0
\$\begingroup\$

What will happen if rotor diameter is increased in an induction motor?

How the torque changes ?

How the performance will change?

How the pf will change ?

What will be the disadvantages?

This was asked in an interview for a scientist position in BARC India. Please help me with this.

I only know that output rating of machine is proportional to (D^2)LN

where D-Diameter L-Axial Length N-Speed

So, I can say that output rating will increase. But I am not able to comment on other factors.

\$\endgroup\$
  • \$\begingroup\$ If nothing is said about any other aspect of the motor design I would say you must assume that nothing else is changed. If that is true, the rotor diameter could only be increased the the extent that the air gap can be reduced. Reducing the air gap would have advantages and disadvantages. That is probably the only thing that be considered without saying that you are looking at an entirely new motor design. \$\endgroup\$ – Charles Cowie Jun 10 '18 at 2:53
  • \$\begingroup\$ sir, I think rotor diameter is increased keeping air gap constant is what they were trying to ask. Because this was a follow-up question after I answered the effect of changing the air gap. \$\endgroup\$ – Nikhil Kashyap Jun 10 '18 at 3:08
  • \$\begingroup\$ Are you then looking at the rotor and stator iron of a larger motor with the winding design of a smaller one? \$\endgroup\$ – Charles Cowie Jun 10 '18 at 3:28
  • \$\begingroup\$ This was a interview question for scientist position in BARC India. I think so. Because they already asked the air gap thing before this. \$\endgroup\$ – Nikhil Kashyap Jun 10 '18 at 3:39
1
\$\begingroup\$

What will happen if rotor diameter is increased in an induction motor?

Well, the rotor diameter will increase... and with that the airgap, as mentioned in the comments, will reduce.

How the torque changes ?

Not much. The flux in the airgap is determined by the stator voltages, and those are not modified.
But the leakage flux in the airgap will reduce a bit and therefore a slightly better coupling between rotor currents and stator flux will be obtained, leading to a slightly higher torque. A second order effect though.

How the pf will change ?

It will increase because the energy in the airgap, which indeed has a lower volume now, will be reduced and with that the magnetisation current, which causes the power factor to be < 1. First order effect.

What will be the disadvantages?

More friction in the airgap, worse cooling, larger torque fluctuations when the rotor grooves pass the stator grooves and as a result higher forces on the windings, higher accuracy requirements, stronger axis needed to better prevent bending, tighter bearing monitoring required to prevent rotor crashing into the stator.

\$\endgroup\$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.