Min. Time bet. Motor Starts
Min. Time bet. Motor Starts
(OP)
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 ..
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 ..





RE: Min. Time bet. Motor Starts
RE: Min. Time bet. Motor Starts
FAQ237-1285: Limits for repetitive induction motor DOL starting
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)' ?
RE: Min. Time bet. Motor Starts
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(2B)+(2B)' ?
RE: Min. Time bet. Motor Starts
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.
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RE: Min. Time bet. Motor Starts
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..."
Which is why I shifted from thermal to mechanical when discussing that transient.
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(2B)+(2B)' ?
RE: Min. Time bet. Motor Starts
Bill
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"Why not the best?"
Jimmy Carter
RE: Min. Time bet. Motor Starts
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.