Specifications

F2
GE Limitamp
®
Medium Voltage Motor Control
Protection & Control
F
Figure F.2 Blown fuse indicator
With this feature, fuses are always bolted in place for correct
orientation and alignment. In addition to providing maximum
reliability, this feature makes it impossible to mount the fuse
in an upside down position which would nullify the trip bar
operation.
Coordination With Other Protective Devices
When Limitamp starters are installed on a given power system,
it is necessary to coordinate the time-current characteristics
of system protective devices, such as Multilin Protective
relays, with those of the starters. Use the time-current
curves included in GE Time Current Curve No. GES-5000 for
this purpose. It includes overload-relay tripping curves, fuse-
melting curves and fuse-clearing time curves.
Surge Protection
The economics of rotating-machine insulation dictates that
the machines be protected from voltage stresses above the
operating level insofar as is reasonably possible. Overvoltage
damages reduce the insulation life. There are many causes
of accidental overvoltage whose effects may be reduced by
protective means. The most prominent causes are:
1. Lightning.
2. Physical contact with higher voltage system.
3. Repetitive restrike (intermittent grounds).
4. Switching surges.
5. Resonance effects in series inductive capacitance circuits.
Switching transients occur in every electrical system. A well-
known phenomenon associated with vacuum interrupters is
current chop. GE utilizes vacuum interrupters constructed
with widely accepted contact tip materials to provide low
chopping currents.
Additional protection against surges for rotating machines
may be economically attractive for system voltage installations
of 2300 volts and above. This consists of a surge capacitor
and lightning arresters.
Lightning arresters reduce the amplitude of the voltage
impulse wave. The surge capacitor further reduces the
amplitude — but in addition, reduces the steepness of the
wave front. It is important to reduce the steepness of the
surge wave front to keep the turn-to-turn voltage stress in
the machine winding to a minimum.
Surge capacitors and arresters should be installed as close
to the machine terminals as possible. Capacitors and
arresters may require a 22-inch wide auxiliary enclosure if
installed in the controller.
Overload Relays
Several types of overload relays are used in Limitamp
Control. Limitamp controllers use thermal-overload relays,
unless other types are specified.
Thermal-Overload Relays
Overload relays provided in Limitamp control have inverse-time
characteristics and are ambient compensated. Limitamp
control utilizes either a thermal-type relay or the solid-state
protective relay. These relays, operating from current trans-
formers in the control equipment, carry current proportional
to the motor-circuit current. When motor overloads occur,
the relay operates to open the main power contactor. The
time required for operation varies inversely with the magni-
tude of the overload. The standard thermal relay should only
be used on motors with starting times up to 10 seconds.
External-Reset Overloads
Some industrial plants do not permit a machine operator to
open the doors of control equipment enclosur
es, this being
reserved for electricians. To make possible overload-relay
reset by operators, it is therefore necessary to provide some
means to do so outside the enclosing case. This is accom-
plished by providing a mechanical-linkage reset mechanism
between the relay and door-mounted reset button.
Where external reset is not absolutely necessary, greater
simplification of relay mounting results, and this is of benefit
to the user because it simplifies maintenance.
Inasmuch as the tripping of an overload device is indicative
of too much strain on the motor, it is preferable that only
experienced and reliable personnel be allowed to reset over-
loads. Such personnel should be capable of realizing whether
it was an unintentional overload on the part of the machine
operator or whether there is an electrical and/or mechanical
defect. The customer should consider this factor, however,
before electing to provide externally reset overloads.
Solid-State Overload Relays
Solid-state overcurrent protection is available as an optional
feature in place of standard thermal ov
erload relays. The
inverse-time characteristics can be adjusted to protect
motors of various characteristics, such as long acceleration
time or short allowable-stall times. Characteristics are accu-
rate and have a smaller error band compared to bimetal
relays. The solid-state overload relay is recommended for