D.C. Shunt Motor

D.C. Shunt Motor

AU: May-09

• In this type, the field winding is connected across the armature winding and the combination is connected across the supply, as shown in the Fig. 4.8.1.

• Let R_sh be the resistance of shunt field winding.

• R_a be the resistance of armature winding.

• The value of R_a is very small while Rsh is quite large. Hence shunt field winding has more number of turns with less cross-sectional area.

1. Voltage and Current Relationship

• The voltage across armature and field winding is same equal to the supply voltage V.

• The total current drawn from the supply is denoted as line current I_L.

I_L = I_a + I_sh

I_sh = V / R_sh and V

= E_b + I_a R_a + V_brush

V_brush is generally neglected.

Now flux produced by the field winding is proportional to the current passing through it i.e. I_sh

Φ α I_sh

Key Point: As long as supply voltage is constant, which is generally so in practice, the flux produced is constant. Hence d.c. shunt motor is called constant flux motor.

Ex. 4.8.1 The back e.m.f. of a shunt motor is 230 V, the field resistance is 160Ω and field current is 1.5 A. If the line current is 37 A, find the armature resistance. Also find the armature current when the motor is stationary AU: May-09, Marks8

Sol. :

Review Questions

1. Draw the circuit model of d.c. shunt motor and state voltage and current relations.

2. A 440 V, d.c. shunt motor has armature resistance of 0.08 Ωand field resistance of 200 2. Determine the back e.m.f. when giving an output of 7.46 kW at 85 % efficiency. 

[Ans. : 438.58 V]

 3. The counter emf of a shunt motor is 227 V, the field resistance is 160 ohm and field current is 1.5 A. If the line current is 39.5 A, find the armature resistance. Also find the armature current when the motor is stationary. 

[Ans.: 0.3421 92, 701.54 A]

4. A 240 V, 4-pole d.c. shunt motor has two-circuit armature winding with 500 conductors. The armature circuit resistance is 0.22 Ω, field resistance is 150 Ω and the flux per pole is 0.02 Wb. Neglect the armature reaction. Determine the speed and torque developed, if the motor draws 16 A from the mains. 

[Ans.: 710.496 r.p.m., 45.792 Nm]