used for almost all the applications where both series and shunt motor combined can be used. In some applications, the series motor will fail but shunt will be useful and in some cases, the shunt will fail but series will be able to perform that task. However, the compound motor will be able to perform properly for both the applications.

Disadvantages

Compound DC motor has many advantages over other motors. However, just like all other motors, the compound motor has few disadvantages, which are been discussed below.

  • Just like other DC motors, a compound DC motor also requires frequent maintenance due to the aging of parts, resulting in high maintenance costs.
  • They are not suitable for the hazardous environment due to spark occurring at carbon brushes.
  • Compared to AC motors, the lifetime of compound DC motor is less due to presence of components expiring with age like brushes.
  • In differential compound motor, the flux of series field and shunt field cancels each other resulting in lower flux and hence low torque. Low torque results in a high speed, which further reduces the torque, and the motor becomes unstable even with a slight change in load.
  • The differential compound motor cannot be used for any practical application due to its unstable behavior with even a slight variation of the load.

Applications of DC compound motor

Due to the ability of the compound motor to provide high starting torque and also speed regulation and control it has many applications involving different areas. Few of the applications involving the use of a compound DC motor are:

  • In Elevators due to the ability of the compound motor to perform better on heavy load changes.
  • In shears and punches to provide high starting torque and better speed control for pressure variations.
  • In steel rolling mills due to the requirement of high starting torque and heavy-duty load.
  • In heavy planers due to the very high fluctuation of the load.
  • In printing press machines and in cutting machines due to the capacity of driving heavy loads.
  • In stamping presses to provide high starting torque.
  • In mixers to provide both the high starting torque and speed control.
  • In different machine tools to manage high load variations and starting torque requirements.
  • In agitator machines to provide speed control.

So in short compound DC motor has many applications in industry and business machines due to its ability to handle high load variations and its capability to provide control over its speed. Many applications are revolving around the speed requirements or load carrying capabilities, and compound DC motor is suitable for both kinds of applications.

Limitation

Compound DC motor is a compound of both series and shunt motor. Therefore, it provides both the properties of series and shunt motor. However, it is a compromise between both the series and shunt motor.

Its motor has a good starting torque as compared to series DC motor due to the presence of the shunt field winding but has slightly poor starting torque as compared to shunt DC motor due to internal losses.

Similarly, compound DC motor can operate at a different speed and also provides better speed regulation as compared to shunt DC motor. However, not as good as series DC motor again due to internal losses and distributions between series and shunt field windings.

In short, DC compound motor has combined properties of both series and shunt motors in one motor at the cost of little compromise of the performance. Therefore, it cannot surpass the good properties of shunt and series DC motors.

Summary

Compound DC motor is a compound of both shunt DC motor and the series DC motor. It takes the benefits of both the motors and combines them. Compound DC motor provides high starting torque and also provides good speed regulation.

However, it is a compromise between both types of motors, which means it has good starting torque as compared to series DC motor but slightly lesser than shunt DC motor. Similarly, it has good speed regulation as compared to shunt but not as good as series DC motor due to different losses and distributions.

It has different types depending upon the connection of and polarity of series field winding and shunt field winding. In cumulative compound DC type, total flux is the addition of series field flux and shunt field flux. More flux results in a more starting torque.

Due to these properties of high starting torque and speed regulation, compound DC motor is been commonly used in many heavy-duty applications like an elevator, heavy planers, printing press and cutting machine.

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