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Section 5: Synchronous motor system
Synchronous motors
Synchronous motors are like other induction motors in that
they have stator windings that induce currents and magnetic
fields in rotor squirrel cage bars. In the synchronous motor,
these squirrel cage bars, or amortisseur windings, are
short-time rated, for starting duty. The synchronous motor
also has externally-powered wound rotor magnets. How the
wound rotor magnets receive their power defines the two
synchronous motor types.
Synchronous motors come in two varieties: brush-type
and brushless. The brush-type motor uses slip rings and
brushes to conduct DC excitation current to the rotor
wound electromagnets. The brushless type uses a separate
set of stator windings and rotor bars to transmit AC to
rotor-mounted hardware for conversion to DC.
Synchronous motor components
Figure 78, Figure 79 and Figure 80 show the essential rotor
DC excitation components of brush-type and brushless
synchronous motors.
Figure 78. Brush-type synchronous motor elements
Figure 79. Brush-type motor slip rings
Figure 80. Brushless synchronous motor elements
A synchronous motor starts like a conventional induction
motor, using a motor starter or VFD and relying on the torque
produced by the stator magnets and squirrel cage bar
magnets for acceleration. As the motor speed approaches its
synchronous speed, an external control system detects this
and energizes the rotor's separate-excitation windings, the
wound electromagnets, pulling the rotor up to synchronous,
or rated speed. Once at synchronous speed, the amortisseur
windings act as damping windings to discourage motor
speed variation, or hunting.
SC9000 EP medium voltage drives IB020001EN—February 2016 www.eaton.com
Appendix D: Optional equipment
75
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