Synchronous Motor Drives Questions and Answers

Answer: A

Correct answer: its speed at a given supply frequency

The synchronous speed is \( 120f/P \), so at 50 Hz a two-pole machine runs at 3000 rpm, a four-pole at 1500, a six-pole at 1000, and so on. Pole number is chosen to give the speed the driven load wants without a gearbox - hence the many-poled machines of low-speed hydro sets. It has no bearing on the power factor, which excitation sets, or on the excitation current itself.

Answer: B

Correct answer: 3,000 rpm

With two poles at 50 Hz,

This is the highest speed obtainable at 50 Hz without a converter, and it is the speed of the large turbo-alternators driven by steam turbines. On a 60 Hz system the same machine would turn at 3600 rpm, which is why turbine sets are built in two speed families.

Answer: C

Correct answer: set the power factor at which it operates

Once the machine is running, the field current is the operator’s handle on reactive power: under-excited it draws lagging current, at the V-curve minimum it runs at unity power factor, and over-excited it supplies reactive power with a leading current. The torque is set by the mechanical load, and the speed by the supply frequency, so neither can be adjusted by the field.

Answer: D

Correct answer: slip poles and lose synchronism

Beyond pull-out the torque angle passes 90 degrees, where the developed torque begins to fall as the angle grows while the load torque does not. The rotor cannot recover, falls back a pole pitch and then another, drawing heavy pulsating current and producing severe torque pulsations in the shaft. It cannot simply run slower, since torque exists only at synchronous speed, and the machine must be tripped and restarted.

Answer: A

Correct answer: both stator and rotor carry their own excitation

A synchronous machine is doubly excited: the stator carries three-phase alternating current and the rotor carries direct current in its field winding, and torque arises from the interaction of the two independently established fields. An induction machine is singly excited, the rotor current being induced by the stator rather than supplied. The term describes where excitation comes from, not the number of windings on any one member.

Answer: B

Leakage flux and the field the armature current itself sets up are lumped into one reactance for steady-state work. Its value in per unit is typically greater than one, which is why the pull-out power is of the order of the machine rating.

Answer: C

Correct answer: a smaller pull-out torque

The pull-out power is

so the synchronous reactance appears in the denominator and a large value directly reduces the overload capability. It also increases the voltage regulation of the machine as a generator. A machine with a small \( X_s \) is stiffer and more robust to load transients, but its short-circuit current is correspondingly larger, so the choice is a design compromise.

Answer: D

Correct answer: Rotor resistance control through slip rings

Rotor resistance control belongs to the wound rotor induction machine, where inserting resistance changes the slip at which maximum torque occurs. A synchronous machine’s slip rings carry direct field current, not a variable resistance in an induced-current circuit. The three genuine methods are induction starting on the damper winding, an auxiliary pony motor, and ramping the frequency up from zero with a converter.

Answer: A

Tens of megawatts driving a propeller at a couple of hundred revolutions a minute is exactly what a many-poled synchronous machine on a cycloconverter does well. Cost and complexity are accepted for the efficiency and the controllability.

Answer: B

Removing the commutator removes the maintenance and the limit it places on voltage and speed, which is why very large variable-speed drives are synchronous rather than dc. The control is more involved rather than less.