3-Phase or Single-Phase Supply

[lpace]

I am in the process of purchasing laundry equipment that is now independent of the phase; it can now operate on either 120v/Single Phase or 208-240V/three Phase since the "motor" unit is an inverter drive.

In the past, from an energy cost and amperage perspective the prefer choice for this equipment would be 208-240V/three Phase. However, the equipment vendor states now that same cost and efficiency of operation can be achieved with 120v/1P.

What's the opinion of the experts: 208V/3P or 120V/1P?

 

[aolalde]

208 Volts- 3Ph means smaller currents. that will make your life and operating costs easier.

 

[jraef]

Actually, if you are using an inverter that can convert 120V 1 phase to 230V 3 phase the motor must be under 1HP, so the issue of smaller currents and current carrying components isn't really going to be applicable.

Your vendor is correct, you really will not see much difference in machine efficiency. There are small percentage losses in the VFD, but now you have a VFD either way so the difference between the two input voltages is negligible. The technology that allows this is called a Voltage Doubler on the input side of the VFD. It is a passive device, i.e. no switching, so no additional switching losses. Only a very slight amount of extra resistance across the diodes involved.

It will take a larger circuit capacity on the input source for the end user and there may be some slight additional losses there, but not really enough to worry about, and technically that is not your "machine's" efficiency

Eng-Tips: Help for your job, not for your homework Read faq731-376

 

[lpace]

Thanks--another variation

I believe the vendor states the equipment can operate at 120V/1P but recommends 208-240V/1P since the HP of the motors run between less than 1 HP up to 4 HP.

What's the preferred supply among manufacturers and the experts:

1. 120V/1P

2. 208-240V/1P

or

3. 208-240V/3P??

 

[ScottyUK]

If you have 3-phase available, use it. If you have to install it for this job but already have single phase, perhaps you could justify single phase on economic grounds.


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I don't suffer from insanity. I enjoy it...

 

[jraef]

4HP from a 1 phase source is a lot of current, around 30A, and 30A 120V circuits are not very common in distribution panels or pre-existing wiring (10ga wire minimum). So from that standpoint 208-230V 1 phase wuld be better because 4HP @ 208V would only require a 20A circuit, very common.

I don't understand the variation on HP though. Are they referring to different models? Because if they mean that this one model sometimes uses 1HP and at other times 4HP (i.e. multiple motors), then as far as you are concerned the total load is 4HP. You must always consider the worst case scenario, so you will need that 30A circuit.

If on the other hand you are reading a sales brochure that is common to several models, then base your decision only on the model you are going to buy. Their recommendation would (should) also be based on the worst case scenario. So if you are buying a 1HP model, either way is fine. If you are buying a 4HP model, go with the 208 or 230V input. 3 phase would be better all around, but if you don't have it, it's a moot point.

Eng-Tips: Help for your job, not for your homework Read faq731-376

 

[lpace]

It's new construction. I will purchase four models (1-4 HP): all four sizes of equipment can run on 120V/1P plus three of the models use inverter drives and can run 208/1P or 208/3P. The entire equipment BOM can run 120V/1P, except for the HVAC (240v/3P).

I've been debating making all the large washers use 240V/3P; my past training says 240V/3P is the right choice for heavy loads, especially over a 7-10 year period.

 

[waross]

I concur with scottyUK. If you have it, use it.
However, the cost of adding 3 phase when it is not needed will be excessive.

 

[LionelHutz]

I would use the 3-phase, if you have it, for the larger machines.

If you use a VFD to change 1-phase to 3-phase then you need more 1-phase input current that the required 3-phase output current. The input current would be 1.73 times the 3-phase motor current.

A 4hp, 120V, 3-phase motor would draw approx. 24A on 3-phase so multiply times 1.73 to give approx. 1-phase current of 40A on the input. On the other hand, 20V, 3-phase would draw about 14A.

Of course, the manufacturer should be able to provide you with the 1-phase and 3-phase currents of the machines you're looking at. When you get into the smaller motors the rated current can vary greatly between different motors so using a s"standard" current from a table means little. Use the manufacturers current values and determine which voltage and phase gives the cheapest, easiest and most logical installation.

 

[waross]

lpace;
Do you have 3 phase power in your plant already?

 

[CJCPE]

I believe that a 3-phase VFD will cause less input harmonic current distortion than a 1-phase VFD. The power system may also more easily tolerate balanced 3-phase distortion.

 

[stevenal]

Suggestion: Consider the redundancy you achieve by running three phase. Many three phase drives can handle phase loss without a hiccup. Size your input circuits to handle the single phase condition. Also check with the utility before deciding on 240V three phase. I know one that will not supply it to a new service.