# Induction motor construction

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 ?

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.

• 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.
– user80875
Commented Jun 10, 2018 at 2:53
• 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. Commented Jun 10, 2018 at 3:08
• Are you then looking at the rotor and stator iron of a larger motor with the winding design of a smaller one?
– user80875
Commented Jun 10, 2018 at 3:28
• This was a interview question for scientist position in BARC India. I think so. Because they already asked the air gap thing before this. Commented Jun 10, 2018 at 3:39

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.