In this article, we will calculate synchronous speed (*N _{S}*) in r.p.m., Number of Poles (

*P*) and Frequency

*(f)*in Hz.

Formulae used to find N_{S}, P and f are given below:

**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/N_{S}

= 120*50/750

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

**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/N_{S}

= 120*50/200

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

**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/N_{S}

= 120*60/1800

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

**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/N_{S}

= 120*50/1000

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

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

Solution:

Synchronous Speed, N_{S }= 120f/P

= 120*50/4

Synchronous Speed, N_{S }= 6000/4 =1500 r.p.m.

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

Solution:

Synchronous Speed, N_{S }= 120f/P

= 120*50/4

Synchronous Speed, N_{S }= 6000/6=1000 r.p.m.

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

Solution:

Synchronous Speed, N_{S }= 120f/P

= 120*60/8

Synchronous Speed, N_{S }= 7200/8 =900 r.p.m.

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

Solution:

Frequency, f = (N_{S} * P)/120

= (750 * 8)/120= **50 Hz**

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

Solution:

Frequency, f = (N_{S} * P)/120

= (1500 * 4)/120= **50 Hz**

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

Solution:

Frequency, f = (N_{S} * P)/120

= (1200 * 6)/120= **60 Hz**