VFD's - An Explanation Please

[goldstar]

Be aware the addition of the VFD may likely cause GFCI tripping. Might be in long term better to get a non VFD option as the VFD would get nuisance tripping issues.
GFCI is required in 210.8(B)(2) for the kitchen area of non dwelling units.

(B) Other Than Dwelling Units.
All 125-volt through 250-volt receptacles supplied by single-phase branch circuits rated 150 volts or less to ground, 50 amperes or less, and all receptacles supplied by three-phase branch circuits rated 150 volts or less to ground, 100 amperes or less, installed in the following locations shall be provided with GFCI protection:

Great. That would have been my next question.

 

[busman]

I think the VFD also needs to limit current thru the motor since lower frequency means lower impedance in the windings.

 

[winnie]

I think the VFD also needs to limit current thru the motor since lower frequency means lower impedance in the windings.

If you are answering the original question of 'what does a VFD do' you are correct.

In general, when the VFD changes the _frequency_ of its synthesized output, it also changes the _voltage_ in a corresponding fashion, to maintain a constant V/Hz ratio and compensate for the changing winding impedance.

Slightly smarter drives add a factor which compensates for the winding resistance separately from the winding inductance.

There are much smarter approaches to controlling the motor, but in the end they all involve tweaks to the frequency or voltage used to operate the motor.

-Jonathan

 

[busman]

If you are answering the original question of 'what does a VFD do' you are correct.

In general, when the VFD changes the _frequency_ of its synthesized output, it also changes the _voltage_ in a corresponding fashion, to maintain a constant V/Hz ratio and compensate for the changing winding impedance.

Slightly smarter drives add a factor which compensates for the winding resistance separately from the winding inductance.

There are much smarter approaches to controlling the motor, but in the end they all involve tweaks to the frequency or voltage used to operate the motor.

-Jonathan

Yes, answering the OP. And yes, what you describe is what I understand also.

Mark

 

[kwired]

If you are answering the original question of 'what does a VFD do' you are correct.

In general, when the VFD changes the _frequency_ of its synthesized output, it also changes the _voltage_ in a corresponding fashion, to maintain a constant V/Hz ratio and compensate for the changing winding impedance.

Slightly smarter drives add a factor which compensates for the winding resistance separately from the winding inductance.

There are much smarter approaches to controlling the motor, but in the end they all involve tweaks to the frequency or voltage used to operate the motor.

-Jonathan

Also remember if you have say a 10 HP,480 volt 60 Hz motor and are running at 30 Hz the motor voltage like you said will be 240 and current will be right about nameplate amps if motor load is right about 5 HP and basically is what the motor is rated for if supplied by 240 volts @ 30 Hz.

 

[garbo]

If you are answering the original question of 'what does a VFD do' you are correct.

In general, when the VFD changes the _frequency_ of its synthesized output, it also changes the _voltage_ in a corresponding fashion, to maintain a constant V/Hz ratio and compensate for the changing winding impedance.

Slightly smarter drives add a factor which compensates for the winding resistance separately from the winding inductance.

There are much smarter approaches to controlling the motor, but in the end they all involve tweaks to the frequency or voltage used to operate the motor.

-Jonathan

Was told by two drive company techs that most drives are set up so voltage is in proportion with output frequency. A 480 volt drive running at 30 Hertz usually has 240 volt output. I know on air handlers you still can put out a good amount of air flow with a tremendous savings in power. In our two 12 story research buildings they surely had over a hundred 4 to 6' wide exhaust hoods with sliding glass to be able to extend your hand into area. One roof exhaust fan might have handled over a dozen exhaust hoods and Johnson Control system would control the exhaust fan speed. If only one hold had the glass door open roof fan might only had to run at 30% .

 

[garbo]

VFD stands for Variable Frequency Drive. A 3-phase motor's speed is controlled not by the voltage sent to the motor, but by the frequency.

Examples:
60Hz = nameplate speed (1750RPM for a 4-pole motor).
30Hz = 1/2 nameplate speed.

In your case, the vendor is using a VFD to convert from single phase to 3-phase power. This works because a VFD takes in AC from the line, rectifies it into high-voltage DC, and then chops that DC up back into a synthesized AC waveform at whatever frequency is programmed. In your case, it'll probably be set to 60Hz, and will be used simply to convert single phase to 3-phase power.

You need the larger circuit because that power has to come from somewhere. For a given amperage, a 3-phase supply can deliver 1.73 times as much power as an equivalent single phase supply.


SceneryDriver

All of the hundred of drives that I PM'ed or troubleshoot & replaced bad boards, capacitor bank , fuses, IGBT'S etc had the output voltage proportional to the output frequency generally speaking. When a 480 volt drive was producing 30 Hertz output would be 240 volts. When troubleshooting I would start testing a drive at 20 Hertz and write down the output frequency, speed, ampere & voltage then I crease it by 5 hertz .

 

[kwired]

Was told by two drive company techs that most drives are set up so voltage is in proportion with output frequency. A 480 volt drive running at 30 Hertz usually has 240 volt output. I know on air handlers you still can put out a good amount of air flow with a tremendous savings in power. In our two 12 story research buildings they surely had over a hundred 4 to 6' wide exhaust hoods with sliding glass to be able to extend your hand into area. One roof exhaust fan might have handled over a dozen exhaust hoods and Johnson Control system would control the exhaust fan speed. If only one hold had the glass door open roof fan might only had to run at 30% .

I think they generally only tweak v/f ratio for boosting torque and even then only for limited time, like maybe to improve acceleration curves, otherwise you get into saturation levels if excitation levels are too high and that leads to overheating. This would maybe be more common on a motor intended to drive a high inertia load that will take time to accelerate, or even time to decelerate if that is to be controlled by the drive as well.

 

[Jraef]

An AC motor’s torque is directly related to the Volts and Hertz that is was designed around, the “V/Hz ratio”. If you want to change the speed of an AC motor without involving mechanics, you have three choices; reduce the voltage, which reduces the torque, but INCREASES the slip and that makes the motor draw more current while doing less work, usually leading to overload, reduce the frequency only, which causes the motor to go into magnetic saturation and leads to overheating, or reduce BOTH the voltage and frequency together, keeping the V/Hz ratio constant. That’s what ALL VFDs do, that’s their primary mission.

Within that basic function, you can “tweak” the algorithm to change the performance. Boosting the voltage boosts the torque, at least temporarily (because it increases the heat in the motor too). Reducing the voltage reduces the torque, which means IF the load it’s centrifugal, like a pump or fan, it can reduce the energy consumption by reducing waste.

Advanced (Vector) drives go even further by accomplishing these tweaks on the fly WITHIN each AC cycle going to the motor. So they can increase the torque while AVOIDING the accompanying saturation that might overheat it.

 

[SceneryDriver]

All of the hundred of drives that I PM'ed or troubleshoot & replaced bad boards, capacitor bank , fuses, IGBT'S etc had the output voltage proportional to the output frequency generally speaking. When a 480 volt drive was producing 30 Hertz output would be 240 volts. When troubleshooting I would start testing a drive at 20 Hertz and write down the output frequency, speed, ampere & voltage then I crease it by 5 hertz .

You're right, garbo. I was trying to keep the basic explanation simple. V/F ratio is important, but often extremely hard to measure in the field due to the way drives "make" the AC waveform. PWM introduces lots of harmonics and often causes volt meters to read incorrectly (unless they're true RMS).

The important takeaway with AC motors is that their speed is proportional to the frequency they're fed with and not (directly) the voltage, as it is with DC motors.


SceneryDriver

 

[garbo]

You're right, garbo. I was trying to keep the basic explanation simple. V/F ratio is important, but often extremely hard to measure in the field due to the way drives "make" the AC waveform. PWM introduces lots of harmonics and often causes volt meters to read incorrectly (unless they're true RMS).

The important takeaway with AC motors is that their speed is proportional to the frequency they're fed with and not (directly) the voltage, as it is with DC motors.


SceneryDriver

I always measured drive ampere at input and used the touch screen display to provide output voltage readings. Several times while troubleshooting the three input amps were almost identical then once I increased speed ( while in hand mode ) pass 35 to 40% readings would be all over the place due to faulty board. We had an old 4 or 500 amp amprobe clamp on amp meter with the red digital displays back in1980 that they told us to take output ampere reading from VFD'S. Readings jumped all over the place while drive speed was set using a pot. ( good old analog days ) I called Amprobe company up and they were nice enough to inform me that that old meter was calibrated to work on 60 Hertz. Don't know why some $300 meters do not read true RMS values. Had a best in class Fluke combination VOM /1,000 volt megger that probably now cost well over $600 and did not read true RMS.