Single phase equipment to power three phase motors

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Natfuelbilll

Senior Member
Project location has single phase power only. Three phase is $$$ away.

What is the largest three phase motor (240V) that you have seen variable speed drives powering?

Mine is 20HP three phase pump.
WEG tells me they can power a 40HP three phase motor.

What have you seen out there?
 

mcclary's electrical

Senior Member
Location
VA
It needs to be engineered properly. I saw a 25 on a brand new compressor, new VFD, single phase source. It had lots of problems starting when loaded.
 

Jraef

Moderator, OTD
Staff member
Location
San Francisco Bay Area, CA, USA
Occupation
Electrical Engineer
You can power any 3 phase pump from a single phase supply. There are issues to contend with however.

When you power a 3 phase motor from a single phase supply, the motor nameplate FLA value will result in a draw from the supply that is 1.732 x that. So if you have a 20HP 230V 3 phase motor with an FLA of around 50A, the 1-phase current to feed it will be at least 87A which, if you have a 100A service or a 200A with a lot of other loads, may be too much.

In addition, you must double the size of the VFD. Not only to cover the 1.732 multiplier on current capacity of the rectifier front-end, but also to provide added capacitance for the DC bus because 1-phase power results in added ripple current. So now for your 20HP motor you must buy a 40HP VFD. Then the code says you must size the circuit for a MINIMUM of 125% of the VFDs MAXIMUM current rating, not the rating of the motor it is running. So a typical 40HP 230V VFD will be rated for 115A max, which means the circuit feeding it must be capable of 143A.

Another "gotcha" is to make sure your VFD is capable of accepting 1 phase input. Most small ones (under 5HP) do, but once you get beyond that there are some that have phase loss protection that cannot be defeated. If you end up with one of those, it will not work, so just be careful.

So for most typical installations, 25HP on a single phase service is probably the practical limit, but only if you have a lightly loaded 200A service.
 

Natfuelbilll

Senior Member
WEG had me use two 1500 uF capacitors on the vfd for the 20hp project.
My next project has a dedicated 400A service. The mechanical guy wants a 75hp motor. WEG does the 40 hp and (just learned yesterday). I am going to propose two 40 HP tied to the load.
 

Natfuelbilll

Senior Member
Ooo 75 hp that is the rating I need.

Single phase 240 - right?

What manufacturer did you work with?

What size utility service was available?

Thanks for your help.
 

Oddest_raindrop

New member
The company I work for does many single phase vfd installations a year. The largest was 100 hp 480 volt with a 200 amp 480v service entrance. This was under special permission from the power company. But we have done 75 hp under 240v. Most of the time we end up with a step up transformer to do so since the feeder is 1000 feet long in many cases. This is all manly for irrigation wells.
 
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