Number of starts in a motor

[luis7006]

Hello,

I would like to receive your feedback about:
How can we determine the number of starts per hour or allowable conscutive startings of a motor?

If we have not available information, how can we specify this??

We have read that NEMA defines a typical 2 colds/1 hot starts, this could be a good reference?

I thank in advance your support

 

[Skogsgurra]

It really depends a lot what size the motor is and what starting method you are using.

So, what is the size, power, motor type, starting method?

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[oldfieldguy]

The motor manufacturer typically has this information for larger motors as part of the documentation package.

For smaller motors, you may have to depend on either direct measurement of internal temperatures via RTD, etc., or by the algorithms for thermal capacity in motor protection devices.

old field guy

 

[jraef]

If you can get hold of a copy of NEMA MG-1 specifications, that information is in there as a minimum. Some manufacturers may exceed it. It does vary greatly by size; small motors may be much much higher. But keep in mind this will only apply to NEMA frame motors which stops at about 250HP 460V.

Anything larger and it is a design spec that may be specific to any given motor only. I once had a customer tell me he got a great price on a 500HP Westinghouse motor and wanted me to determine how often it could be started with a soft starter. Turns out the motor was application specific and was rated to start 1 time in no less than 24hrs.

 

[itsmoked]

no less than 24hrs

Yak!

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[jraef]

Yeah, I think that's why he got such a bargain on it. Also, it was 500HP 4160V with a 2-1/8" shaft. It was designed for a ventilation fan specific to the aluminum industry so they ran continuously, only starting if there was a power failure.

He wanted to put it on a Banbury Mixer (rubber shredding machine in the tire industry). It wouldn't have lasted a day.

 

[Skogsgurra]

Horror story about starting motors.

There is this customer with his heat pump. And the 11 MW synchronous motor being DOL started on 10 kV line using the (shorted by thyristors) excitation winding for starting.

For some reason, the rotor temperature supervision and start blocking relay do not work, have been tampered with - or never existed. Operators come and go and most of them don't know anything about motors.

I first met this miserable machine in 2003. It was then dead and I saw it in the winder's shop. Dismantled and getting its rotor repaired. My task was to verify that the thyristor device that shorts the rotor winding worked and also verify that the voltage, where it switches off to let the DC excitation in, was correct.

The machine had had the same problem before. And this september, we met again. Three major repairs in about ten years. Quite expensive an excersise.

I volunteered to check it up and found that:
1 the cos(phi) transducer was set to work vith D connected VTs - but ran on Y connected VTs with neutral connected
1a which resulted in cos(phi) transducer having a working range from full capacitive to just under 1(cap)
2 the supplier had told the operators to run at a fixed cos(phi) being 0.975(cap) because "the grid could not accept reactive power"
3 the operators did not know - or did not care - to wait for the machine to cool down between starts
4 the device to prevent too many starts in too little time did not work
5 every time the motor failed, it did so on the third start attemt
5a the motor failed on third start attempt and that was about one hour after the first attempt
6 motor manufacturer says it can be started twice in a one hour period and then the next start had to wait six hours

I still haven't made them understand what they do wrong and that such a machine shall be treated with some care. There is no electrical engineer any more, so no one understands what I say and no one seems to care either - except when the machine dies next time. I have now installed a recorder there so we can see what happens next time. A fast one so we can see the sine waves and their phase relations, the excitation current and the speed. I hope that helps.


Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[oldfieldguy]

Another horror story:

One of the local refineries installed an 8,000 horsepower motor as the driver for a coke-cutting pump. We fed it from a 13,800 volt bus through a captive 13,800 – 4160-volt captive transformer, using the 13,800-volt circuit breaker as a motor controller. As the high voltage power systems guy, I saw to the commissioning of the new circuit breaker, the protective relaying, including differential protection for the transformer and a Multilin relay for the motor protection.

The motor itself was located a thousand feet away through the unit. We tested for the normal winding resistance and insulation integrity stuff. I was confident of the integrity of all the electrical components in the system.

Comes the day we’re supposed to run the motor for the first time. We have the usual crowd: electrical, mechanical and production folks all over the place to see the new toy run. I station myself in the switchgear building near my pet circuit breaker.

First start is a “bump” for rotation check. Radio communications from my end to radio on the other end, and they punch the start button and the stop button in rapid sequence, just enough to see the shaft turn. It’s correct.

It’s time for a run to check vibration, etc. Another quick radio conversation and I and the client’s electrical engineer tell them that we’re ready at our end. We hear the breaker close and watch the current jump up and settle down below FLA. “Good,” I think.

Wrong! The radio screams “Shut it down, shut down, shut it down!!!” I slap the emergency stop and the breaker dutifully opens.

The radio sounds again. “It smells like a barbecue out here…” For my friends who may not be familiar with American barbecue, it usually involves cooking over a smokey WOOD fire.

I take the radio at my end. “Let me ask a silly question,” I say. “When you got this motor in, did you also get a pallet with a couple of wooden crates about a foot and a half square on it?”

Radio pops. “Yeah. We thought that was spare parts. They’re still at the warehouse.”

I replied, “Those are your bearings. This is a sleeve-bearing motor. It’s shipped with a wooden block supporting the shaft inside the bearing housing. That’s where the smoke comes from.”

The motor was torn down for inspection and the shaft was found to have warped from a few seconds of running 3600 RPM on oak blocks instead of the sleeve bearings..

The moral of the story: There are likely several. I, being a poor ol' contractor with a scope of work limited to the electrical power system, never looked at what work was performed by the other disciplines during the course of the project. Maybe I should have...


old field guy

 

[unclebob]

I must admit. Sometimes, in large projects, I stick to my part and don't get involved in other's area... May be I should.

 

[itsmoked]

I always get as involved with the whole thing as possible.. I try to wallow in the project. Why? Techno junkie addict.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[Skogsgurra]

Nom oblige, Smoked!

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[oldfieldguy]

Smoked, skogs-

I understand, but as a contractor working in some of these facilities, they can be very parochial in their approach, as in "You're the electrical guy. We have mechanical EXPERTS here for that."

It's sad to contemplate how it works out... Especially when the "darling" of the mechanical department doesn't ask questions about things...

old field guy

 

[itsmoked]

I realize that oldfieldguy but I generally go for the, "Wow that's really interesting! So how does that work"? Which seems to always work, as apposed to, "Did you check x?"

Skogs I'm not getting that connotation.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[Skogsgurra]

You know the saying "Noblesse oblige"? Meaning that "If you are of noble rank, then you are obliged to behave like one".

So, "Nom oblige" means that your name (itsmoked) obliges you to behave like one that doesn't fear catastrophes - like smoke and fire. And obviously you don't.


Gunnar Englund

http://www.gke.org

--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[itsmoked]

Thanks for the explanation. Much better answer than what I was guessing.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[jraef]

I too have a very difficult time biting my tongue when I am on-site for one reason and see something completely different happening. Especially if I am apparently the only one recognizing it. I just had this very discussion about start-per-hour at a startup today. I was there for commissioning of a VFD with a Soft Starter for bypass. VFD went fine, soft starter needed reprogramming so we had to est it for the user. After 2 tests I said we needed to wait for a long time to test again. The pump supplier said no, I was wrong, go ahead and test it the third time (he needed to go home). Naturally, the temperature sensors tripped the MPR and it blocked start... for 6 hours! Now they are all screwed (but I was done so I got to leave!).

Last year I got into a fight with a "pump expert" when a pump failed to do the job. I was hired to "tweak a 100HP drive" because the PID loop was apparently not working right and the tank level was rising too fast. When I arrived, the VFD output was pegged and the tank kept filling. Without even breaking out a slide rule (homage to Skogs), I determined in about 3.2 seconds that the pump was too small. Turns out they were replacing a 400HP full speed pump and motor. Hmmm....

Pump guy said it was the VFD's fault, I didn't know what I was talking about because I was not an ME. He insisted on fighting with me on this in front of the customer, which essentially succeeded in making himself look more and more like an idiot.

 

[oldfieldguy]

I am quite familiar with the "It's something wrong with the motor" syndrome among mechanical types. In my new capacity, I get to stick my head into those conversations and try to set things on the correct path.

In the story I cited above, I was just a dumb ol' contractor hired to set up the electrical power stuff that the plant staff didn't have people qualified to do.

They had their own "qualified" mechanical people and basically told me that I didn't need to concern myself with the mechanical side of things at all.

I would have happily overseen the commissioning of the mechanical aspects of the motor had it been within my scope. As it turned out, I was working on other stuff inside the substation when they supposedly completed their "mechanical checks". They probably saved a few hundred bucks by not giving me the work.

I was there on site when they pried the lid open on one of the crates containing an uninstalled bearing. I was there when they had the post-failure meeting and read the report that the shaft run-out NOW was outside allowable tolerances.

I think I showed admirable self-control by refraining from the mention of "I told you so".

old field guy