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Allen Bradley Motor Controller/Starter Bucket - With HMCP Cutler-

Hammer 30 Amp Circuit Breaker

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Motor starting problem and high motor inrush currents

Electric motor protection device depends on the

accurate and correct selection of overload(s), fusesand/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.

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, ar

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).

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Figure 1: Current waveform showing an asymmetrical inrush

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.

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Figure 2: First cycle current can differ greatly depending on what point on the voltage wave the circuit is energized

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.

SOURCE: Rockwell Automation | Allen-Bredley – High Efficiency Motor Protection

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