rhenders
Member
- Location
- Alabama, USA
Question: I have a 20 HP, 208V, 3 Phase, 3600 RPM electric motor driven by a Variable Frequency Drive (VFD). If I run that motor at 1700 RPM, will it still make the full 20 HP?
Same HP at Lower RPM?
- Thread starter rhenders
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ron
Senior Member
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- New York, 40.7514,-73.9925
I believe using the formula HP=(RPM * T) / 5252, that when you reduce rpm, the hp used reduces linearly.
kwired
Electron manager
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- NE Nebraska
No, unless you tweak voltage/frequency patterns, and if you do you shouldn't change them to that large of an extent, you will only have half voltage when running at half frequency, but current can still be up to full load rating current, so if you are drawing full load current rating you are only drawing about half the power you would at full voltage and effectively have only about 10 hp output instead of 20.
Think of it this way - (disregard friction or other losses for a moment) if you are running at half the speed on a constant torque load you are only doing half the work, on a variable torque load you are doing even less work as the torque requirement decreases as speed decreases.
gar
Senior Member
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- Ann Arbor, Michigan
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- EE
150417-1001 EDT
rhenders:
You should have been able to answer that question yourself, but is there more to the real question.
Depending upon on the motor characteristics you might be able to load the motor to 20 HP for a short time. Thermal characteristics would determine how long and how often. Also would depend upon the motor speed torque capability. Is the peak torque capability somewhat greater than 2 times rating? The VFD would have to be able to supply the necessary current.
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rhenders:
You should have been able to answer that question yourself, but is there more to the real question.
Depending upon on the motor characteristics you might be able to load the motor to 20 HP for a short time. Thermal characteristics would determine how long and how often. Also would depend upon the motor speed torque capability. Is the peak torque capability somewhat greater than 2 times rating? The VFD would have to be able to supply the necessary current.
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DarylH
Member
- Location
- San Marcos, CA
Right, HP is a function of torque times RPM divided by 5252.
Regardless of how much torque or horsepower the motor makes, they will be equal at 5252 RPM.
There are lots of references to consult on the net, but here's one I found with a quick Google search that explains it fairly well and includes a derivation toward the end of the page.
http://www.epi-eng.com/piston_engine_technology/power_and_torque.htm
- Location
- San Francisco Bay Area, CA, USA
- Occupation
- Electrical Engineer
Here's a problem though, you are thinking that your load requires HP, when in fact MOST loads require TORQUE. The term "HP" is really just a shorthand expression of "x amount of torque at y speed". So yes, when you lower the speed by changing the frequency of an AC induction motor, by definition you lower the "HP".
But the beauty of a VFD is that the motor will produce the same TORQUE at 1700RPM as it did at 3600. So chances are really good that if your machine was DESIGNED around a 20HP 2 pole (3600sRPM) motor, and you want to slow it down to be the equivalent of a 4pole motor running at 1700RPM, then a VFD is just the ticket.
However...
If the problem you are trying to solve is that you NEED a 20HP 4 pole motor, and all you have is a 20HP 2 pole motor plus a VFD, that will NOT solve your problem. The 2 pole motor already had half the torque of an equivalent 4 pole motor if the HP is the same, therefore slowing the 2 pole motor to half speed still gives you half of the torque.
But the beauty of a VFD is that the motor will produce the same TORQUE at 1700RPM as it did at 3600. So chances are really good that if your machine was DESIGNED around a 20HP 2 pole (3600sRPM) motor, and you want to slow it down to be the equivalent of a 4pole motor running at 1700RPM, then a VFD is just the ticket.
However...
If the problem you are trying to solve is that you NEED a 20HP 4 pole motor, and all you have is a 20HP 2 pole motor plus a VFD, that will NOT solve your problem. The 2 pole motor already had half the torque of an equivalent 4 pole motor if the HP is the same, therefore slowing the 2 pole motor to half speed still gives you half of the torque.
kwired
Electron manager
- Location
- NE Nebraska
they used to work, I just tried to post what you likely were looking for and my symbol just showed up as a question mark - so I deleted it and replaced with what you see now.
junkhound
Senior Member
- Location
- Renton, WA
- Occupation
- EE, power electronics specialty
what gar said:
20 HP for a short time. Thermal characteristics would determine how long
If you use vfd capable of providing the current needed for 20 HP at 1750/1800 rpm, you can run at full 20 HP at half speed, with enough cooling.
Likewise, at 3600 rpm, if you cool the motor well enough, you can run it at 40HP with a 40HP vfd.
example at home for pressure washer, needed 10 HP and only had a 184T frame 5 HP motor: added a 1/3 hp furnace blower ducted to blow thru thru and around the motor, the 5 HP motor pulls 47 amps driving 4gpm, 4000 psi pump, almost 10 HP. Have run for 5 hours straight OK. Not the greatest efficiency, but works.
20 HP for a short time. Thermal characteristics would determine how long
If you use vfd capable of providing the current needed for 20 HP at 1750/1800 rpm, you can run at full 20 HP at half speed, with enough cooling.
Likewise, at 3600 rpm, if you cool the motor well enough, you can run it at 40HP with a 40HP vfd.
example at home for pressure washer, needed 10 HP and only had a 184T frame 5 HP motor: added a 1/3 hp furnace blower ducted to blow thru thru and around the motor, the 5 HP motor pulls 47 amps driving 4gpm, 4000 psi pump, almost 10 HP. Have run for 5 hours straight OK. Not the greatest efficiency, but works.
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