Question

A DC shunt motor with an armature resistance of 0.3Ω and a field resistance of 120Ω is connected to a 240 V supply. At no-load conditions, the motor draws 9 A.

(a) Find the rotational losses. (Hint: At no load, all the developed power is lost as rotational losses).

Answer 1

The rotational losses in a DC shunt motor at no-load conditions with the given parameters are approximately 2135.7 W. This is calculated by subtracting the power lost in the armature from the total electrical power input to the motor.

Step-by-step explanation:

The student's question relates to the calculation of rotational losses in a DC shunt motor at no-load conditions. The armature resistance is given as 0.3Ω and the field resistance as 120Ω. The supply voltage is 240 V and no-load current is 9 A. Rotational losses can be calculated by determining the power developed by the motor minus the electrical losses in the armature and the field.

First, the total electrical power input to the motor at no load is:

Power input = Voltage × Current = 240 V × 9 A = 2160 W

Next, we calculate the power lost in the armature:

Power loss in armature (Pa) = I2 × Ra = 9 A2 × 0.3Ω = 24.3 W

Power loss in the field (Pf) is negligible in no load condition because it is significantly less compared to the armature's power loss and the total input power. Therefore, the rotational losses are essentially the total power input minus the power lost in the armature.

Rotational losses = Power input - Power loss in armature = 2160 W - 24.3 W ≈ 2135.7 W