Single phase vs 3 phase AC motor

Hi guys,

I'm just looking around for some small motors for a project <100W. Can anyone briefly describe what is the advantage and disadvantage of 1 phase vs 3 phase motor?

As I know 1 phase motor may required higher amp, while 3 phase required lower amp to drive the same load. Correct me if i'm wrong. Not sure if any other factors to consider between these 2 types of motor.

Thank you, guys.

You can reverse a 3-phase motor "instantly".
Reversing a single phase motor requires that you apply a delay that is long enough to make sure that the motor has come to a standstill before starting in the other direction.

also single phase motors don't take the stress of of been continuously started and stopped like a 3ph would, and you can also ramp a 3ph motor with a VSD meaning less load and also depending on what your driving less stress on that as well, and also control the speed at which it runs at

rob

In my view, the two most important considerations are, first, single phase motors are difficult to start and often develop very little torque when starting. It is critical, when using single phase motors, to consider this when selecting what type of single phase motor to use (there are about a dozen types each with different capabilities). Second, as already mentioned, any need to reverse must be considered carefully. Some single phase motors cannot be reversed, others can only be reversed when stopped, and others will reverse on the fly just like a three phase motor.

Of course, at these small sizes, three phase motors are more expensive to wire up and may even be more difficult to purchase.

Be careful and good luck.

Oh no -- 3phase are not more difficult to purchase. We purchase them all day every day. They're extremely common.

That said, 100W is 1/8HP. These might be difficult to find in 3 phase... That's a mighty small motor fir 3 phase.

To help you specify the right motor, flyers, we need to know more about your application... Give us some more clues about what you want to do with that motor.

i think thats what dick means 3ph at such a small size.

Also 3 phase motors are cheaper to run becouse they draw less amps

3 phase might be cheaper, but it's because of efficiency not amp draw.

100 W 3 phase will be a little hard to find especially if you want 120 volt. 230 volt will be a lot easier
Take a look here
https://www.surpluscenter.com/item.asp?UID=2156010811545116&item=10-2115-G&catname=electric

Never bought motors from them so cannot guarentee anything.

Little oversize for his needs but he probably will not burn it up easily.
Dan Bentler

Originally posted by hd_coop:
3 phase might be cheaper, but it's because of efficiency not amp draw.

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While I agree, this begs more explanation. When I first read this I inferred hd_coop to mean that there is no difference in amp draw between a given horsepower of single phase and three phase motor. But I really don't think that is what he means.

What I THINK he means is that even though the amp draw is lower you can't simply say that it draws less power. A three-phase motor is drawing power from all three phases to some degree during each electrical cycle while the single phase motor is only drawing power from one phase.

The equation for power consumption in a single phase resistive circuit is V * A. In a three-phase resistive circuit it is V*A*1.73. So at a given voltage one would expect the current in the single phase circuit to be 1.73 times that in the three phase circuit to achieve the same power level.

Keith

One minor add on to Kamenges
1 HP = 746 watt
regardless of single vs 3 phase vs DC
In addition to Dick DV comments I would add that 3 phase is simpler to maintain - no start caps starting switches etc.

Dan Bentler

leitmotif said:

. . .I would add that 3 phase is simpler to maintain - no start caps starting switches etc.

Click to expand...

Which is why three phase is generally less expensive.

joeparrish said:

Also 3 phase motors are cheaper to run becouse they draw less amps

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My initial knee jerk reaction was to say we don’t pay for energy consumption on amps alone. We buy it by the Watt, KW, MW, Etc.
My father-in-law just bought a 220V 3 HP air compressor he was telling me how much cheaper it is to run a 220 vs. 110 single phase. I asked him if he could do math? His response was of course I can. Can you? My (not so friendly) response was I will not be letting my kids do math homework with you. I tries to explain the W= E*I and half the current and double the volt don’t change the product. I don’t know why he doesn’t like me that much.

flyers said:

Hi guys,
Not sure if any other factors to consider between these 2 types of motor.

Click to expand...

It depends on combination of line power available, control hardware, need to reverse motor, and need to control its speed/acceleration/deceleration.
The key decision is whether an AC inverter is used.
Specifically:
- With single-phase only available as line power, the only way to use 3-phase motor is to use AC inverter.
- AC inverter should be used for variable speed application or when controlled acceleration/deceleration is necessary.
With AC inverter, 3-phase motor should be used.
If motor reversing is needed, a low-end AC inverter is comparable by cost with reversing contactor system (which does not allow variable speed/acceleration/deceleration).
AC inverters feature integrated thermal overload protection for motor and do not contain mechanical power contacts.
Low-power inverters typically may be powered from single phase.
Generally speaking, AC inverter+3phase motor is always preferred.

Amperage should not be a consideration in such low-power application.

Interesting, maybe we should find out what the OP actually wants to do. A DC motor might make the most sense.