QUESTION ONE:
a) Sketch an electromagnetic wave as it travels through space showing the axes of electric field (E), magnetic field (B) and distance (d).
b) A strain gauge measures 20 mm along its active length and has an unstrained resistance of 125 Ω. When put under strain the length changes to 20.032 mm and the resistance goes to 125.4 Ω. Calculate the Gauge Factor of the strain gauge.
c) Sketch the frequency response of the circuit shown on the axes given. Calculate the frequency at which there is maximum change in Vo.
d) In a four wire three phase supply the currents are as follows:
Ia=5 cis(0) A
Ib=5 cis (-120) A
Ic=0
Deduce from a phasor diagram the value of the neutral current
e) Sketch on the axes given, the torque-speed curve of an 400V, 50Hz, 4 pole induction motor and label upon it the following points:
i) Synchronous speed (Give the value of this speed)
ii) Starting torque
f) Sketch a circuit which may be used to vary the speed of a dc motor. The circuit must provide speeds greater than the rated speed and less than the rated speed.
g) Explain using a diagram how a capacitor start motor operates.
h) Explain the action of a commutator in a DC motor.
i) Explain the advantages of using feedback in a control system. In the block diagram below G = 10 and the input =1. Calculate the value of the output.
j) Label the voltages at the points i) and ii) in the circuits below.
VC=15V and VE= -15V.
QUESTION TWO
a) A 10kW, 400V, 50Hz, 4 pole, three phase induction motor runs at a power factor of 0.78 lagging and has a mechanical loss of 500W. The stator losses are 400W. The motor runs at a speed of 1440 rev/min.
i) Calculate Rotor power loss
ii) Calculate the motor Efficiency
iii) Estimate the stator current if the torque on the shaft doubles.
QUESTION THREE
A 200V, shunt connected DC motor has a field resistance of 350Ω and an armature resistance of 1.2Ω. The machine consumes 7A and runs at 1000 rev/min and delivers power to a 800W to a water pump.
Calculate
i) Generated back Emf.
ii) The Efficiency of the motor/pump combination.
iii) Mechanical loss
iii) Torque upon the shaft.
QUESTION FOUR
In the system shown below calculate the currents Ia, Ib, Ic and In
