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 3.3 BRUSHLESS CONTROLLER
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3.3 BRUSHLESS CONTROLLER 3.3.1 DESCRIPTION
A brushless controller provides functions relevant to starting and protecting a brushless synchronous motor. To
understand the functional requirements, it is necessary to review the construction of a brushless motor.
3.3.2 BRUSHLESS MOTOR REVIEW
A brushless motor is like a conventional slip ring motor in that it has rotor-mounted field poles. These poles
must have DC supplied to their windings so they can "lock" onto the rotating stator field and run in synchro-
nism. Also, like the slip-ring motor, amortisseur windings are built into the rotor pole tips to provide acceleration
and damping torque during starting and normal operation. During start, the motor accelerates to near synchro-
nous speed. When the rotor is close enough to synchronous speed for the field poles to pull the rotor into syn-
chronism, DC is applied to the main field and the rotor pulls into step, normally operating at a power factor
equal to or more leading than unity.
The brushless motor has, as its name implies, no brushes or slip-rings. Instead, it contains a rotating exciter
with stator mounted DC windings and the armature winding on the rotor. A rotor-mounted, solid-state rectifier
converts the AC from the exciter to DC for the main-field poles. The silicon controlled rectifiers (SCRs) and
control circuitry are rotor-mounted, along with the field discharge resistor, to control the application of DC to the
main field at proper rotor speed and angle. The schematic below shows that the actual field control is provided
with the motor and is not part of the motor controller.
Figure 3–14: ROTATING RECTIFIER EXCITER WITH SYNCHRONOUS MOTOR