DickDV - 50Hz motor on 100Hz?

I'm looking for DickDV or any drive gurus.

I'm seeking the answer to this question posted over at Ron's site, in different words.

If you have a 50Hz motor common 3phase motor and run it at 100Hz via an inverter, for what reason would you not expect it to run twice as fast? Would it stall for any reason at this freq.?

For clarity, assume the motor's driven load is in perfect condition and correctly sized. Just concentrate on the drive.

Yes it can stall if the total length of wiring exceeds a certain distance when powered with an invertor. If the motor original design RPM was low, say 450 or 900, the motor will easily stall above a certain frequency even with just a small load. This is based on experience, I have had problems in the past. On a 900 RPM motor with the invertor running at 70 Hz., the motor just slowly came to a stop. To test the wiring an invertor itself, we changed the motor to an 1800 RPM motor and it would run up to 90 Hz. I'm no engineer so I don't know the reasons for this. I would only guess that inductive reactance or magnetic flux in the motor causes this.

Humm, never thought about that.

One other point that someone told me though. Even if one could run that motor at twice the speed with twice the HZ, it probably would tear itself apart, as it most likely was not balanced or built to handle those RPMs.

You also are operating above base speed, and are losing torque (moving out of the constant torque region, and into the constant horsepower region).
Eventually, the motor won't have enough torque to continue increasing speed.

loss of torque.

Horsepower = (torque * RPM)/ 5252

Do the math: Let's say you have a 10 HP motor that runs at 1800 rpm. That motor will produce about 30 lbs of torque. So lets increase the rpm on that same motor to 3600. Now you've got a 10 hp motor producing about 15 lbs of torque.

Simply stated; you cant have your cake and eat it too...

My guess the application in question has a constant load, like a hydraulic pump, blower or air compressor. If it were a rolling load like a dolly, train, lift or maybe a conveyer, the load enertia would decrease the necessary torque to keep it rolling at higher speeds.

Rotor flux is proportional to applied voltage divided by applied frequency. We call this "volts per hertz." Above nameplate speed/frequency, you do NOT go above nameplate voltage, so your numerator remains constant as your denominator doubles. Therefore, you get about half the flux in the rotor.

Rotor flux times stator current is approximately your torque. As you lose torque, your rotor slows down, moving you higher on the torque-speed curve, because your stator current increases due to less counter (back) electro-motive force. In other words, you get more slip, and thereby more stator currentto provide the requisite torque to push the load at this lower than double speed (doubled as the synchronous formula would dictate, less than due to the increased slip).

In other words, you develop enough torque to keep the motor turning at some lower frequency by increasing the stator current to account for the decreased rotor flux.

This can have many effects, from "everything works fine" to "the motor stalls and overheats." It really depends on too many factors to give you a "one size fits all" answer.

The frequency says the motor (load) will go twice as fast.

The decreased rotor flux says you get less torque per unit of stator current.

To increase torque, you get more slip and therefore more stator current.

Where does everything balance, and at what point/speed does your motor run? Unknown without more information.

Unload the motor completely, and you need very little torque, so everything runs fine at twice the speed (assuming the centrifugal forces can be handled by the rotor and the speed can be handled by the bearings).

Load the motor at base speed, and continue to increase the load as speed increases, and you'll "stall" somewhere between base speed and twice base speed.

silva.foxx said:

For clarity, assume the motor's driven load is in perfect condition and correctly sized. Just concentrate on the drive.

Click to expand...

The issue is that the motor and drive is improperly sized for what your trying to do, or the load is not in perfect condition. If you want to keep the existing drive, then get a 3600 rpm motor. A 3600 RPM motor might be a hard find, so you'll probably have to upsize the drive & the motor so you can deliver more current without damaging them.

Since the motor field rotation speed is determined by frequency only, running a motor above its base speed will cause the rotor to try to spin up to the higher speed. Clearly, it will need to have adequate torque to get to the higher speed.

That indicates that we need to focus on the motor torque curve over the motor base speed and compare it to the load torque curve at these same speeds. If, at any point, the motor matches or falls below load speed, further acceleration is impossible and the speed will not increase beyond that point.

It is true that virtually all motors operate with a constant torque characteristic from zero to nameplate base speed but, above nameplate speed, the torque will be reduced by the inverse of the overspeed ratio. So, for example, if you are up to 70 Hz on a 60 Hz motor, the speed will be 70/60 times nameplate but the torque will be 60/70 times normal rated torque. This behavior continues up to a point where the motor internals no longer can deal with the higher frequencies. At that point, the torque starts to fall off even faster and stalling follows soon after. Each motor design is different in this respect--some inverter duty four pole motors are guaranteed to keep torque in this proportion (constant hp) up to double base speed while many others, especially slow speed motors and early designs, will not hold this proportional relationship up to even 80hz on a 60hz motor.

A further complication is that, while continuous rated motor torque follows the above inverse ratio rule, the short term overload torque available from the motor comes down as the SQUARE of the overspeed ratio. So, at double base speed on most motors, there is no overload capacity left at all.

As to mechanical issues, any NEMA or IEC motor with a cast aluminum rotor excluding two pole designs can be taken to at least 90 Hz without any real balance or bearing concerns. On two pole motors, I quit at 4500rpm mainly for cooling fan concerns.

The bottom line is that using the motor overspeed capabilities is good design practice as long as the limitations are taken into account. For myself, under 100hp and excluding two pole motors and increasing torque loads, I try to design the power train so full speed on the load is 90Hz on the motor. Lots of good things happen when you follow that rule.

Hope this helps answer your questions.