Starting Protection - GE SPM Series Instruction Manual

3 SYNCHRONOUS MOTOR APPLICATIONS
The amortisseur, or cage winding of a synchronous motor, is probably the element most susceptible to thermal
damage. Its function is essentially operative only during starting, and there are limitations on space available
for its construction onto the rotor. Hence, it is usually made of lighter material than the cage winding of an
induction motor. The cage is also vulnerable to overheating should the motor be allowed to run out of synchro-
nism with no excitation. In this case, it runs as an induction motor at some value of slip which will produce cage
current that develops running torque. However, the cage of a synchronous motor is not designed for continu-
ous operation. Therefore, an important protective function of the controller is to prevent overheating of the
cage winding both during starting and running out of synchronism.
Monitoring the starting condition of a synchronous motor can be accomplished by looking at the frequency of
induced field current, the same procedure used to accomplish synchronizing. Motor designers always place a
limit on the time a particular motor can be allowed to remain stalled ("allowable stall time"). An accelerated
schedule can then be established for the motor in terms of running time at any speed less than synchronous as
a percent of allowable stall time. Increased air circulation from the rotor fan reduces the heating rate as the
motor accelerates. Frequency can be measured directly as an indication of speed, and the designer's curves
for speed versus time can be used for protection by software that integrates the time-speed function. The fig-
ure below shows the typical cage heating protection characteristics during acceleration.
The time-speed function shown in below is determined internally by SPM software. The motor speed is deter-
mined from the induced field voltage frequency. The programmed values for maximum allowable stall time and
50% speed run time determine which characteristic of protection is required from the family of curves.
The SPM will cause a TRIP operation and display "SQL CAGE TRIP" if it calculates that the thermal limit of the
cage winding is reached. The SPM will also prevent an attempted restart if it calculates, from learned start
experience, that the cage winding has not had sufficient cooling time to allow a successful start. In this case
the message "START INHIBITED! Ready in xxx min" will be displayed
GE Power Management
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Figure 3–8: AMORTISSEUR WINDING PROTECTION
SPM Synchronous Motor Protection & Control
3.2 COLLECTOR-RING MOTORS

3.2.3 STARTING PROTECTION

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