Specifications

F6
GE Limitamp
®
Medium Voltage Motor Control
Protection & Control
F
On-Line Field Adjustment
The manual potentiometer is normally mounted on the door
and allows an operator to adjust the motor field current
while the motor is running. This provides the convenience
over the SFC type exciter of not having to shut down the
motor and physically move cables between several taps on
the exciter transformer.
Field Current Regulation
The field current regulator module also employs a manual
potentiometer for adjustment of the field current. However,
the regulator provides a closed loop control so that the VFC
DC output is automatically adjusted to maintain the set-point
field current as set by the manual potentiometer. This feature
allows the operator to set the field one time at a desired field
current. The field current will then be regulated to compensate
for field resistance changes due to field winding heating or
system voltage fluctuations. The leading reactive power
contribution of a synchronous motor is related to the level of
field current. If it is desired to maximize the contribution of
leading reactive power from the synchronous motor at all
shaft loading conditions, set the field current as high as pos-
sible without exceeding its nameplate rating. Field current
regulation is the ideal choice for maximizing the leading
reactive power because it allows the operator to set the field
current very close to rated and not worry about the current
“creeping” higher or lower from the potentiometer setting.
Power Factor Regulation
Power factor r
egulation is an excellent choice for applications
requiring field forcing, which is applying DC excitation above
its rating for a short time. Many drives, such as chippers, are
subject to transient impact overloads many times the motor
rating for short time intervals. By forcing the field, the syn-
chronous motor can be enabled to deliver shaft torques
above the rating without “pulling-out” of synchronism and
shutting down. GE tests on chipper drives have demonstrated
that the power factor regulation option can provide the rapid
field forcing feature to prevent disruptive motor “pull-out.”
Power factor regulation operates on the principle that the
motor running power factor is a good predictor of motor pull
out. Before a motor pulls out of step (as a result of high shaft
loading from a hard or oversized log entering a chipper), the
power factor dips in the lagging direction drastically. By set-
ting the regulator such that it boosts excitation as the power
factor dips more lagging than the regulator set point, the
motor running power factor is held to a “healthy” level and
motor “pull-out” is avoided. Power factor regulation also allows
the field excitation power to be conserved when the motor is
running lightly loaded or unloaded. This not only allows
energy conservation but also deeper no-load cooling of the
motor windings, so the motor runs cooler for a given level of
RMS loading. Power factor regulation can help regulate the
power system voltage by minimizing reactive power swings
over a wide range of motor loads.
Brushless Synchronous Control
The GE-Multilin SPM is also designed for use with brushless
synchronous motor
s. It provides timed field exciter application,
power factor and pull-out protection and starting/stall pro-
tection. Included with the standard brushless synchronous
motor Limitamp controller is a variable exciter field supply
consisting of a door mounted variable autotransformer and
rectifier for on-line exciter voltage control.
Fixed-Tap Field Resistor
A fixed-tap field resistor may be used for separate DC source.
This resistor, when supplied with the Limitamp panel, is
mounted on top and is connected directly in series with the
synchronous-motor field as a means of adjusting field current.
The resistor is continuously rated with taps to adjust field
current 10-percent above and below rated full-load field current
for rated power factors in approximately 2 1/2-percent steps.
Other Options
Exciter Voltage Check Protection, field circuit continuity check
protection and field ground pr
otection are also available on
some applications.
Control Circuits
Control Power Transformer
Control power transformers used in Limitamp starters are
single-phase, air-cooled, core-and-coil construction with
high-voltage windings covered to prevent contamination by
dust and dirt. Those furnished in standard panels have a 25-
kV Basic Impulse Level (BIL) rating. 750VA is standard in a basic
controller. Transformers 2 kVA and above are optional, and
above 3 kVA may require an auxiliary enclosure for mounting.
When specified, a 500VA, 60-kV BIL rated control transformers
can be furnished, but will require special space consideration.
Omission Of Control Power Transformer
A lineup of starters can use a common control power trans-
former or other source of control power. In either case, the
power source and control circuit must be provided with
interlocking relays so the loss of either will shut down all
operating motors. Control bus is required in all controllers if
a common source of control power is used.
A single source of control power results in some disadvantages:
(1) Unless each panel is provided with a fused control switch,
troubleshooting must be done with live wires in the panel;
(2) a single controller, if relocated independent of the lineup,
will require modification to add a control transformer and
fuses; and (3) the loss of control power will cause shutdown
of all machines.
Timing Relays
Solid State and or Electronic timing relays close or open a