motors

is it safe to run any AC motor over 60hz, if so how much.

""is it safe to run any AC motor over 60hz, if so how much.""
Safe?? well depends.
How much?? till you blow the rotor.

Single phase don't think so. Kinda hard to connect to 3 phase VFD and make work.

3 phase synchronous well you could BUT -- (see below)
3 phae wound rotor well you could BUT -- (see below)
3 phase induction squirrel cage well that is safest BUT
beyond 70 HZ or so I would surely check with mfr to find maximum rotor RPM.
ALSO beyond 60 HZ -- torque drops so it is easy to get into overload at higher freq.

Dan Bentler

quebmaint, also see http://www.patchn.com/motors.htm
Great info and tutorials on everything motors.

another

From Ron`s site.
http://www.sea.siemens.com/step/default.html

atchn:

Often, the motor bearings are the most critical limiting factor. The mfg. should be able to provide max RPM dta for you. I have some "inverter duty" motors that provide RPM limits right on the data plate. One of them ir rated for 5000 RPM which would be ~171Hz. Don't exceed mfg. specs, or you will probably trash the motor/machine.

For quick reference, you can easily take any NEMA frame 4 pole or slower induction motor to 90Hz without asking any questions. Bearings are not an issue as the same bearings go in two pole motors as go in lower speeds.

Rotor balance can be an issue in frames over about 360T at 90Hz but rarely.

You must be aware that hp peaks at motor base speed and will not exceed that level at any overspeed point. In some motors, the hp may even drop a bit by 90hz.

Overload capacity goes from about 220% down to only about 100% at 90Hz.

Special built motors or IEC motors may follow different rules. Check with the manufacturer.

We have motors running from 2 to 100 Hz. Mostly without problems. Our mechanical department calculates the loads etc. I must say that most of our motors are larger than they need to be for the load the have to pull.

Two pole motors (3600 rpm nominal @ 60Hz) are going to withstand less overspeed than 4-pole (1800 nominal @ 60 Hz) motors.

Always check with the manufacturer for the specific motor model you are using.

With the motors available today, overspeeding one should not be necessary. Get one that is designed to go to the RPM you need.

You must be aware that hp peaks at motor base speed and will not exceed that level at any overspeed point. In some motors, the hp may even drop a bit by 90hz.

Overload capacity goes from about 220% down to only about 100% at 90Hz.

Click to expand...

Generally it is safe to run inverter duty rated motors up to 90 Hz.
I have a current app where I am considering running 1200 RPM motor
at 90 HZ (1800 RPM). I am using a Westinghouse motor and it is OK to run it at those speeds. You have to be careful though, a standard V / Hz motor drive will produce a rated torque at 1200 RPM. Since HP = RPM X Torque you will be losing torque above 1200 RPM as motor will maintain the HP.
You can use a vector drive which will provide uniform torque.
This is significantly more costly.

Originally posted by Jiri Toman:

You can use a vector drive which will provide uniform torque.

Click to expand...

Be careful with that. Nothing is free; you have to give to get. AC motors are designed to produce a back EMF very close to rated voltage at rated load and rated RPM. The only way to go faster with the same available voltage is to decrease the rotor flux. This decreases the back EMF for a given motor speed. But this will also decrease the available torque from the motor. The only way to get that torque back is to increase the rotor flux again, which would require a higher voltage at the drive to counter the back EMF.

Generally, when you run above rated speed you are in the constant horsepower region of motor operation. Torque will drop as speed increases (due tot he needed reduction in rotor flux to keep the back EMF in check) and horsepower stays constant.

Keith