In this article, we will calculate synchronous speed (S) in r.p.m., Number of Poles (P) and Frequency (f) in Hz.

Formulae used to find N­S, P and f are given below:

1. A 3-phase 50Hz, alternator runs at 750 r.p.m. Find the number of poles of the machines?

Solution:

Number of poles, P = 120f/NS

=   120*50/750

Number of poles, P = 6000/750 = 8 poles

1. A 50 Hz synchronous motor runs at 200 r.p.m. The number of salient poles on the rotor are:

Solution:

Number of poles, P = 120f/NS

=   120*50/200

Number of poles, P = 6000/200 = 30 poles

1. A 60 Hz synchronous motor runs at 1800 r.p.m. The number of salient poles on the rotor are:

Solution:

Number of poles, P = 120f/NS

=   120*60/1800

Number of poles, P = 7200/1800 = 4 poles

1. A 50 Hz synchronous motor runs at 1000 r.p.m. The number of salient poles on the rotor are:

Solution:

Number of poles, P = 120f/NS

=   120*50/1000

Number of poles, P = 6000/1000 = 30 poles

1. A three phase induction motor is wound for 4 poles and is supplied from a 50 Hz system. Calculate synchronous speed.

Solution:

Synchronous Speed, NS = 120f/P

= 120*50/4

Synchronous Speed, NS = 6000/4 =1500 r.p.m.

1. A three phase induction motor is wound for 6 poles and is supplied from a 50 Hz system. Calculate synchronous speed.

Solution:

Synchronous Speed, NS = 120f/P

= 120*50/4

Synchronous Speed, NS = 6000/6=1000 r.p.m.

1. A three phase induction motor is wound for 8 poles and is supplied from a 60 Hz system. Calculate synchronous speed.

Solution:

Synchronous Speed, NS = 120f/P

= 120*60/8

Synchronous Speed, NS = 7200/8 =900 r.p.m.

1. An 8-pole alternator runs at 750 r.p.m. Find the frequency of the alternator.

Solution:

Frequency, f = (NS * P)/120

= (750 * 8)/120= 50 Hz

1. A 4-pole alternator runs at 1500 r.p.m. Find the frequency.

Solution:

Frequency, f = (NS * P)/120

= (1500 * 4)/120= 50 Hz

1. A 6-pole alternator runs at 1200 r.p.m. Find the frequency.

Solution:

Frequency, f = (NS * P)/120

= (1200 * 6)/120= 60 Hz