Which of inv-duty motors w/VFD is prefarable - increasing or reducing 4

[vlad05]

Hello,
we are going to use AC 15 to 60 Hz Constant torque inverter duty motors w/VFD for for rotary screw air compressors.
Motors: from 50hp to 250hp. Lincoln motors company offers 2 pole motor 3600rpm - max 3600rpm and 4 pole motor 1800rpm -
max 3600rpm. Power supply -460v/60Hz.
Full air demand is @2500-2600rpm of motor

My question to you, as the experts, is: which motor is preferable for this application and what is the theoretical
basis for it?

 

[electricpete]

Thx jraef. I like it when things come together.

Is it also true that if we choose 2-pole then we have to choose a horsepower rating 3600/2500 larger than what is drawn by the load at full speed (2500rpm) so that rated torque of the motor is high enough?

 

[jraef]

electricpete
Again, it depends. Since a compressor is a constant torque load, he really should look only at his torque and speed requirements, then select the motor HP and base speed that fits these criterea. In the days before VFDs, he had no choice but to use a 3600RPM motor. So his selection MAY have had a higher HP rating than a 4 pole run overspeed with the VFD. Since we do not know his torque requirements at 2500RPM, we can't yet answer your question. Quando Omni Flunkus Moritati

 

[electricpete]

Thanks.

If that his constant-torque load draws 100hp at 2500rpm, that means we need a 100hp 4-pole or a 3600/2500*100 ~ 150hp 2-pole. (assuming we don't have concerns about cooling at low speed) Right?

(Just trying to make sure I understand it)

 

[jraef]

OK, I think I see where you are coming from. I'm not being thick about this, but I apparently missed vlad05's post of 2/24 where he did mention a HP...

"... Constant torque on motor shaft mantains constant pressure,for example, 125psi or, say, @100hp provides 450cfm of air @125psi. If VFD is in frequency range 0-60Hz, as I understood from Jraef explanation, I have no problem;
if a frequency is in 60-120Hz range, compressor still can
build up 125psi, but must drop an air delivery @ same power consumption or on electrical language: decreasing the torque @ same motor hp. Any comments? ..."

Even so, he still did not mention that he had chosen a 100HP motor based on 2 pole or 4 pole, and it makes a difference. If he chose based on 4 pole, then yes, the 2 pole motor would have needed to be oversized to compensate. If he based that on a 2 pole, then no, because the 2 pole motor must have given him the required torque at 2500RPM.

Now, given the TONE of his response, I can see where it is a reasonable to ASSUME that he based that on the torque specs of a 4 pole motor, so YES, your hypothesis is true in that case.

But again, what happens when we ASSUME?

I'm not a compressor guy, but somewhere I have formulae for calculating motor requirements for compressors. I'm going to check his numbers a little.

Vlad05, if we have not yet bored you into oblivion, please help out a little here.
Quando Omni Flunkus Moritati

 

[electricpete]

Thanks jraef. I think he is looking at a range of applications from his original post.

The conclusion I'm reaching is that if we are comparing two motors for this application then the 2-pole needs a higher horsepower rating than the 4-pole. In my mind it erases the inherent price advantage in 2-pole over 4-pole that we sometimes assume for non-vfd applications.

good discussion.

 

[jbartos]

Suggestion: Also, visit this Forum posting:
Speed Reducer or Increaser, which is better ?
thread 237-40604

 

[coldbrew]

vlad05

On a variable speed drive application, the torque of the motor falls off roughly as a square at speeds above the 60hz rpm rating of the motor (constant hp range, torque is proportional to(v/hz)squared). Since this type of compressor is a constant torque application, you could experience problems at the higher speeds. For me, the preferable method would be to size the motor for the max rating of the compressor, speed and horsepower required, and use the VSD to lower the speed to your operating range. The VFD will allow the motor to apply its rated torque for your entire speed range under this condition.

Regardless of which method you use, as jraef pointed out, you will have cooling problems at the lower speeds.

 

[coldbrew]

vlad05

I missed that these are inverter duty motors rated for constant torque applications and therefore may be suitable for operating at the lower speeds without external cooling.
Sorry.

 

[GusD]

I don't want to add any more fuel to what already is a huge BONFIRE (quite personal at times I may add).After reading all of the expert's knowledgeable answers,I was asking myself, where did I go wrong in all my years dealing with electric motors.
I have a question that most of you should be able to help me with?
How many installations are there, where we use 4 pole motors ( VFD or other) to operate at speeds such as 2500 rpm.Perhaps I have been working in all the wrong places,I have not seen one yet.I don't doubt that someone somewhere,used the 4 pole motor option for whatever reason.I believe these cases would be very few.
Thanks a lot Gents

knowledgeble experts answers GusD