Specifications

B4
GE Limitamp
®
Medium Voltage Motor Control
Controllers
B
Latched contactors are interchangeable mechanically with
the standard non-latched versions, both from latched to
non-latched, and vice versa. However, in each case, it is
necessary to change the wiring in the control circuit to the
contactor coil or coils and to change the enclosure door to
accommodate the manual latch release knob.
Application Notes — Vacuum Contactors
Switching Transients and Vacuum Contactors
Voltage transients when transmitted downstream can be
harmf
ul to motor insulation systems. The transients occur in
most electrical systems and are usually due to switching
surges or lightning strikes. Vacuum contactor switching is only
one source of voltage transients. For these reasons GE recom-
mends that customers install surge capacitors and arresters
at the motor terminals for vacuum as well as airbreak contactor
applications. The surge capacitors reduce the steepness of
the voltage transient wavefront, thus reducing the stress on
the motor insulation.
Vacuum contactors have proven their suitability as a reliable
and safe means of controlling motors, transformers, and
capacitor loads. This has been demonstrated by a very good
track record over a period of more than 20 years in vacuum
Limitamp equipment and much longer in GE Power-Vac
switchgear equipment.
Also, an independent Electric Power Research Institute (EPRI)
study, investigating the reliability of vacuum switching devices
a number of years ago, concluded “... motors switched by
vacuum devices had failure rates which are no higher than
those for motors switched by air or air-magnetic devices.”
Chopping Transients in Vacuum Limitamp
The vacuum switching device is among the best switching
device available because it most frequently interrupts load
currents in an “ideal” fashion — that is, when the load current
is at zero. However, there is a probability that some switching
operations may produce voltage transients due to chopping.
Chopping is a phenomenon that occasionally occurs as the
current through a contactor pole is interrupted during a
contactor opening operation.
To understand the nature of chopping, a little understanding
of what occurs as a vacuum contactor interrupts current is
necessary. When the operating coil of a vacuum contactor
is de-energized, kick-out springs in the contactor cause the
armature to open and force the vacuum interrupter tips to
part. Any current that is flowing through the tips at the instant
of parting continues to arc across the open tips. This arcing
continues until the sinusoidally varying current approaches
zero. As the polarity reverses across the open tips, current
ceases to flow because all charge carriers in the arc disappear
during the zero-crossing, leaving in its place a very high
dielectric vacuum space. Chopping occurs just before the
current zero crossing because the arc becomes unstable
under the light current conditions and prematurely interrupts
the current. The instantaneous level of current change when
this interruption occurs is called the “chop” current. The
magnitude of the resulting voltage transients is the product
of the “chop current” and the load surge impedance.
GE employs special metallurgy in its tip design to minimize
chopping. The tip material consists of a sintered tungsten-
carbide material that is impregnated with silver. The tungsten
provides long life in hot arcing conditions, and the silver provides
for low chop currents. In chop current tests performed on GE’s
400 ampere vacuum contactors, it was found that the load surge
impedance had significant effect on the average chop current.
For example, tests with a surge impedance of 1000 ohms
yielded average chop currents of 1.2 amperes but only 0.28
amperes with 4500 ohms surge impedance. These levels of
chop currents cause little concern for motor insulation systems.
If motors are expected to be “jogged” or frequently switched-
off while accelerating up to speed, surge suppressing devices
discussed earlier should be seriously considered to minimize
the effects of long term motor winding insulation degradation
due to multiple re-ignition transients that can occur while
interrupting motor inrush currents. Multiple re-ignitions are
surges of arcing current across an opening vacuum inter-
rupter tip that occur in the first few micro-seconds after the
tips part. Multiple re-ignitions are virtually non-existent while
interrupting normal motor running currents.
Vacuum Interrupter Integrity
The loss of interrupter integrity due to loss of vacuum is a
potential concern because the v
acuum interrupter ceases to act
as an interrupter if vacuum is lost. Vacuum Limitamp interrupters
are tested three times during the manufacturing process for
vacuum integrity. Historically, this process has reliably
eliminated loss of vacuum during normal product operation.
To maintain integrity, annual hipot checks are recommended
as part of a user’s normal preventative maintenance practice.
The recommended hipot test voltage is 20 kV AC RMS for the
400 ampere and 800 ampere contactors. The hipot procedures
are described in equipment instructions GEH-5305 and
GEH-5396.
AC vs. DC Hipot
The AC hipot is recommended for vacuum interrupters because
DC hipot may indicate problems with a good interrupter. The
reason for this is complex, but in essence there may be
microscopic gap broaching “anomalies” across the open