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SYNCMTR1
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
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
• •
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