Topics: Electric motor, Induction motor, Electric motors Pages: 10 (2102 words) Published: January 23, 2013

Synchronous Motor Characteristics

Copyright 2004 Kilowatt Classroom, LLC.

Synchronous Motors are three-phase AC motors which run at synchronous speed, without slip. (In an induction motor the rotor must have some “slip”. The rotor speed must be less than, or lag behind, that of the rotating stator flux in order for current to be induced into the rotor. If an induction motor rotor were to achieve synchronous speed, no lines of force would cut through the rotor, so no current would be induced in the rotor and no torque would be developed.) Synchronous motors have the following characteristics: • • A three-phase stator similar to that of an induction motor. Medium voltage stators are often used. A wound rotor (rotating field) which has the same number of poles as the stator, and is supplied by an external source of direct current (DC). Both brush-type and brushless exciters are used to supply the DC field current to the rotor. The rotor current establishes a north/south magnetic pole relationship in the rotor poles enabling the rotor to “lock-in-step” with the rotating stator flux. Starts as an induction motor. The synchronous motor rotor also has a squirrel-cage winding, known as an Amortisseur winding, which produces torque for motor starting. Synchronous motors will run at synchronous speed in accordance with the formula: 120 x Frequency Synchronous RPM = Number of Poles Example: the speed of a 24 -Pole Synchronous Motor operating at 60 Hz would be: 120 x 60 / 24 = 7200 / 24 = 300 RPM

Sync Motors

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Synchronous Motor Operation • • The squirrel-cage Amortisseur winding in the rotor produces Starting Torque and Accelerating Torque to bring the synchronous motor up to speed. When the motor speed reaches approximately 97% of nameplate RPM, the DC field current is applied to the rotor producing Pull-in Torque and the rotor will pull-in -step and “synchronize” with the rotating flux field in the stator. The motor will run at synchronous speed and produce Synchronous Torque. After synchronization, the Pull-out Torque cannot be exceeded or the motor will pull out-of-step. Occasionally, if the overload is momentary, the motor will “slip-a-pole” and resynchronize. Pull-out protection must be provided otherwise the motor will run as an induction motor drawing high current with the possibility of severe motor damage. Advantages of Synchronous Motors The initial cost of a synchronous motor is more than that of a conventional AC induction motor due to the expense of the wound rotor and synchronizing circuitry. These initial costs are often off-set by: • • • Precise speed regulation makes the synchronous motor an ideal choice for certain industrial processes and as a prime mover for generators. Synchronous motors have speed / torque characteristics which are ideally suited for direct drive of large horsepower, low-rpm loads such as reciprocating compressors. Sheet 1 Synchronous motors operate at an improved power factor, thereby improving overall system power factor and eliminating or reducing utility power factor penalties. An improved power factor also reduces the system voltage drop and the voltage drop at the motor terminals.


Synchronous Motor Construction

Copyright 2004 Kilowatt Classroom, LLC.

2000 Horsepower Synchronous Motor In Refinery Service

Characteristics and Features • The rotation of a synchronous motor is established by the phase sequence of the three-phase AC applied to the motor stator. As with a three-phase induction motor, synchronous motor rotation is changed by reversing any two stator leads. Rotor polarity has no effect on rotation. Synchronous motors are often direct-coupled to the load and may share a common shaft and bearings with the load. Large synchronous motors are usually started acrossthe-line. Occasionally, reduced voltage starting methods, such as autotransformer or part-winding starting, may be employed.

Sync Motors

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