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