Question

If the dimensions of all the parts of a synchronous generator and the number of field and armature turns are doubled, then the generated voltage will change by a factor of ________

(a) 1

(b) 2

(c) 4

(d) 8

This question was posed to me during a job interview.

Question is taken from Generated EMF in portion Principles of Electro mechanical Energy Conversions of Electrical Machines

Answer 1

Right answer is (d) 8

To explain I would say: E = √2πfrNΦ, Φ = flux per pole = 4/pBplr

dimensions are doubled ⇒ l^1 = 2l and r^1 = 2r ⇒ Φ^1 = 4Φ

No. of turns doubled ⇒ N^1 = 2N, fr changes only when the poles are changed ⇒ fr^1 = fr

⇒ E^1=√2πfr(2N)(4Φ) = 8E

The generated voltage will change by a factor of 8.