MOTOR TRIP ON RUSH CURRENT

[NEW ENGINEER AT OIL REF]

Hello everyone,
I have 1200A, 480V Westinghouse breaker, protecting a 450HP (1800rpm) pump.
it trips on inrush current.
How to fix this problem, should I replace my breaker??
Thank you

 

[tom baker]

It may not be the motor, but the load.
Measure the inrush current with a peak reading ampmeter
What are the megger readings historically and currently?
What type of breaker, IT only, TM or electronic?
Does the beaker have adjustments and if so what are the settings?
What type of overloads?
Has anything changed with the load, belts bearings, head?
What type of pump?

 

[Jraef]

Easiest thing to start with is the magnetic trip settings of your 1200A breaker. The magnetic inrush current on a motor that big is going to be in the neighborhood of 5500 to 7500A, so make sure the mag trip is set high enough. The NEC allows 800 to 1300% of the motor FLA, higher (up to 1700%) if it is deemed an “energy efficient” motor that will not allow lower settings. Often times the factory default setting is the lowest on the adjustment setting, so on a 1200A breaker that may be 6000A, which could easily be too low.

If the breaker does not have adjustable mag trips, I would not be using that for a motor load.

 

[retirede]

Is this a new installation, or something that used to work, but suddenly started having problems? The answer will aid in troubleshooting.

 

[David Castor]

Common problem. Increase the magnetic trip pickup setting, as Jeff suggested. I assume the motor and cable has been meggered?

 

[topgone]

Change the short-time setting (magnetic trip) of CB to 4X I_n , I_n being the setting of your LT. Say, you have set your I_n = 650A, set mag trip to 2,600A (4X) with a delay a bit longer than your accel time.

 

[JoeStillman]

New breakers come from the factory with the instantaneous set to minimum. If you crank it up, you might be chasing the problem upstream, depending on what feeds the 1200A breaker. Check the time-current curves for your breakers and make sure the upstream breaker has the instantaneous (magnetic) trip set higher than the ones downstream.

Your breakers might be in this file here...

 

[gar]

230314-0901 EST

I don not believe you are talking about "in rush" current, but you really mean "starting current". AC induction motors show virtually no "in rush" current.

Starting current to an AC induction motor is moderately higher than running current during the time to accelerate the motor to running speed. The duration of this high starting current will be a combination of the frictional and inertial motor load. Even an unloaded motor may have considerable inertia load requiring many cycles to accelerate the motor to running speed.

When I can find my plots of motor starting current with some varying inertia and friction loads I will reference them here.

.

Hello everyone,
I have 1200A, 480V Westinghouse breaker, protecting a 450HP (1800rpm) pump.
it trips on inrush current.
How to fix this problem, should I replace my breaker??
Thank you

 

[David Castor]

gar: I'm not sure why type of motors you are referring to, but a typical squirrel cage induction motor started across the line with have a locked-rotor current that is 5 to 6 times the rated full load amps. A NEMA motor nameplate will have locked-rotor code that gives an approximate range for the locked-rotor amps. In addition to the locked-rotor amps, there can also be asymmetrical inrush current (dc offset). The magnitude of this inrush current depends on the voltage phase angle when the motor is energized but can be as much as 1.6 times the locked-rotor current.

 

[don_resqcapt19]

230314-0901 EST

I don not believe you are talking about "in rush" current, but you really mean "starting current". AC induction motors show virtually no "in rush" current.

Starting current to an AC induction motor is moderately higher than running current during the time to accelerate the motor to running speed. The duration of this high starting current will be a combination of the frictional and inertial motor load. Even an unloaded motor may have considerable inertia load requiring many cycles to accelerate the motor to running speed.

When I can find my plots of motor starting current with some varying inertia and friction loads I will reference them here.

.

There actually is a very high current transient inrush of 10-20 times the motor full load current. This inrush is often what trips the magnetic part of the breaker. This high current is only about a half cycle long, and then it transitions into locked rotor current that decreases over time to normal running current.
Bussmann Motor Protection
From the above link:

When an AC motor is energized, a high inrush current occurs. Typically, during the initial half cycle, the inrush current is often higher than 20 times the normal full load current. After the first half-cycle the motor begins to rotate and the starting current subsides to 4 to 8 times the normal current for several seconds.

 

[gar]

230321-1007 EDT

I have found some of my previous comments and plots, and one of these threads is at
https://forums.mikeholt.com/threads/motor-trip-on-rush-current.2573170/ .

In my language "starting current" and "in-rush current" are quite different items, and when viewed on a scope look very different.

In the book used for the class I took on AC machinery the wording "inrush current" was never used. Rather "starting current" was always used to describe the initial current to get a motor up to running speed. That book was "Alternating-Current Machinery" by Bailey and Gault. When I started school at Michigan Bailey had retired, and Gault had died. I did know Bailey's wife. Bailey was an early pioneer in the electrical industry. Before becoming an electrical engineering professor at Michigan he was an associate with Henry Ford at an electrical power plant in Detroit in the early 1890s.

Bailey was at least the developer, and possibly the inventor, of the single phase capacitor run induction motor. This work was done in the 1920s as a result of a request by Detroit Edison for a motor to use as a refrigeration compressor motor. As a result of this development Tecumseh Products of Tecumseh, MI, was able to develop a fully enclosed refrigeration compressor and motor. Production of such refrigeration has now existed since the early 1930s. I have a freezer that has run for more than 50 years with no service of any kind. Good proof of the value of that invention.

Transformers and induction motors are quit different.

In a standard transformer one wants very tight coupling between primary and secondary. to get efficient energy transfer. When a transformer is turned off a very high residual flux state may remain in the core. When power is reapplied to the transformer the phasing may force the flux even higher causing a high current pulse.

In an induction motor there has to be a fairly large air gap in the magnetic path to allow the motor to rotate. This alone would account for a reduction in the residual flux at turn off. However, I believe there is gradual de-magnetizing of the core as a result of the alternating flux and gradual flux reduction as the rotor slows down.

I am not in the motor business, but I have never seen inrush current in any tests that I ran.

.

.

 

[Jraef]

Gar, your link loops back into this same thread…

 

[gar]

230321-1725

Thanks Jraef.

The location is

1-phase induction motor starting current

160215-2133 EST Many tend to lump motor starting current into the term inrush current. The starting current to an induction motor is not comparable in form to the inrush current to a cold incandescent bulb or a transformer core driven into saturation. In a motor there is a moderate air gap in...

forums.mikeholt.com

It is useful to see how the current phase angle shifts as the motor reaches full speed.

..

 

[topgone]

230321-1007 EDT
(snipped)

Transformers and induction motors are quit different.

(snipped).

IMHO, transformers and motors are similar at the instant when the rotor is not turning (locked condition) and the motor windings are supplied with rated voltage. You have the motor windings acting as primary and the rotor windings as the shorted secondary!

 

[gar]

23032q-2020 EDT

topgone:

There is a very major difference between an induction motor, and a transformer.

A transformer magnetic core has a very small equivalent air gap in the core, and a transformer has no rotating magnetic element. Transformers are run at a magnetic core flux level that takes them partially into saturation every cycle. The kind or magnitude of load on the transformer determines the residual flux level in the core at turn off. The timing of the next application of power to the transformer will determine whether it is driven further into saturation or not, and thus the magnitude of the peak turn on current.

An induction motor has a relatively large air gap in the magnetic circuit between primary ( field coils ) and the secondary ( the rotor coil ). Also a motor has has a slowly decaying and oscillating magnetic field as the motor armature slows down. Thus, residual flux in the motor core is probably close to zero when the motor comes to rest. I have never run experiments on this subject, but based on what I see for starting current this is my conjecture. I have never seen a motor starting current that would imply otherwise.

.

.

 

[ptonsparky]

Is it wrong to use inrush when the discussion is motors? Possibly, but we all know we are dealing with motors. Not transformers.

 

[David Castor]

A starting induction motor has both locked rotor current and asymmetrical inrush current. These are two separate things. This is why the NEC allows the pickup setting for motor short circuit protectors to be set well above the maximum symmetrical locked rotor current. This isn't controversial.