Single phase VFD

[gonzo]

This is probably a silly question but-----
Do they make a VFD with 230V single phase in to 230V single phase out?

 

[markba633csi]

Yes, they do make variable frequency single phase power supplies
What is the application?
-Mark

 

[gonzo]

I want to drive the 2hp motor on my mill/drill. to get variable speed.

 

[markba633csi]

With single phase capacitor start motors the speed is typically fixed. Is that what you have?
You would need to change the motor to 3-phase to use a VFD

 

[MikeInOr]

The 3 legs of 3 phase current can be seen as 3 seperate sine waves 120 degrees apart. So 2 legs of a 3ph VFD will be 120 degrees apart resulting in under voltage across those 2 legs. So I would guess trying to control a single phase motor off of 2 legs from a 3ph VFD will not work very well.

 

[rabler]

The 3 legs of 3 phase current can be seen as 3 seperate sine waves 120 degrees apart. So 2 legs of a 3ph VFD will be 120 degrees apart resulting in under voltage across those 2 legs. So I would guess trying to control a single phase motor off of 2 legs from a 3ph VFD will not work very well.

Not true. Put a voltmeter across any two legs. You'll get rated voltage. Yes, the three legs are separated by 120 degrees phase angle, but the voltage is spec'd as the voltage between any two legs.

 

[mksj]

Single phase is single phase, in the US split phase but there is no stagger between waves. There are VFD's that will drive single phase motors, for them to be somewhat effective you remove the capacitors and the centrifugal switch, and they use the start winding wired to the VFD for another phase. How well this works, and the speed range is still a question. They are also limited to certain types of single phase motors depending on the VFD design. Some examples below.

VFD’S for Single Phase Motors?

www.wolfautomation.com

Single Phase VFD, control speed for single phase motors | ATO.com

www.ato.com

 

[MikeInOr]

Not true. Put a voltmeter across any two legs. You'll get rated voltage. Yes, the three legs are separated by 120 degrees phase angle, but the voltage is spec'd as the voltage between any two legs.

I am correct. "The formula is volts times the square root of 3, which happens to be rounded off to 1.732. For 2 lines each carrying 120 volts, the calculation for this is 120 volts times 1.732, and the result is rounded up to 208 volts."
Please read: https://www.raritan.com/landing/three-phase-power-explained

240v single phase into a VFD will result in 208v output across any 2 legs of the generated 3 phase output.
220v single phase into a VFD will result in 190v output across any 2 legs of the generated 3 phase output.

 

[rabler]

Mike,
I taught college level Electrical Engineering. While power systems wasn't my particular research area I'm familiar enough with the basics to be on firm ground.

I am correct. "The formula is volts times the square root of 3, which happens to be rounded off to 1.732. For 2 lines each carrying 120 volts, the calculation for this is 120 volts times 1.732, and the result is rounded up to 208 volts."
Please read: https://www.raritan.com/landing/three-phase-power-explained

That's voltage measured from neutral times the square root of 3. So three phase @ 120V to neutral will measure 208V between any two legs. You've picked an example but failed to realize that it is not the usual case. That is ONE possible three phase configuration. A three phase that measures 240V between each leg is a different, higher voltage, is more common, and is what most VFDs and three phase motors are built around, (A fair number of 3phase motors are rated to run on both 208V and 240V three phase in the same configuration). You can also get into the 440V class 3phase power, motors and vfds.

This gets into the difference between delta and Y configurations, and is non-trivial. There are a lot of variation of three phase power, and it's uses.

240v single phase into a VFD will result in 208v output across any 2 legs of the generated 3 phase output.
220v single phase into a VFD will result in 190v output across any 2 legs of the generated 3 phase output.

A VFD converts voltage in from AC to DC and then from DC back to AC, at just about any voltage the designer want. Often you can tweak the voltage within some range from nominal through the program settings. I happen to have a VFD that uses 120V single phase and generates 240V three phase for a small motor. See for example:

 

[MikeInOr]

The original question was running a single phase motor through a VFD. Using a 3 phase VFD will result in an under voltage situation for the motor when using 2 legs of the generated 3 phase output the VFD. Very simple.

240v single phase into a VFD will result in 208v output across any 2 legs of the generated 3 phase output.
220v single phase into a VFD will result in 190v output across any 2 legs of the generated 3 phase output.
Is correct!

The output voltage will be the difference of the two 3 phase legs which are 120 degrees offset from each other. The 120 degree offset is what you are failing to take into account.

If the two output legs were 180 degrees offset from one another the difference (the output voltage) would be the SUM of the two legs. The output legs are not 180 degrees from each other, they are only 120 degrees offset from each other.

Mike,
I taught college level Electrical Engineering. While power systems wasn't my particular research area I'm familiar enough with the basics to be on firm ground.


That's voltage measured from neutral times the square root of 3. So three phase @ 120V to neutral will measure 208V between any two legs. You've picked an example but failed to realize that it is not the usual case. That is ONE possible three phase configuration. A three phase that measures 240V between each leg is a different, higher voltage, is more common, and is what most VFDs and three phase motors are built around, unless you get into the 440V class motors/vfds. This gets into the difference between delta and Y configurations, and is non-trivial.

Sorry you are wrong here:

Totally wrong. A VFD converts voltage in from AC to DC and then from DC back to AC, at just about any voltage the designer want. Often you can tweak the voltage within some range from nominal through the program settings. I happen to have a VFD that uses 120V single phase and generates 240V three phase for a small motor. See for example:

The DC voltage from a single phase rectifier will be DC voltage = AC voltage divided by the square root of 2. Always! Rectifiers do not boost the output DC to any voltage you want. It is always AC voltage/1.4 for single phase AC.

240v AC -> rectifier = 157v DC out.