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Can a VFD lower 240V to run 208 motor properly?

Garwood

Diamond
Joined
Oct 10, 2009
Location
Oregon
Can I use a VFD to run a 208V 3 phase heat pump compressor from 245V single phase power?

Some VFD's I have setup have voltage parameters. I have never had a situation where motor voltage is lower than line voltage, never tried this.If I set the voltage to 208 will it actually output 208 volts?
 
VFD has no problem outputting lower voltages, trying to go higher can be a problem. Also running a high load compressor isn't a great thing from single phase power.
 
So it just works eh? I don't need more power. It's an expensive sealed refrigeration compressor so I don't want to cook it.
 
most VFD's take the difference from the low part of the sine wave and high end and convert that to a DC bus that stores the power. in 3 phase and large power draws that one leg will always be in the dead spot of the sine wave on single phase and can cause the electronics to fry under heavy load. Light loads can handle it better like 1 hp or so and are also much cheaper purchase smaller vfd's.
A heat pump, you would be cheaper to buy the correct voltage pump instead of a vfd.
 
most VFD's take the difference from the low part of the sine wave and high end and convert that to a DC bus that stores the power. in 3 phase and large power draws that one leg will always be in the dead spot of the sine wave on single phase and can cause the electronics to fry under heavy load. Light loads can handle it better like 1 hp or so and are also much cheaper purchase smaller vfd's.
A heat pump, you would be cheaper to buy the correct voltage pump instead of a vfd.

I've not had any issue using quality 1-3ph VFD's to drive heavily loaded 3 phase motors running pumps all day, everyday.

I've scoped a lot of rectifiers in power supplies feed single phase power and never seen what you're talking about unless caps are bad.

And no, prices are not as you say. 5 ton 240V single phase Copeland compressor is $1200+++. 240V 3 phase is $800+ surplus price. Quality 1ph-3ph 5HP VFD is $700.
 
If the VFD is DESIGNED for it, you are golden. Risk is on them. That seems to be what you are referencing

Otherwise, you are kinda guessing, and can run into problems with the DC bus capacitors, possibly with the rectifiers, or maybe just a "low voltage" trip.
 
If the VFD is DESIGNED for it, you are golden. Risk is on them. That seems to be what you are referencing

Otherwise, you are kinda guessing, and can run into problems with the DC bus capacitors, possibly with the rectifiers, or maybe just a "low voltage" trip.

Are you referring to a VFD lowering the output voltage from 240 to 208 or using a single to 3 phase VFD?

My concern is about the 240/208 aspect, not single to 3 phase. I don't understand how a VFD can lower the output voltage, my google searching has given a lot of poor quality results so I figured ask here if anyone knows if this will work. If you can properly power a 208 motor (229 volts max in this case) from 245 volt supply using a VFD to lower the voltage.

Thanks!
 
I don't understand how a VFD can lower the output voltage...
It's the same way as an audio amplifier turns the volume up or down: transistors. Just like one of your spindle or axis drives, you have IGBTs (Integrated Gate Bipolar Transistors) taking a DC power supply and varying the output to each pole, depending on load.

The motors are just inductive-resistive loads. There really isn't a "voltage" they can handle. There is a current they can handle (watts). The amount of watts you shove through the wires, affects how much mechanical energy they can make (HP) and some lost to heat. The ability to shed the excess heat is what sizes the motor for a suitable load.

If you tell the VFD to output a certain voltage, it will simply regulate the number of watts it shoves into the motor and will check its own math by monitoring the voltage. I would imagine the ability to control these various things will depend on the features of the VFD. Some might only control things at fixed values, determined by the internal design and others may have parameters you can set for whatever you want.

It may be that your refrigeration system might even be fine with the higher voltage. You're not increasing the speed of the pump. The mechanical load won't change. The excess voltage might just result in a lower 'amps' number. Of course I/we don't know if there are other controls and things expecting 208 vs 240, though I suspect they also would be fine with it.

TL;Not-Doc-Brown: since you're using a VFD anyway, if it allows you to match the output voltage, do that and you shouldn't get in trouble.
 
Will add...less issues with 3ph compressors than single.
Yes the start components are outside the can, however I changed many more singles due to electrical failures than 3 ph.

I would have no issues using a quality vfd to run that can. I would keep start time at one second.
During normal operation the pressures will equalize anyway when it shuts off and startup load is low.
If you would develop a problem where the system is short cycling ....not sure which would fail first, vfd or can
Keep wire runs short, use THHN stranded wire of ample size.
 
Will add...less issues with 3ph compressors than single.
Yes the start components are outside the can, however I changed many more singles due to electrical failures than 3 ph.

I would have no issues using a quality vfd to run that can. I would keep start time at one second.
During normal operation the pressures will equalize anyway when it shuts off and startup load is low.
If you would develop a problem where the system is short cycling ....not sure which would fail first, vfd or can
Keep wire runs short, use THHN stranded wire of ample size.

Thanks a bunch! I forgot you were big into HVAC stuff! I should have asked in the other place lol. I have more questions...
 
Are you referring to a VFD lowering the output voltage from 240 to 208 or using a single to 3 phase VFD?

My concern is about the 240/208 aspect, not single to 3 phase. I don't understand how a VFD can lower the output voltage, my google searching has given a lot of poor quality results so I figured ask here if anyone knows if this will work. If you can properly power a 208 motor (229 volts max in this case) from 245 volt supply using a VFD to lower the voltage.

Thanks!
I was thinking about the single to 3ph for that specific bit. For the voltage deal, in more extreme cases of reduction, the current might be a problem as far as getting the same power, since the VFD is current limited, and the same power is more current at lower voltages.

As for the rest, the VFD puts out pulses of voltage, that "average out" to the voltage wanted. For instance, since most 240V VFDs operate from a DC bus of about 340V, the pulses are nearly 340V. But if the pulse is only "on" half the time, it amounts to 170V, not 340V as far as the motor is concerned. (very simplified)

The VFD varies the pulse width to get the voltage which is wanted. it works fine.

The only issue is that as mentioned, the actual pulses are still 340V or so. That system might not work well for extreme cases, like trying to operate a 24V motor with a 240V VFD. The VFD "might be" able to do its part, but the motor might not "take" the high pulse voltage.
 
Wierd 3P at my family's shop. 2 120's, and a 220. Pretty much 240 pole to pole, give or take.

Added a transformer for one machine to get 208. Expected to see a weird 104/104/??? on the output. Nope, 120/120/120 ... 208 everywhere.

Took me awhile to figure out its pairing input legs (not stepping each down and passing them thru) and the phase angle between them, 90/90/180, is what makes it 'just work.

Gonna be, whenever ..., they're my only option ... other than 'tens of thousands of dollars' to run it a half mile from the main line.

That outta get me all the PP's I'd ever need ... assuming I can pull enough 240 off the pole to feed them.

'Hope for the best, but be prepared for the worst'
 
................

Took me awhile to figure out its pairing input legs (not stepping each down and passing them thru) and the phase angle between them, 90/90/180, is what makes it 'just work.

................................
The 90/90/180 you mention is the same setup as is present with the old "Scott-T" connection to change 2 phase to 3 phase (or the reverse).

In fact the voltages and phases from an RPC are the same as you would find in the Scott-T connection. What's actually "under the hood" differs depending on whether you have a wye or delta idler motor, but the result is mathematically the same.

It is also the same as "farm 3 phase", also known as "stinger 3 phase", "lighting tap 3 phase" and some other names.

The difference with an RPC, and with at least some of the Phase Perfect units, is whether the 3 phase neutral is a true neutral (as with wye), or an imaginary floating neutral, as with farm 3 phase etc. The RPC, and some PP units as I understand, have the 3 phase "neutral" not grounded, so it is "imaginary", and has a voltage to ground. You need a transformer to re-reference it to ground as wye.

Getting 3 phase with a large VFD, as some have done, is a bit different, but again, there is not a true "neutral" unless re-referenced to ground through a transformer.

In all cases, however, the line-to-line voltages will be what they need to be.
 
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