What is DC Shunt Motor? Working, Diagram & Applications

Connection diagram of DC Shunt Motor

In the DC shunt motor, the armature and field winding are connected in parallel as shown in Fig. 1. The parallel combination of the two windings is connected across a common dc power supply.

Fig. 1: DC Shunt Motor.

The resistance of the shunt field winding (R_sh) is always much higher than that of the armature winding (R_a).

Voltage and Current relations in DC Shunt Motor

From Fig. 1 we can say that,

\[\text{I }=\text{ }{{\text{I}}_{\text{a}}}+\text{ }{{\text{I}}_{\text{sh}}}\]

Where,

I = Total current drawn from the voltage source

I_a = Armature current

I_sh = Field current.

Therefore the field current will be,

\[{{\text{I}}_{\text{sh}}}=\frac{\text{V}}{{{\text{R}}_{\text{sh}}}}\]

The supply voltage is given by,

V = Voltage across armature = Voltage across field.

\[=\text{ }{{\text{E}}_{\text{b}}}+\text{ }{{\text{I}}_{\text{a}}}{{\text{R}}_{\text{a}}}+\text{ }{{\text{V}}_{\text{brush}}}=\text{ }{{\text{I}}_{\text{sh}}}{{\text{R}}_{\text{sh}}}\]

Thus,

\[\text{V }=\text{ }{{\text{E}}_{\text{b}}}+\text{ }{{\text{I}}_{\text{a}}}{{\text{R}}_{\text{a}}}+\text{ }{{\text{V}}_{\text{brush}}}=\text{ }{{\text{I}}_{\text{sh}}}{{\text{R}}_{\text{sh}}}\]

Neglecting the drop across brushes we get

\[\text{V }=\text{ }{{\text{E}}_{\text{b}}}+\text{ }{{\text{I}}_{\text{a}}}{{\text{R}}_{\text{a}}}\]

The field current I_sh always remains essentially constant. Since V and R_sh both are constant. Hence the flux produced also remains constant, because field current is responsible for the generation of flux.

\[\Phi \propto {{\text{I}}_{\text{sh}}}\]

This is why the DC shunt motor is also called as the constant flux motors.

Speed–torque characteristic curves for a DC shunt motor

Selection of DC Shunt Motors for a Particular Application

D.C. shunt motor is used to drive pumps, lathes machine tools, prime mover for generators due to constant speed characteristic.

DC Shunt Motor Applications

Characteristics of DC Shunt motor indicate that it is a nearly constant speed motor having its starting torque (torque at N = 0) 30% to 40% more than its rated torque and the speed of shunt motor can be adjusted over a wide range. Therefore, such a motor can be used for the following applications:

1. Various machine tools such as lathe machines, Drilling machines, milling machines etc.
2. Printing machinery
3. Paper machines
4. Centrifugal and reciprocating pumps
5. Blowers and fans etc.

Summary of DC Shunt Motor