DC Shunt Motor: Connection Diagram, Torque and Speed Equation

DC Shunt Motor

There are two types of windings in DC machine;

  • Armature Winding
  • Field Winding

According to the connection of armature and field winding, the DC motors are classified into three types;

In this article, we will discuss the DC shunt motor.

Connection Diagram of DC Shunt Motor

In the DC shunt motor, the armature winding and field winding is connected in parallel. The connection diagram of the DC shunt motor is as shown in the figure below.

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DC power supply is connected between these two windings.

The resistance of shunt field winding (Rsh) is always higher than that of armature winding (Ra). Because the number of turns of field winding is greater than the number of turns in the armature winding.

In the field winding, the cross-section area of the wire is smaller than the armature winding.

Voltage and Current Relationship

The current passes through the armature winding is Ia and current passes through the shunt winding is Ish.

The total supplied current from the DC source is I. And this current is divided in to both windings.

As shown in the connection diagram;

    \[ I = I_a + I_{sh} \]

Shunt field current;

    \[ I_{sh} = \frac{V}{R_{sh}} \]

Where,

V = Supplied DC voltage

Rsh = Shunt field resistance

    \[ V = I_{sh} R_{sh} \]

The supply voltage is given by;

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

If we neglect the bush drop, the supply voltage is given by;

    \[ V = E_b + I_a R_a \]

The field current depends on the supplied voltage V and shunt field resistance Rsh. Both quantities are constant. Hence, the shunt field current is also constant.

The flux produced in the motor depends on the field current. In the shunt motor, the field current is constant. Therefore, the flux produced in the motor is also constant. So, this motor is also known as a constant flux motor.

    \[ \phi \propto I_{sh} \]

Torque Equation of DC Shunt Motor

Torque produced in the DC motor is proportional to the current passing through the armature winding and flux.

Torque Equation of DC Motor

    \[ T \propto I_a \phi \]

For the DC shunt motor, the field current is constant. And hence, the flux is also constant.

So, the torque produced in the shunt motor is proportional to the armature current only.

    \[ T \propto I_a \]

Speed Equation of DC Shunt Motor

For DC motors, the speed depends on the below equation,

    \[ N \propto \frac{V - I_a R_a}{I_{field}} \]

For the DC shunt motors, the field current is constant. Hence the speed of this motor depends on the equation below,

    \[ N \propto V - I_a R_a \]

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