Do VFDs adjust output PWM for changes in line voltage?

Curious about something:

Say you have a VFD that is designed for a line voltage of 200-240AC, 3PH. At 240V, the DC bus voltage will obviously be higher than at 200V, and therefore the output pulses will be a higher voltage.

My question is- does the VFD compensate for this change in DC bus voltage, and adjust the PWM duty cycle? Or does it assume it always has the same bus voltage, as if it were running full load with a line voltage of say, 220V?

The VFD in question is an AB PowerFlex 523, but I'm curious if this applies to VFDs in general.

Thanks!

rupej said:

Curious about something:

Say you have a VFD that is designed for a line voltage of 200-240AC, 3PH. At 240V, the DC bus voltage will obviously be higher than at 200V, and therefore the output pulses will be a higher voltage.

My question is- does the VFD compensate for this change in DC bus voltage, and adjust the PWM duty cycle? Or does it assume it always has the same bus voltage, as if it were running full load with a line voltage of say, 220V?

The VFD in question is an AB PowerFlex 523, but I'm curious if this applies to VFDs in general.

Thanks!

Click to expand...

I would think that good ones do this, but I'm sure there are some really cheap ones that don't. I've seen some low end motor controllers with really interesting behaviors....

Don't know about that model specifically.

VFD's adjust their output to control the motor. The incoming line voltage doesn't really matter at all, as long as it is within tolerance.

The best way to look at it, is even inexpensive low horsepower VFD's can often run off of Three phase, single phase, or even DC.

So just one addition. MODERN STANDARD Voltage Source Inverter (VSI) PWM VFDs do not adjust the line voltage, it is just rectified and smothed DC at whatever level corresponds to the incoming AC (let’s leave motor regenerative energy out of this discussion for clarity). The front-end rectifier is just a full wave diode bridge followed by a capacitor bank, sometimes an inductor between them. There is nothing in that which has the ability to adjust anything. Output RMS voltage to the motor is controlled solely by adjusting the PWM pattern via the transistor firing. In that endeavor, the maximum output voltage going to the motor is in fact dictated by the input line voltage; the drive can adjust the output RMS to any level LOWER than the line, but is limited by the maximum line input voltage. Max. E out = E in. The A-B PF 520 series falls into this category by the way.

CURRENT SOURCE Inverters (CSI) on the other hand DO adjust the DC bus voltage on the rectifier end via phase angle control using thyristors. For the most part, nobody is using CSI technology in LV drives any more, but it is still used in MV drives because it is easy to implement Line Regenerative Braking, useful for braking large loads (where one would typically use MV motors) as well as harmonic mitigation.

More recently, drive mfrs are releasing new designs of Low Voltage VSI “Active Front End” (AFE) drives that are, for all intents and purposes, two back-to-back PWM transistor inverters; an Active PWM inverter doing rectification of the AC line to DC, then the standard back-end output inverter for recreating AC to the motor. In these AFE drives, you do have the ability to actively control the DC bus level for a number of reasons. With that control, one benefit is to allow the drive to “boost” the line voltage by about 10%. That then allows the drive to provide FULL voltage to the motor even when the line side dips by 10%. So in this type of drive, Max. E out = E in +10% (roughly). AFE LV drives have been around for a long time for line regenerative applications, but the latest generation have added harmonic mitigation capabilities that have increased the sales volume and brought the cost down significantly to where it is now competitive against other low harmonic options (like 18 pulse). MV PWM VSI AFE drives for now are too complicated to make and far too expensive to be practical.

rdrast said:

VFD's adjust their output to control the motor. The incoming line voltage doesn't really matter at all, as long as it is within tolerance.

Click to expand...

But the incoming voltage has to matter, because that's what directly determines the voltage of the PWM signal. We all know that the VFD can control the apparent RMS voltage of its output by adjusting the PWM pattern, but is it smart enough to "trim" the PWM pattern based on changes to its DC bus voltage? If not, then the output RMS voltage would greater at 240V input than 200V input.

rupej said:

But the incoming voltage has to matter, because that's what directly determines the voltage of the PWM signal. We all know that the VFD can control the apparent RMS voltage of its output by adjusting the PWM pattern, but is it smart enough to "trim" the PWM pattern based on changes to its DC bus voltage? If not, then the output RMS voltage would greater at 240V input than 200V input.

Click to expand...

No, it can't ADD voltage that isn't there. Yes, the DC bus is higher than the AC line RMS voltage, but at the same time the AC RMS output voltage has to be reconstructed the same way, so the maximum output you can get is still whatever your input voltage is. If you have 200V in, the highest you can get out is 200V, then anything LOWER than that is adjusted in the PWM pattern.

I'm not talking about adding output voltage without raising the input voltage. Let me ask a different way:

If you have a 230V input to the drive, and the drive is connected to a 230V motor, the drive will output a PWM pattern to generate 230VRMs at 60Hz if it is programmed correctly. Okay, now you move the drive to a 240V input. Does the PWM pattern stay the same? Or does it lower the "duty cycle" to account for the increased input voltage?

rupej said:

I'm not talking about adding output voltage without raising the input voltage. Let me ask a different way:

If you have a 230V input to the drive, and the drive is connected to a 230V motor, the drive will output a PWM pattern to generate 230VRMs at 60Hz if it is programmed correctly. Okay, now you move the drive to a 240V input. Does the PWM pattern stay the same? Or does it lower the "duty cycle" to account for the increased input voltage?

Click to expand...

If you have the drive programmed to put out 230V at 60Hz, you can feed the drive with any voltage from 230V up to whatever the input tolerance it. The DC bus voltage will float according to the line voltage increase, but it doesn't affect the output voltage.

So for example on a PF520 drive, a 240V rated drive will handle a DC bus of up to 405VDC (which equates to an input of up to 290VAC) before it will trip off to protect itself. But the output will remain at the programmed value of 230V, because the drive will alter the PWM pattern to adjust that output RMS voltage in order to maintain the proper V/Hz ratio as determined by the motor nameplate data you entered.

That same drive will stay active down to a DC bus of 190VDC (equating to 150VAC input), but the output will have to be 150VAC at that level. In most cases, drives will then automatically lower the output frequency commensurately, because its job is to maintain the desired V/Hz ratio.

Ah, thanks, that answers my question. I appreciate you taking the time to write all of that out.

The only thing that surprises me is that you said a drive cannot produce a higher RMS output than its input. I would have thought that since the input is rated 200-240V, it could produce a 230V RMS output from any voltage within that range.

rupej said:

I'm not talking about adding output voltage without raising the input voltage. Let me ask a different way:

If you have a 230V input to the drive, and the drive is connected to a 230V motor, the drive will output a PWM pattern to generate 230VRMs at 60Hz if it is programmed correctly. Okay, now you move the drive to a 240V input. Does the PWM pattern stay the same? Or does it lower the "duty cycle" to account for the increased input voltage?

Click to expand...

I want to preface my response by admitting that I don't consider myself to be a drive expert, but what you are failing to consider is that these drives utilize closed loop control by monitoring their own output voltage, current and frequency in order to ascertain that they are delivering the appropriate V/Hz ratio as requested by their speed reference input.