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?

 

[GordS]

From a surge point-of-view switching a motor off is similar to switching a motor on - the surge just changes sign. Motors have a surge capability whose magnitude is a function of the surge rise time - 1 pu at .02 usec and 4.5 pu at 5 usec (Red Book Fig 45 which refers to old motors, newer insulation systems are even better). So as long as you don't exceed this envelope, you do not stress the motor. In most case, there is enough capacitance in the cable feeding the motor to keep the surge voltage at safe levels - I have seen a paper and done a study that show about 50m of cable is usually adequate.

 

[electricpete]

Thanks Gord. Your comments that closing surge and opening surge strengthen's edison's comments. I don't get the comment about the opening being a minus of the closing and it doesn't seem follow from simple wave behavior: O->VL increasing front reflects/doubles and becomes 2VL (forward wave +VL, reflected wave +VL). VL->0 decreasing front reflects/doubles and becomes 0 (incident wave is 0 and reflected wave is -0). But the source voltage proposed during opening was affected by the di/dt. Maybe this di/dt voltage plays no role at all?

I'll have to think about it some more.

 

[electricpete]

Once again: isn't it true that vacuum breakers (without suppresion) create high di/dt upon opening which can produce dangerous surges?

If so, then we expect any similar effect will be lower on mech breakers due to longer arcing time. But also we then expect that the magnitude of current at time of opening is important in determining the opening surge.

 

[electricpete]

I did find some literature (epri 4862v1) which seems to address this questions.

For vacuum circuit breakers during interruption, multiple restrikes can occur resulting in escalating voltage which can reach 10 p.u. (1 p.u. = VLL* sqrt(2)/sqrt(3) = peak line-to-neutral voltage).

For air mechanical circuit breakers, the interrupting medium does not have as high dielectric strength and the same multiple restrike scenario is not possible.

From the whole context of their discussion I think they are saying that for air breakers, the only surge of concern occurs during closing, but for vacuum breakers there are surges of concern on both opening and closing.

 

[electricpete]

just to clarify,

for vacuum breakers (only), the magnitude of the surge on opening IS heavily dependent upon the current flowing prior to opening. People with vacuum breakers should probably avoid the quick bump if at all possible (let it run for 5 seconds).

for air breakers, any opening surge should be far less than 2 pu.

 

[jOmega]

EP:

Many decades ago, when I was a young inexperienced lad working on the test floor at a Westinghouse Electric plant, a wiring error on a control panel inadvertently connected the load side of the panel input circuit breaker to the line side of the L3 terminal of the breaker... Since the wiring was bundled... it wasn't obvious.

When 460/60/3 power was applied, the 4/0 cable connecting the panel to the distribution board came about 2-feet off the concrete floor..... before the breakers tripped.

It was at that moment that I learned about the torque force produced by the field around the wire resulting from the short circuit current passing thru it.

Motor engineers will tell you that the same torque forces exist within motors when subjected to DOL starting currents. Part of the varnishing and lacing applied to the windings, is to restrict their movement under such conditions. Now, while you can lace the wires outside the slots, you cannot lace the wires within the slots.... and they will find a way to move... ..

Interesting things that go on inside the motor housing that we don't see ... and don't think about .... until a failure occurs.... and we try to find a reason...

Kind regards,

jO

 

[jbartos]

Suggestion to the previous posting: Apparently, there are different qualities of electrical motors around. One type of electrical motors has conductors in slots placed at random, other type will have the winding consisting of conductors carefully laid without any room for movements and beginning turn far away from the ending turn. Visit

http://www.lincolnmotors.com/pdf/tb-100.pdf

etc. for more info

 

[edison123]

pete,

refer this site for retrike voltages due to VCB's without surge protection

http://www.electricityforum.com/et/issue0401/i04_refiner.htm

 

[jOmega]

JB,

The only motors that I know of that use an ordered winding pattern were Lincoln Electric.... They used machines that looked like a sewing machine lying on its side that was made by a company in Milwaukee that is no longer in business. The coils were actually wound into the stator slots.

The vast majority of motor manufacturers use a random wound method for producing stator coils. And then they pound them (force them) into the slots.

As to your claim about them not being able to move..... BullPuckey.... !

jO

 

[electricpete]

jO
Today form wound motors are widely used for medium voltage (2kv and above), random wound for low voltage (<600vac).

But your comments were useful... I did not specify which type of motor I was considering.

 

[jOmega]

Peter,

Your correct. My comments were predicated upon low voltage type motors.

FYI ..

ABB uses form wound in their larger low voltage motors (HXR type).

It's more expensive as you can imagine.... but there are
benefits as well that justify the added cost.

Form wound stators rarely, if ever, experience dv/dt failures when supplied from power darlington and IGBT type inverter bridges of the PWM and vector type inverter bridges.

 

[jbartos]

Suggestion: Also, IGCTs are more suitable for the motor controls. Visit

http://www.ewh.ieee.org/r1/schenectady/sep24_1999.html

http://www.manufacturing.net/ctl/index.asp?layout=article&articleid=CA189352

http://www.electricalline.com/images/mag_archive/14.pdf

etc. for more info

 

[jOmega]

JB,

What LOW VOLTAGE VFD manufacturers are presently using IGCTs in their products today ?

 

[jbartos]

Suggestion: Presently, IGCTs are mostly used in medium voltage drives. Visit

http://www.controldesign.com/Web_First/CD.nsf/ArticleID/MFEY-4N3KZA/

http://www.controldesign.com/Web_First/CD.nsf/ArticleID/MFEY-4MTL5C/

http://www.tc.umn.edu/~chris143/WGI8/Files/PEBB Developments.pdf

etc. for more info
Presently, there are a few manufacturers of IGCTs. They were introduced by ABB that is using them in medium voltage drives only.

 

[jOmega]

JB,

That's my point; IGCTs were developed for use in medium voltage drives.

So, when you made your comment: &quot; Suggestion: Also, IGCTs are more suitable for the motor controls.....&quot;

You lost me on that one... because I couldn't see any relevance to the discussions in this thread.

JB, there is nothing in the previous posts about IGBTs ... VFDs....etc..... that would seem to warrant your comment.

So, please tell me why you felt compelled to make that comment about IGCTs and how it relates to the discussion in this thread which, unless I missed something, is about bumping a motor, and what harm can be caused by doing so....


Thanks.

p.s.

FYI JB, medium voltage drives usually have a filter in the output that knocks down the dv/dt... and renders as close as possible, a sine wave to the motor.

Without such filtering .... lots of motor failures.
Fast switching and medium voltage = very high dv/dt rates.

 

[sparky1976]

Hi,

Just a silly question, In motor junction box (where you connect the motor lead to cable feeeder) in MV motors usually have some kind of arrester to to protect motor from switching surge, is that count on this thread ?

 

[jbartos]

Suggestions to jOmega (Electrical) Sep 26, 2003 marked ///\\JB,
That's my point; IGCTs were developed for use in medium voltage drives.
///Yes, agreed by ABB.\\So, when you made your comment: &quot; Suggestion: Also, IGCTs are more suitable for the motor controls.....&quot;
///Since the original posting is not limited to any voltage level.\\You lost me on that one... because I couldn't see any relevance to the discussions in this thread.
///Please, just recall the &quot;excessive voltage surges&quot; context in original posting with respect to IGCTs that were designed to reduce those by somewhat slower switching with respect to IGBTs.\\JB, there is nothing in the previous posts about IGBTs ... VFDs....etc..... that would seem to warrant your comment.
///Please, see my previous statement.\\So, please tell me why you felt compelled to make that comment about IGCTs and how it relates to the discussion in this thread which, unless I missed something, is about bumping a motor, and what harm can be caused by doing so....
///Addressed above.\\Thanks.
p.s.
FYI JB, medium voltage drives usually have a filter in the output that knocks down the dv/dt... and renders as close as possible, a sine wave to the motor.
///Yes, true. I am familiar with those.\\Without such filtering .... lots of motor failures.
///Yes, agreed.\\Fast switching and medium voltage = very high dv/dt rates.
///Yes, that is why there are IGCTs invented thanks to ABB.\\

 

[cerpico]

This is a common practice for large motors as well as small motors where I used to work. The nuclear electrical industry motors ranged anywhere from 480V up to 6.9KV We bumped the motors for rotation uncoupled. I empahasize UNCOUPLED. We found the effects which you have spoken of to be negligable. I would not recommend bumping a motor for roation check while coupled to its work output. Especially if the leads had been removed and relanded.

 

[cerpico]

This is a common practice for large motors as well as small motors where I used to work. The nuclear electrical industry motors ranged anywhere from 480V up to 6.9KV We bumped the motors for rotation uncoupled. I empahasize UNCOUPLED. We found the effects which you have spoken of to be negligable. I would not recommend bumping a motor for roation check while coupled to its work output. Especially if the leads had been removed and relanded.

 

[electricpete]

thx cerpico. That is the main purpose of our bump test... to verify rotation BEFORE we couple.

We did have one instance of pump somehow got coupled and started with motor leads reversed. It wasn't pretty.