GE SPM Series, Instruction Manual

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GE Power Management SPM Synchronous Motor Protection & Control 3-
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3 SYNCHRONOUS MOTOR APPLICATIONS 3.1 OVERVIEW
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Figure 3–4: ANGULAR DISPLACEMENT OF ROTOR
It has been established that salient-pole torque near synchronous speed is a function of both slip and field-dis-
charge resistance. The combined effects of cage torque and salient pole torque for a typical motor are shown
below. The effect of a higher value of discharge resistance on a medium-torque motor are shown in Figure 3–
6: FIELD DISCHARGE RESISTANCE – MEDIUM STARTING TORQUE. Obviously, without salient-pole torque
the motor would cease to accelerate certain loads at some point on the speed axis.
Figure 3–5: MOTOR TORQUE VS. SPEED
The upper limit of the discharge resistance is governed by the other function of the resistor, which is reducing
field voltage to safe levels during starting. As the discharge resistance increases so does the induced voltage,
and at some point this voltage would be damaging to insulation or other components in the field circuit. Solid-
state excitation and control components in the field circuit have had the effect of making the discharge resis-
tance and its voltage effect more significant. There is a greater sensitivity to field voltage tolerance levels with
solid-state components.
HIGH-STARTING-TORQUE MOTOR MEDIUM-STARTING-TORQUE MOTOR