# Rotor and stator flux in 3 phase Induction motor

## Main Question or Discussion Point

The rotor and stator flux speed are stationary with respect to each other in 3 phase induction motor. What will happen if there is relative speed between them? How these two fluxes interact with each other to develop a torque?

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cnh1995
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What will happen if there is relative speed between them?
There isn't. They rotate at the same angular speed at any loading condition, hence they always appear stationary to each other.

How these two fluxes interact with each other to develop a torque?
I think that would require a pretty big post to explain. I believe you can find the explanation in any standard electrical engineering book. If you are still unclear, you can post some specific queries.
See if this video helps.

Last edited:
Noaha and narcissus_papyra
There isn't. They rotate at the same angular speed at any loading condition, hence they always appear stationary to each other.
Doesn't this indicate that there is no slip? If slip is zero, there is no torque developed in an induction machine. I think this is mistaken.

Doesn't this indicate that there is no slip? If slip is zero, there is no torque developed in an induction machine. I think this is mistaken.
Slip is the *mechanical* speed difference between rotor & stator. As loading changes, slip also x hanged, but both magnetic fluxes revolve at synchronous speed for any load value.

x hanged
???

Slip is the *mechanical* speed difference between rotor & stator.
In most motor applications, the stator is stationary, i.e., non-rotating. Thus the difference between the rotor and stator would come down to simply shaft speed, but that is not correct the slip.

cnh1995
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but that is not correct the slip.
???

Slip is the difference between the speed of the stator flux (synchronous speed) and mechanical speed of the rotor. I think you are confusing the speed of rotor flux with the mechanical speed of the rotor body (or shaft).

This is right. But what I want to know is how it will effect the machine if their angular speeds are not same? Like what will the effect on torque developed or efficiency.

cnh1995
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But what I want to know is how it will effect the machine if their angular speeds are not same?
It never happens unless there is some fault in the motor, like when one or two of the stator phases get open circuited, or when there are broken rotor bars etc. That results in an uneven torque and rotor vibrations. There may be some other abnormal conditions that can cause unbalanced magnetic pull. But that is a whole different topic and is taken care of while designing the motor.

Under normal working conditions, the physics of the motor does not let it happen.

Noaha
It never happens unless there is some fault in the motor, like when one or two of the stator phases get open circuited, or when there are broken rotor bars etc. That results in an uneven torque and rotor vibrations. There may be some other abnormal conditions that can cause unbalanced magnetic pull. But that is a whole different topic and is taken care of while designing the motor.

Under normal working conditions, the physics of the motor does not let it happen.
That was informative. Thank you.

jim hardy
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But what I want to know is how it will effect the machine if their angular speeds are not same?
Practical, Intuitive approach here so i may be all wet .................

Isn't it basic math that the product of two sinewaves of different frequencies averages to zero ?

Were it even possible to have the proposed different frequency fluxes they'd produce zero net torque so the machine would not turn.

You see this on a washing machine motor when trying to start it with the wrong start winding energized.
It just hums(really loud) and gets hot.
That's because the start and run windings with their different #s of poles make their respective rotating fields at different frequencies , not same frequency with just differing phase that's necessary to produce a rotating field

@Dr.D @cabraham - does that make any sense ?

old jim

???
Typo, sorry, should read "changed". Anyway, the revolving fields both have an angular speed equal to synchronous speed. For 4 poles at 60 Hz, that is 1800 rpm. Call this 100%. The rotor mechanical speed is a little less, around 98%, for example, which is 1764 rpm. The slip is 2%.

Claude

product of two sinewaves of different frequencies averages to zero
Even though this is basic, this is something new for me. With this I understood why the stator and rotor flux should have same speed. Thank you.
Pardon me for being weak in basics but I have just started learning about these machines and I still learning. Queries like these may come from my side.Sorry for inconvenience.

You see this on a washing machine motor when trying to start it with the wrong start winding energized.
It just hums(really loud) and gets hot.
That's because the start and run windings with their different #s of poles make their respective rotating fields at different frequencies , not same frequency with just differing phase that's necessary to produce a rotating field
Are you talking about single phase induction motor here?

mmasba
-when a 3-phase supply applies on the stator winding the current will flow through it and a rotating magnetic field will produce.
- Rotating magnetic field is a constant magnitude but rotate at synchronous speed (NS).

Ns=120 F/P

Where: P is number of poles & F is the frequency
-this field across the air gap between stator and rotor and cut the rotor conductors which as yet are stationary.
-due to the relative speed between the rotating flux and stationary conductors, an E.m.f induced in the rotor according to Faraday's law.
- due to rotor short-circuit, the current will flow through the rotor and another field will be generated
-then the torque will be produced
Slip
-
It is the difference between rotor speed and synchronous speed
-the relative speed between the field of stator and rotor is the key of the motor rotating
-If the rotor speed(n) reaches synchronous speed (ns) the relative speed will be zero and no current will flow in rotor and no torque produce