DC Compound Motor

DC Compound Motor

A DC motor with both types of windings, i.e. shunt field winding and series field winding, is known as a DC compound motor or compound DC motor.

DC Shunt Motor

Both field windings are wound on each pole. The flux is partially provided by the shunt field and partially provided by series field winding.

DC Series Motor

The DC compound motor is further classified into two types;

  • Long shunt compound motor
  • Short shunt compound motor

The shunt field winding gives an advantage of constant speed, and series field winding gives an advantage of high torque when a motor is started under a heavy load. The compound motor has both windings and achieves both benefits.

Long Shunt Compound DC Motor

When the shunt field winding is connected in parallel with a series combination of armature and series field winding, this motor is known as a long shunt compound motor.

The connection diagram of the long shunt compound motor is as shown in the figure below.

"Long

The mechanical load is connected with the shaft of the motor.

Armature resistance: Ra
Series field resistance: Rs
Shunt field resistance: Rsh

Voltage and Current Equations

The voltage and current equation for the long shunt compound motor is as derived below.

The total current drawn from the DC supply is I.

    \[ I = I_{sh} + I_s \]

The armature winding and series field windings are connected in series. Therefore, the same current will flow through both windings.

    \[ I_s = I_a \]

    \[ I = I_{sh} + I_a \]

Shunt field current;

    \[ I_{sh} = \frac{V}{R_{sh}} \]

Supply voltage;

    \[ V = E_b + I_a R_a + I_a R_s + V_{brush} \]

    \[ V = E_b + I_a (R_a + R_s) + V_{brush} \]

Short Shunt Compound DC Shunt Motor

When the shunt field winding is connected only with the armature winding, the DC motor is known as the short shunt compound DC motor.

The connection diagram of this motor is as shown in the figure below.

"Short

Voltage and Current Equation

The total current is passes through the shunt field winding.

    \[ I = I_s \]

From the connection diagram,

    \[ I = I_s = I_a + I_{sh} \]

Supply voltage,

    \[ V = E_b + I_a R_a + I_s R_s + V_{brush} \]

    \[ V = E_b + I_a R_a + I R_s + V_{brush} \]

Shunt field current,

    \[ I_{sh} = \frac{V_{sh}}{R_{sh}} \]

    \[ I_{sh} = \frac{V-I R_s}{R_{sh}} \]

From above equation,

    \[ I_{sh} = \frac{E_b + I_a R_a + V_{brush} }{R_{sh}} \]

The long shunt and short shunt compound motors are further classified into two types;

  • Cumulative compound
  • Differential compound

Cumulative Compound

If series and shunt windings are connected in such a way that the fluxes produced by them always add or assist each other, then this type of compound motor is known as cumulative compound DC motor.

Differential Compound

If series and shunt windings are connected in such a way that the fluxes produced by them always oppose each other and try to cancel each other, then this type of compound motor is known as differential compound DC motor.

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