The rotating field of a three-phase machine has a magnitude that is
Answer: A
Adding three sinusoidally varying fields displaced by a hundred and twenty degrees in both space and time gives a resultant of constant magnitude that revolves. It is that constancy which makes the three-phase machine so much smoother than the single-phase one.
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A single-phase winding fed with alternating current produces
Answer: B
The two components of equal magnitude produce equal and opposite torques at standstill, which is exactly why such a machine will not start by itself. Making the field elliptical with an auxiliary winding gives the necessary net torque.
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The speed at which the field of a three-phase machine revolves depends on
Answer: C
Correct answer: the supply frequency and the number of poles
\[ N_s = \frac{120f}{P}. \] The revolving field is produced by three windings displaced in space and fed by currents displaced in time, and how fast the resultant travels round the bore depends only on how fast the currents alternate and how many pole pairs the winding is arranged into. Neither the load nor the applied voltage enters into it.
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The direction in which the field of a three-phase machine revolves is reversed by
Answer: D
Correct answer: interchanging any two of the supply connections
Swapping two phases reverses the sequence, and the resultant field then travels round the bore the other way. Reversing all three restores the original sequence and changes nothing - a useful check on whether an apparent reversal has actually been made.
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An induction machine is described as singly excited because
Answer: A
The rotor current is induced by the revolving field rather than supplied, which is what makes the cage rotor possible and the machine so simple. A synchronous machine is doubly excited, with direct current fed to its rotor.
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The rotor of a cage induction machine consists of
Answer: B
Correct answer: bars in slots short-circuited by end rings
The cage is the simplest rotor there is: uninsulated bars, usually aluminium cast in place, joined at both ends by rings. It carries no external connection, has nothing to wear, and its ruggedness is the reason the cage machine is the most common electrical machine in industry.
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Rotor bars of a cage machine are skewed relative to the shaft in order to
Answer: C
A skew averages the interaction between stator and rotor slots over the length of the machine, which removes the tendency to lock at particular positions and quietens the machine. It slightly reduces the useful torque.
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A wound rotor induction machine differs from a cage machine in having
Answer: D
Correct answer: a three-phase rotor winding brought out to slip rings
The wound rotor carries a proper three-phase winding whose terminals reach the outside through slip rings, so external resistance may be inserted. That is its whole purpose: resistance in the rotor circuit raises the starting torque and allows some speed control, at the cost of brushes to maintain.
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The air gap of an induction machine is made as small as mechanically practicable in order to
Answer: A
The gap dominates the reluctance of the magnetic circuit, so a smaller gap needs fewer ampere-turns to establish the flux and gives a better power factor. Bearing tolerances and rotor deflection set the practical floor.
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The stator winding of an induction machine is distributed over several slots per pole per phase in order to
Answer: B
Correct answer: produce a more nearly sinusoidal field distribution
A winding concentrated in one slot per pole per phase would produce a stepped, strongly harmonic field. Spreading it over several slots sums the contributions with a small phase displacement between them, which attenuates the harmonics and leaves a distribution much closer to a sinusoid.
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