Choosing a motor

[nrogers]

Hi,

I am replacing a 6-pole motor which is now to be controlled by a VSD. The motor runs a waste water sewage pump with a max required speed of 1000 rpm. Is there any reason why I should not use the cheaper 4-pole motor and use the VSD to obtain the necessary speeds?

Thanks.

 

[RajT]

The 6 pole m/c would have given you a rated speed of 1200 rpm
at rated volts and freq.Running at 1000rpm would not have impaired the cooling a great deal so all was well.
Using a 4 pole m/c which has a rated speed of say 1750 ( with 3% slip), when running at 1000rpm ( ie half speed) may impair proper cooling. A word with the motor supplier would be prudent.

 

[jOmega]

Since you post the speed as 1000 rpm ... either you've a 60 Hz supply and motor with a lot of slip ..or you are stating the synchronous speed for a 6-pole 50 Hz motor on a 50 Hz supply.

While on the outside, it would seem reasonable that you could operate the 4-pole motor at 1000 rpm with a VFD ... what is NOT considered is the T O R Q U E requirement.

A 50 HP 6-pole motor on 50 Hz power develops about 353 N-m of torque (260 ft-lbs).

A 50 HP 4-pole motor on 50 Hz power develops about 235 N-m of torque (174 ft-lbs)

What has just been defined is the maximum RATED torque that the motor will produce.

In the above example, you would need a 75 HP 4-pole motor to be able to supply the same value of rated torque as the 50 HP 6-pole motor.... and of course, you'd need a 75 HP rated VFD.

So, to answer your question, you can see that if you replace the six-pole motor with a 4-pole motor of the same power rating, you certainly can operate it at 1000 rpm ..but you can't get the torque out of it. The motor would be overloaded if you try...and so would the VFD because it would have to source the overload amps to the motor.

One other clue that gets overlooked: look at the frame size of the two motors in the same power rating... you will find that the 6-pole motor is in a larger frame size than the 4-pole ... and that's because he torque comes out of the iron. The 4-pole in a lesser frame size, is rated for less torque. The six pole in greater frame size, is rated for greater torque.

Hope this helps

jO

 

[jraef]

thread237-46696 will give you plenty of additional insight into this issue.

Quando Omni Flunkus Moritati

 

[Marke]

If the pump is coupled to the motor by belts, then you could use a four pole motor and change the pully ratio to give you the same torque output as with the six pole motor.
If the pully ratio was 1:1 with the 6 pole motor, you would change the pully ratio to 3:2 so that the pum speed was 1000 rpm while the motor speed was 1500 rpm.
Best regards,

Mark Empson

http://www.lmphotonics.com

 

[jbartos]

Suggestion: Sewage plant pumps are rarely run over the pulleys. If they were, it would be a reasonable solution. Supposing that the 4-pole motor is selected. The motor will have to have larger HP rating. If it ran at lower speed due to VFD, it would be running less efficiently. The efficiency versus speed curves should be evaluated for 6-pole motor in comparison to 4-pole motor.