I.B. 48008
Page 8
Effective 11/97
SYSTEM PROTECTION (Continued)
circuit. These contacts are available at terminals SQ1 and
SQ2 of Terminal Board 1 (TB1). See Figure 6.
RLY1 (see Figure 6) becomes energized about 50 millisec-
onds after power is delivered to the printed circuit board
and proper operational status of the controller is estab-
lished. RLY1 and its associated Light Emitting Diode
(LED1) remain energized as long as the built-in +15 VDC
voltage source remains above a prescribed level and
three-phase power to the controller is present. The
normally-open (NO) contacts of RLY1 should be used in
series with the control circuit. Thus loss of control power
will result in shutdown of the motor.
ROTARY SWITCH SETTINGS
Rotary Switch 1 (SW1) is a ten-position selector switch
which determines the percent slip (1 to 9%) at which the
controller is to apply DC power to the synchronous motor
field to begin synchronization. Unless experience with a
particular motor suggests otherwise, set SW1 at 5%.
Rotary Switch 2 (SW2) is a ten-position selector switch
which determines the period (0 to 9 seconds) during which
the motor must accelerate to a slip condition of 75% or
less. If the motor does not accelerate to the 75% slip or
less within the set time period, a stalled rotor condition is
presumed to exist and the incomplete sequence relay
(RLY3) will be energized. Unless experience with a
particular motor and its load suggests otherwise set SW2
at 5 seconds.
START WINDING PROTECTION
The most critical protection for synchronous motors is that
for the windings used to start the motors. These windings
are short-time rated for starting duty only and are most
vulnerable under locked-rotor conditions. Optimum
protection provides for stall protection while still permitting
slip protection. Squirrel-cage bar protection is required on
motor start-up while ensuring that proper sequential
synchronizing occurs and that motors will operate continu-
ously in synchronism. The protection system must detect
and operate for a condition of prolonged subsynchronous
operation beyond the thermal capability of the starting
windings. See Figure 7.
Once synchronous motors have been initially and suc-
cessfully synchronized, loss of synchronism or pullout is
detected by the presence of induced slip current or AC
voltage superimposed on the DC excitation source. In
brush type controllers, the same system used for field
application is also employed for pullout protection where
field sensing is used for both functions.
Fig. 6 Control Relay Terminals
Because of the vulnerability of synchronous motors, the
best practice is to provide for immediate shutdown on
pullout except for those installations where positive
protection against all combinations of operating hazards is
assured.
MOTOR OPERATING HAZARDS
Some of the major operating hazards for synchronous
motors are operating abuse, low line voltage, low excita-
tion current, and excessive shaft load. The pullout torque
capabilities of synchronous motors are a function of stator
and field power. Supplementary protection for synchro-
nous motors may be provided through the use of field
voltage and current relays and stator frequency relays.
Extreme care must be exercised, especially with large
synchronous motors, if attempts are made to reconnect
motors to the line after momentary power interruption.
Reconnection of line voltage that is substantially out of
phase with open-circuit motor terminal voltage can result
in extremely high current and torque surges capable of
creating system disturbances and mechanical damage.
Additional hazards for synchronous motors are jogging,
too frequent starting, stalling and excessive accelerating
times. Any of these conditions are serious hazards to
start windings. Even the best protective system may not
protect under such extreme operating conditions.
