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Registered Member #190
Joined: Fri Feb 17 2006, 12:00AM
Location:
Posts: 1567
A simple three phase motor would have three coils with one N-S pole. If we double this a few times (3, 6, 12, 24) and there are 24 coils one would need 8 N-S poles. Can we have a three phase motor with *any* multiple of three and use 42 coils and 14 poles, or would this not work properly?
Registered Member #8
Joined: Thu Feb 02 2006, 04:34AM
Location: Harlowton, MT, United States
Posts: 214
The more poles the lower the speed of the motor. For example a 60Hz 1750rpm 3 phase induction motor would have 3600(cycles per minute)/1800(the actual synchronous speed, not including the slip necessary for induction to take place) = 2, times the minimum 3 poles. That means the 1750rpm motor would have 6 poles, where a 3450rpm one would have the minimum 3 poles. The same applies to single phase motors, for example a 1750rpm single phase induction motor has 4 poles where a 3450rpm one has the minimum 2 poles. A 60Hz 3 phase induction motor with 42 poles would have 1/(42/3) = 1/14th the maximal 3600rpm at 60Hz, or somewhat less than 3600/14rpm, perhaps 220-250rpm or so depending on design. So in a word, yes, they can have any number of poles, it will just decrease the rotational speed and increase the torque. For synchronous motors, the rpm is just the exact synchronous speed, for example 3600rpm for a 3 pole 60Hz 3 phase motor, 1800rpm for 6 pole motor, 227rpm for a 42 pole motor, etc, though synchronous motors are rarely encountered in most applications.
Registered Member #72
Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659
I scrapped a very old front-loading washing machine a while ago, the motor dated from way before electronic speed control, and was a very elegant solution to both wash and spin operation.
It was an induction motor with 32 laminated salients, with a total of 4 windings in two 2-phase sets. The first phase connected to to the supply, the second phase from a run capacitor.
The first winding set was wound on each pole piece, giving a slow 8 mains cycles per revolution for wash agitation. The 2 phases could be reversed for reversing the action. The second winding set was wound onto groups of 8 poles, giving a one mains cycle per revolution for spin. It was big and heavy, low power to weight ratio, but a nice solution of its day.
So you can have multiple poles, even in the same motor at the same time. To generalise to N phases, have a multiple of N poles.
Registered Member #8
Joined: Thu Feb 02 2006, 04:34AM
Location: Harlowton, MT, United States
Posts: 214
Every washing machine motor I have ever seen, old or new (here in the US) is a two-speed design, and I have several. They have a four pole set and a 6 pole set of run windings, for operating at either 1750rpm or 1050rpm, as well as a thinner set of windings for the capacitor start. 3 phase motors usually have multiple windings on the same poles which can be seriesed or paralleled, or wired in delta or wye, to run on a variety of different 3 phase sources, but I cannot say I have ever come across a multi-speed one (though I'm sure they exist).
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three phase motors
