Specifications

F5
GE Limitamp
®
Medium Voltage Motor Control
Protection & Control
F
Undervoltage Protection
NEMA defines undervoltage protection as a device whose
principal objective is to prevent automatic restarting of
equipment.
Instantaneous undervoltage protection is inherent to the
standard 3-wire control circuits, since the contactor will drop
out and stay out on loss of voltage.
Time-Delay
Time delay undervoltage protection (TDUV) for a Limitamp
controller can be provided to prevent shut-down of a motor
on adjustable duration voltage dips below the adjustable
dropout voltage. With either time-delay or the standard
instantaneous undervoltage protection, the motor remains
disconnected until the operator restarts the motor.
Automatic Restart
GE offers an auto-restart relay which automatically restarts
the motor following a momentary outage. It also can be set
to delay contactor r
eclosure following power restoration so
the motor is not re-energized out-of-phase due to residual
field or cage rotor currents,
Ride Through
This optional feature provides control to maintain a standard
electrically-held contactor in the closed position through
momentary control voltage outages up to 20 cycles (333
milliseconds). Also, protective relay operation and/or normal
stop control is configured to allow the contactor to open
without the ride-through delay.
Synchronous-Motor Control and Excitation
Synchronous Motor Control
GE Limitamp synchronous-motor controllers are offered for
both brush-type and brushless synchronous motors. As a
standard, both brush-type and brushless synchronous
motor controllers are equipped with the GE-Multilin SPM
solid-state field application and protection module. This
microprocessor-based module provides basic synchronous
motor control and protection functions including squirrel-
cage starting protection, power factor and pull-out running
protection, and field application control to maximize pull-in
torque (for brush-type machines only). Digital displays of
motor running line current and power factor are featured
along with a keypad for entering set-point parameters.
Available options are field loss protection, exciter voltage
check protection, field amps display, exciter volts display,
incomplete sequence protection, and power factor regulation
(when used with compatible SCR type variable field exciters).
Exciters For Brush Type Motors
For synchronous motors equipped with sliprings and brushes,
Limitamp is offered with a variety of excitation options.
Single-phase solid-state exciters can be integrated in the
controller NEMA 1 ventilated enclosure up to 9 kW (exciters
must be derated for non-ventilated enclosures). Larger
exciters require auxiliary enclosures that can be placed in
the common bussed line-up with the Limitamp controllers.
Two basic types of exciters are available:
• SFC (fixed excitation with adjustable tapped transformer)
• VFC (on-line adjustable excitation by manual or automatic
means)
Fixed Excitation
The basic exciter offering is a single-phase, tapped-transformer,
static field contactor (SFC). The SFC is a solid-state switching
device consisting of silicon controlled rectifiers (SCRs) in a
bridge circuit for rectification of AC power to DC. Additional
SCRs are provided to switch the field discharge resistor. During
starting, the SFC switches the field discharge resistor on so
that the induced field current from the motor field is passed
through the discharge resistor. The field discharge resistor is
also switched on to discharge the field current when DC is
removed at motor shutdown and if, during normal motor
operation, the motor field generates a high voltage surge
above approximately 600 volts, such as would occur if the
motor “slips” a pole. When the motor has accelerated to near
synchronous speed, the GE-Multilin SPM module signals the
SFC to apply DC to the motor field, the SFC switches the field
discharge resistor off and causes the SCRs in the rectifier
bridge to turn on, resulting in DC being applied to the motor
field. The bridge SCRs are gated “full on” so that they emulate
a diode rectifier bridge. The voltage of this DC field supply is
determined by the tap connection of the customized trans-
former that feeds AC power to the rectifier bridge. This
transformer has secondary taps arranged so that the DC
voltage can be adjusted in 5% increments from 70% to
130% of the transformer nominal secondary voltage by
changing connections at the transformer tap.
Variable Excitation
Another exciter offering is the electronic variable field contactor
(VFC). The VFC is available in single- or thr
ee-phase versions.
Three phase VFC exciters are recommended for sizes 20 kW
and above (125 VDC fields), and 25 kW and above (250 VDC
fields). Like the SFC (above), the VFC controls the switching of
the field discharge resistor and DC to the field depending on
inputs from the GE-Multilin SPM. The difference is that the
gating of the rectifier bridge SCRs can be controlled by vary-
ing an analog voltage at its control input. This allows on-line
control of the DC exciter voltage by any of several means:
1. Manual control via a door-mounted potentiometer.
2. Automatic control via the field current regulation module.
3. Automatic control via the GE-Multilin SPM equipped with
power factor regulation.