What is the role of rotor bars in single phase induction motors?

[ranju]

Homework Statement

I studied that as the single phase motor speeds up the backward induced field currents are larger than the forward field currents. So , the corresponding opposing rotor mmf causes backward field to be greatly reduced whereas the smaller forward currents leads to enhancement of forward flux wave..
I am not getting this fact as if the backward current is higher so even the backward flux should be greater . Then , on what basis the forward flux is greater?

Homework Equations

The Attempt at a Solution

The backward field currents has a lower power factor whereas the forward currents have higher power factor .[/B]

 

[jim hardy]

Ahhhh single phase motors

i don't have right now a good word picture to paint for you.

I've done this hands on experiment hundreds of times :
energize a single phase motor with start winding disconnected
hear and feel it hum as locked rotor amps vibrate the windings
give it a spin either direction, watch it smoothly accelerate and grow quieter and feel vibration diminish.

Clearly something cancels out nearly all of one of those counter-rotating fluxes.Here's a link with some mathematics trying to explain it
but to me he does it in a roundabout manner
http://ee.lamar.edu/gleb/tps/Lecture%2003%20-%20Special-Purpose%20Motors.pdf

Since, in that equivalent circuit, resistance is where torque is produced , most of the torque will appear in the upper resistor

I am not getting this fact as if the backward current is higher so even the backward flux should be greater . Then , on what basis the forward flux is greater?

His mathematical explanation, same link

I am not getting this fact as if the backward current is higher so even the backward flux should be greater . Then , on what basis the forward flux is greater?

Hmmm... see if these words help you accept what we empirically know to be true... and what the author cited above asserts:

Remember an induction motor is essentially a transformer with shorted secondary.
Since in a transformer secondary mmf cancels primary mmf, backward component of current in rotor cancels backward component of primary mmf...

actually i think it only cancels most of it. Single phase motors do have small pulsating torque at 2x line frequency.

Any help ?

Clearly my thinking on this subject needs some polishing. I look forward to your improved presentation.

old jim

 

[jim hardy]

I'm still trying to reconcile what happens at the rotor... a physical picture not an equation

The key to an intuitive picture i think lies iwith envisioning the rotor bars and their relation to the stator poles.
I might try to draw something when get back home...Envision a simple two pole machine with one turn stator windings, one per pole at 12 o'clock and 6 o'clock.
Freeze frame your thinking to instant of maximum flux, stator voltage zero crossing
Next , envision the rotor bars in plane 3 o'clock-9 o'clock; clearly they're perpendicular to flux so link it all
and they're the ones that cancel stator flux

next envision the rotor bars in plane 11:59 o'clock-5:59 o'clock not quite aligned with poles
they link very little flux
but what happens if the rotor is turning slowly ?
Flux linked by 3-9 o'clock loop is not changing
but flux linked by 11:59-5:59 o'clock loop is changing considerably.. Lenz will try to do something about that...

old gray cells are not quite there yet

how're your younger ones ?

old jim