GE SPM Series, Instruction Manual

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SPM Synchronous Motor Protection & Control GE Power Management
3.2 COLLECTOR-RING MOTORS 3 SYNCHRONOUS MOTOR APPLICATIONS
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3.2.2 RELUCTANCE TORQUE SYNCHRONIZING
A lightly loaded synchronous motor connected to a low inertial load may pull into synchronism before the rotor
poles are externally magnetized. This is commonly known as reluctance torque synchronizing. This magnetiza-
tion can result in sufficient torque to hold the salient poles in direct alignment with corresponding stator poles
and run the motor at synchronous speed. However, when load is applied, the rotor begins to slip since the
torque developed is only a fraction of rated torque under separate excitation. Furthermore, the rotor is polar-
ized by the stator flux under this condition and can therefore be polarized in any direct axis alignment; occur-
ring each 180°. External excitation forces pole-to-pole alignment in only one orientation of the direct axis.
Should the rotor pull in to synchronism 180° away from the normal running alignment, external excitation will
build up rotor flux in opposition to the stator flux. As the external excitation builds up, correct alignment of rotor
to stator occurs by slipping one pole and the motor will then run in normal synchronism.
The Field Application Control must respond in such a way as to proceed with proper application of excitation in
the event the motor does synchronize on reluctance torque. The following diagram demonstrates how the SPM
automatically responds to reluctance torque synchronizing.
Figure 3–7: RELUCTANCE TORQUE MOTOR MAGNETIZATION
CORRECT
ORIENTATION
180°
DISORIENTATION