Question

What is the torque equation in terms of B, Ic, l, Zr (r= mean air gap radius)?

(a) Bav*Ic*l*Zr

(b) Bav*Ic*l/Zr

(c) Bav*Ic*Zr/l

(d) Can’t be expressed

The question was asked during an internship interview.

Enquiry is from EMF and Torque Production topic in section Circuit Model, Emf and Torque of DC Machines

Answer 1

Correct choice is (a) Bav*Ic*l*Zr

To explain I would say: Avg. conductor force f= Bav*l*Ic. Here, Bav= Average flux density over pole, l= acyive conductor length. Thus, torque T= Z*f = Bav*l*Ia*Z. This torque is constant because both the flux density wave and current distribution is fixed in space at all times.

T developed= Bav*Ic*l*Zr  (Here, r= mean air gap radius).