Single phase induction motors (cap run)

[VadimR]

I understand there are different types of induction motors. We use single phase 2 pole and 4 pole cap run motors which have a pretty good balance between forward and reverse torque. We are able to get motors where they adjust the coils to get the forward and reverse toque to be different, up to 20% different in some cases (say CW is 20% more torque than CCW) all without changing the frame size nor as, I understand, the amount of copper used.

So the question is: Can you wind a motor so that it is unidirectional? Can you cut the copper use in half vs. a standard dual direction motor? Or go down in frame size for the same horsepower motor? What exactly do they do to the motor windings when they have more torque in one direction?

Thanks!

 

[anorlunda]

ping @jim hardy

 

[jim hardy]

There's a decent introduction to single phase motors here in the Wikipedia entry

https://en.wikipedia.org/wiki/Induction_motor
and a bit more here

https://www.allaboutcircuits.com/textbook/alternating-current/chpt-13/single-phase-induction-motors/

Can you wind a motor so that it is unidirectional?

I've encountered single phase motors that are not reversible
but only because the manufacturer saved money by connecting the windings together permanently on the inside.You should understand some basics about single phase motors, and i encourage you to get an old washing machine motor to tinker with.

A single set of windings cannot make a rotating field.

So - that's why a single phase motor won't start without a start winding - the rotor doesn't know which of the two oppositely rotating phasors to chase.
You can try that, open the start winding and energize.
It'll just hum and draw too much current.
But give the shaft a spin and watch - it'll take off and accelerate to running speed, in whichever direction you started it with that twist of your wrist.
That's a good way to test washing machine motors you've salvaged for DIY projects.

So they add a second winding that's offset somehow from the other winding.
Sometimes it's physically offset giving spatial offset
sometimes it just gets current that's not in phase with that provided to the other winding, giving electrical offset
sometimes both kinds of offset are used.but you must have that offset in order for the motor to start.

Now some motors disconnect the second winding after start,
others leave it connected

but the basic fact is you'll have to have two windings to achieve single phase start and run, irrespective of direction.

So no, there's no way that I'm aware of to save copper by building a unidirectional motor.

You can save a few inches of wire by not bringing out separate wires for start and run windings, that's all.

Sorry for the unscientific answer
but the question belies a meager understanding of motor basics
and i tried to introduce the concept ..

reading tutorials and searching on keywords in them will lead OP to understanding.

I hope this is a start.

We had a fun thread on single phase motors a couple years ago , check it out
https://www.physicsforums.com/threa...g-machine-motor-to-power-a-grain-mill.833300/
old jim

 

[jim hardy]

What exactly do they do to the motor windings when they have more torque in one direction?

That i would like to know myself.
I suspect they use a combination of spatial and electrical offset

but i'd never before now thought about intentionally building in that characteristic.

A fellow learns something every day - even at my age !

old jim