Induction motor and Lenz's law

In summary: It shows how an induction motor works:In summary, the shown below, Figure 1, is an interaction of magnetic fields of stator and rotor of a squirrel cage induction motor.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.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
  • #1
PainterGuy
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TL;DR Summary
I was reading on squirrel cage induction motor and couldn't understand how Lenz's law comes into play in this scenario.
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!
 
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  • #2
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.
 
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  • #3
Thank you!

scottdave said:
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
PainterGuy said:
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.
 
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  • #5
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!
 
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  • #6
Famous demonstration of the same principle:
 
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  • #7
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
Sometimes only animated 3D visualization helps. Have a look at the following video starting at about 3:00.

 
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Related to Induction motor and Lenz's law

1. What is an induction motor?

An induction motor is an AC electric motor that works on the principle of electromagnetic induction, where a rotating magnetic field is created to induce a current in the rotor, causing it to rotate.

2. How does an induction motor work?

An induction motor consists of a stator, which contains the primary winding, and a rotor, which contains the secondary winding. When an AC current is supplied to the stator, it creates a rotating magnetic field, which induces a current in the rotor. This current in the rotor interacts with the magnetic field and causes the rotor to rotate.

3. What is Lenz's law and how does it relate to induction motors?

Lenz's law states that the direction of an induced current will always oppose the change in the magnetic field that caused it. In induction motors, this means that the rotor will rotate in the opposite direction of the rotating magnetic field created by the stator, as the induced current in the rotor tries to oppose the change in the magnetic field.

4. What are the advantages of using an induction motor?

Induction motors are known for their simplicity, reliability, and low cost. They also do not require any physical contact between the stator and rotor, making them maintenance-free. Additionally, they are more efficient than other types of motors, such as DC motors.

5. How are induction motors used in everyday life?

Induction motors are used in a variety of everyday appliances and machines, such as washing machines, refrigerators, air conditioners, fans, and pumps. They are also used in industrial applications, such as conveyor belts, compressors, and machine tools.

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