GE SPM Series Instruction Manual

Also see for SPM: Instruction manual

Contents

3- 10SPM Synchronous Motor Protection & Control GE Power Management3.2 COLLECTOR-RING MOTORS 3 SYNCHRONOUS MOTOR APPLICATIONS33.2.5 POWER FACTOR (PULL-OUT) PROTECTIONSynchronous motors are designed to run at constant speed and drive shaft loads from torque derived from themagnetic poles on their rotors magnetically linking opposite stator poles. Whenever the rotor turns at a speedless than that of stator rotating field, the motor is said to be slipping poles. Slip can occur with the field polesmagnetized while running in synchronism from the following four major causes.1. A gradual increase in load beyond the pull-out capabilities of the motor.2. A slow decrease in field current.3. A sudden large impact load.4. A system fault or voltage dip lasting long enough to cause pull-out.Loss of synchronism with field applied will create intense pulsations in torque at the motor shaft each time astator pole passes a rotor pole. Corresponding pulsations occur in line current. Both types of pulsations can bedamaging. Torque pulsations can break a shaft, coupling, or other mechanical elements, and current pulsa-tions can interfere with smooth power system operation. Slipping poles with field applied is always unaccept-able for a synchronous motor, therefore some means must be provided to prevent this condition fromoccurring.One of the most reliable indicators of synchronous and asynchronous (out-of-step) operation is the motorpower factor. Power factor is related to the phase angle between voltage and current. Synchronous motors sel-dom, if ever, operate continuously at lagging power factor. Synchronous motors run at either unity or somevalue of leading power factor. Lagging power factor appears when the motor load angle increases beyondrated, becoming almost fully lagging (90°) as the motor slips out-of-step. Therefore, lagging power factor canbe utilized to initiate action to prevent slipping.Torque and power pulsations during slip can be reduced by removing field current to the rotor poles. The motorwill then run essentially as an induction motor on its amortisseur winding. Slip with the field current removed istolerable to the load and power system but intolerable for any length of time to the motor amortisseur windingitself, since the winding is designed with limited thermal capability and for short-time operation. Motor PowerFactor during induction motor operation (that is with field removed) is always lagging. However, the degree towhich the current lags the voltage is less than at pull-out when field poles are excited. Lagging power factorcan again be utilized as an indicator of "slip" during induction motor operation.For synchronous motors, power-factor monitoring can be employed to guard against pull-out or loss of fieldconditions.