75 HP or 100 HP

[MWeb7]

I have a vacuum system with a 75 HP pump running on 480 with a SF of 1.15. FLA is 86.

The motor is running overloaded at/above the service factor load (~99-100+). We had a contractor look at the pump and give us three recommendations:

Rewind the motor to a 1.25 SF
Buy a new motor with a 1.25 SF
Buy a 100 HP motor to handle the load

We have had the motor fail many times and recent testing resulted having the largest difference in resistance between T leads be about 0.72% (readings in mOhms: 83.2, 82.6, 82.8). Resistance to ground is over 200Mohms for each T lead. The motor is only a few years old and has run for 12 hours every 36 hours. Is 0.72% a big enough difference to be concerned?

I am weary about buying a new motor/rewinding a motor for a higher SF just to run it above the FLA rating. Would this cause potential problems such as the current ones?

 

[electricpete]

Also I was making assumption as Dick said that there is still substantial margin between steady state operation and peak torque... bounce the 33% mentioned upgrade against typical 250% peak torque. The peak torque concerns generally relate to momentary overload rather than steady state thermal performance.

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[electricpete]

One more thing to add - being near breakdown torque does not imply being near saturation (other than localized tooth /leakage reactance saturation associated with high currents alluded above). The reason that the ratio of current to torque increases as we increase load toward breakdown torque is that the power factor is decreasing due to increasing rotor frequency resulting in increasing rotor reactance.

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[electricpete]

The last comment - I was not sure why saturation was being tied to breakdown torque. Now I think I see the point was that if you try to increase flux level during rewind to increase breakdown torque, that will push you toward saturation. That is true... the options are to push the core to slightly higher flux levels (use up some of existing magnetic margin) or more likely to use up some of the substantial margin between steady state operation and breakdown torque which exists in original design... as long as we don't expect the motor to be exposed to temporary overloads that will exceed the new breakdown torque.

Sorry for repeating myself and Dick. I'm done now...

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[DickDV]

Sorry, Scotty. Indeed, it is you!

And, electricpete, I am fascinated by your more detailed analysis of my comment about increasing motor capacity. I've watched motor shop people do this but have precious little understanding of exactly what's happening.

You, of course, have the details and I defer to your view of this. It sounds like you believe this type of redesign is possible.

 

[electricpete]

Thanks. I probably made it more complicated than needed with my rambling, but I agree with everything you said. I think 133% uprate (75hp->100hp)is a distinct possibility for a small motor like this, but a competent shop (like edison and others) would have to look at what they have to work with in that specific motor to see what can be done... and if there was an off-the-shelf that would definitely be the preferred option.

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[edison123]

I agree with scotty on the new motors having not much margin for an upgrades since everything has already been "optimized" already. I have done many uprates of older motors (typically pre 70's) when the designers were very conservative and allowed liberal margins, both in the current density and insulation thickness. For LV motors, I do only class H since I have found the class H insulation is not only thermally superior but also electrically. Typical uprates vary from 10 to 25% and in some pre 50's machines, it was as high as 50% (the conductors were individually cotton/empire cloth covered).

As for magnetic loading, nothing much to play around since almost all motors are designed around the knee of the saturation curve and any steep increase in flux density (like over 10%) will cause excessive no-load currents, which the clients don't like.

Muthu

http://www.edison.co.in

 

[electricpete]

It sounds like you are saying there is not a lot of thermal or magnetic margin in today's design. That is a little different twist than what I understood Scotty to say... it sounded to me that he suggested the core flux density would necessarily be challenged through an uprate. I don't see that to be the case....if thermal margin were available there is no need to increase the flux density (provided that margin between breakdown and momentary overload is maintained).

But it is a minor distinction. The more relevant question is whether it can realistically be done and I'll defer to your opinion on that.

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[ScottyUK]

Hi ePete,

I think I had in my mind that the motor would be operating with a higher flux density to achieve the higher output, but in retrospect I think others are considering continuous operation much higher up the slip curve than would normally be permissible and maintaining stator flux density somewhere near nominal.

I am not 100% certain how well the rotor would cope with the higher power given its limited ability to reject heat to the outside world. The first thing to experience problems might be the bearings as heat is conducted out along the shaft, although the rotor would obviously contribute considerable heat to the stator through the shared cooling air circuit.


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If we learn from our mistakes I'm getting a great education!

 

[electricpete]

Thanks - that makes sense.

NEMA MG 1-12.39.1 specifies minimum breakdown torque of 200% FLT for motors in this size range, and typically motors exceed the required minimum by quite a bit (the example 60HP motor I just posted in the other thread on power factor wiggles has breakdown torque around 350% of full load torque). So I suspect that from among the multiple constraints, breakdown torque is not a limiting constraint in the design... for example if you tried to lower breakdown torque to the minimum you might bust an efficiency target. For pump and fan loads of this size in NEMA world I don't think there would be a problem uprating the motor 33% without changing breakdown torque if there is thermal margin.

Good points about the rotor etc. It is not a simple review and usually ends up getting a lot less attention/expertise than the OEM's would throw at a new motor. And for a small motor like this, I imagine the economics of large engineering review become unattractive pretty quick

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[DickDV]

Guys, I don't know if the application needs all 100hp. We were talking 100hp because that's the next size up.

If he only needs, say 84hp, then we have an easier time with the redesign.