Maximum Voltage That Can be Applied to a Motor

I was reading about 87Hz technique. I saw we can configure the drive for a motor 400Y/230D as 400V /87hz. I know v/hz ratio should be kept constant and when we configure drive as 400V and 87 Hz and wire it delta this ratio is still constant. But there will be 1.7 times more voltage applied to motor windings. Is it ok for winding. Is there no risk of damaging insulation ? For example Can we apply 800V and 174 Hz to a motor without damaging assuming that we can source it?

I have never done it, but it has been done to get considerable more HP/kW power out of a motor.

Supposedly if the voltage to HZ ratio is kept it can be more than double the motor rated voltage and the motor will run smooth and cool with the extra power.

The first time I heard about it I was very doubtful, but it is done without causing any problem for the motors.

What is the motor's nameplate rating? 400v/50hz (Wye) / 230v/50hz (Delta)?

If so, wiring it Delta (230v/50hz) you can run 400v/87hz ok. Just be sure the drive can handle the 230v nameplate current, and the rotor is rated for the higher RPM. This is a common 'trick' we've used for years on spindles and such for higher speed applications while still having constant torque.

There's not a real insulation issue, since the voltage to ground and the turn to turn voltage isn't any higher than when wired for the 400v rating...

Load side Voltage on a VFD generally cannot exceed line side voltage. Above rated frequency, the Volts/Hz ratio is no longer applicable and the drive operates at a constant output voltage to the motor.

Tom, in this case they are operating a motor connected for 230V from a 400V drive. So the drive has the available voltage to keep the V/Hz ratio constant at frequencies above base frequency of the motor. And, as already stated, since the motor is designed to handle 400V anyway, insulation taring shouldn't be an issue.

Keith

kamenges said:

Tom, in this case they are operating a motor connected for 230V from a 400V drive. So the drive has the available voltage to keep the V/Hz ratio constant at frequencies above base frequency of the motor. And, as already stated, since the motor is designed to handle 400V anyway, insulation taring shouldn't be an issue.

Keith

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But he also asked about applying 800 V. If he applied that to a standard VFD he would probably let the smoke out. I assumed he was trying to get 800 V out from 400 V in. (Shame on me for making assumptions!)

800V was a just random number to emphasize the question in my head. The point that I don't understand is if the motor is rated for 400V Y Connection and if I wired it delta 400V then there will be 1.7 times more voltage applied to windings. It seemed to me a bit risky for insulation of motors.

bodoo23 said:

The point that I don't understand is if the motor is rated for 400V Y Connection and if I wired it delta 400V then there will be 1.7 times more voltage applied to windings. It seemed to me a bit risky for insulation of motors.

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When a winding works with AC, the impedance is almost entirely inductive, the resistive part is very small, and that impedance increases with frequency and that compensates the increase in voltage.

bodoo23 said:

800V was a just random number to emphasize the question in my head. The point that I don't understand is if the motor is rated for 400V Y Connection and if I wired it delta 400V then there will be 1.7 times more voltage applied to windings. It seemed to me a bit risky for insulation of motors.

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How so? If you have 400V, you have 400V, not some higher amount. The whole point of the "87Hz technique" as you call it is to take advantage of the motor being capable of being connected for the lower voltage pattern and using it at the higher one, but that does not make the actual voltage any higher, it's still 400V, and because the motor was designed to be useable at 400V, you have not violated anything.

You cannot however take a motor only rated for 400V and operate it at some higher voltage such as 4800V, the insulation would bit be rated for that.

Gene Bond said:

What is the motor's nameplate rating? 400v/50hz (Wye) / 230v/50hz (Delta)?

If so, wiring it Delta (230v/50hz) you can run 400v/87hz ok. Just be sure the drive can handle the 230v nameplate current, and the rotor is rated for the higher RPM. This is a common 'trick' we've used for years on spindles and such for higher speed applications while still having constant torque.

There's not a real insulation issue, since the voltage to ground and the turn to turn voltage isn't any higher than when wired for the 400v rating...

Click to expand...

Exactly and if I may break it down a bit for beginners like myself:


When running a motor in V/Hz or constant torque mode the drive regulates the voltage to increase at the same ratio with the increase in frequency (speed). If a 50Hz motor is wired delta for VAC230 and controlled by a 230VAC drive you have 230/50 = 4.6volta per 1Hz. So for every increase of 1Hz the drive increases the output by 4.6 volts. That allows you to control the motor speed, and maintain constant ratio V/Hz (constant torque), up to 50Hz but beyond that the drive can not maintain the ratio because it is limited by the power supply of 230VAC.
So if you raise the frequency to 60Hz you still have only 230VAC and the consequence of that is loss in torque.

On the other hand if you still wired the motor delta for 230VAC and controlled it instead with a VFD rated for 400VAC then it will be able to maintain that ratio up to 87Hz which comes out to 400/87 = 4.6V per 1Hz, same as 230/50 and that way you can maintain constant torque up to 87Hz; of course beyond 87Hz you start loosing torque because the drive will not be able to increase the voltage above its supply of 400VAC


Edit: I think I just got your question, you mean to ask what if we follow the same principal and apply it to a motor wired for Y 400VAC? I don't know, that's a good question for the experts to answer. Can we get a 600VAC VFD and run a 400VAC motor, are the motors rated that tightly or is there a range of say 230-800VAC or something like that?


Edit: And now I'm officially hijacking the thread :
What if we have 400VAC Y motor and control it with 600VAC up to 33Hz, does that mean we're still running the motor within specs and getting extra torque? Is that something anyone would do or have I lost the plot?

If the motor was rated 400v Wye, 50hz, running it up to 600v, 75hz would be kosher for the flux and torque, but may be excessive for the insulation. Chances are on a modern motor it'd be fine, but you should check with the manufacturer...

I dont think the voltage applied to winding is same when it is wired 400V and delta. I mean if it is wired 400V and star , 230 will be applied to each winding of motor and it is same if wired it delta and the line voltage is 230V. But if we apply 400V and wire it delta then 400V will be applied each windings.