Induction motor and Lenz's law

  • Thread starter PainterGuy
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  • #1
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Summary:

I was reading on squirrel cage induction motor and couldn't understand how Lenz's law comes into play in this scenario.

Main Question or Discussion Point

Hi,

The shown below, Figure 1, is an interaction of magnetic fields of stator and rotor of a squirrel cage induction motor.


1576045637197.png

I believe the current is coming out of the squirrel cage bar on the left and entering on the right as is shown below, Figure 2. I have assumed that the bars highlighted in green are shown in Figure 1 and "x" represents current entering the bar and "." shows current coming out of bar.

1576047198453.png


Lenz's law states that the direction of the current induced in a conductor by a changing magnetic field is such that the magnetic field created by the induced current opposes the initial changing magnetic field.

I don't understand how the induced magnetic field in rotor bar opposes the initial rotating magnetic field of stator. Could you please help me with it?

Thank you!
 

Answers and Replies

  • #2
scottdave
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It's hard to see it what is happening due to motion when right next to the rotor bar. Imagine when the N stator has moved to the 10 o'clock position, and the S stator has moved to 4 o'clock position. If you draw the magnetic lines all the way across, are the cage's fields working with the stator or against it?

You can also use the right hand rule to figure out the direction of force due to the external field and the induced current.
 
  • #3
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Thank you!

It's hard to see it what is happening due to motion when right next to the rotor bar. Imagine when the N stator has moved to the 10 o'clock position, and the S stator has moved to 4 o'clock position. If you draw the magnetic lines all the way across, are the cage's fields working with the stator or against it?
This would assume that the induced magnetic field of rotor bar lags behind the rotating magnetic field of stator. Anyway, please have a look on the attachment. I understand that the attached drawing is really bad but I hope it captures what you said.
 

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  • #4
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Thank you!



This would assume that the induced magnetic field of rotor bar lags behind the rotating magnetic field of stator. Anyway, please have a look on the attachment. I understand that the attached drawing is really bad but I hope it captures what you said.
This is precisely how induction machines work. They are "asynchronous", which means the rotor is NOT synchronized with the rotating stator field.

For an induction machine to produce torque, the rotor must spin slower (or faster if generating) than the stator field rotating speed, this difference in speed means the rotor bars are seeing a changing field which induces a voltage across the bars, and since the DC resistance is low this will cause currents to flow which cause the rotor to interact with the stator field and produce torque.
 
  • #5
Svein
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Back in high school we had a really inventive physics teacher. In order to demonstrate such a machine, he drew a circle on a piece of plywood, mounted three coils equidistant along the circle and stuck a knitting needle through the center of the circle. He then took an empty aluminum can, punched a hole in the center and pushed it onto the knitting needle. When he connected the coils to a three-phase mains, the can spun like crazy!
 
  • #6
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Famous demonstration of the same principle:
 
  • #7
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Hi,

I have been struggling to understand the plot shown below for a squirrel cage induction motor. I'd really appreciate if you could help me with it. At the moment I don't understand what's going on.

Flux density wave is resulting from stator's windings and it produces bar currents and voltages. Are bar currents and voltages in phase with flux density wave? I'm not sure if they are in phase. Look at the point marked "K" which represents the peak of flux density wave. Doesn't it look that bar currents and voltages are actually leading the flux density wave by a small margin?

Do those 'circles' under the horizontal axis show rotor currents?

Why is magnetomotive force wave lagging the flux density wave?

Here is another related animation: http://people.ece.umn.edu/users/riaz/animations/sqmovies.html

induction_squirrel.jpg

Source: http://people.ece.umn.edu/users/riaz/animations/squirrelcage.html
 
  • #8
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Sometimes only animated 3D visualization helps. Have a look at the following video starting at about 3:00.

 

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