# 240 VOLTS MOTOR VS 120 VOLTS ONE

Discussion in 'General Electronics Chat' started by Rookie2002, Apr 30, 2009.

1. ### Rookie2002 Thread Starter New Member

Apr 30, 2009
1
0
I'm wondering if this is true. A co-worker ( Heating and Ac trade) told me the other day that a 240 volts motor has more poles than a 120 volts one. So, it would rotate slower than the 120 volts. one. I know that speed is a inversely proportional to the number of poles in the stator.More poles ,slower thew RPM. But, Is that true? a 240 volts motor really has more ( He said " twice as much ') poles than the 120 volts one.? Where in the name tag of a motor can I read the number of poles a motor has?
We were troubleshooting a blower in a furnace which was not moving enough air. Come on! I also know that we always play by the 10% rule. You know + or - 10%. He said that an enexperinece tech. installed a 240 volts blower motor instead of a 120 volts one. By the way, this motor was installed in 2006 , how come it lasted so long?
I'm very new in this field. He's all the years of expeirence in this. Was him pulling my leag?
Please could you educate me on this subject or point me to a web site that I can read about it.
Thanks Rookie

2. ### mik3 Senior Member

Feb 4, 2008
4,846
70
The number of the poles is determined by the desired output speed of the motor. Usually a 240V system works with 50Hz and a 120V system with 60Hz. This may be a reason to change the number of poles in the motors but I don't think 50Hz motors have twice the number of poles of 60Hz motors.

3. ### SgtWookie Expert

Jul 17, 2007
22,201
1,809
In North America, almost all AC power is 60Hz except for aircraft, which is 400Hz.

Frequently, AC motors designed for this market have jumpers so that they can be operated from either 120v or 240v, as long as the jumpers are set properly.

4. ### greendean Member

Dec 20, 2008
19
0
yes.

more poles, less rpm.

Synchronous Speed = (120 x frequency) / # of poles.

All power in the US is 60hz. Whether it be 120, 208, 277, 480, 347, or 600 volts.

So a 208 volt motor with 2 poles will spin twice as fast as a 208 volt motor with 4 poles.

So he is right that more poles less speed.
But he is wrong saying voltage has anything to do with it.

There is so mouch to know about motors and the control of them that this post could go on for hours.
But I kept it to a bare minimum.
Very Bare!