5 H.P., 50Hz, 3-phase, 440 V, induction motors are available for the following r.p.m. Which motor will be the costliest?
A. 730 r.p.m.
B. 960 r.p.m.
C. 1440 r.p.m.
D. 2880 r.p.m.
Answer: Option A
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P=120f/N
For 720,p=8
960,p=6
1440,p=4
2880,p=2
For 720 rpm poles were more hence cost is more
Poles are not mentioned
Ns=120f/p
=120*50/8
=750 rpm
Then how A is answer
For 730rpm numbers of poles are 8 which is more than B,C,D rpm. If the poles are increased, the motor cost will increase. There will be more coils, more slot insulation materials, more labor involved in assembly, typically a larger frame size meaning more materials in the manufacturing of the motor enclosure and iron.
Formula:-Ns=120f/P
Plz solve it