Min. Time bet. Motor Starts 1

[AKassem]

Hello Every Body ,

I'm facing a situation of too much motor starts per day for For Siemens LV Motor, 75 KW , 3000 rpm , 400V on an Air Compressor Application ..
What is the minimum allowed time between DOL motor starts ( or the mumber of starts per day) to protect the motor and the contactor also , knowing that the ambient temprature is around 40 degC & the motor starts successively from HOT state ..
I know this time depends on motor temp at starting and the ambient temp. and inertia of load .. but an approximate time will be helpful ..
What will be the time in case of using star/delta starter ?
Which Standards to refer for this subject ?

Thank You All for your help ..

 

[KllrWolf]

Starts per day is something that is part of the motor design. If the starts per day is not on the nameplate, then contacting either your supplier or the manufacturer with the model number of each part you are concerned about should get you the information you are wanting, along with answering any other questions about your application.

 

[electricpete]

NEMA provides some detailed recommendations in the first link here:
faq237-1285

It applies to DOL start. I don't recall seeing any discussion of extrapolating to delta-wye start in the document. Depending on load torque and timing, delta-wye start can certainly be more severe thermally. And mechanically there is that extra transition with the potential for out of phase if delta is not open long enough before wye closes. It would seem prudent to reduce the limit for wye delta compared to those published dol start limits.

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(2B)+(2B)' ?

 

[electricpete]

Whoops. I reversed the roles of delta and wye in my previous post. You know what I meant.

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(2B)+(2B)' ?

 

[jraef]

The first link in the FAQ that Pete showed is a direct copy from NEMA MG-1, the design guide for AC motors. Siemens for the most part designs their motors to be able to meet specs anywhere in the world, so yours will likely be designed to meet that, but only Siemens can definitively answer that question.

As to DOL vs Star-Delta, as I was taught it makes no difference in terms of the starting capability of the motor, the energy it takes to accelerate a load from a dead stop is fixed. All that changes is that in reduced voltage starting, you trade the peak height for the time width on that energy curve, but the area of the curve, which determines the heat in the motor, remains constant. I agree that there are other nasty transient consequences to Star-Delta starting and I abhor it as a starting method, but I don't think the effects of those transients cumulatively increase the overall acceleration energy, or at least not significantly. But neither will Star-Delta help out, so there is no good reason to use it. If you want to use a reduced voltage starting method, use a Solid State Starter. Still, no difference in the thermal effects on the motor, but it is a better way to do it.

the only starting / control method that does alter the thermal characteristics is to use a VFD. With a VFD you can accelerate without ever exceeding the motor nameplate FLA, assuming you can take as long as necessary to do so. This is because with a VFD you are always only applying the acceleration torque that is absolutely necessary, there is no excess energy in the motor. thermally, there are other issues with using a VFD that need consideration and there are a myriad of other energy related issues that get added to the mix, some good, some bad. But that is one way to increase the number of starts-per-hour.

"Dear future generations: Please accept our apologies. We were rolling drunk on petroleum."
— Kilgore Trout (via Kurt Vonnegut)
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[electricpete]

Jeff - good comments. It's not clear exactly whether your response was to original poster or as some kind of correction to me. In the latter case, I offer this response:

As to DOL vs Star-Delta, as I was taught it makes no difference in terms of the starting capability of the motor, the energy it takes to accelerate a load from a dead stop is fixed. All that changes is that in reduced voltage starting, you trade the peak height for the time width on that energy curve, but the area of the curve, which determines the heat in the motor, remains constant

The energy is the same regardless of voltage for unloaded motor start. That is approximately true for low load torque levels as well. But the approximation breaks down if load torque becomes a substantial portion of reduced-voltage motor torque (which is why I said "depending on load torque..."

I agree that there are other nasty transient consequences to Star-Delta starting and I abhor it as a starting method, but I don't think the effects of those transients cumulatively increase the overall acceleration energy, or at least not significantly

Which is why I shifted from thermal to mechanical when discussing that transient.





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(2B)+(2B)' ?

 

[waross]

We had a large compressor that was limited to three starts per hour. I put a twenty minute timer on it that forced it to run for at least twenty minutes each time it started. Then it was always ready for a start. We cycled the unloader to control the pressure.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[LionelHutz]

There are standard which give starts per hour and/or time between start numbers for each motor size when a certain load is applied. Unfortunately. the numbers don't mean as much once you change from this specified load and apply your load instead. A motor may be specifed as 2 start per hour and with NEMA they seem to use a certain rated ineria. So, when you use this motor on your load with 1/10th the inertia you could get away with a few more starts per hour. I have yet to see this addressed in any papers or standards, likely because it's too dependant on the motor design. Heck, even the motor manufacturer would likely have difficulty answering if you asked them.

I agree with what electricpete posted. A longer motor and load acceleration time means more thermal heating for the motor. Reduce the motor voltage and therefore the torque so the motor will barely accelerate and you can cause double or triple the motor heating compared to a full-voltage start. Installing any kind of reduced voltage starter will not help thermally.