# Questions about 3-phase AC motors

Discussion in 'General Electronics Chat' started by Macnerd, Jun 28, 2015.

1. ### Macnerd Thread Starter Member

May 22, 2014
67
2
I am curious about this.

I recently watched a program on TV about the Tesla car. It has a 3-phase AC motor. A 3-phase motor is like a 3-cylinder internal combustion engine.

Granted the electronics to control a multi-phase motor would be complicated.

While I watched the TV show, I wondered if it is feasible to make a 4-phase, 6-phase, 8-phase, 12-phase motor. I'm sure that it would be impractical, but is it feasible?

Would the motor be more efficient if it has a higher frequency - 120 Hz, 240 Hz, 480 Hz?

2. ### #12 Expert

Nov 30, 2010
17,840
9,178
A three phase motor is not like a 3 cylinder engine because, no matter how many cylinders you have, the energy is still delivered in pulses. A three phase motor has continuous, constant power as the phases overlap. More phases will not fix it because it isn't broken.

High frequency motors need a lot less iron, but efficiency is always in the details. You can still design a lousy 400 Hz motor.

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3. ### MaxHeadRoom Expert

Jul 18, 2013
12,917
3,551
It is not a question of phases, but poles and applied frequency for induction types, There are 3phase controllers (VFD's) that currently operate 3ph induction motors from 0 rpm to 120Hz and above.
Also look up PM pole, ECM, BLDC and the Fischer-Paykel outrunner motor for more information. (currently used in washing machines).
Another example is a BLDC servo that is typically 8 pole maximum, but with PID control is capable of extremely fine positioning resolution.
Locomotives have used 3 ph induction traction motors since the 80's.
Max.

Last edited: Jun 28, 2015
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4. ### crutschow Expert

Mar 14, 2008
16,209
4,333
The reason we use 3-phase is that it is the minimum number of phases where the sum of power of the all the phases is a constant value, the same as DC (if you sum the instantaneous power of the three phases over one cycle it equals a constant). This means the power delivered by a 3-phase motor is also constant-power with no power or torque fluctuations.

Single-phase and 2-phase do no have this characteristic and their instantaneous power varies over a cycle.
That's why 3-phase motors are quieter and smoother than single-phase motors.

Certainly you can make a motor with more than three phases but it just adds complexity and wires without any other advantages.

Sep 25, 2013
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6. ### Biff383 Active Member

Jun 6, 2012
50
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I watched the video....now my head hurts. Thanks a lot. I've been working with these things for 30 years, I hope they don't change them on me now.

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