I.B. 48008
Page 2
Effective 11/97
SYNCHRONOUS MOTORS (Continued)
changing the field current can change the power factor at
which the synchronous motor runs or the reactive current
drawn from the AC line.
A synchronous motor cannot start with its DC pole
windings excited. Voltage is applied to the DC winding
only after the motor has been accelerated to a speed
which is over 90 percent of synchronous speed. With a
slip of less than ten percent, the DC poles will jump to
synchronous speed when DC voltage is applied and will
lock onto the rotating magnetic field produced by the
three-phase alternating current in the stator.
Applying DC at the most advantageous time is the job of
the motor controller which uses feedback signals to opti-
mize the transition and thereby minimize the disturbance to
the power system at the time of synchronization.
MOTOR FIELD EXCITATION
In this controller, the DC power for the excitation of a
synchronous motor field is obtained from a solid-state
power supply (exciter). Provisions are made for field
current adjustments. Usually, field current is set to
optimum values only after the motor field has reached
maximum operating temperatures. On cold start-ups, the
field current may be 20 to 40% high initially but will
decrease to normal as operating temperatures are
reached. The field current is usually maintained as set
during motor operation except where the field current
regulator option is included for those applications in which
VAR, power factor, or the field current itself is being
automatically adjusted.
THE MOTOR CONTROLLER
As explained in the companion instruction material, each
Ampgard
®
full-voltage motor starter (controller) consists of
one nonload-break isolating switch, one contactor, current-
limiting fuses, a set of current transformers, and some
form of overload protection. A reduced-voltage starter
includes all of the components of a full-voltage starter plus
one or two additional contactors and related control
components. A synchronous motor controller is either a
full-voltage starter or a reduced-voltage starter which
includes a source of DC and the additional controls
needed to start a synchronous motor, operate it at syn-
chronous speed, and protect it.
THE MARK V CONTROLLER
The Mark V is a field power supply and controller that
blocks the field voltage during subsynchronous operation
and applies the field voltage during synchronous operation.
The field voltage is adjusted by a potentiometer during
operation.
Fig. 1 Synchronous Motor Components
This solid-state field power source functions without
regard to the phase sequence of the three-phase power
supply, which may be at either 50 or 60 hertz. The field
power supply and controller contains synchronizing
circuitry with means to adjust the synchronizing slip-
frequency from 1 to 9 percent (SW1) and the permissible
stalled-rotor time from 0 to 9 seconds (SW2). It has pole
slippage sensing, incomplete sequence detection and
shutdown in the case of exciter low voltage or phase loss.
Figure 2 shows the solid-state field power supply panel
designed to control the application of either 125 VDC or
250 VDC to the field of a synchronous motor. The panel
consists of a printed circuit board (synchronizing control
board), protective fuses, and an assembly of power
thyristors (silicon controlled rectifiers, SCR’s) arranged to
convert 120 or 240 VAC, three-phase, to 125 or 250 VDC,
respectively. The direct current (DC) is supplied directly to
the field of the synchronous motor. The synchronizing
control board shown in the lower portion of Figure 2
appears enlarged in Figure 3.
Characteristics
Each Mark V controller has a maximum DC current
capability (50, 100, or 200 amperes), a rated field supply
voltage of 125 or 250 VDC, and the ability to withstand a
field output (induced) voltage of up to 1500 volts during
the time that the motor is coming up to rated speed, i.e.,
