Single phase induction motor

• Engineering
• 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?

The Attempt at a Solution

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

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 [Broken]

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?

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

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cnh1995
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