Class 4 (4th Engineer) MET 📅 Nov 2022

Exam Question

(a) Explain the Speed-Current Characteristics of D.C motor. (6)

(b)A 4-pole, 32 conductor, lap-wound d.c. shunt generator with terminal voltage of 200 volts delivering 12A to the load has Ra=2 and field circuit resistance of 200 ohms. It is driven at 1000 rpm. Calculate the flux per pole in the machine. If the machine has to be run as a motor with the same terminal voltage and drawing 5A from mains, maintaining the same magnetic field, find the speed of the machine. (10)

Reference Answer

### Part (a): Speed-Current Characteristics of D.C. Motor
The speed-current characteristic of a D.C. motor is the relationship between its speed (N) and the armature current (Ia) it draws. This characteristic is fundamental to determining the motor's suitability for different applications. The relationship is derived from the basic D.C. motor equations:
1. **Back EMF Equation**: The supply voltage (V) applied to the armature must overcome the back EMF (Eb) and the voltage drop across the armature resistance (Ra).
$$ V = E_b + I_a R_a \implies E_b = V - I_a R_a $$

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