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Category: Plant Basics
Question: What is a synchronous motor?
Keywords: v1i12,synchronous,motor,definition
Answer: A synchronous motor is an electric motor driver that is in unison, or in step, with the phase of the alternating current operating it. It is constructed of essentially 2 elements:* A stator (armature);* A rotor (field).It may have either the armature or the field revolving, although most synchronous motors are of the revolving-field type. The stationary armature is attached to the stator frame and field magnets are attached to a frame which revolves with the shaft. The field rotor coils are "excited" by direct currents - either from a small d.c. generator (usually mounted on the same motor shaft & called an exciter), or from some other d.c. source. It is not inherently a self-starting motor, but must necessarily be equipped with some form of auxiliary starting device, such as a squirrel cage winding. For a synchronous motor pulling a constant load, there is a single value of field current that will give unity Power Factor at the motor terminals. Increasing or decreasing the field current from this value will give a power factor less than unity - increasing the field current will give "leading" power facor, decreasing the field current will give "lagging" power factor. Thus, you can vary (or control) the imposed power factor - depending on the load.In a squirrel cage (induction) motor the secondary or squirrel cage winding takes the place of the field winding found in a synchronous motor. As in a synchronous motor, a rotating magnetic field is set up by the currents in the stator. There is one important difference between the synchronous and induction motor: the rotor of the latter does not rotate as fast as the rotating field in the armature. For a squirrel cage motor, the motor slows down when loaded an amount just sufficient to produce the increased current to meet the required torque. The difference in speed for any given load between synchronous and load speed is called the "slip" - usually expressed as % of the synchronous speed. The synchronous motor is a constant speed machine - there is no slip. That is one reason why synchronous machines are favored in large sizes for steady, constant (low) speed operations - such as reciprocating compressors.Mechanically, the synchronous motor requires carbon brushes for the exciter and these are subject to wear. The squirrel cage motor has an air gap and no wearing parts on the electric side. Of course, both types have bearings on their rotating shafts.Reference: CERP Message Board, Mr. Art Montemayor


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