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High Efficiency Motor Protection
Industry White Paper
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High Efficiency Motor Protection
High Efficiency Motor Protection - An
Overview
Electric motor protection depends on the accurate selection of overloads,
fuses and/or circuit breakers. Over the years the protective devices have
been selected according to the applicable code requirements with only
minimal nuisance tripping.
However, in recent years, the problem of nuisance tripping due to the high
motor inrush currents that occur during motor starting has gained
increased attention.
In order to avoid the problem of nuisance tripping, application engineers
have been forced to either set the HMCP magnetic circuit breaker above
code requirements or take a step backward and exchange the HMCP
circuit breaker for an inverse time circuit breaker. Both scenarios have the
disadvantage of sacrificing the close coordination protection for which
HMCPs were initially designed.
The National Electric Code (NEC) was changed slightly in 1996 to
address this problem.
The problem stems from the fact that the NEC allows certain settings for
HMCPs (currently 800% of full load current, 1100% for design E motors)
based on the motor’s locked rotor current (LRC), which is generally
600% to 700% of full load current (FLC). However, with high efficiency
motors the inrush current may exceed the 800% of FLC. Also, the
application voltage may be over the nominal by 3 to 5%.
These factors will cause the initial inrush current to be much higher than
usual. Additionally, one other phenomenon that will exacerbate the
situation is that the initial peak inrush current will not be symmetrical.
High Efficiency Motor Protection
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Why is Inrush Current So Much Higher
Than LRC?
The basic answer is...LRC is not the only component of inrush current.
This raises the question: “What else is there?”
LRC is a steady state current. That is, it remains constant so long as the
rotor is not moving. Motors, however, are highly inductive loads. Like all
inductive loads they generate an initial transient (short lived) response
which causes the load to draw more current.
The steady state LRC is symmetrical when voltage is near zero. The
initial transient response raises the LRC curve so that it is no longer
symmetrical - thus giving it the name “asymmetrical offset.” This
asymmetrical offset usually lasts only a few cycles as the current settles to
a normal steady state LRC, which dies off as the motor begins to rotate
(refer to Figure 1).
Figure 1: Current waveform showing an asymmetrical inrush.
Peak inrush current
Current
LRC
Time
Motor “Inrush” Current when Starting Voltage is Zero
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High Efficiency Motor Protection
The asymmetrical offset is dependent mainly upon at which point on the
voltage wave the circuit is energized. If the circuit is energized at a
voltage maximum, there is no asymmetrical offset and the inrush current
is essentially the LRC for that current phase. However, if the circuit is
energized when the voltage is zero the initial inrush current is made
completely asymmetrical, that is, shifted from the nominal current axis
(refer to Figure 2). This makes the inrush current greater than the LRC for
that current phase. Also, in a three phase system, the odds of one of the
phases being at or near voltage zero when starting a motor is very high.
This explains the source of nuisance tripping. Considering the actual
asymmetrical inrush current could be, according to NEMA
manufacturers, as much as two times the LRC. A HMCP circuit breaker
(that is set based on the LRC) and is used with a high efficiency motor
will experience nuisance tripping during energizing. Thus, the inrush
could be 18 times the FLC -- much higher than the 13 times FLC that the
HMCP circuit breaker may be set to by the NEC.
Figure 2: First cycle current can differ greatly depending on what point on the voltage wave the
circuit is energized.
Current
Current
Voltage
Voltage
High Efficiency Motor Protection
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So What Do I Do About It?
If your motor control center equipment has already been installed, one
may have few options:
• Choose a HMCP circuit breaker with a higher instantaneous trip range.
• Substitute a thermal magnetic circuit breaker with a higher
instantaneous trip range.
If you are still in the planning stages, making a few additional
considerations now can save you a lot of headaches down the road.
• Get a complete set of specifications from the motor manufacturer and
be sure to request data on the actual maximum inrush current along with
the FLC and/or LRC ratio data.
• Specify motors with inrush to FLC ratios that would prevent you from
violating the NEC.
• Make certain the motor is built to NEMA standards.
• Do not exceed the nominal voltage by more than 2 or 3%.
• Encourage the National Fire Protection Association (NFPA) to further
address this issue in future editions of the NEC.
Publication PCP-1.8 - March, 1998
 1998 Rockwell International. All rights reserved. Printed in USA.