Induction motor rotor design features

In summary, the rotor end ring blades on single phase induction motors are likely made for cooling of the stator winding ends that extend out of the stator. The closer the aluminum bars on the rotor are to the stator magnetic field, the more efficient the motor may be, but this may also be affected by the inductance of the rotor winding and the depth of the rotor bars. Older motors may have had the aluminum bars embedded deeper for mechanical strength, while newer motors may have extended them to the surface for improved performance. The shape and depth of the rotor bars can also be used to vary the speed-torque characteristics of the motor.
  • #1
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I had to repair (change bearings) on a single phase induction motor recently and I recalled an age old question I've had.
What are the rotor end ring blades made for ?
The only reasonable answer I can come up with is - they are for cooling of the stator winding ends that extend out of the stator?

Induction%20Motor%20Rotor.png


And one additional question. I've seen on older style induction motors the rotor aluminum bars, that form single loop circuits in the rotor , not visible on the rotor surface , instead they are some mm or so embedded into the rotor and the surface is completely covered by the electrical steel lamination outer surface. On newer rotors I see the aluminum bars extend to the very surface like in the picture above.

Now I would think that the closer the rotor bars are to the stator magnetic field the more flux passes through them and stronger the induction and the motor more efficient for a given size, is that true?

I assume in the older designs they did not extend the aluminum bars to the surface due to machining and mechanical issues?
 
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  • #2
One of the things that determine starting torque in an induction motor is how far down into the rotor the copper or aluminum bars are buried. Most torque occurs when they are close to the surface.
 
  • #3
Averagesupernova said:
One of the things that determine starting torque in an induction motor is how far down into the rotor the copper or aluminum bars are buried. Most torque occurs when they are close to the surface.
And I assume that is because the further axially inwards you go in the rotor the weaker the flux becomes therefore less induced current.
So it seems the earlier motors had the aluminum bars embedded deeper simply for mechanical strength possibly as otherwise it seems detrimental to performance
 
  • #4
artis said:
And I assume that is because the further axially inwards you go in the rotor the weaker the flux becomes therefore less induced current.
So it seems the earlier motors had the aluminum bars embedded deeper simply for mechanical strength possibly as otherwise it seems detrimental to performance
The inductance of the rotor winding(s) goes up as the conductors are buried deeper. This increases inductive reactance. Startup is when the induced current in the rotor is at the highest frequency. The higher inductive reactance limits rotor current moreso than if the rotor conductors are closer to the surface. Naturally, higher rotor current gets us higher torque.
 
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  • #5
artis said:
And one additional question. I've seen on older style induction motors the rotor aluminum bars, that form single loop circuits in the rotor , not visible on the rotor surface , instead they are some mm or so embedded into the rotor and the surface is completely covered by the electrical steel lamination outer surface. On newer rotors I see the aluminum bars extend to the very surface like in the picture above.

How about this from Wikipedia:

https://en.wikipedia.org/wiki/Squirrel-cage_rotor#Structure
The shape and depth of the rotor bars can be used to vary the speed-torque characteristics of the induction motor. At standstill, the revolving magnetic field passes the rotor bars at a high rate, inducing line-frequency current into the rotor bars. Due to the skin effect, the induced current tends to flow at the outer edge of the winding. As the motor accelerates, the slip frequency decreases and induced current flows at greater depths in the winding. By tapering the profile of the rotor bars to vary their resistance at different depths, or by constructing a double squirrel cage, with a combination of high and low impedance rotor in parallel the motor can be arranged to produce more or less torque at standstill and near its synchronous speed.
 
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