Any electrically minded people?

[Sam Nichols]

I'm looking at three-phase motors and I understand that a three phase motor will not start on two phases BUT if the motor is already running and it loses a phase, it will continue to run.

Now the bit that I don't really understand is why the motor will continue to run at the same speed on two phases.

Can anyone explain this to me?

You can assume I have relatively good knowledge of electrical fundamentals

[AdamR28]

Momentum? A bit like a 3 cylinder car engine.

(That is a complete guess by the way, using logic, nothing knowledge-based )

[Sam Nichols]

I really hope it's that simple, Adam!

[monkeyseemonkeydo]

Another guess here (I probably should know but don't) is that the remaining two phases continue to turn the motor at the same speed as it was before (assuming it's not under much load) but it would subsequently have less torque available and would be more likely to stall.

[Kevinfor]

As far as I know it would continue to run at the same speed, but if you put any increased load on it, it will lose speed pretty quick OR it would put a lot more stress on the remaining 2 phases as they now have to provide 50% more power each.

How it reacts is down to what type of motor it is and how it was designed, I think.

[monkeyseemonkeydo]

Diagram time. Because the individual phases drive the individual stators (?) if you knock one out of the loop the motor should continue spinning (though possibly less smoothly than before) but with load it will be more likely to lose speed and stall.

[Sam Nichols]

I get you, is that actually the reason or is that your opinion? What you're saying does makes sense!

I have an exam on electrical fundamentals tomorrow and it's not actually particularly important that I know why this works, I just have to know the fact that it does. However, that's not the way I like to learn shit, I don't see the point in just learning facts when I can put TF's endless expanse of knowledge to good use!

[monkeyseemonkeydo]

That's my educated guess .

Edit: From a knowledgeable sounding dude on vintage-radio.net

A three phase motor will not start on only two phases. If it is running and only lightly loaded it will continue to run on two phases. If heavily loaded it will stall or overheat.

The simple way to run a three phase motor off single phase is to connect the single phase supply to two of the phase connections (it may be necessary to use a transformer to get the right voltage) and then use a capacitor connected from either (but not both) of the two phase connections to the third phase connection.

The problem is to get the right size of capacitor to balance the three phases. The ideal size of this capacitor depends on the size of the motor, the inductance of the windings, the AC frequency, and also the motor load but with a fan this load should be more or less constant, so this is probably not a problem.

The other problem is that because the starting current is much higher than the running current you may need a much larger capacitor to start the motor so you usually have two capacitors, a starting and running capacitor, the starting capacitor being switched out when the motor has started. Here again you might be lucky with something like a fan which has a low starting torque that it might start with just the run capacitor.

And from answers.yahoo:

"When one phase is lost, a loaded three phase motor cannot start or may stall under load...the current flowing in the remaining winding(s), will increase to 600% of the nameplate rating...Winding insulation subjected to locked rotor current may fail in as little as 15 to 90 seconds. This winding insulation damage is permanent and cumulative. Motors should not be started during phase loss and if they stall, they must be disconnected immediately."

"the life of a motor operating with a 3% continuous voltage unbalance (at full load) will be cut in half."

[dezmtber]

its right.

electric motors work in asimmilar pattern to car engines really

a 3 cyclinder engine will only fire one piston at a time to produce kinetic enregy in the form of the turning prop.

the coils on your 3 phase motor are in effect 3 pistons all putting there bit into turning the prop

so just like a car if you loose one piston it sometimes contuies to run till it stalls when the broke piston starts to cause friction and load on the engine.

when a coil goes on the electric motor you get a flat spot in the rotation but momentum keeps it going

[manuel]

with accurate position sensing and some smart control I don't see why you cant start a motor on 2 phases (other than why would you want to) with no load.

[pete_s]

Mmmmm... I did a bit of 3-phase theory at college. My educated opinion (guess) is that if you look at the start of the graph the red and blue lines are cancelling each other out so effectively you only have one winding when the motor switches on that's delivering any power. However, if you remove one of the lines (i.e. blue) at any point after the beginning the phases never cancel out again.

To see why it will keep on running until the load changes I think you'll need to graph the phase of the current.

Edited April 25, 2013 by pete_s

[AdamR28]

Dammit, a sensible, scientific answer