# Thread: Single phase and Three phase motors

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## Single phase and Three phase motors

im a college student studying HVAC, and I was really trying to understand the difference between single phase and three phase motors in the hvac industry

2. that is a very loaded question. basically, a single phase motor uses 2 lines of incoming power and a three phase motor uses 3 lines of incoming power.

a single phase motor is usually used for low horsepower applications where as 3 phase motors are used for larger horsepower applications.

3. Were you looking for something more specific? Jay gave a good answer for how vague your question was.

4. 3phase is more efficient although initial cost is more. This video may help http://m.youtube.com/watch?v=07uXnc1C5CA

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Originally Posted by RIEIO
im a college student studying HVAC, and I was really trying to understand the difference between single phase and three phase motors in the hvac industry
The motor used is matched to the building power available, so a commercial unit installed where 3-phase power is available will contain 3-phase motors, while a residential unit will have single phase motors.

The big difference between the two motors is that 3-phase motors use the phase rotation of the incoming power itself to create the rotating magnetic field, while single phase motors cannot. They must use an additional component (such as a capacitor) to create the necessary phase shift for starting, and in some cases running as well. This creates additional complexity and points of possible failure for the single-phase motor.

Very cool old video btw core_d.

6. .

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Also, 3phase motors have poles mechanically separated by 120 degrees, which coincides with the phase angle offset of 120 degrees per phase. 3 phase motors do not have starting aids (relays and caps) because the starting torque is so much greater than single phase.

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8. Originally Posted by timtanguay
Also, 3phase motors have poles mechanically separated by 120 degrees, which coincides with the phase angle offset of 120 degrees per phase. 3 phase motors do not have starting aids (relays and caps) because the starting torque is so much greater than single phase.

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Actually they do have relay systems to start them. Look up wye delta starter.

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Just hang on Rieo... If your in college they will explain that to you - it might be coming later in the course. It's been my experience that college explains things to such a depth that alot of it is useless. So if they don't explain that basic difference in an ac/refer education that you are paying thousands of dollars for... sue the crap out of them and get your money back bud...

10. Originally Posted by genman
Just hang on Rieo... If your in college they will explain that to you - it might be coming later in the course. It's been my experience that college explains things to such a depth that alot of it is useless. So if they don't explain that basic difference in an ac/refer education that you are paying thousands of dollars for... sue the crap out of them and get your money back bud...
Genmen, good to see in the forum. You have dug up every thread I have been a part of in the electrical forum. Perhaps you could start your own thread either learning or teaching.
Hope to see you in the future.

11. Well, as i know 3phase motors are for industrial usage, factories...
And with a single phase motor you can power home equipment

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Good thread! Always someone post something that I can learn from.

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A single phase motor may have as few as two magnetic poles, 3450 RPM. In the most common brushless variety the reversal of ac current reverses the magnetic field poles. Expansion, and collapse of magnetism induces current in the aluminum bars in the rotor. Current in these bars conducts through the rotor ends. This induces magnetic field at 90 mechanical degrees to the field poles. Magnetic attraction, and repulsion cause the rotor to move. Some means of starting is needed. Often, this is a second field winding with a switch to turn it off when it speeds up.

Three phase motors have at least 6 poles. Distance between field, and rotor magnetic poles is less. As the three phases are sequential the magnetic field rotates. These motors need no start winding, and direction of rotation can be reversed by switching any two leads. They offer more torque with a smaller package. Three phase motors are cheaper to build.

Not all buildings are provided with three phase power supply. For some, in rural areas, three phase installation can be very expensive.

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