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synchronous motor[′siŋ·krə·nəs ′mōd·ər]
a synchronous machine that operates as a motor. The stator of a synchronous motor has a polyphase (most often, three-phase) armature winding. On the rotor is a field winding with the same number of poles as the stator winding. The stator winding is connected to an AC power source, and the rotor winding, in most synchronous-motor designs, is connected to a DC source. As a result of the interaction of the magnetic fields of the stator and the rotor, a torque is produced that causes the rotor to turn in synchronism with the field-strength vector of the stator’s magnetic field.
The excitation of a synchronous motor can be performed by a DC generator sharing a common shaft with the motor (seeELECTRICAL MACHINE EXCITER) or by a thyristor rectifier (seeCONVERSION TECHNOLOGY). The operating reliability of synchronous motors with dynamoelectric exciters is lower than that of motors using thyristor rectifiers. Synchronous motors of low power—up to 2 kilowatts—are sometimes excited by means of permanent magnets or the reactive current of the stator (reluctance motors without a field winding on the rotor).
Three different methods of starting synchronous motors are used. In the first method, the synchronous motor, with the load disconnected, is brought up to synchronous speed by a low-power auxiliary motor. In the second method, the stator-wind-ing voltage is increased smoothly. The third method, which is the most common, is based on induction-motor principles. Here, an electromagnetic torque results from the interaction between the magnetic field of the stator and the field from the current induced in a starting winding or in the body of the rotor. In this method of starting, the starting winding is either short-circuited or closed through a discharge resistance.
When the rotor reaches a steady speed close to the synchronous speed, the field winding is opened and is connected to a DC source. The synchronizing torque pulls the motor into synchronism. Stable synchronous operation of the motor is possible when the electromagnetic power is equal to the mechanical (braking) power. If the load power exceeds the electromagnetic power, the motor goes out of synchronism and stops. The motor’s synchronous operation can also be disrupted by a drop in the power-supply voltage or a reduction in the field current.
Unlike induction motors, synchronous motors are able to operate under a given load with various power factors (cos ø). As the field current is increased, the power factor rises. At a certain current value, the power factor becomes equal to unity. A further increase in the field current causes the motor to supply reactive power to the power circuit. Thus, reactive power can either be supplied to the power circuit (overexcitation) or drawn from it (underexcitation), depending on the value of the field current. A synchronous motor that is designed to generate reactive power and operate without a load is known as a synchronous condenser.
Synchronous motors are used in electric drives where speed adjustment is not required when there are no substantial overloads on the motor shaft. Such motors are used, for example, to drive pumps, compressors, and blowers.
REFERENCESyromiatnikov, I. A. Rezhimy raboty asinkhronnykh i sinkhronnykh elektrodvigatelei, 3rd ed. Moscow-Leningrad, 1963.
See also references under .
M. I. OZEROV
An alternating-current (ac) motor which operates at a fixed synchronous speed proportional to the frequency of the applied ac power. A synchronous machine may operate as a generator, motor, or capacitor depending only on its applied shaft torque (whether positive, negative, or zero) and its excitation. There is no fundamental difference in the theory, design, or construction of a machine intended for any of these roles, although certain design features are stressed for each of them. In use, the machine may change its role from instant to instant. For these reasons it is preferable not to set up separate theories for synchronous generators, motors, and capacitors. It is better to establish a general theory which is applicable to all three and in which the distinction between them is merely a difference in the direction of the currents and the sign of the torque angles. See Alternating-current generator, Alternating-current motor. For special types of synchronous motors See Hysteresis motor, Reluctance motor