Question 1a

Using the equivalent circuit of a separately excited DC motor, illustrate what affects the speed of such a motor.

Question 1b

A 28 V separately excited motor has the following specifications:
Armature resistance = 0.5 ohms
Voltage constant = 0.7 V/rad/s,
Torque constant = 100 Nm/s
Field current = 10 mA

Question 1b(i)

Calculate the speed when the developed torque is 10 Nm.

Question 1b(ii)
Demonstrate how the torque of the motor can be increased without reducing the speed.

Question 1c
Infer from the results you have obtained from Q1a, explain how speed control can be achieved in a permanent magnet-brushed DC motor.

Question 1d

Compare the advantages and disadvantages of using brushless DC permanent magnet motors with using permanent magnet brush DC motors in drones.

Question 2a

Distinguish between AC synchronous motor and AC induction motor with reference to their speed, torque, advantages, and disadvantages.

Question 2b

Figure Q2(b) shows an equivalent circuit of a 3-phase 8 pole 115 V 400 Hz aircraft induction motor. Interpret the equivalent circuit and explain how Rm affects the performance of the motor.

Question 2c
For the motor in Figure Q2(b), given that Xs = 0.24 ohm, Rs = 3.5 ohm, Xi’ = 0.6 ohm, Rr = 0.5 ohm, Xm = 16 ohm and there is negligible core losses. The motor speed at full load is 5850 rpm. Demonstrate how you would calculate Q2c(i) to Q2c(v).

Question 2c(i)
The synchronous speed.

Question 2c(ii)
The slip.

Question 2c(iii)
The stator current and power factor.

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