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Polyphase induction motors

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carrotSnack

Sat Nov 29 2014, 02:39PM

Registered Member #4115 Joined: Fri Sept 30 2011, 01:13PM
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Other than complexity, what effect does changing the number of phases in a polyphase induction motor have?
If I could have inverters generating any arbitrary number of phases what would there be a difference between a 2-phase and 3-phase motor? What about beyond 3-phase?

Wolfram

Sat Nov 29 2014, 03:03PM

Registered Member #33 Joined: Sat Feb 04 2006, 01:31PM
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Posts: 971

Torque ripple goes down as the number of phases goes up.

BigBad

Sat Nov 29 2014, 04:23PM

Registered Member #2529 Joined: Thu Dec 10 2009, 02:43AM
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I think 2 phase inverters have to have bigger capacitors because the currents aren't balanced. With 3 phase inverters the overall current is the same at all times.

hen918

Sat Nov 29 2014, 06:25PM

Registered Member #11591 Joined: Wed Mar 20 2013, 08:20PM
Location: UK
Posts: 556

BigBad wrote ...

I think 2 phase inverters have to have bigger capacitors because the currents aren't balanced. With 3 phase inverters the overall current is the same at all times.

3 phase motors don't need capacitors at all, because each phase pushes the rotor in the same direction so it starts spinning that way.
Single phase motors will need at least a starting capacitor which creates another phase to push the motor in that direction, without that capacitor the motor will just vibrate at 50Hz unless you turn it by hand, then you run the risk of it taking your fingers off!

Shrad

Sat Nov 29 2014, 07:34PM

Registered Member #3215 Joined: Sun Sept 19 2010, 08:42PM
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rotating field ;)

think of a single phase motor like a single cylinder two stroke motor... it can start backwards if launched that way

carrotSnack

Sun Nov 30 2014, 01:42AM

Registered Member #4115 Joined: Fri Sept 30 2011, 01:13PM
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Thanks for the replies.
For a 2 phase motor, if there are two inverters then there is no need for capacitors as they are used to generate two phases from one correct?
If we fed two motor with the same input power, one having more phases, its torque ripple will be less but is its average torque any different?

How does increasing the number of phases effect a linear induction motor if the pole size is fixed? It seems it would increase the synchronous speed but would it have an effect on the force?

Dr. Slack

Sun Nov 30 2014, 01:55PM

Registered Member #72 Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
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2 or 3 phase?

I think 2 phase motors ought to be entirely adequate, after all, there are a lot of single phase motors that perform OK, and they should be simpler to make than 3 phase.

However, 3 phase supply is the norm, so motors are made to match. Why is 3 phase the preferred form of distribution rather than 2 phase? Because 3 phase can drop the neutral. If you send 2 phase down equal sized wires, you need 4. If you send it down 3 wires, one carries a current 1.414 times that of the other two. Three balanced equal sized wires running 3 phase carries more power per copper area than any other configuration.

If factors other than economics of transmission dominate, like cogging, or a specific ratio, or lowest cost for a small motor, then other numbers of phases get used. But for a reasonable sized motor, the system cost is lowest when it matches the native format for power transmission.

Shrad

Sun Nov 30 2014, 02:05PM

Registered Member #3215 Joined: Sun Sept 19 2010, 08:42PM
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3 phase motors can be driven without neutral due to the rotating field, but in practice they will usually need to evacuate current to earth or neutral due to differences between phases as well as induction induced currents in the rotor body

a rotor axle can even draw arcs to the motor body if it has micro fissures and it is usually solved by applying a brush to the axle

klugesmith

Sun Nov 30 2014, 10:42PM

Registered Member #2099 Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1716

Funny that should come up on 4hv, the day after I stumbled on literature about shaft voltages and bearing currents. Their existence had never occurred to me, except for "ball-bearing motor" demonstrations.

Turns out it has become a significant problem with the advent of HF-switching VFD's, and can rapidly damage ball bearings unless mitigated, for example as Shrad mentioned.

BigBad

Mon Dec 01 2014, 05:03AM

Registered Member #2529 Joined: Thu Dec 10 2009, 02:43AM
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hen918 wrote ...

BigBad wrote ...

I think 2 phase inverters have to have bigger capacitors because the currents aren't balanced. With 3 phase inverters the overall current is the same at all times.

3 phase motors don't need capacitors at all, because each phase pushes the rotor in the same direction so it starts spinning that way.

Approximately, but not quite. If you're doing a d-class type inverter then the PWM pulses draw down on the capacitor across the DC bus; if you have 3 phase this capacitor can be fairly small; but it still needs to be there. With 2 phases you need to hold the volts up over a complete cycle of the motor, rather than just the PWM cycle.

Also, the DC bus needs to be fed somehow. If it's fed from a single phase you still need a pretty big capacitor or something to smooth it out after rectifying it.

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