Does anyone know if you can drive a 3 phase motor faster with a VFD than it goes with line voltage?
[OP]
Member
Does anyone know if you can drive a 3 phase motor faster with a VFD than it goes with line voltage?
Member
If I understand the question correct,I believe that you can only go as high as what the hz rating for the motor.Try going to www.practicalmachinist.com and scroll down to VFD's----Carroll
Last edited by Carroll Courtney; 01-23-2010 at 10:44 PM.
Member
Originally Posted by kenneth kayser
Yes you can by exceeding 60 hz. 10-20% overrun is usually considered OK. Once you get a lot higher the motor may physically rip itself apart. Heat and wear are obviously also issues.
Contributor
Kenneth,
The VFD that I use on my lathe will output frequencies higher than 60 hz so it can run an attached motor faster than the rated speed. I don't know if a slower running motor (say 1725 rpm) pushed to higher speeds will damage the motor or not and I don't think anyone other than the manufacturer can answer that question with certainty. I've read that others have run their motors at higher speeds. Just understand that doubling the motor speed quadruples the loads on the rotating motor components. Personally, I might be tempted to run a 1100 or 1725 rpm motor to modestly higher speeds but I would not be tempted to run a 3750 rpm motor at higher speeds.
Dennis
Contributor
If the VFD will provide frequency beyond the rated frequency for the motor, yes, you can drive the motor faster than line frequency. There's a limit, perhaps twice the line frequency, and the motor will have less torque at the higher speeds.
The motor appears as an inductor to the VFD, and the reluctance decreases as the frequency decreases. So the VFD decreases the input voltage to maintain the rated current under load. When you go above the rated frequency, the reluctance of the motor increases and to maintain rated current under load you'd have to increase the voltage beyond the rated voltage. Since that would be dangerous, the voltage is kept at rated voltage, which decreases the current in the motor at load (at higher frequencies) so the torque is decreased.
At least that's what I remember from my motors books.
Mike
[Heat should not be a problem at higher than rated frequency as long as you don't overload the motor. The current in the motor will be lower than at rated frequency and the cooling fan will be operating at the motor RPM so it will be moving more cooling air. The heat problem actually occurs at lower than rated frequency because the fan is running too slow to keep the motor cool (for ordinary induction motors). The higher RPM should not be a mechanical problem to the motor. Universal motors run at 10,000 to 20,000+ RPM without any ill effects. I'd expect the bearings of an ordinary induction motor could handle the higher speed without any problems at all. I do remember there were some issues raised in the motors book about overspeeding but I'll have to check to see what they were.]
Last edited by Mike Henderson; 01-23-2010 at 11:14 PM.
Go into the world and do well. But more importantly, go into the world and do good.
Contributor
I often wondered the same thing, if I could speed up my old dust collector motor. But since I've replaced the old 3 phase unit with a single phase Clear Vue I guess I'll never get to know first hand how it would have worked.
+1 on the practical machinist site for VFD help. I put one on my jig bore and the guys there were very helpful.
The Plane Anarchist
Member
Mike's comments on motor operation are correct. However, when speaking of universal motors, one must remember that they are designed to run at a high rotational speed. Their high horsepower ratings are a result of the high RPM. Common three-phase AC motors (and large DC motors) are not designed for these speeds. As was mentioned, 10-20% overspeed is reasonable, and 30% is probably not a concern. Only the manufacturer would know if your motor could withstand operation at twice rated speed. At high rotor speeds, the increased centrifugal forces can lead to a mechanical failure of the rotor. The cast cooling fins of the rotor can come off and make contact with the stator end turns, the outer surface of the rotor laminations can fail allowing the aluminum (or copper if you have an expensive motor) to pull out and contact the stator, etcetera.
For this reason, DC drives generally have some form of speed feedback and overspeed protection to limit the armature speed if the field voltage collapses. AC motors don't have a field winding, so the cause of an overspeed condition is generally different. AC drives do however employ software settings to limit maximum frequency to protect the motor and the driven equipment.
Todd
Member
I also run the 2hp 3 phase motor on my CNC Mill for extended periods (8+ hours continously) at 120HZ to get speed sufficent for smalll carbide end mills.
Have been doing this for 4 years with no ill effects. The motor whine is different but he motor does not heat or act funky at all.
gary
Contributor
Yes, many VFDs will go as high as 400 hz. The limiting factor is the bearings and to some extent the motor innards which can come unglued at high rpm. At reasonable overspeeds of 50% or so you should have no problems, especially with slower motors such as a 1800rpm motor. I regularly run my 1800rpm 3ph equipped lathe at 90hz for extended periods. On the other hand, I'd be more concerned with running your motor at low (10-20hz) range for extended periods as the motor isn't spining fast enough to cool well without some outside assistance.
Mike
Contributor
Just a comment, but a fan application is not a good one to try to push the RPMs beyond the rated frequency. The reason is that a fan load varies with RPM - the faster you run it, generally the more HP you need to push the fan.
If your motor is matched to the fan, so that it's developing rated HP at rated RPM, you will push it into overload by overspeeding it.
Mike
Go into the world and do well. But more importantly, go into the world and do good.
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You can over speed most any small 3 phase motor 30 to 50 percent as long as you have properly configured the drive using the nameplate information on the motor. Be sure you enter the nominal operating speed of the motor on the name plate such as 1750 or 3500. The drive will allow the motor to produce full rated torque below and up to the rate speed. This is called the constant torque mode. Above base speed the drive will start reducing the torque the motor is allowed to produce. This is called the constant HP mode. This is what allows AC drive and motor systems to give DC drive performance. The drive does this by adjusting the voltage, current, and frequency that is delivered to the motor. It all goes back to HP= T x RPM / 5252. If you continue to produce full torque above base speed then the motor is actually producing more HP than it was design for. Needless to say, it will greatly reduce the life of the motor. To be safe you should consult the manufacture to see want they recommend as the absolute max speed if you intend to go over 30%.
[OP]
Member
Thank you all very much. It seems that 100% overspeed should be no problem. I cannot imagine that an induction motor rotor could not handle this. Significantly lower than synchronous speed might be a problem. I think the iron losses go up and certainly the fan will move much less air. For amateur use I think 90-200% speed should be no problem, but I think one should be sure the motor has overload protection.
I wonder how much torque is lost at 70% speed. Might be a lot. The magnetic field of the stator will be reduced and the field produced by the rotor will be reduced even more. The voltage will have to be reduced to avoid overheating the stator coils. I wouldn't surprise me to see the torque drop below 50% of rated.
Contributor
Most VFDs will maintain constant torque below the rated frequency. The VFD does reduce the applied voltage below the rated frequency, but the reluctance of the motor also decreases so the current remains constant. And torque is proportional to the current. This may not apply all the way down to very low frequencies (a couple of hertz) - it depends on the VFD.
Since horsepower is proportional to torque and RPM, the HP will decrease below rated frequency, even though torque is constant (because the RPM decreases).
Above the rated frequency, the torque decreases with increasing RPM, but the HP output of the motor remains more or less constant because the RPMs increase.
Mike
Last edited by Mike Henderson; 01-26-2010 at 4:38 PM.
Go into the world and do well. But more importantly, go into the world and do good.
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