speeding up a motor
- Thread starter jaykool
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The available torque will reduce by about 25%. Not a big deal if it is sufficient to drive the load.
I can't say for sure but I really don't think bearing problems would be an issue.
If the motor has a shaft mounted fan, and many do, motor cooling will be improved at the expense of the increased power required to drive that fan. That will reduce a little of the available power for the driven load.
GeorgeB
ElectroHydraulics engineer (retired)
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- Greenville SC
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- Retired
That 78HP is what the LOAD will present to the motor at 1.25*base speed. The 40HP motor, if like most, will be rated for constant torque below 60Hz, and for constant power above 60Hz.
SO ... if your load follows that law, you would need a 75HP motor (3600RPM@60Hz, 4500RPM@75Hz)
- Location
- San Francisco Bay Area, CA, USA
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- Electrical Engineer
The affinity law applies to centrifugal loads, such as pumps and fans. If you have a 40HP motor on a centrifugal pump and you increase the speed to 75Hz, the PUMP will require 1.95X the HP it required at 60Hz. So if the pump only required 20HP at 60Hz and you had a 40HP motor on it, no problem. But if the pump was using 30 of that 40HP (75%, which is common), then at 75Hz it is going to need almost 60HP. Forget the breaker issue for now, you will overload the motor.
Why don't you provide us a FULL description of what you have and what you want to do so we can point you in the right direction.
Why don't you provide us a FULL description of what you have and what you want to do so we can point you in the right direction.
I hope this makes sense...this is a blower motor application...I am told that it has characteristics of a centrifugal pump and a positive displacement pump (curve is in between the two).
The vendor rep told the project manager that at 60Hz the motor should be seeing 21.8HP...and increasing to 75Hz will increase to 38.5HP.
I plan on taking amp measurements and verifying at 60Hz that we are drawing close to 30A. If this is the case, I will inform the PM that we can run motor at 75Hz safely.
Thanks for all the help.
The vendor rep told the project manager that at 60Hz the motor should be seeing 21.8HP...and increasing to 75Hz will increase to 38.5HP.
I plan on taking amp measurements and verifying at 60Hz that we are drawing close to 30A. If this is the case, I will inform the PM that we can run motor at 75Hz safely.
Thanks for all the help.
- Location
- San Francisco Bay Area, CA, USA
- Occupation
- Electrical Engineer
If you have a 40HP motor that the mfr says is good to 90Hz and a 40HP drive with a load of 38.5HP at 75Hz, you should be good to go. program the protection in the motor as a 40HP with the standard nameplate FLA. A 100A feeder should be fine, assuming you have the correct size conductors of course and that the VFD is OK with being protected by that large of a breaker (or has its own breaker or fuses with it).
I agree with Jraef. If the load is 38.5HP at 75 Hz, you should be OK.
I have just a minor niggle though........
All the fans and centrifugal pumps I have dealt with have been cube law loads. Applying that to your blower taking 21.8 HP at 60Hz equates to about 44HP.
That said, I don't know what kind of blower it is.
- Location
- San Francisco Bay Area, CA, USA
- Occupation
- Electrical Engineer
I'd guess probably a new "screw" blower like the Atlas Copco ZS, kind of a cross between a lobe type ("roots") PD blower and a screw compressor. I've seen them discussed but have never connected to one yet.
Could be, I agree.
My only knowledge and experience of such blowers has been with supercharged IC engines.
My current car has an exhaust driven turbo charger.
The previous was a supercharged Mercedes. The compressor was driven by the crankshaft.
A relatively small engine even by UK standards but still a top speed of over 140 mph.
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