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?

 

[Lewish]

From my point of view, and please observe this caveat, that I don't work with motors that large, but I would say that the 2 pole motor is your only choice for this application. When a motor is driven a ways above its synchronous speed, you loose the constant torque capability.

If someone has a diffent point of view, I welcome it.

 

[jraef]

Lewish is right. All that the manufacturer is saying there is that the bearings on the 4 pole motors are the same as on the 2 pole motors, so IF you want to overspeed them it is OK. The concept however is a problem with relation to torque. Constant torque is maintained by applying 7.67 Volts/per/Hertz. At 60Hz then, you will be applying 460V because 460/60 = 7.67; at 30Hz you will be supplying 230V, still 7.67V/Hz.

Above 60Hz (actually whatever the rated frequency is), an AC motor will operate in contant HP mode since no more voltage is going to be available to keep torque the same. For instance if you take your 4 pole motor and operate it at 120Hz to get 3600RPM, but you still only have 460V available, your V/Hz ratio just went down to 3.83 (460/120), or 1/2 the ratio of the 2 pole motor at 60Hz. In reality, your switching losses also increase rapidly above 60Hz, so your available torque drops off even faster. At 3600RPM then, your 4 pole motor will produce less than 1/2 the torque of your 2 pole motor.

Did this explain it for you? Quando Omni Flunkus Moritati

 

[jbartos]

Suggestion: The lower speed rated motor working at the higher speed, with manufacturer's permission, is supposed to be the better solution since the higher speed motor working at the lower speed may or may not get enough cooling and the efficiency may be smaller.

 

[electricpete]

All good points. To add to Lewish and jraef's discussion. Torque vs slip is dependent upon volts per hz squared. If you are at 150% speed (frequency), v/f is at ~ 66% and torque vs slip curve is ~ 4/9. So breakdwon torque may be perhaps 4/9 of normal-frequency breakdown torque. Assuming normal frequency breakdown torque were 200% of rated , that would move 150%-frequency breakdwon torque down to 4/9*200%=88% of rated. But at 150% speed and full load, the full-torque is 66%. So at 150%, speed the margin between full-load torque and breakdown speed is approx (88%-66%).


Regarding overspeed capability, NEMA MG1 Table 12-5 has specific requirements for general purpose squirrel cage motors. AT <=30hp a 4-pole motor can be run continuously up to 2700rpm, but at >=40hp a 4-pole motor is limited to 32300rpm. There may be another section in that standard that gives other guidelines for motors for variable speed service. But I would check this very carefully.... an error in overspeeding could be dangerous.

Another consideration for the lower speed motor is that if you limit the motor to rated torque for mechanical considerations, then the power is derated according to the speed.

One final consideration, initial cost of a 2-pole motor is lower than a 4-pole motor of same horsepower.

 

[Lewish]

Interesting points electricpete. I buy 3 HP Hyundai motors, inverter duty, which are UL listed and certified to 6000 rpm. I think this may be the exception, however. These are 4 pole, 1760 nominal rpm motors at 230 volts. We limit their maximum speed to 4800 rpm. That gives us a comfortable safety margin over their rated capability.

 

[jbartos]

Question to the last electricpete posting. What is the basis for: AT <=30hp a 4-pole motor can be run continuously up to 2700rpm, but at >=40hp a 4-pole motor is limited to 32300rpm.

Why would 30hp motor be permitted to run continuously up to 2700RPM and 40hp motor be permitted to run continuously up to 32,300RPM?

 

[wired1]

I'm not sure what you mean by &quot;constant torque invertor duty motors&quot; Does that mean the compressor requires a certain torque from the motor? I can tell you that the constant torque region of the motor is from &quot;min speed&quot; to rated. After that, torque falls off (unless your drive can compensate).
So from a process standpoint, it looks like you should go with the motor that will operate in the region where torque is predictable. This also ensures the smallest torque/framesize ratio. I presume you are also considering the cooling requirements of the motor when operating at a low speed. In any event, the mtr mfr should be able to answer the question of motor performance at the required speed.

 

[vlad05]

Thanks to All.
I had the same opinion, as Lewish & supporters, and now received your confirmation.
On wired1 question about constant torque for air compressors.
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?

 

[electricpete]

jbartos - there was a typo in my info, should be 3200rpm per the referenced NEMA table.

Now that I re-read the original post, he has already checked the max speed of his motors and found both the 2-pole and 4-pole have max speed of 3600rpm.

I stumbled upon an article whose title appears to address this exact issue. &quot;Optimum Pole Configuration of AC Induction Motors Used on Adjustable Frequency Power Supplies&quot;

At first glance I'm not sure I understand the whole point of the article.... what are the contraints and what are they optimizing.

 

[vlad05]

electricpete - where can I find the article you have mentioned &quot;Optimum Pole Configuration of AC Induction Motors Used on Adjustable Frequency Power Supplies&quot;?
Thanks.

 

[electricpete]

I'm sorry, I intended to post the link.

http://www.reliance.com/prodserv/motgen/b7100_1.htm

(I'm still not sure exactly what the main point is).

 

[electricpete]

The punchline of the article seems to be the graph at the end which gives optimal number of poles vs torque. Since your full load torque is in the range 100-500 ft-lbf, this chart would tell you 4-pole is best.

But the whole analysis is based upon motor design parameters. I would think that the price, efficiency and power factor numbers for the alternate motors would tell the story and the more detailed analysis shown in the article doesn't add anything useful beyond that.

That's my take after reading the article for 5 minutes more. Maybe I'm missing something.

 

[electricpete]

You guys tell me if I am right on this.... Let's say it is a 100hp application, then the two motors we should compare are:

#1 - a 2-pole motor with horsepower rating of 100 * (2500/3600)

and

#2 - a 4-pole motor with horsepower rating of 100.

The reason for derating the 2-pole motor is to prevent operating above rated torque.... is this required?

Now the question why the 4-pole motor was ruled out by Lewish and others. I understand we will increase frequency but not voltage so decrease volts per hz. In my discussion above I conservatively assumed 150% increase in speed and 200% normal-speed breakdwon torque and it still seemed like there was some margin between breakdown torque and operating torque. I repeat it here:

If you are at 150% speed (frequency), v/f is at ~ 66% and torque vs slip curve is ~ 4/9. So breakdwon torque may be perhaps 4/9 of normal-frequency breakdown torque. Assuming normal frequency breakdown torque were 200% of rated , that would move 150%-frequency breakdwon torque down to 4/9*200%=88% of rated. But at 150% speed and full load, the full-torque is 66%. So at 150%, speed the margin between full-load torque and breakdown speed is approx (88%-66%).

Is this an unacceptably small margin:? If it were a non-reciprocating load would the smaller motor be acceptable?

 

[electricpete]

My assumed minimum locked rotor torque of 200% rated under standard frequency applies to NEMA motors up to 200hp. At 250hp the minimum goes to 175%.

 

[electricpete]

I meant NEMA design B (general purpose), of course.

 

[electricpete]

I think jbartos and wired1 brought up another important consideration of cooling requirements. The lower speed motor operated above it's rated speed has no special cooling considerations. The higher speed motor operated below it's speed has reduced cooling. That is not usually a concern for a variable torque load (like centrifugal pump), but may be a concern for constant torque load where you can get appreciable power requirement at low speed.

 

[vlad05]

The normal operating speed 2500rpm I need to provide specific torque, which give me needed pressure on air compressor output. If motor will run at 150% frequency or 90hz the torque at full load will only, as you said, 66% of torque I need to keep my pressure at rated level, wich means
I have to compromise compressor air delivery to keep pressure on same level, or oversize the motor... Am I right?

 

[electricpete]

Absolutely right in my opinion. My #1 should have said
#1 - a 2-pole motor with horsepower rating of 100*(3600/2500) (not 2500/3600)

Hopefully the typo is evident from my comment that the 2-pole motor is &quot;derated&quot;.

 

[electricpete]

OK, now I see that I said something different than you.

To my way of thinking there is no need to derate the lower speed 4-pole motor operated at higher speed, as long as we maintain some margin between operating torque and breakdown torque.

It is true that the breakdown torque decreases, but that means the slip will increase slightly to allow the motor torque to match the load torque. This will always occur as long as we have maintained some margin (for temporary overload) between operating torque and breakdown torque. Once we exceed breakdown torque the motor is no longer capable of handling the load by increasing it's slip.