D.C. Series Motor

D.C. Series Motor

• In this type of motor, the series field winding is connected in series with the armature and the supply, as shown in the Fig. 4.9.1.

• Let R_se be the resistance of the series field winding. The value of R_se is very small and it is made of small number of turns having large cross-sectional area.

1. Voltage and Current Relationship

• Let I_L be the total current drawn from the supply.

• Supply voltage has to overcome the drop across series field winding in addition to E_b and drop across armature winding.

Key Point: In series motor, entire armature current is passing through the series field winding. So flux produced is proportional to the armature current.

Φ α I_se α I_a for series motor

Ex. 4.9.1 A 4 pole, 250 V, d.c. series motor has a wave connected armature with 200 conductors. The flux per pole is 25 mWb when motor is drawing 60 A from the supply. Armature resistance is 0.15 Ω while series field winding resistance is 0.2 9. Calculate the speed under this condition.

Sol. :

Ex. 4.9.2 A 4 pole series motor has 944 wave connected armature conductors. At a certain load the flux per pole is 34.6 mWb and the total mechanical torque developed is 209 Nm. Calculate the line current taken by the motor and the speed at which it will run with an applied voltage of 500 V. Total motor resistance is 3 Ω.

Sol.

Review Questions

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

2. A d.c. series motor is running with a speed of 800 r.p.m. while taking a current of 20 A from the  supply. If the load is changed such that the current drawn by the motor is increased to 50 A, calculate the speed of the motor on new load. The armature and series field winding resistances are 0.2 Ω and 0.3 Ω respectively. Assume the flux produced is proportional to the current. Assume supply voltage as 250 V. 

[Ans.: 300 r.p.m]

3. A d.c. series motor developing 40 Nm torque is subjected to the conditions that make field flux to decrease by 30 % and armature current to increase by 15 %. Calculate the new torque. 

[Ans.: 32.2 Nm]