# relation between number of poles and speed

3 replies to this topic

### #1 SAMER

SAMER

Intermediate Member

• Members
• 10 posts

Posted 12 November 2005 - 12:48 PM

Hi
I would like exactly why the synchronous speed decrease when number of Poles increase and the demonstration of the below equation:
Ns=120*f/p

### #2 chaterpilar

chaterpilar

Member

• Full Member
• 171 posts

Posted 13 November 2005 - 07:05 AM

Dear samer,

All loads moved by electric motors are really moved by magnetism. The purpose of every component in a motor is to help harness, control, and use magnetic force. When applying an AC drive system it helps to remember you are actually applying magnets to move a load. To move a load fast does not require more magnets, you just move the magnets fast. To move a heavier load or to decrease acceleration time (accelerate faster) more magnets (more torque) are needed. This is the basis for all motor applications.

Now when you have less poles the interchange of magnets is fastest and hence the speed is more.

Chaterpilar

" If you tell the truth, you don't have to remember your lies ! "

### #3 SAMER

SAMER

Intermediate Member

• Members
• 10 posts

Posted 15 November 2005 - 01:42 PM

where is the demonstratin of the formula Ns=120*f/p?

### #4 GGOSS

GGOSS

Senior Member

• Moderator
• 407 posts
• Location:Melbourne

Posted 16 November 2005 - 12:01 AM

Hello SAMER

I would have thought that the information provided by chaterpilar (excellent work) would have been sufficient for you. As that does not appear to be the case, the following should help you further.

Definitions:
Ns = Synchronous speed
120 = Engineering constant
f = frequency
p = number of poles

It is clear from the above, syncrounous speed is inversly proportional to number of poles. Take for example;

2 pole motor, 50Hz supply
Ns = (120 x 50) / 2 = 3000 RPM

4 pole motor, 50Hz supply
Ns = (120 x 50) / 4 = 1500 RPM

Regards,
GGOSS

#### 0 user(s) are reading this topic

0 members, 0 guests, 0 anonymous users