USING A VFD TO CONVERT 1 PHASE TO 3 PHASE

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doncloutier

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osceola wi
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contractor
I AM WIRING A 3 PHASE 480 VOLT 50 HP MOTOR NAMEPLATE AMPS 65. I WANT TO USE A VFD FOR A PHASE CONVERTOR. I HAVE ON SITE 240 VOLT SINGLE PHASE. IM PLANNING TO INSTALL A 240 VOLT TO 480 VOLT SINGLE PHASE TRANSFORMER TO FEED LINE SIDE OF VFD AT 480 VOLT SINGLE PHASE
. MY QUESTIONS ARE #1-HOW DO I PROPERLY SIZE VFD? DO I GO UP 2 SIZES AS A RULE OF THUMB . #2-DO I SIZE SINGLE PHASE TRANSFORMER TO THE SIZE OF THE 50 HP MOTOR (65 AMPS X 1.73 =112.45 AMPS ) 240 X 112.45/1000 =27.12 KVA TRANSFORMER OR TO THE HP RATING OF THE VFD.(I THINK I SIZE TO THE VFD RATED HP)THATS 2 SIZES BIGGER .#3-IF I FUSE TRANSFORMER AT 125% ON PRIMARY SIDE WOULD I HAVE TO INSTALL OCP BETWEEN LOAD SIDE OF VFD (480 3 PHASE) AND MOTOR AT 175%. THANKS HOPEFULLY NOT TO CONFUSING
 
LarryFine

LarryFine

Master Electrician Electric Contractor Richmond VA
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Henrico County, VA
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Size VFD to motor, transformer to VFD, etc.

You need to start at the load, work backward from there, never reducing capacity or creating a weak link.
 
don_resqcapt19

don_resqcapt19

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The general rule of thumb is that the HP rating of the drive used for the power conversion must be at least 173% of the driven motor horsepower.
 
Besoeker3

Besoeker3

Senior Member
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UK
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Retired Electrical Engineer
don_resqcapt19 said:
The general rule of thumb is that the HP rating of the drive used for the power conversion must be at least 173% of the driven motor horsepower.
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Yes, that would work. SQRT3 might be better. But my concern is using a 50HP motor for single phase. I know my background is industrial so we would rarely using single phase lower that about 1kW.
 
don_resqcapt19

don_resqcapt19

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Based on the standard transformer ratings I would be looking at a 75kVA 240 to 480 volt single phase transformer, and a drive with a 100 hp rating.
 
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hillbilly1

Senior Member
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North Georgia mountains
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Owner/electrical contractor
Being a single phase service, the poco transformer would probably be no larger than a 25 kva, so as others have said, it might pay off checking with the poco to see if they have the capacity before spending a lot of money.
 
tortuga

tortuga

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Besoeker3 said:
Yes - that would be a concern for me too. I wonder if the poster realises that?
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Agreed, but its a 240 volt service.
If this is a new service that is terrible planning.
Keep in mind allot the 'service planner' or engineering tech' at most utulities I have dealt with are not real EE's and are just kinda used to the same thing over and over again.
Not really good at out of the box stuff like 480 single phase for rural pumps.
 
kwired

kwired

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Location
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If this is the major load on the service and single phase supply is all that is easily available, maybe see if you can get 480 single phase service and maybe you can use much smaller transformer to derive any 120/240 that might be needed for other loads.

I agree with others in that when supplying drive with only single phase you need to derate the drive - by factor of square root of three (square root of 3 is the amount the current will increase to get same output power) though many just round that up to factor of 2. Many times that factor of 2 is the next size available anyway. 50 hp motor would need to use a 100 hp drive.

If you have a step up transformer and is two wire to two wire, you can protect secondary side with primary overcurrent protection - something to keep in mind, but pay attention to details and what may or may not work, though you won't have the starting surge like you have with across the line starting either. If you ground the midpoint of the secondary then you still have a three wire secondary even if not using the neutral for any load.
 
kwired

kwired

Electron manager
Location
NE Nebraska
electrofelon said:
I hooked up a phase converter to run a 30HP motor off a residential 240 single phase service. We didn't ask POCO but prolly should have. I'm sure they would not have been thrilled.
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If you had somewhat stout service to begin with and had light loading on it at the time you were using that particular motor - might not be all that much noticed by you or the neighbors. Starting surge possibly the worst, and depending on driven load it may not even demand that much while up to speed. Some commercial/industrial wood working tool - probably need to feed material at high rate of speed, at a maximum depth and be using max cutting surface design (if a cutting tool) to even come close to needing all 30 HP capability of the drive motor.
 
kwired

kwired

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Location
NE Nebraska
Electrical Underground said:
I have used the Square D Altivar drives for running 3 phase 480v motors straight from 240v single phase source. Works very good and for 8+ years now no problems.
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Are they dual voltage motors and you reconfigure them for low volts?

Is it drive designed to step up voltage to 480? I have seen one that was designed to do that. Otherwise typical off the shelf drive will have 240 in 240 out/480 in 480 out.

You can run 480 volt motor at 30 Hz 240 volts (which is what volts should typically be at 30 Hz even if you didn't tweak any settings) and it will be fine, but you get half speed and half nameplate HP.
 
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Electrical Underground

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Location
Southeast
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Contractor
kwired said:
Are they dual voltage motors and you reconfigure them for low volts?

Is it drive designed to step up voltage to 480? I have seen one that was designed to do that. Otherwise typical off the shelf drive will have 240 in 240 out/480 in 480 out.

You can run 480 volt motor at 30 Hz 240 volts (which is what volts should typically be at 30 Hz even if you didn't tweak any settings) and it will be fine, but you get half speed and half nameplate HP.
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Yes, the Square D altivar drives will convert 240 single phase to 480v 3 phase. Just verify HP ratings they do this to .
 
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