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This question was asked in a interview for Scientist position.

For a given T-slip characteristics of an induction motor, I want the same maximum torque but at a lower slip value?

  1. Is it even possible?

  2. If possible, then what will be its advantages and disadvantages?

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slip per torque is proportional to magnetic field. so a weaker field will cause more slip. (I'm not speaking accurately it might be a square relationship etc.)

Most motors are already operated with the stator near magnetic saturation (else there would be excess iron in the stator) so it's not practical to increase the magnetic field.

one approach that may pay off is to use a different type of motor - the permanent magnet types do not slip in normal operation.

another approach would be to use a larger motor or to add another motor possibly coupling to back shaft of the existing motor.

another would be the replace aluminium in the rotor with a better conductor like copper or silver. As Nail Tosun comments cooling the rotor also reduces its resistance and thus reduces slippage a little.

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  • \$\begingroup\$ How is slip per torque is proportional to the magnetic field? I only got the last method you mentioned. Can you please elaborate on other methods. As resistance decreases, slip at maximum torque will decrease. \$\endgroup\$ – Nikhil Kashyap Jul 1 '18 at 7:14
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    \$\begingroup\$ squirrel-cage rotors combine "iron" laminations and a conductive aluminium cage structure, replacing the aluminium with copper would make the rotor more efficient, (also much more expensive), and reduce slip. \$\endgroup\$ – Jasen Jul 1 '18 at 8:10
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    \$\begingroup\$ induction motors do not have rotor windings as such the conductive part of the rotor is cast in place onto the magnetic part, the squirrel cage part is usually aluminium. \$\endgroup\$ – Jasen Jul 1 '18 at 9:54
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    \$\begingroup\$ Effective cooling will decreases rotor resistance and and decreases maximum torque slip. Moreover changing rotor resistance doesn't change maximum torque of the machine. We use external resistance bank to increase starting torque (moving the maximum torque point.). Reverse process also valid \$\endgroup\$ – Nail Tosun Jul 1 '18 at 15:28
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    \$\begingroup\$ Thank you for adding my comment but you misspelled my name :) \$\endgroup\$ – Nail Tosun Jul 2 '18 at 17:04
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I am not sure what the question is really getting at. But there are several different standard NEMA designs for induction motors. Here is a graph from "TORQUE CHARACTERISTICS of NEMA DESIGN A,B,C,D & E MOTORS".

torque_vs_speed

The text also says that NEMA design E motors have 0.5% to 3% slip, which is less than any other type, although from the graph, it appears that design B motors are almost as low.

Anyway, one possible answer to your question could be to specify a NEMA design E motor.

Here is where I found the document.

http://industrialelectricalco.com/wp-content/uploads/2014/01/nema-abcde-torque-curves.pdf

Another possible answer is to use a synchronous motor, which has no slip, but may require special starting circuitry. Synchronous motors don't HAVE to be made from permanent magnets. You can also add slip rings and wound rotors so that the rotor can be magnetized by rectified AC current.

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  • \$\begingroup\$ In the same design, he wanted me to do some modifications. Like changing material of winding or adding some load. \$\endgroup\$ – Nikhil Kashyap Jul 1 '18 at 7:18
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    \$\begingroup\$ Maybe you can investigate how designers implement Design E motors. \$\endgroup\$ – mkeith Jul 1 '18 at 16:49
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As you commented on Jasen's answer, slip at which max torque occurs is ratio of rotor resistance to rotor leakage reactance. You can decrease rotor resistance in SCIM by changing material (eg using copper instead of aluminum) or increasing thickness of rotor bars. This costs more and takes more space.

It is easily achieved in slip ring motors though.

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  • \$\begingroup\$ In slip ring motors, you can only increase resistance right ? I think we can't decrease it as you can only connect extra in series. \$\endgroup\$ – Nikhil Kashyap Jul 1 '18 at 17:57
  • \$\begingroup\$ Obviously you can't have value lower than that with 0 external resistance. \$\endgroup\$ – Deep Jul 1 '18 at 18:23
  • \$\begingroup\$ @NikhilKashyap, don't focus on the wrong thing. What Deep is saying is that lowering rotor resistance will reduce slip (assuming leakage is held constant). Seems very likely this is what the answer to the interview question should have been. Reduce rotor resistance. \$\endgroup\$ – mkeith Jul 1 '18 at 20:30
  • \$\begingroup\$ No, it was not. Initially there was no external resistance connected. So, you can't decrease it using external resistance. \$\endgroup\$ – Nikhil Kashyap Jul 2 '18 at 8:03
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Yes it is possible.

If you have already external resistance bank connected to motor

Changing external resistance bank can change maximum torque point over slip. However, these control method only valid for wound type rotor induction machine. Since there is no slip ring in squirral cages we cannot connect resistor bank.

Following formulas can be derived by using Thevenin equivalent circuit of induction motor. I will not deeply derive this formulas, but you can find it in Fitzgerald Kingsley electric machinery book.

Thevenin Equivalent circuit of induction motor

Maximum torque formula for induction motor is following;

$$T_{max}=3\frac{0.5V_{th}^2}{\omega_s}\frac{1}{R_{th}+\sqrt{R_{th}^2+(X_{th}+X_2')2}}$$

As you see from the formula, \$T_{max}\$ is not dependent on rotor resistance, it is mostly related with stator side.

$$s_{Tmax}=\frac{r_2'}{\sqrt{R_{th}^2+(X_{th}+X_2')^2}}$$

Notice that \$s_{Tmax}\$ is dependent with \$r_2'\$ which is rotor resistance.

This graph can be constituted by using these formulas on MATLAB environment:

rotor resistance

If there is no external resistor bank

  1. You can more effectively cool the machine. This is also will reduce rotor resistance and \$s_{max}\$ as well.

  2. Delta-Wye switch

    Most of the induction machine has delta-wye switch for effective starting. The work principal is the same. Changing wye to delta connection can also decrease rotor resistance and \$s_{max}\$.

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  • \$\begingroup\$ This way you can increase rotor resistance and thus increase slip. But, you can't decrease it. \$\endgroup\$ – Nikhil Kashyap Jul 1 '18 at 16:09
  • \$\begingroup\$ Changing rotor winding wye to delta can help. It again reduces rotor resistance without changing maximum torque. \$\endgroup\$ – Nail Tosun Jul 1 '18 at 18:24
  • \$\begingroup\$ @Nail - Hi, I've fixed some MathJAX syntax (on this site, the inline MathJAX must have "\$" at the start & end, not just "$"). One point isn't completely clear, so I hope my attempt at clarification is correct. Are you saying that "if the induction motor has wound type rotor" then "changing external resistance bank" can be used? And "if the induction motor has [...] squirrel cage rotor, this control technique doesn't work"? If that is correct, then I hope my edit has indeed made that clearer. If you believe my edit is unclear, or if I've misinterpreted you, please edit your question. Thanks \$\endgroup\$ – SamGibson Jul 1 '18 at 18:43

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