Circuit breaker, Motor overload – GE Industrial Solutions AF-650 GP MultiPulse Drive Panel User Manual

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MULTIPULSE DRIVE PANEL

5.2 Circuit Breaker

The optional circuit breaker in the AF 600 FP &
Bypass panel is a solid state Spectra RMS

molded case circuit breaker from GE. The
purpose of the circuit breaker is to provide
short circuit protection and over current for
the panel components, field wiring and the
motor. The breaker uses a solid state sensing
element which calculates the true RMS

value

of the current every cycle, resulting in a
tripping action which is very fast. For this
reason the NEC allows the trip level to be set
at 700% of the motor FLA to allow for the high
locked rotor current which the motor draws
during across-the-line starts (see NEC section
430-52 and Table 430-152 of the NEC).
Because instantaneous trip circuit breakers
are so fast acting, the NEC actually allows trip
level settings up to 1300% of motor FLA if it
can be shown to be required by engineering
evaluation. When a motor is started across
the AC line it can draw a very large peak
current for the first half-cycle which can cause
nuisance tripping in instantaneous trip
breakers set at 700%. GE Motors & Industrial
Systems has conducted analyses, which show
that certain models can require this 1300%
trip level to avoid nuisance tripping on across-
the-line starts. Generally the trip level will
need to be higher for premium efficiency
motors than for standard efficiency ones.

The rating plugs for the circuit breakers in the
panels have been chosen to allow trip level
settings up to 13 times the current ratings of
the panel. If the trip level is set at 13 times the
motor FLA and the circuit breaker still trips it is
probably due to the fact that your motor has
very high efficiency and therefore exhibits the
highest inrush currents. Possible corrective
actions are:

1. Make sure there is not an actual short

causing the trip.

2. Try starting several times. The inrush level

is a function of the point on the AC line
when the breaker is applied. Try enough
times and you’ll probably apply power at a
favorable point.

3. Try another rating plug. Perhaps the

tolerances on the one included with your
breaker are unfavorable.

4. Start the motor with the adjustable

frequency drive and then switch over to
bypass mode once the motor is up to
speed. This will not trip the breaker
because the drive “soft-starts” the motor
by applying a very low voltage and
gradually ramping up.

5. For NEMA E motors (super premium

efficiency) the NEC 1996 version will allow
a trip setting of 1700% of motor FLA.

5.3 Motor Overload

The overload relay in your panel is the C-
2000

Class 20 or Class 30 fixed heater type

from GE which is panel mounted. Class 30
overload relays are utilized for panels rated
60Hp and above at 208 VAC plus 150Hp and
above at 460 VAC. The overload relay carries
all motor current whether it is running from
the drive or across the AC line (bypass mode).
Whereas the panel circuit breaker or input
fuses is intended to provide short circuit
protection for the panel components, field
wiring and the motor, the overload relay’s
function is to provide continuous long term
overload protection against loads which draw
current in excess of the motor’s rating.

According to GE ED&C publication DET-034B
entitled C-2000

Contactors & Starters

Technical Information; the trip level dial on the
contactors should be set as follows.