OT: v/f vfd pattern

I have a general question on the theory of a vfd.

I understand that when set to V/hz, the drive will bring the volts and hz up together when speeding up.

My question is, when running a motor over 60hz, when does the output voltage equal base voltage or max output voltage?

For example, if I had a vfd on a 480V line and had it set to a max hz of 75hz. Would the output voltage be at 480V at 60hz, then just increase the hz up to 75 or would the drive scale the whole curve so that it would not hit 480V until it was at 75hz?

The reason I prompted this question. I had a customer today that told me that he had a vfd that has 495V incoming. He claimed that he was getting 795,800 output voltage. First, I do not think that is electrically possible (please correct me if I am wrong). Second, I don't know if he had an RMS meter or not.

He said they were running this motor at 120hz.

So I am trying to understand this and I want to make sure I am right. That even over 60hz even on a standard v/hz pattern, there is no way he could have had more than 495V out??

The motor failed after only a few months which is why he called us and trying to figure it out.

The v/hz ratio will stay the same if you just command the drive to go beyond 60hz. It won't automatically change the v/hz ratio from 0-60hz.

It also will not increase the voltage beyond 60hz, which is why your available torque goes down beyond that.

700V is about what's on the DC bus. My guess is your customer was using a cheap meter, and his motor failed because it was running twice as fast as it's rated for and pulled itself apart.

What's the increase in centrifugal force on an object that doubles its rotational velocity? 4x? It's been a while...

lesmar96 said:

My question is, when running a motor over 60hz, when does the output voltage equal base voltage or max output voltage?

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It should be at rated output voltage when you hit the frequency that you told the drive that the motor is rated for, in the setup. This is normally 50 Hz or 60 Hz. I have not tried to set it to 90 Hz or 120 Hz to see what happens.

The reason I prompted this question. I had a customer today that told me that he had a vfd that has 495V incoming. He claimed that he was getting 795,800 output voltage. First, I do not think that is electrically possible (please correct me if I am wrong). Second, I don't know if he had an RMS meter or not.

He said they were running this motor at 120hz.

So I am trying to understand this and I want to make sure I am right. That even over 60hz even on a standard v/hz pattern, there is no way he could have had more than 495V out??

The motor failed after only a few months which is why he called us and trying to figure it out.

Click to expand...

You can measure more voltage out than in, because there is a standing wave on the motor terminals due to the square waves that the drive sends as output reflecting when they see the change in impedance between the cable and the motor. But the reflected waves increased voltage you normally need an oscilloscope to see.

With an RMS meter, I have seen a difference of 10 or 20V between input and output, never as much as the customer is seeing.

Is there is a transformer between the supply and the VFD? This is done by OEMs sometimes to use one of their standard parts (VFD or Motor) for application in another country. We often see 460VAC US VFDs supplied to us here in Canada, with a step-down transformer in the cabinet so that our 575 VAC can be fed to the panel. But the panel is seldom sized to get rid of the heat that the transformer generates.

The right solution is to run the motor at rated speed and use the gearbox, or the belt drive and sheaves, to run the load at the desired speed. Or put in a motor that turns at your desired speed, by replacing an 1800 rpm motor with a 3600 rpm motor. It appears that something was not sized properly for this equipment.

As for likely motor failure causes, if the customer is running a 60 Hz motor at 120 Hz and is near rated current, the fan is not cooling enough, so the motor is likely failing due to overheating. P = I*I*R. In this case, R is actually X, since part of it is resistance and part is the inductance of the motor. The motor inductance is directly related to frequency. So the motor could be heating up more than the fan can take away, even though the fan is moving much more air. The heat still has to travel through the stator in order to have the blowing air remove it.

Another possible failure is the bearings. If the bearings are set up to run at 1800 rpm and they are running 3600 rpm, they are likely to fail much sooner

All of that said, I have a 10 HP motor that has been running in the 90 - 120 Hz range for 10 years now. It was over-sized to begin with, and is only running about 60% current at 120 Hz. So over-speeding equipment is a problem some of the time. It does give you more risk for failure.

lesmar96 said:

I have a general question on the theory of a vfd.

I understand that when set to V/hz, the drive will bring the volts and hz up together when speeding up.

For example, if I had a vfd on a 480V line and had it set to a max hz of 75hz. Would the output voltage be at 480V at 60hz, then just increase the hz up to 75 or would the drive scale the whole curve so that it would not hit 480V until it was at 75hz?

The reason I prompted this question. I had a customer today that told me that he had a vfd that has 495V incoming. He claimed that he was getting 795,800 output voltage. First, I do not think that is electrically possible (please correct me if I am wrong). Second, I don't know if he had an RMS meter or not.

He said they were running this motor at 120hz.

So I am trying to understand this and I want to make sure I am right. That even over 60hz even on a standard v/hz pattern, there is no way he could have had more than 495V out??

The motor failed after only a few months which is why he called us and trying to figure it out.

Click to expand...

Not sure if this is what your customer is doing, but there is a scheme where you can use a 480V VFD, wire the motor connections for 240, set the Volts Hertz curve to deliver 240 volts at 60 Hz and 480 volts at 120 Hz. To do this you need a vfd capable of delivering the amperage the motor requires at 240 volts. There are also bearing issues. For reliability issues this should never be done on a "stock" 3600 rpm motor. For motors under 10 hp and 1800 rpm this rarely an issue.

When the volts hz curve is configured this way there will be no appreciable loss of torque above 60 hz.

A few thoughts on this
To answer the first question
“when running a motor over 60hz, when does the output voltage equal base voltage or max output voltage?”

The effective VFD output voltage will remain constant at the set motor base voltage when operating above the base speed. The motor will change over to a constant HP motor operation. From the base frequency up the motor output torque will go down as the frequency go up.
You can operate a 400V 50hz motor at 480V 60hz without any problems the volts to hertz ratio is the same for both.

As for measuring the output voltage on a VFD don’t even try any reading you will get will be meaningless unless you are using very high end lab equipment not normally use in the field. You have to remember the output voltage of a VFD contains both AC and DC components. It also madders if you are measuring the output with a motor connected or not the effective output voltage will change with different conditions also motor loading will effect the voltage reading. Multiple motors, line reactors
A true RMS meter is a good start but it will still be off. The reading on the VFD display will be better than what you will read with a meter.
As for reading over 800 volts on the output of a 480 volt VFD cold be right the normal buss voltage of a 480v VFD is about 824 V again depending on the incoming line voltage. The buss loader transistor normally turns on at about 850v and off below about 830v
One thing I always have remind people is you must use a voltmeter or a multi meter rated for 1000 Volts when working on any VFD. I have seen many people use a standard meter rated at 600v they get away with it for while but when it blows up in your hand you will change your mind. Remember the buss voltage is over 820v DC supplied from the buss caps. If it blows it could discharge the buss cap energy into the meter in your hand before it would end, not something I would want.
I actually had a 1000 volt rated meter blow up testing a drive fortunately I was not holding it at the time.
The case split open the smoke came out. I sent it back and the manufacture determined it was defective and replaced it. This was on a good meter with the full rating imagine what could happen with a meter rated for less.

As for the idea of using a 480V VFD on a motor wired for 240V to get around the constant HP problem of operation above the base frequency. It’s a very bad idea. I sound’s good in theory. We don’t work in theory but with the reality of the way things are. What you are doing is to double the motor HP that would also double the motor current. It would both over heat the motor and over current the motor windings burning them out. I know some of you are thinking of just limiting the motor current in the VFD. If you limit the motor current you also limit the motor voltage the two work together (if you change one the other will also change) so then again you would cause the motor to operate in constant HP mode not a real gain there. Also most VFD’s don’t support current limiting, only some of the higher end VFD’s do. And they limit the current by limiting the effective output voltage. Again you are back were you started no gain.


Keep in mind with a fluid load on a motor ( Fan or water/oil) there both considered a fluid load to a motor.
The worked needed to move the fluid goes up by the log of the pump speed. As small increase in pump speed will cause a large increase in flow volume. Increased flow volume = increase in power required to move the fluid.
Another thing to keep in mind when dealing with VFDs and motors is the motor rating
A TEFC ( Totally Enclosed Fan Cooled ) frame motor running at lower speed may tend to overheat at higher loads the motor fan is not moving the air necessary to cool the motor. You may have to supply an external cooling fan. A TENV frame may be a better choice for both higher speed and lower speed operations. They dissipate the heat much better.

What was the failure mode of the motor?

From my limited experience with running motors over nameplate speed, 90 Hz is about the maximum safe frequency unless the motor specifically allows a higher speed.

GaryS said:

It also madders if you are measuring the output with a motor connected or not the effective output voltage will change with different conditions also motor loading will effect the voltage reading.

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I did not realize that it could measure differently with or without a motor connnected. the customer did say he was checking it without a motor connected.

I agree that using the vfd readout would be a whole lot more accurate.

KB1GNI said:

What was the failure mode of the motor?

From my limited experience with running motors over nameplate speed, 90 Hz is about the maximum safe frequency unless the motor specifically allows a higher speed.

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We haven't seen the motor here yet. He said that it just hums. So it could be mechanical or electrical failure.

Do you make the 90hz rule for electrical or mechanical reasons? I understand that for torque reasons, yes.

I did get some data from the shop that indicates that it was a winding failure. Seems to have an open winding and shorted to ground.

I know heat has already been mentioned. Is there any other reason with the customer running it at 120hz would have ruined the windings?

I appreciate your input and answering my questions. Thanks!

I didn't make the rule, but I have seen it mentioned when I was looking into running a motor over nameplate. It is for both - mechanically, the rotor or fan could fail catastrophically and come apart. Electrically, there are spikes that get worse as the frequency increases over nameplate which could cause the windings to fail prematurely.

That begs another question - was this an inverter duty motor or a general purpose one?

Your motor failure is probably first turn failure. It's common on noinverter duty motors.
you are usually safe to double the base speed of a motor or 120hz I have in past had motors custom balanced for over that precision bearing.
the Black Max motor in the lower HP sizes comes from the factory labeled for a max rpm of 5000 on 4 pole 1800 motor

For another data point, the motor I was dealing with most recently was a 2 pole ~3450 rpm motor with a listed maximum safe speed of 5400 - which correlated to right about 90 Hz.

KB1GNI said:

Electrically, there are spikes that get worse as the frequency increases over nameplate which could cause the windings to fail prematurely.

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This is one thing I was wondering. Can you please elaborate more? I assume voltage spikes?

The system needs to be redesigned so that they don't need to run so fast, i.e. change the gearing. But I thought this was a good opportunity to learn more about electrical and vfd theory.

Also this motor is 1800rpm. The manufacturer says it is good to run to 4000rpm, which is over even 120hz

Here's some reading material that should answer a lot of questions:

https://www.machinedesign.com/motorsdrives/how-reflected-waves-damage-ac-motors