Motor inrush > LRA

[Cold Fusion]

Background:
40hp, 4pole, Design B, Code G, 47.7FLA, meets MG-1 standards for "energy efficiency", factory test data shows LRA at 287A - which is consistent with a Code G (both NEC and MG-1).

Motor will occasionally trip (on startup) the instantaneous CB when set to 600A. Does not trip when CB set to 800A. There does not appear to be any thing wrong with the motor. Megger is fine, phase voltages are pretty even (within in a few percent), running current is about 60% FLA and also within a few percent.

The Question:
If the motor is spinning backwards when started, will (can) this increase the inrush current?

I can see a failure mode where this could happen. But, I can't get at the motor right now to see if this is happening - hopefully I'll be able to get at it later.

And I don't have any reference books here to go through a motor model to see how I might expect it to behave - next week I will.

Another interesting piece: I called the mfg engineering and asked about reverse rotation on starting affecting the inrush current. Response was, "This is outside of the operating envelope. We have no data on that scenerio."

Any insight will be graciously appreciated.

cf

edited to fix the factory LRA and FLA

 

[Besoeker]

The Question:
If the motor is spinning backwards when started, will (can) this increase the inrush current?

I don't think it would affect inrush or starting current very much.
For reverse rotation the starting time be longer, of course.

 

[bth0mas20]

I would question the system that the motor is connected to. Is the motor started under load? Are their any other protective features other than the Breaker.....Why arnt the overloads tripping before the breaker?

 

[Cold Fusion]

(cut) Is the motor started under load? (cut)

It's a pd pump, so it always starts under load. However, some times the line is pressured up and that would require extra torque to turn the pump. I'm not seeing why this would affect the LRA or inrush. "Shaft not moving" is the same as "shaft chained down" as far as an instantaneous CB is concerned.

(cut) Are their any other protective features other than the Breaker.....(cut)

Yes, the instantaneous CB is part of a listed combination controller exactly per the NEC. So there are overloads.

(cut).....Why arnt the overloads tripping before the breaker?

Well, it should be because the current is high enough to trip the instantaneous CB, and still below the overload T-C trip curve.

cf

 

[Doug S.]

Is it SqD?
When our facility was new I took the time to do all the calcs and set the trips. ( Rather than just guesstimate. ) ALL, yes, all utilized tubs in our MCC tripped on start up. It was a rather embarrassing start-up. I double checked my calcs and they were good. Humbled, embarrassed, and pressed for time I gave up and bumped them all up. No more problems.
I had noticed similar instances in the past w/ SqD, but this was 30+ units at once running pumps, air handlers, fans, and compressors. This confirmed to me the "problem" was the trips being over sensitive.

Good Luck,
Doug S.

 

[cadpoint]

Background:
40hp, 4pole, Design B, Code G, 47.7FLA, meets MG-1 standards for "energy efficiency", factory test data shows LRA at 287A - which is consistent with a Code G (both NEC and MG-1).

Motor will occasionally trip (on startup) the instantaneous CB when set to 600A. Does not trip when CB set to 800A. ...

I have a question and its no answer! If this motor is so BIG and specific, why doesn't it have a dual element fused breaker in front of the 600 Amp breaker?

 

[GeorgeB]

It's a pd pump, so it always starts under load. However, some times the line is pressured up and that would require extra torque to turn the pump. I'm not seeing why this would affect the LRA or inrush. "Shaft not moving" is the same as "shaft chained down" as far as an instantaneous CB is concerned.

Starting time will be longer under more load, and "instantaneous" isn't really, just almost.

My area of specialty is hydraulics (fluid power). When practical, we greatly prefer to start with the outlet vented, even if only for less than a second ... longer is better and depends on motor start time which can extend to 2 or 3 seconds on larger motors or with higher inertia loads. Can you possibly commute the outlet and inlet (or outlet to reservoir) for a second or so on startup?

 

[mull982]

The Question:
If the motor is spinning backwards when started, will (can) this increase the inrush current?

I dont see how a motor by itself can increase the inrush current by spinning backwards since all of the internal motor paramaters are the same but just in the opposite direction.

The inrush current is based on a locked rotor condition and will never go above the LRC because this value assumes that the rotor is locked. The LRC may persist for a longer amount of time however if the motor cannot start the load.

Depending on the application that the motor is connected to, I can see the application itself having an effect on the starting current with the motor running in reverse. However remember the starting current will never be higher than the LRC for the motor rather it can only exist for a longer time if the motor cannot turn the load when running in reverse.

 

[Goroon]

Motor inrush is nomialy 6 to 8 times nameplate depending on application. Maximum Locked-Rotor Current in the tables of 430 are used to size disconnects to accomidate the inrush in the disconect rating (also look for a LRA value on contactors, Knife switches...] The act of "Plugging" a motor, starting while turning reverse of run rotation, can result in inrush currents of 10 to 12 times nameplate. all this is based on design and application. The damage to the CB and its reliabilty would be my next concern from repeated resetings.

 

[Jraef]

Major semantics issue going on here.

1) "Inrush Current" is the current that flows for a few cycles only to establish the magnetic flux in the motor. Essentially until a motor stator induces a current flow in the rotor cage and the lines of flux in the rotor cage start to pass through the stator fields to create inductance, the stator windings are nothing more than a short circuit. Current flow in that moment is then limited only by the resistance in the winding conductors themselves. By the way, this is essentially EXACTLY the same kind of Inrush Current you see in transformers when you first energize them. In older and more standard motor designs, that amount of Inrush Current was typically 700-1100% of the rated FLA.

2) "Starting Current" is the peak current after the inrush is over that the motor will pull in order to accelerate until it gets to about 90% of full speed. If the shaft is "chained down" or otherwise unable to move, the starting current will remain at the Locked Rotor Amps / Current (LRA/LRC) until some protective device interrupts the circuit. That is the current referred to in the NEMA Code Letter of the motor.

When we are speaking of having breakers adjusted higher to allow for high "Inrush Current" in newer energy efficient motors, it is referring to the true Inrush Current as described in 1) above, NOT the Starting Current described in 2). EE motors use techniques to lower the copper and iron losses in the motor when it is running. The side effect of this is lower resistance in the windings, meaning the true Inrush Current (that flows for a few cycles only) is a lot higher than what people were used to, sometimes as high as 2000%! So breakers that typically came set to 700 or 800% FLA to allow for typical inrush current started nuisance tripping. For that reason, the NEC has made allowances to provide for higher settings of the magnetic trips of Instantaneous Trip Circuit Breakers when it is demonstrated that a specific motor is not going to start without nuisance tripping.

 

[Besoeker]

Major semantics issue going on here.

1) "Inrush Current" is the current that flows for a few cycles only to establish the magnetic flux in the motor. Essentially until a motor stator induces a current flow in the rotor cage ...

It's there until steady-state is established in the stator field. It exists whether it is a cage or wound rotor machine even if, in the case of the wound rotor machine, the rotor is open circuit.

 

[glene77is]

1) "Inrush Current"
2) "Starting Current"

I think if these two terms as being the same phenomenom.
In a sense, I see all the current terms as being related,
but I see them observed at different times during operation.

I say it this way:
(1) we handle the Initial current with the Magnetic OCPD (800%),
(2) we handle the Short-term Overload current with a Thermal OCPD (175%),
(3) we handle the Long term Running current with Overloads (100%).

I'd like your comments on my thinking.

.

 

[chaterpilar]

Check before you calibrate.

If you have a clampmeter with MAX current recording facility ( eg Fluke 337) check the max amperes drawn.

I anticipate it to be around 300 amps, and once you know that figure then you can set the breaker settings.

cheers.

 

[VinceS]

"Motor will occasionally trip (on startup) the instantaneous CB when set to 600A. Does not trip when CB set to 800A. There does not appear to be any thing wrong with the motor. Megger is fine, phase voltages are pretty even (within in a few percent), running current is about 60% FLA and also within a few percent."

If... Check NEC 430.52(C)(3) Instantaneous trip CB's Exception #1, You have a limitation of no more than 1700% of FLA for a Design Class B Motor.

Then... If your FLA is 47.7 your MAX could be over slightly 800A, using the CB specified and FLA @ 1700%. This is why it works on the 800A setting. I agree with using the Fluke 337, but try the inrush feature. I use this to set CB's in just this instance...

 

[Cold Fusion]

(cut)The act of "Plugging" a motor, starting while turning reverse of run rotation, can result in inrush currents of 10 to 12 times nameplate. all this is based on design and application. (cut)

Goroor -
Thanks for your response. Would you have a reference I could look at?

cf

 

[Cold Fusion]

(cut)the true Inrush Current (that flows for a few cycles only) is a lot higher than what people were used to, sometimes as high as 2000%! (cut)

Jraef -
Thanks for your response. Could you recomend any references?

I did get a chance to connect a Dranetz PX-5 and record a couple of starts (shaft stopped - not rotating backwards) And there is the cutest little bump to about 450A for about 20ms then the current settles down to expected LRA, about 300A.

This of course would be a graphic representation of the inrush you speak of and the difference between inrush and LRA.

Now, if Goroon has a reference to a model for plugging I think I'll have what I'mlooking for.

cf

 

[Cold Fusion]

Girls and guys -
I truely appreciate the spirit of all your help for selecting instantaneous CB settings. However, I do have a good grasp of NEC 430.52 and that's not what I'm asking about.

Thanks to some of the discussion here, I now have the issue of inrush separated from LRA. So the question is still pretty much the same:
The Question:

If the motor is spinning backwards when started, will (can) this increase the inrush current?

I got a look at a Chapmans Electrical Machinery today and it mentions plugging, but it does not show a model. However, it alludes to there being enough information for me to figure it out - Maybe I'll get some time tomorrow - or I'll get lucky and one of us will give me a jumpstart on where to look.

cf

 

[Ken Harris]

CEMF Issues

CEMF Issues

Have you considered the CEMF when the motor is rotating in reverse. If this is a pump, try having the pipefitters install a Suction and discharge check valve in the lines to prevent the pressure from counter rotating the motor when shut down. It should reduce your inruch current and alleviate start-up loading