The outer cage of a double cage rotor is made of
Answer: C
Brass or bronze bars of modest section give the resistance that produces the starting torque, while the inner cage of copper carries the running current. The reactance of the inner cage is what keeps the current out of it at standstill.
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A deep bar rotor achieves a similar effect to a double cage by
Answer: D
At the high rotor frequency of standstill the current crowds into the part of the bar nearest the air gap, where the leakage reactance is least, so the effective resistance is high. It is cheaper to make than a true double cage.
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An induction machine driven above synchronous speed
Answer: A
Correct answer: acts as a generator and returns power to the supply
Above synchronous speed the slip is negative, the rotor current reverses, and the torque opposes the rotation - the machine takes mechanical power in and returns electrical power to the supply. This is the induction generator, used in wind turbines and whenever an overhauling load drives the machine.
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An induction generator connected to a network draws from it
Answer: B
Having no field of its own, it needs the network to supply the reactive current that establishes the flux. Standing alone it requires capacitors, and controlling the voltage and frequency of such an arrangement is awkward.
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A self-excited induction generator uses
Answer: C
The capacitors and the machine's magnetising reactance form a resonant pair, and the voltage builds up from residual magnetism much as in a dc shunt generator. Voltage and frequency both vary with load, which limits the arrangement.
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Harmonic torques in an induction machine arise from
Answer: D
A non-sinusoidal field distribution contains harmonics that revolve at sub-multiples of synchronous speed and produce their own torque-slip curves. Winding design that suppresses them is what avoids the resulting dips in the characteristic.
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An eight-pole induction machine on a 50 Hz supply has a synchronous speed of
Answer: A
Correct answer: 750 rpm
\[ N_s = \frac{120 \times 50}{8} = 750\ \mathrm{rpm}. \] The pole number and the synchronous speed are inversely related: two poles give 3,000 rpm on a 50 Hz supply, four give 1,500, six give 1,000 and eight give 750.
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That machine running at 720 rpm has a slip of
Answer: B
Correct answer: 4 per cent
\[ s = \frac{N_s - N}{N_s} = \frac{750 - 720}{750} = \frac{30}{750} = 0.04. \] Four per cent is a typical full-load slip for a machine of moderate size; larger machines run at less, small ones at more.
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An induction machine takes 30 kW from its supply, with 1.2 kW of stator copper loss and 0.8 kW of core loss. Its air gap power is
Answer: C
Correct answer: 28 kW
Air gap power is what remains of the input after the losses in the stator: \[ 30 - 1.2 - 0.8 = 28\ \mathrm{kW}. \] Everything that crosses the air gap is then divided between the rotor copper loss and the mechanical power, in the ratio \( s \) to \( 1-s \).
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If that machine runs at 3 per cent slip, its rotor copper loss is
Answer: D
Correct answer: 0.84 kW
The rotor copper loss is the slip fraction of the air gap power: \[ 0.03 \times 28 = 0.84\ \mathrm{kW}. \] This relation is exact and it is the reason slip and efficiency are so closely tied - every per cent of slip is a per cent of the air gap power turned into heat in the rotor.
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