Efficiency

[Microwatt]

This question was on test that a condo association came up with when looking for a maintenance man.

Which is more efficient, a single phase or three phase motor?

How would you answer it?

 

[peter d]

Three phase is definitely more efficient for the operation of motors.

 

[iwire]

Three phase is definitely more efficient for the operation of motors.

I agree with Peter.

 

[peter d]

I agree with Peter.

OK, now the hard part. We must explain why.

IIRC from my textbooks on this matter, if you look at the three sine waves of a three phase wave form, they coincide such that there is very little up/down variation from peak to peak of each wave form. This is very conducive to motor operation. But I cannot explain this in a more precise technical manner so hopefully someone else will.

 

[marti smith]

Single phase motors need a method of turning the rotor to create a magnetic field (small appliances, whatever), and 2 and 3 phase motors, their rotating phase currents cause field attraction to those currents, and so the rotor turns due to the presence of all three phase currents (current induced in the rotor). This theory (horribly explained by myself) simply is what makes polyphase motors more effecient. Where are those brainiacs when we need them?

 

[iMuse97]

Single phase motors need something (ie. a capacitor) to create the phase shift needed for operation, while three phase motors already have this set up in the three phases, all 120* different from one another.

 

[shermansfather]

If a 3 phase motor is in fact more efficient that difference would be realized as extra heat. Does that bear out anyones experience ?

 

[cpal]

If a 3 phase motor is in fact more efficient that difference would be realized as extra heat. Does that bear out anyones experience ?

not sure about the heat but as marti and iMuse stated 3 P motors are self starting they additionally have fewer parts to fail (cenbtrifugal sw) (no capacitors) or start windings to burn out. They additionally are smaller per HP and require less infrastructure to supply (smaller conductors, fuses etc. per HP) .

I'm interested on opinions regarding speed control and torque development??

 

[hurk27]

A 3-phase motor draws 1.732 X less amperage, then a comparable single phase motor, thus reducing wire size, also a 3-phase motor has a much shorter spin up to run speed time, so it will use less energy for starting up.

 

[SG-1]

Check out this 3-Phase site: http://www.3phasepower.org/3phaseelectricity.htm

Great article on 2 Phase that takes you back to the dinosaur days of electricity in the US at the very bottom of the page. If you like history.

 

[LarryFine]

I guess it also depends on how you define "efficiency". Is there a difference in HP/kw between the two types?

 

[flashlight]

Check out this 3-Phase site: http://www.3phasepower.org/3phaseelectricity.htm

Great article on 2 Phase that takes you back to the dinosaur days of electricity in the US at the very bottom of the page. If you like history.

Fascinating article. Thanks for link.

 

[SG-1]

Fascinating article. Thanks for link.

You are Very Welcome

 

[Rick Christopherson]

The primary difference relates to the instantaneous power of the two systems. The instantaneous power from a single phase system is more or less sinusoidal, with periods of peak power, periods of low power, and even periods of negative power (depending on the powerfactor).

The instantaneous power from 3-phase is always constant.

In the graph below, you might remember seeing part of this when I described the instantaneous single phase power in a previous discussion. The voltage and current curves represent the single phase current and voltage with a 0.6 powerfactor. The dashed purple line is the single phase instantaneous power.

I used the same graph, but added the B-phase and C-phase power based on the original voltage and current, and then I added all three of these power values together. The total instantaneous power is the sum of the three phase powers, and is shown by the straight dashed line at the top of the graph.

 

[Besoeker]

I guess it also depends on how you define "efficiency".

Output power/input power is a pretty good definition.

 

[iwire]

I guess it also depends on how you define "efficiency". Is there a difference in HP/kw between the two types?

Per the tables in Article 430

10 HP 230 volt motor

Single phase 11,500 watts

Three phase 10,075 watts

5 HP 230 volt motor

Single phase 5,824 watts

Three phase 5,469 watts

(BTW I did not include PF in any of the above)

 

[cschmid]

This question was on test that a condo association came up with when looking for a maintenance man.

Which is more efficient, a single phase or three phase motor?

How would you answer it?

Three phase

I would not read to much into it.

 

[charlie b]

The instantaneous power from 3-phase is always constant.

I once worked out the trigonometric derivation of power in a single phase and a three phase system. Indeed, the three phase power is constant. If anyone wants a copy of the trig worksheets, send me your email address via a PM.

 

[Besoeker]

I once worked out the trigonometric derivation of power in a single phase and a three phase system. Indeed, the three phase power is constant.

I think that's true for only the case of a perfectly balanced three-phase load.
With any imbalance in either amplitude or phase, you get a twice supply frequency component.

 

[charlie b]

I think that's true for only the case of a perfectly balanced three-phase load.

Agreed. My math solution involved only trig, not symmetrical components. I used a couple trig identities that would not work in an imbalance situation. But it was intended to address a load consisting of one three-phase motor. That makes the assumption of a balanced load a relatively good one.