surge due to "bump" test for motor rotation? 13

[electricpete]

During motor retest following motor removal/replacement, we confirm the direction of rotation by momentarily energizing the motor just long enough to get it rotating.

A fairly knowledgeable engineer recently told me that this can cause excessive surge stresses to the motor. Specifically, the full starting current is still flowing at the time the breaker is re-opened. He said that the interruption of this higher-than-normal current caused excessive voltage surges which could over time damage the insualtion. v=L*di/dt and the i in this case is ~ 6xFLA.

By the way we have mechanical circuit breakers.

I don't know enough about surges and circuit interruption to evaluate this concern. I am skeptical for the following reasons: I was under the impression that this bump test was fairly common practice. Therefore if it were creating severe surges, I would think I would have heard more about it.

What do you guys think?

 

[jbartos]

Suggestion: The motor bumping can be so considerate to the motor that the motor barely makes one revolution. If on no load there is practically no impact on the motor except a voltage spike. Then, transient voltage suppressors might be used for the motor bumping to protect the motor winding insulation.

 

[jOmega]

... and JB, the consequence of /b]NOT[/b] making one revolution is WHAT ????

Suggestion: Instead of a motor, JB, think of it in terms of a three phase transformer.

Suggestion: re-read my post of 20 & 21 Sept. to learn of other stresses that occur within the motor every time the windings are connected to line power when the rotor is in a dead-stop state.

JB, your proposal about utilizing a suppressor, while technically correct, lacks practical merit for the following reasons:[li]How many JOULEs must it be capable of handling ?[/li][li]How large (physical dimension) a unit would be required ?[/li][li]And how expensive would such a suppressor be as a percentage of original motor cost ?[/li][li]How much cost would such a suppressor add to the installation cost of the motor ?[/li]

New motors, can certainly withstand an occassional "bump test". Older motors experience insulation and bracing deterioriation as a consequence of heat and time; and so, are more succeptible to experience a failure during the "bump test".

However, I offer the following for consideration: [ul] Of the total number of motors in service that are more than 5 years old, what percentage experience internal failures during a &quot;bump test&quot; [ul][li]10% ?[/li][li]1% ?[/li][li]0.1% ?[/li][li]0.01% ?[/li][li]<0.01% ?[/li][/ul][/ul]

I think the answer will show why adding a &quot;suppressor&quot; to the motor connections is not a practical consideration. Also, while the suppressor would knock-down du/dt stress to the motor insulation, it does absolutely nothing to releive the mechanical stress that the windings are subjected to.

 

[jbartos]

Suggestion to the previous posting: I did not mean the permanent voltage suppressor. I meant some portable or mobile one.
I disagree with the reasoning that covers costs. Often, the motor is engaged in some process that has some value. Any downtime may prove to be costly. Therefore, a device as transient voltage or surge suppressor saving the motor insulation against damage or insipient puncture is very justified and required. Obviously, any fan for a dog house, for example, may or may not need such a suppressor.

 

[jOmega]

JB,
You are more than welcome to disagree.

Perhaps you have not yet learned that the majority
of such decisions are based in economics. Downtime
not being at the top of the list UNLESS... there is a high probability of damaging the motor every time it is subjected to a &quot;bump test&quot;.

Question: How many motors do you personally know of that have suffered an insulation failure consequent to a &quot;bump-test&quot; being performed ?

The TVSS that you propose, even in a portable configuration is not inexpensive, and the COST/BENEFIT RATIO numbers do not justify the expense in the majority of applications. The proof of this claim is in the reality that facilities having a dedicated TVSS for connection to a motor whenever a &quot;bump test&quot; is performed is unheard of.

Do you have one at your place of employment? Is there a procedure that mandates its use whenever anyone perfoms a &quot;bump test&quot; on a motor ?

If so, [ul][li]What is the largest size motor you would bump ?[/li][li]What Voltage(s)?and, [/li] [li]how many JOULES is the TVSS rated for ?[/li][/ul]

Also, as I have said a couple of times in this thread, failure from insulation du/dt stress is not the only failure mode. The mechanical stress that the windings are subjected to also causes failure and would not be prevented by a TVSS.

Look at the numbers, JB. Most motors are bumped at installation to verify direction of rotation. Of the hundreds of thousands of motors out there.... how many do you know of that have failed ? Is the number large enough to justify the expense of a TVSS ? Really ?????

Also, consider EASA shops; how many motors do you think they have bumped in the last 50 years ? What percentage do you think have suffered a failure as an immediate consequence thereof ? Can't be very many; they're still doing it today, and I know they don't connect a TVSS to a motor whenever they do it.

Can't be much of a concern to those learned folks, you think ? At least not enough to justify the cost of having one in every shop.

I'd guess they don't disagree.

 

[dazmac]

Hi Pete,

It's common practice here in the UK too ! However, in my experience, I uncouple the motor and give it a longer run, about 10 seconds. This is mainly because the contactor can be quite remote from the motor I'm testing and I need time to run from the control panel to it and check the rotation as it is coming to a stop !

If the contactor is local to the motor then I do a bump just as you do ! never caused me any problems.

 

[jOmega]

Daazmac:

1st preference is to bump motor when it is uncoupled from its mechanical load.

However, there are those, I'm afraid to say, that get lazy or rushed, and choose to ignore such logic.

The only other time that one could be excused from bumping a motor that is coupled to its load.... is when there is 100% certainty that neither machine nor personnel will be endangered as a consequence there of.

Can't think of any reason what-so-ever to not allow the motor to run up to speed, and then catch the rotation direction as it decelerates or coasts to a stop.