Motors; Single phase vs 3 phase

[mshields]

Our rule of thumb is to provide 3 phase feeders for all motors 1/2 horse power and above.

I have a situation where we are feeding a duplex pump with two 3/4HP motors that can run at the same time. The contractor wants to provide 120V instead of 208V (for reasons I won't bore you with). I want to be reasonable and say yes, but I'm wondering at what size motor or combination of motors should I draw the line and say we must have 3 phase. Obviously it's a judgement call but I'd be interested in your thoughts

Thanks,

 

[kingpb]

Rules of thumb are great for general applications, but in some instances flexibility needs to be administered. It sounds like if it's a duplex pump, it may be a sump pump? In that case, isn't the controller included with the pumps? So, does it really matter.

If it is a process load, then there may be other considerations, or if it is going to require a special starter for the application. etc. But, the long and short, it is OK to be flexible.

 

[jtester]

I agree, be flexible. 5 hp single phase motors are readily available. I often design single phase to 5 ton HVAC units. The vast majority of residential wells in my area are 1-3 hp single phase.

Jim T

 

[petersonra]

IMO, requiring 1/2 HP motors to be 3 phase is adding pain and cost to a job that serves no real purpose. Most specs I see require them above 1 HP, but there is generally room for special cases.

 

[ducks1333]

The main reason to use 3 phase motors is that they are more efficient than single phase motors. In the long term is is an operating expense issue.

 

[steve66]

Sometimes it is difficult to get 1/2 HP motors three phase. 1 HP and above sounds a little better. That still fits on a 30A circuit at 120V, and a 15A circuit at 240. Even at 1HP, sometimes you may have to settle for a single phase.

 

[Jraef]

Although it is generally considered accurate that 3 phase is "more efficient" than 1 phase, that is not always true. Some 1 phase motors are more efficient that some 3 phase standard motors. That said, most NEW 3 phase motors are now required to be "energy efficient" versions, and as such are almost always more efficient than 1 phase equivalents. As of yet there is no standard for an "energy efficient" 1 phase motor, so every mfr is on their own. In fact, there is a lot of disagreement as to testing methods anyway, whereas the test methods for 3 phase motor efficiency are very tight. So for all practivcal purposes one should consider 3 phase motors to be slightly more efficient that 1 phase. The transmission of electro-motive force to the load is significantly smoother as well, although that issue would be irrelevant on a pump.

All THAT said, your application presents some different issues. You said that your choices are 120V or 208V single phase, which is technically not really correct. If you have 208V, that means you have 208Y120V 3 phase service; there is no such thing as a 208/120V single phase service, only 240/120V. So if you have 208V 3 phase available, I would go with that option hands down.

First off, 1 phase motors have caps and possibly moving electrical parts that can wear out; 3 phase motors do not.

Secondly, if you were to use 208V on a 1 phase motor designed for 240V (it will NOT be designed for 208V 1 phase, no such thing), then you will be running the motor at roughly 10% undervoltage, which will put additional thermal stress on the windings, translating to lower service life.

Lastly, in using 2 poles of a 3 phase service as single phase, the phase relationship between the poles is 120 degrees. In a 1 phase service it is 180 degrees. This means if supplying a 1 phase motor with 2 phases from a 208V 3 phase system, the torque will be even more discontinuous than if supplied with a regular 240V 1 phase supply. That will also translate to thermal stress on the windings.

Bottom line, go 3 phase if you can or 120V 1 phase if you cannot.

 

[hillbilly]

Bottom line, go 3 phase if you can or 120V 1 phase if you cannot.

Me too...If three phase is readily available, I use 3 phase.
Hands down, it's better short and long term for powering motors.
I've never had a problem getting 3 phase motors down to 1/4 hp.
Of course there is (or used to be) a lot of textile plants around here and they use a lot of small 480/240/3 phase motors, so the distributors stock them.
There's no 208 3 phase in this area (that I've ever seen).
steve

 

[kingpb]

If you have 208V, that means you have 208Y120V 3 phase service; there is no such thing as a 208/120V single phase service, only 240/120V.

You are incorrect on two counts. First, a 120/208Y volt service which, is a 1-ph, 3-wire open-wye service is possible. Second, in accordance with IEEE standards, a 120/240 volt service is 1-phase, 3-wire, where as a 240/120 volt service is a 3-phase, 4-wire high-leg delta system.

The importance of accurately stating voltages is paramount to any technical discussion, as it can obviously mean different things to different people, especially when stated incorrectly.

 

[steve66]

Lastly, in using 2 poles of a 3 phase service as single phase, the phase relationship between the poles is 120 degrees. In a 1 phase service it is 180 degrees. This means if supplying a 1 phase motor with 2 phases from a 208V 3 phase system, the torque will be even more discontinuous than if supplied with a regular 240V 1 phase supply.

Not true. If you use 2 poles of the 208, the motor only sees one voltage, and it is a sine wave just like 120V single phase would be. So using 208 V doesn't make the torque any more "discontinuous" than 120V.

 

[charlie b]

There are two other advantages to three phase motors. One is that you get a balanced loading, as opposed to having load on one phase and not on another.

The other is that the power drawn by a three phase motor is constant. That is, it does not vary as a function of time.

By contrast, 60 times every second a single phase motor goes through the cycle of drawing a little power, then more power, then its max power, then less power, then no power, then provides power back to the source, the draws no power, and repeats. Imagine participating in a 10K run/walk for charity, and having to alternately walk, run, walk, sit down, and repeat every 10 yards. That would be as hard on your body as the power cycling is hard on the motor.

My rule of thumb is to draw the line at 1HP. But I have no technical basis for putting the line there, and am open to having the situation dictate otherwise.

 

[petersonra]

As far as efficiency goes, this is almost always obscured by reality. often these small motors are on intermittent duty (like sump pumps) and may only run a few minutes at a time once or twice a week.

The amount of electrical energy used is so small that the tiny amount of money that might have been saved by using a more efficient motor is just not noticeable.

A long time back I discussed this at length with a customer who had bought a system from the company I worked for and showed him that the extra cost he incurred by changing to higher efficiency motors was going to take him like 200 years to make back in energy savings. He agreed with the numbers but still insisted on the higher efficiency motors. We gladly charged him extra for them. Maybe not all that surprising that he turned down a proposal we made using VFDs that would have had about a one year payback.