HELP! 3 phase 120/208 to 3 phase 240

Location
Colorado
Occupation
Electrician
I need 3 phase 240v to feed a machine for a customer. motor only has 240v on name plate and that's what customer wants. Motor only pulls under 12 amps at 240. I have a 120/208 panel in the same room with space available. I thought there would be a transformer that could take me to 240 but I cant find one anywhere online. I'm fairly new at this and not very smart. Any help would be greatly appreciated. Customer is in a rush now because he just got the machine in and was supposed to have power to it already
 

jim dungar

Moderator
Staff member
Location
Wisconsin
Occupation
Engineer
Do not carry the neutral to the 240V load. Use two transformers in a open-delta configuration. You do not need exactly 240V, 230-236 would be close enough.
 

Electricmo

Member
Location
Missouri
Occupation
Lineman
Not to sure where you are at but it appears your existing service is a wye connected configuration. Most likely there is a existing 3 bank transformer service supplying a 120/208 voltage for existing equipment. Your needing a delta connection to get 120/240 voltage with a 208 wild leg. Dry type secondary transformers will most likely be needed. Not enough info supplied.
 

LarryFine

Master Electrician Electric Contractor Richmond VA
Your needing a delta connection to get 120/240 voltage with a 208 wild leg.
The only thing that would make a delta have a high leg is bonding the center tap of the opposite winding; not required here. Plus, a standard transformer requires bonding headaches of its own.

If the equipment doesn't require the neutral, two buck-boost transformers can get real close to 240v 3ph as an open delta. The voltages-to-ground will be imbalanced, but that doesn't matter.

If the equipment does require the neutral, three buck-boost transformers can get real close to 240v 3ph as a wye. The voltages-to-ground would be balanced, which matters to such loads.

In either case, buck-boost transformers are more economical here, because each one only has to have the kva capacity to supply the load current at the voltage difference, not the current of the entire load.

In a nutshell, you wire the primary and secondary in series, which creates an auto-transformer, a single winding with a tap near one end. Many units come with dual-voltage primaries and secondaries.

You wire the two primaries in either series or parallel to suit the source voltage, and the secondaries in either series or parallel to suit the desired voltage increase or decrease. The link in post #2 tells you how to select and connect them.

If you apply voltage to the two end terminals, you receive a reduced voltage between the tap and the far end. If you apply voltage to the tap and the far end, you receive a reduced voltage between the two end terminals.
 

Electricmo

Member
Location
Missouri
Occupation
Lineman
Without changing existing panel and equipment voltage you are correct Larry. He can’t get the voltage he needs without secondary transformers on his new circuit. I thought I stated that but Sometimes I’m not to clear, at least that’s what my wife says, LOL.
 

Electricmo

Member
Location
Missouri
Occupation
Lineman
My apologies. I thought you were specifically suggesting a 208v-to-240v 3ph transformer.
I understand. Also in my line of business we always center tap a winding to supply a neutral to the panel. If the electrician doesn’t need a neutral he doesn’t have to run it from the panel for all we care.
 

jim dungar

Moderator
Staff member
Location
Wisconsin
Occupation
Engineer
If the equipment does require the neutral, three buck-boost transformers can get real close to 240v 3ph as a wye. The voltages-to-ground would be balanced, which matters to such loads.
The wye neutral point of a step-up buck-boost might not be usable for the load, as it will be closer to 136V than to 120V.
 
Location
Colorado
Occupation
Electrician
Thank you for all the input. I am getting power from a 120/208 3phase 4 wire panel. Sounds like I need the 2 buck boost transformers then. Don't need a neutral just 3 phase 240. Feeding a fused disconnect. Then feeding a ball mill that has a control cabinet. Cabinet has a vfd and it has a contactor with single phase 240v coil. Feeds a 3 phase 240v motor that says it draws 11 amps and a cooling fan that draws 1 amp.
 

hillbilly1

Senior Member
Location
Atlanta,Ga
Thank you for all the input. I am getting power from a 120/208 3phase 4 wire panel. Sounds like I need the 2 buck boost transformers then. Don't need a neutral just 3 phase 240. Feeding a fused disconnect. Then feeding a ball mill that has a control cabinet. Cabinet has a vfd and it has a contactor with single phase 240v coil. Feeds a 3 phase 240v motor that says it draws 11 amps and a cooling fan that draws 1 amp.
May have to go with a three buck boost setup, the vfd may not like the unbalanced voltage a two transformer gives. Our VFD gurus may chime in if its true or not.
 

jim dungar

Moderator
Staff member
Location
Wisconsin
Occupation
Engineer
May have to go with a three buck boost setup, the vfd may not like the unbalanced voltage a two transformer gives. Our VFD gurus may chime in if its true or not.
With balance 3-phase loading and open delta will provide balanced voltages.
It the only single-phase is the contactor coil, there shouldn't be any problems, but a single phase cooling fan might cause some problem as it is 10% of the VFD load. Again not a problem if the transformer is oversized.
 

winnie

Senior Member
With the 2 transformer setup and balanced loading, the line-line voltages will be balanced.

However the line-ground voltages will not (and cannot) be balanced. This can be a problem for VFDs because they usually have input side transient protection that is wired line-ground.

This is a problem in 480V systems that are corner grounded (or ungrounded systems that become grounded during faults). I am not sure how much of a problem it would be for the particular system being discussed; the L-G voltages would nominally be 120V, 149V, 149V

This is such a small system I'd go with the 3 transformer setup; according the the calculator in post 2 we are talking about three 0.250kVA units.

-Jon
 

GeorgeB

ElectroHydraulics engineer (retired)
Location
Greenville SC
Occupation
Retired
The wye neutral point of a step-up buck-boost might not be usable for the load, as it will be closer to 136V than to 120V.
dumb engineer here ... if 3 buck-boost wired boost are used, cannot the 208 junction continue to be used for 120, and the boosted 240 be used for the 240-3-60 load. The grounded "center" neutral still exists and can be used with both the 120 (obviously single phase), 208 (single and/or 3 phase), and 240 (3 phase).
 

jim dungar

Moderator
Staff member
Location
Wisconsin
Occupation
Engineer
dumb engineer here ... if 3 buck-boost wired boost are used, cannot the 208 junction continue to be used for 120, and the boosted 240 be used for the 240-3-60 load. The grounded "center" neutral still exists and can be used with both the 120 (obviously single phase), 208 (single and/or 3 phase), and 240 (3 phase).
The wye neutral would simply be a point that is referenced to a 'line'. If you are using one of the boosted legs, you change your refernce point and the resulting output.

Yes you could run one of the original 208V Lines and the common center conductor, but that would be a separate circuit using a common neutral.
 

Jraef

Moderator
Staff member
Location
San Francisco Bay Area, CA, USA
Occupation
Electrical Engineer
Do you need to do anything at all though? With a VFD on the motor, we can assume it’s there to turn down the speed. The VFD is fine with 208V, it just can’t put out more than you put in. If you just feed 208V to the VFD, you can get the motor to deliver full rated torque at up to 90% speed, the VFD will take care of giving the motor the correct V/Hz ratio. If they never need to run faster than that, leave it as is.

You might need to swap out the contactor coil, but that’s dirt cheap. Ask the customer how they will be using the ball mill.
 

Todd0x1

Senior Member
Location
CA
Is this an imported machine? It seems odd to have a 240v 3ph machine with a vfd in it. Usually the smaller VFDs say to not use them on anything other than a solidly grounded wye service.

You say the motor nameplate says 240v, but what does the VFD dataplate say? Could the VFD be a single phase 240v input model?
 
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