Induction Motor Efficiency at Reduced RPM
Induction Motor Efficiency at Reduced RPM
(OP)
Hello all:
Bit of intro: I am a gyro gearloose (mechancal) as opposed to a wireloose, but I have a lot of past experience in controls/electrical design/analysis etc, so if the reply is not too technical, I can probably follow.
A source of great frustration for me for several years has been the inability to get estimates of induction motor efficiency and power factor for motor loads less than nameplate and especially at less than full rpm. Replys from salemen to design guru's are always silence, audible head scratching, or someone saying that someone looked at that years ago and they don't remember what the results were.
Motor manufacturers always give eff and PF numbers for (usually) half to full motor loading, but it is always at full rpm. However, the whole point of an inverter drive system is to run at something different than nameplate line frequency/speed.
We have an application in the chemical process industry where knowledge of the process torque demand is extremely valuable for design optimization. Motors that drive the process (through gear reducers...process is typically from 2-4 rpm final) run from 30 hp all the way to 450 hp, and I am designing a system now requiring 750 hp. I have data for past installations, but a lot of it is only motor nameplate, and process volts, amps, and frequency. We also have rpm, but the method of measureing is probably not as good as recording freq and estimating slip. Recently, through site testing, we have discovered that some makes of inverters will give an accurate indication of electrical torque, and I have some limited data on this for two larger systems, which solves the PF problem for these, but still leaves the efficiency issue. Motor manufacturer's are willing to test at a (hefty) price, and there are devices that can measure torque (of which I may have succeeded in getting a client to pay for on a new job). But,all this does not solve digesting the past data where some real value lies.
I realize that motors vary all over the map, but is there someway to at least estimate, with range of uncertainty known, the change in effeciency and PF at less that full motor rpm's? Are there rules of thumb? Are there models...and if so, what needs to be known to trim them?
thanks in advance!
John
Bit of intro: I am a gyro gearloose (mechancal) as opposed to a wireloose, but I have a lot of past experience in controls/electrical design/analysis etc, so if the reply is not too technical, I can probably follow.
A source of great frustration for me for several years has been the inability to get estimates of induction motor efficiency and power factor for motor loads less than nameplate and especially at less than full rpm. Replys from salemen to design guru's are always silence, audible head scratching, or someone saying that someone looked at that years ago and they don't remember what the results were.
Motor manufacturers always give eff and PF numbers for (usually) half to full motor loading, but it is always at full rpm. However, the whole point of an inverter drive system is to run at something different than nameplate line frequency/speed.
We have an application in the chemical process industry where knowledge of the process torque demand is extremely valuable for design optimization. Motors that drive the process (through gear reducers...process is typically from 2-4 rpm final) run from 30 hp all the way to 450 hp, and I am designing a system now requiring 750 hp. I have data for past installations, but a lot of it is only motor nameplate, and process volts, amps, and frequency. We also have rpm, but the method of measureing is probably not as good as recording freq and estimating slip. Recently, through site testing, we have discovered that some makes of inverters will give an accurate indication of electrical torque, and I have some limited data on this for two larger systems, which solves the PF problem for these, but still leaves the efficiency issue. Motor manufacturer's are willing to test at a (hefty) price, and there are devices that can measure torque (of which I may have succeeded in getting a client to pay for on a new job). But,all this does not solve digesting the past data where some real value lies.
I realize that motors vary all over the map, but is there someway to at least estimate, with range of uncertainty known, the change in effeciency and PF at less that full motor rpm's? Are there rules of thumb? Are there models...and if so, what needs to be known to trim them?
thanks in advance!
John





RE: Induction Motor Efficiency at Reduced RPM
Power Factor will not be an issue on a VFD.
David Castor
www.cvoes.com
RE: Induction Motor Efficiency at Reduced RPM
thread237-273230: VFD partial load efficiencies
The paper link posted by BigInch has some good data.
RE: Induction Motor Efficiency at Reduced RPM
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RE: Induction Motor Efficiency at Reduced RPM
David:
For my purpose, PF curves are very important. The old data that I have are (VFD outputed)volts, amps, and frequency. Without motor PF, one cannot separate real power from apparent, and without eff, one cannot get the deliverd mechanical power from the recieved real elecrical power.
RE: Induction Motor Efficiency at Reduced RPM
If you are trying to use volt and amp data measured at the VFD output, I would be extremely suspicious of the accuracy of the data. Most measurement equipment is not capable of providing much useful data from the output of a PWM drive. The VFD can do a much better job of computing motor kW load than can be done with external measurements on the drive output.
David Castor
www.cvoes.com
RE: Induction Motor Efficiency at Reduced RPM