Increase VFD output voltage?

einnh · Jul 12, 2016

If a VFD (PowerFlex) is running on a 440VAC/3p supply (I know, odd voltage). Can you still use with a 460VAC/3p motor?

According to the tech specs for the 755 drive it states:

Page 11 - Drive full power = nominal motor voltage (460VAC) to drive rated voltage (480VAC) + 10% of drive (48VAC)

Page 11 - drive operating ragne = lowest nominal motor voltage (460VAC) - 10% (46VAC) to drive rated voltage (480VAC) + 10% (48VAC) output is linearly derated when actual line voltage is less than nominal motor voltage

It sounds like I can operate with a derating on the drive because: 460VAC - 46VAC = 414 VAC.

I'm contacting AB TechConnect about thought I'd ask here as well

einnh · Jul 12, 2016

Motor has a 1.15SF So that should eat up the 5% voltage difference.

keithkyll · Jul 12, 2016

My first reaction is the difference is too small to worry about. More info in this link.
Looks like this was written without a VFD in mind. A VFD forgives a lot of sins, including low voltage.

Aabeck · Jul 12, 2016

Almost anything rated for a voltage from 440 to 480 will operate on that "family" of voltages. 440 & 460 motors and drives are wired to 480 regularly. I remember only once seeing a unit that was strictly 440 and needed a transformer installed on a 480 machine.

On the same respect, there is no intrinsic difference for ratings of 220 to 240, but 208 is different and should not be wired to 220 to 240.

And frequently in European machines 380 and 400 are interchanged.

jraef · Jul 12, 2016

einnh said:
If a VFD (PowerFlex) is running on a 440VAC/3p supply (I know, odd voltage). Can you still use with a 460VAC/3p motor?

According to the tech specs for the 755 drive it states:
Page 11 - Drive full power = nominal motor voltage (460VAC) to drive rated voltage (480VAC) + 10% of drive (48VAC)
Page 11 - drive operating ragne = lowest nominal motor voltage (460VAC) - 10% (46VAC) to drive rated voltage (480VAC) + 10% (48VAC) output is linearly derated when actual line voltage is less than nominal motor voltage
It sounds like I can operate with a derating on the drive because: 460VAC - 46VAC = 414 VAC.

I'm contacting AB TechConnect about thought I'd ask here as well

Two different concepts here:
1) Will the drive FUNCTION? That's what that spec is speaking to for the most part. The part about de-rating the output is mostly having to do with when you are OUTSIDE of the motor design specs (see below).

2) What will the maximum output be?
[FONT=&quot]As a general rule, any basic "6 pulse" VFD cannot create voltage potential, i.e. "V-in = V-out", because [/FONT][FONT=&quot][FONT=&quot]the DC bus [FONT=&quot]voltage[/FONT] is always the same as the peak (not RMS) [FONT=&quot]voltage[/FONT] of your AC input; RMS x 1.414.[FONT=&quot] H[/FONT][/FONT]ow a drive works is that it rectifies the incoming AC to pulsating DC using diodes, which only conduct at those peaks. That creates ripples in the DC, so the drive smooths it out with inductors and/or caps, then fires transistors from the DC bus in a PWM pattern as 3 phase pseudo-AC to the motor. In the interstitial stage of smoothing with caps and inductors there is a very slight boost of the DC bus voltage above the normal peak of the rectification, but it's nothing to get excited about. The DC bus voltage still dictates the maximum AC RMS output voltage available from the PWM output. [FONT=&quot]So if, after rectifying, your DC bus [FONT=&quot]voltage[/FONT] is 6[FONT=&quot]22[/FONT]VDC instead of 6[FONT=&quot]78[/FONT]VDC (if it was 480V), the maximum RMS that you can [FONT=&quot]create with the PWM output will be 440V; 622 [FONT=&quot]divided by 1.414.[/FONT][/FONT][/FONT][/FONT]

[FONT=&quot]Active Front End (AFE) versions of drives are capable of boosting the output vo[/FONT][FONT=&quot]l[/FONT][FONT=&quot]tage above the input voltage however, because of the inverter based active rectifier. So you would be able to provide a 460V RMS out to the motor from a 440V RMS input. [/FONT][FONT=&quot]That alone however may not be a good reason to upgrade to a drive topology that will be roughly twice as expensive. Motors typically are designed[/FONT][FONT=&quot] (per NEMA and IEC standards)[/FONT][FONT=&quot] to accept a voltage that can be +-10% of nameplate rating while still providing rated torque. So that motor will be fine with a 440V output from the VFD.[/FONT]
[FONT=&quot]
[/FONT]
[FONT=&quot]
[/FONT]

einnh · Jul 12, 2016

jraef said:
Two different concepts here:
1) Will the drive FUNCTION? That's what that spec is speaking to for the most part. The part about de-rating the output is mostly having to do with when you are OUTSIDE of the motor design specs (see below).

2) What will the maximum output be?
[FONT=&quot]As a general rule, any basic "6 pulse" VFD cannot create voltage potential, i.e. "V-in = V-out", because [/FONT][FONT=&quot][FONT=&quot]the DC bus [FONT=&quot]voltage[/FONT] is always the same as the peak (not RMS) [FONT=&quot]voltage[/FONT] of your AC input; RMS x 1.414.[FONT=&quot] H[/FONT][/FONT]ow a drive works is that it rectifies the incoming AC to pulsating DC using diodes, which only conduct at those peaks. That creates ripples in the DC, so the drive smooths it out with inductors and/or caps, then fires transistors from the DC bus in a PWM pattern as 3 phase pseudo-AC to the motor. In the interstitial stage of smoothing with caps and inductors there is a very slight boost of the DC bus voltage above the normal peak of the rectification, but it's nothing to get excited about. The DC bus voltage still dictates the maximum AC RMS output voltage available from the PWM output. [FONT=&quot]So if, after rectifying, your DC bus [FONT=&quot]voltage[/FONT] is 6[FONT=&quot]22[/FONT]VDC instead of 6[FONT=&quot]78[/FONT]VDC (if it was 480V), the maximum RMS that you can [FONT=&quot]create with the PWM output will be 440V; 622 [FONT=&quot]divided by 1.414.[/FONT][/FONT][/FONT][/FONT]

[FONT=&quot]Active Front End (AFE) versions of drives are capable of boosting the output vo[/FONT][FONT=&quot]l[/FONT][FONT=&quot]tage above the input voltage however, because of the inverter based active rectifier. So you would be able to provide a 460V RMS out to the motor from a 440V RMS input. [/FONT][FONT=&quot]That alone however may not be a good reason to upgrade to a drive topology that will be roughly twice as expensive. Motors typically are designed[/FONT][FONT=&quot] (per NEMA and IEC standards)[/FONT][FONT=&quot] to accept a voltage that can be +-10% of nameplate rating while still providing rated torque. So that motor will be fine with a 440V output from the VFD.[/FONT]
[FONT=&quot]
[/FONT]
[FONT=&quot]
[/FONT]

Thanks for the info. It looks like the our design will function with the next size up drive from AB and the current motor with the calculated deratings.

DickDV · Jul 12, 2016

To add a bit to jraef's good comments, the only time the 460V motor will "starve" for voltage will be right about at full speed, probably 57-60hz. That is because at all lower frequencies, the VFD output is reduced below the input voltage in order to keep the volts per hz ratio constant. So there will be no lack of voltage to the motor at any speed below about full speed.

At full speed, as jraef has described, there will be some shortage of voltage but the reduction of motor output torque should be negligible.

I suppose its a rather extreme example but you could run a 460V motor on a 230V drive and you would have full torque available up to 30hz. Above that frequency, the voltage cannot continue to rise above 230V so the V/Hz ratio would begin to fall off with a corresponding reduction in motor output torque.

When you enter the motor nameplate data into the drive, it sets up a V/Hz profile which attempts to apply the nameplate voltage to the motor at the nameplate frequency. Let's just say 460V at 60hz. Following that profile as the motor slows down, the voltage would be 307v at 40hz, 230v at 30hz, 158V at 20hz, 79v at 10hz, and unless the profile is altered in the parameters, there would be 0v at 0hz. Don't look for any torque at 0hz!

Hope that makes the output voltage picture a little clearer.

Tom Jenkins · Jul 13, 2016

As a point of reference, I believe NEMA requires motors to operate properly at voltage +/- 10% of nameplate voltage. Obviously lower voltage requires higher current to provide the same power. Torque is usually a function of current.

Increase VFD output voltage?

einnh

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einnh

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keithkyll

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Aabeck

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jraef

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einnh

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DickDV

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Tom Jenkins

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