Effective 11/97
I.B. 48008
Cutler-Hammer
Instructions For Ampgard
®
Mark V Solid-State,
Brush-Type, Synchronous Motor Controllers
DANGER
HAZARDOUS VOLTAGE.
READ AND UNDERSTAND THIS BOOKLET AND ITS
RELATED INSTRUCTION MATERIAL FOR FULL-
VOLTAGE CONTROLLERS IN THEIR ENTIRETY
BEFORE INSTALLING OR OPERATING THE
CONTROLLER. SEE TABLE 1.
INSTALLATION, ADJUSTMENT, REPAIR AND
MAINTENANCE OF THIS TYPE OF EQUIPMENT MUST
BE PERFORMED BY QUALIFIED PERSONNEL. A
QUALIFIED PERSON IS ONE WHO IS FAMILIAR WITH
THE CONSTRUCTION AND OPERATION OF THIS
EQUIPMENT AND THE HAZARDS INVOLVED.
SYNCHRONOUS MOTORS
Polyphase synchronous motors have stators similar to
squirrel-cage induction motors and most have rotors with
DC slip-ring circuits which must be energized for normal
operation. The controller described in this booklet is for a
synchronous motor with slip rings and brushes.
Synchronous motors operate at constant base speeds
corresponding to line frequency and the number of ma-
chine poles (revolutions/min = 120 x frequency/number of
poles). They are employed primarily to obtain high pullout
torques, constant operating speeds, or generation of
leading reactive VA (VAR) for system power-factor correc-
tion. They require conventional AC polyphase power
sources for their stators and suitable DC power sources
for their rotor fields.
For normal operation, synchronous motors must be
brought to near full operating speed, at which point the DC
power is connected to the rotating field through brushes
and slip rings. The motors are accelerated to their syn-
chronizing speeds by means of either built-in start wind-
ings or external auxiliary drives. Nearly all conventional
synchronous motors now manufactured have built-in rotor
starting windings. Such starting windings are also referred
to as squirrel-cage windings, pole-face windings, damper
windings, or amortisseur windings. Start windings are
actually squirrel-cage induction bars located in the faces
TABLE I. REFERENCE MATERIAL
Contactor
Ampere
Instruction
Type
Rating
I.L. or I.B.
Type SJA
360A
I.B. 48002
Type SJA
720A
I.B. 48005
Type SJD
360A
I.B. 48004
Type SJO
360A
I.L. 16-200-33
Type SJO
720A
I.L. 17047
Type SJS
360A
I.B. 48003
of the DC rotor poles. They produce accelerating torque
only and have short-time intermittent duty ratings. As
start windings, they become inoperative at synchronous
speeds but serve to dampen any tendency of the rotor to
oscillate in angular position with relation to the stator field.
The starting of synchronous motors involves two basic
switching functions. The first is the energizing of the
stator to produce breakaway torque and acceleration to
near synchronous speed, the second is the energizing of
the DC rotor field at the optimum speed and rotor-stator
pole relationship. For motors having built-in start wind-
ings, the same equipment considerations are required as
for full-voltage or reduced-voltage starting methods used
for squirrel-cage induction motors. All factors relating to
the stator circuits are identical.
Synchronous motors have two torque characteristics,
starting torque and running or synchronous torque. The
first is determined by the squirrel-cage design and the
“slip” as the motor accelerates from zero to near synchro-
nous speed. “Slip” is expressed as a percent fraction
where the numerator is the difference between the syn-
chronous speed and the non-synchronous speed, and the
denominator is the base speed, all speeds expressed in
revolution per minute (rpm). The running torque character-
istic (at synchronous speed) is produced by the magnetic
fields created by the DC field coils in the rotor which link
with the rotating fields produced by the AC current in the
stator windings. See Figure 1.
The DC field coils are energized via two slip rings and
brushes. The DC voltage applied to the field coils can be
varied to produce the desired level of direct current which
in turn produces a magnetic field through each pole which
can be varied. Once at synchronous speed,
