Which is better for three phase motor application: VFD with single phase input or use true 3 phase input

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Mr. Pickle

Mr. Pickle

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Rogers MN
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Electrical
Have a project where we have a single phase service. We are using two VFD's to convert the single phase to three phase for two motors. We are being told we would be better off by upgrading the service to 3-phase so we don't need the VFD's for phase conversion. Anyone know if there is actually any advantage in doing so?
 
Mr. Pickle said:
Have a project where we have a single phase service. We are using two VFD's to convert the single phase to three phase for two motors. We are being told we would be better off by upgrading the service to 3-phase so we don't need the VFD's for phase conversion. Anyone know if there is actually any advantage in doing so?
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You probably would still need vfds
 
Do you need to control or vary the speed of the motors?

How large are these motors?

If you need to control the speed of the motors, then you need a VFD anyway.

If the motors are large, the utility might require you to have 3 phase service.

If the motors are small, it is likely not worth the cost of a 3 phase upgrade.

-Jon
 
winnie said:
Do you need to control or vary the speed of the motors?

How large are these motors?

If you need to control the speed of the motors, then you need a VFD anyway.

If the motors are large, the utility might require you to have 3 phase service.

If the motors are small, it is likely not worth the cost of a 3 phase upgrade.

-Jon
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No

7.5HP
 
If the utility dies not have 3 phase available at your service point, you typically have to pay for them to run their cables to the nearest one. That can be VERY expensive.

Then when you go from a single phase service to a 3 phase, you get a Demand Meter, and often a VAR Meter, so now the user must contend with worrying about peak demand charges and their power factor.

Lastly, the cost of going to 3 phase means a new service panel and running all new cables to any 3 phase loads.

All in all if you already have the VFD setup and it’s working for them, the cost to change would not have a very good return in investment.

Also, ask whomever suggested this why they think that would be better? If they think it will save significant energy, they are mistaken. Yes, the VFD loses about 3% in the conversion process, but only while the motor is actually operating. Considering all the other costs to change to 3 phase, it might take a lifetime to recoup them with that little bit of energy savings.
 
Jraef said:
If the utility dies not have 3 phase available at your service point, you typically have to pay for them to run their cables to the nearest one. That can be VERY expensive.

Then when you go from a single phase service to a 3 phase, you get a Demand Meter, and often a VAR Meter, so now the user must contend with worrying about peak demand charges and their power factor.

Lastly, the cost of going to 3 phase means a new service panel and running all new cables to any 3 phase loads.

All in all if you already have the VFD setup and it’s working for them, the cost to change would not have a very good return in investment.

Also, ask whomever suggested this why they think that would be better? If they think it will save significant energy, they are mistaken. Yes, the VFD loses about 3% in the conversion process, but only while the motor is actually operating. Considering all the other costs to change to 3 phase, it might take a lifetime to recoup them with that little bit of energy savings.
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Well the equipment is all getting replaced with new any which way. The utility is saying the cost to upgrade would be about 2K for utility costs. Since it is all getting replaced with a cost of 70-90K, the thought was to go ahead and spend the extra 2K if it makes for a better system.
 
Maybe I should also add that at one of the sites, the distance between motor and control panel/VFD is about 200'
 
To the supply, a VFD isn't a motor load, it's a power supply, so any advantage to converting to 3ph would be the same as any other: more power available for a given current.

If you're upgrading to 3ph anyway, it might be worthwhile. If not, it's not.
 
Mr. Pickle said:
No

7.5HP
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Mr. Pickle said:
Well the equipment is all getting replaced with new any which way. The utility is saying the cost to upgrade would be about 2K for utility costs. Since it is all getting replaced with a cost of 70-90K, the thought was to go ahead and spend the extra 2K if it makes for a better system.
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If the cost is only $2K to upgrade to 3 phase, and you don't need to control motor speed, then I'd lean more toward getting 3 phase and running the motors directly.

_However_ I'd question that $2K upgrade price. Are you going from 120/240V single phase to 208/120V three phase? The utility might only charge 2K for their part of the upgrade (which is quite low), but you will have costs all over the plant for the change. Are there any loads that need the full 240V, for example? If the utility switches to 3 phase, will the motors be happy at 208V? (Many are, some are not.)

Also ask if the three phase service is being charged at a higher tariff rate. It might be more expensive to use 3 phase over time.

At 7.5 Hp, you could plausibly use 240V single phase motors, 230V three phase motors with VFDs for phase conversion, or 208-230V three phase motors on 208/120V supply.

I would strongly avoid any 'high leg delta' three phase service, if that is what is being offered for the upgrade.

-Jon
 
The big question is how large is the new total load calc, and how much of that load, or how many loads (motors, etc) would benefit from 3ph ?
 
If you are going to redo the whole thing, I think three-phase makes a lot of sense especially if any 240 volt single phase loads will also operate on 208 single phase. You wouldn't even have to change any existing panel boards.
 
What is the motor horsepower? My poco limits the max size of a single phase motor, and may require a VFD or SS for large 3 phase motors.
 
Frank DuVal said:
How do they do that? See if you are buying a 7.5 HP air compressor at Tractor Supply with drone surveillance? :unsure:
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If you do a large single phase motor and it causes a voltage drop that someone complains about, they look at your demand and find out what you have done. I had Seattle City Light do that to me at my shop. We were under the impression that they wouldn't know when we put in a 10HP single phase overhead crane motor (the hoist), because it wouldn't be used all that often. They did. Came knocking at the door wanting to know if we had applied for a permit to add that much load (we had not). What we got out of it was a "free" upgrade to an Open Delta 3 phase 4 wire system if we changed the crane to 3 phase, which we did. Then a month later we got the new bill with demand charges and PF penalties that we hadn't had before. So much for "free"...
 
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