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?

 

[jbartos]

Suggestion: Visit

http://www.controldepot.net/vfd1.pdf

for Figure 20 (this is a big *.pdf file)

http://www.saftronics.com/pdf_fils/library/AN_VFD_GEN018.pdf

etc. for motor data at higher speeds

 

[jraef]

electricpete, I enjoy watching how you pose questions, then ammend them, then answer them yourself if we ignore you long enough!

All kidding aside, now I will attempt to mix it up even further. A "trick" often played in a situation like this is to strap a dual volatge 4 pole motor for the low voltage winding, then program the VFD to provide full voltage at the doubled frequency. This way, the VFD provides 460V at 120Hz, but the motor is wired at 230V, so it does not provide full torque until is sees the 120Hz 460V from the drive! This way, as long as the operating range is near the top end of the speed curve, the motor is running fast enough to keep cool without as much loss in torque at the higher speeds.
Chew on that one for a bit. Quando Omni Flunkus Moritati

 

[electricpete]

Hi jraef - I'm not ready for your trick yet. I'm still trying to understand your and Lewish' basic objection to the 4-pole motor. I view it as a learning experience for me.

1 - You have suggested above that the 4-pole motor is unsatisfactory at twice speed because it is limited to 1/2*rated torque (or slightly less due to switching losses). But 1/2*rated torque at twice speed is after-all still full power. The only objection I see is the breakdown torque issue. At 150% speed there is still some margin as computed above. Is the small margin to breakdown torque (stall) the issue?

2 - Can a motor safely be operated continuously above nameplate rated torque... let's say 150% rated torque? (referring to the higher speed motor operating at lower speed... would require higher than nameplate torque to achieve nameplate horsepower).

 

[elhead]

Hi gang

I hve used jraef's "trick" a few times in the past with success (mostly distant past - remember six-step drives?). My experience was limited to smallish (under 10HP) motors. I believe I have done it with a PWM drive, but still in this horsepower range. Twenty years ago Parajust called this hookup 'Superdrive'. One does need to realize that the drive will have to be sized for twice the motor's listed HP. Another way to look at is is that you can get double the motors' rated power (full torque at twice nominal speed)

 

[AussiePete]

Sorry if this is a dumb question but wouldn't a 2 pole motor at 60hz with the same HP give you the same rpm result as the "trick" above and the 2 pole motor is a cheaper motor.

 

[jbartos]

Suggestion: Please, notice the frequency range posted in the original posting, namely: 15 to 60Hz at constant torque. 15Hz is a bit on lower side for an induction squirrel cage motor; however, it may still be good for a synchronous motor with good cooling at 15Hz.

 

[d23]

vlad05, All:

Getting to 15 hertz will be a cooling problem for a squirrel cage motor. The original post doesn't ask, but I've been told that rotor design will limit most two pole squirrel cage motor's to 65 hertz max. Is this correct?

 

[electricpete]

d23 - From NEMA MG-1 Table 12-5 and 12-6 it can be seen there is no continuous overspeed capability required for large 2-pole general purpose motors above approx 50hp. But I'm not sure how it related to the original question.

 

[RadarRay]

If I was going to do this application I would use a 4 pole motor. I don't see any torque disadvantage of using a 4 pole motor over a 2 pole motor.

Here are the numbers.
Tq for any given HP motor is (5250*HP)/base speed. So assuming a 100HP motor, the 3600rpm motor outputs 145ft-lbs. It does this, with a vector drive at least, from 0 speed to 3600 rpm. Thus the torque output at 2500 will be 145 ft-lbs.

The 1800 RPM motor has a torque output of (5250*100)/ 1800 =291 Ft-lbs, at base speed, which is twice as much. So you now have to get the motor to 2500. The relationship between HP and torque is, HP is proportional to Torque Times Speed. So Speed goes up, torque goes down, roughly proportionally. Using this the approx torque at 2500 rpm is 291* (1800/2500) =209 Ft-lbs, which is more than what the 3600 motor can put out.

You do have to worry about the break down torque, but with all the motors I have seen it happens above 150% of base speed. And I will say I speak mostly of Marathon motors, but you can always get the curves from the motor manufacturer if you are unsure. Actually, I would make that "should" not "can".

I have run 4 pole Marathon motors up to 3600 RPM with no problems in an application, but the torque I required was well within the limits of what the motor could put out at that speed.

One of the limitations of running a motor above base speed is the load on the bearings. In my app above the motor was direct coupled to a gear box. If I was using timing belts I could not have done this because of the additional side loading of the belt pulleys.

Lastly, while many a load calc gives you a result in HP, I always verify what I am doing with the Torque calculations. This is especially true if I am going to run above base speed.

 

[vlad05]

radarray- why do you think that same equation T=(5252 x HP)/rpm does not work for 3600rpm motors when you reduce a speed?
Per Lincoln motors company diagram for VFD: in frequency range from 0 to 60hz - motor has A constant torque range while HP range has changed from 0 to 100% or proportionally
to frequency, whoch means energy saving if motor runs @2500rpm keeping the torque at needed level; in frequency range from 60 to 120hz - HP range is constant while torque has reduced proportionally from 100% to approximately 25% at 120hz. In this case we have no energy saving, if 4 pole motor runs @2500rpm. The energy saving, basically, is only advantage of using the VFD for air compressors.
Please, comment.

 

[RadarRay]

Note in my original equation its (5250*HP)/ BASE SPEED. You cannot put any speed into the equation. Base speed on a 4 pole motor is, depending on slip a little under 1800 rpm and for a 2 pole motor under 3600rpm. So this gives the rated torque available. Look at my post again with this in mind.

My post was to explain what torque you would get at what speeds. I don't usually worry about energy usage, so I may not be the best one to answer that, at least without some thought. I don't think anyone got the message that your concern was enerqy usage.

Note from your last post, I can see that from the curves at 200% speed it looks like the breakdown torque becomes an issue on the 4 pole motor. At least it would be an issue if you were going to run that fast.

 

[electricpete]

There is quite an extensive NEMA Adjustable Speed Application Guide pulbished by NEMA and available for free at:

http://208.156.24.64/index_nema.cfm/1427/0DDF742A-8790-4A21-88D139CFC069648B/

My simple reading of this document is that it supports Lewish, jraef's and vlad's comments that the higher speed motor operated below it's 60hz speed is generally the correct choice FOR A CONSTANT TORQUE load.

The lower speed motor operated above it's 60hz speed would generally be correct choice for a variable torque load such as a centrifugal pump.

My apologies for adding to the confusion on this thread. It took me awhile to catch on to the fact that this is a constant torque load.

 

[electricpete]

By the way you will be asked to register after you click download... it is free.

 

[electricpete]

Page 52 of that document shows tables that show motor operating below base speed for const and variable torque applications and above base speed for constant horsepower applications. That is what I based my previous comments on.

And it is understood that the motor motor is limited at a constant power above base speed (up to 150% or more) and limited at a constant torque below base speed. At first glance it makes sense we want to operate in the constant torque region for a constant torque load. (assuming we have evaluated cooling concerns unique to constant torque loads at low speeds).

But now I think I have given an incorrect conclusion to imply that 4-pole is not acceptable. (they are both acceptable)

Let’s say the load can range in speed from 1800-2400rpm (rounder numbers), with constant torque that equates to 100hp at 2400rpm.

Now let’s say I put a 100hp 1800rpm motor in there and operate above rated speed (motor limited to constant power 100hp).

The rated torque of that motor is 100*5250/1800.

At 1800rpm, load torque = 100*5250/2400
At 1800rpm and 90hz, Motor can deliver it’s rated torque times (1800/2400)
Ie motor can deliver (100*5250/1800 ) * (1800/2400) = 100*5250/2400
No problem, motor can meet the torque, and power requirement at full speed.

Now at 2400rpm, load torque is constant, still 100*5250/2400
Motor has no problem meeting this since it is below the motor rated torque 100*5250/1800.

Bottom line… it looks to me like either mode would work. So maybe it boils again down to a question of economics (motor and controls purchase price and efficiency). That is what vlad05 has been looking at. OK jraef stop laughing. I think I'm done now.

 

[electricpete]

Note done yet. I jumbled up my post. Same conclusion, calculations as follows:

The rated torque of that motor is 100*5250/1800.

Look at 2400rpm case:
At 2400rpm, load torque = 100*5250/2400
At 2400rpm and 90hz, Motor can deliver it’s rated torque times (1800/2400)
Ie motor can deliver (100*5250/1800 ) * (1800/2400) = 100*5250/2400
No problem, motor can meet the torque, and power requirement at 2400rpm

Look at 1800 rpm case:
Now at 1800rpm, load torque is constant, still 100*5250/2400
Motor has no problem meeting this since it is below the motor rated torque 100*5250/1800.
Motor also has no problem thermally supplying the horsepower which is ~ 67hp.

 

[electricpete]

If my meaning is not clear from all of the above, it is that I think either motor is acceptable. Can Lewish or jraef or anyone explain why there would be any difficulty using a 4 pole motor for this application?

 

[electricpete]

I guess my discussion amounts to a repeat of what radarray has said. I think his input was useful for me as a reader... other posts seemed to indicate 4-pole was not acceptable.

vlad said: "in frequency range from 60 to 120hz - HP range is constant while torque has reduced proportionally from 100% to approximately 25% at 120hz. In this case we have no energy saving"

I don't understand that comment. Is it possible you are assuming that the motor operating in it's constant horsepower mode will always draw that horsepower? That is certainly not true, only the limit of maximum power in that speed range is constant. Actual power is determined by the load.

 

[electricpete]

I vote radarray a star... I think he has answered many questions raised in the thread.

His conclusions as I understand them:

The 4-pole is acceptable since it is within it's torque and HP limits throughout the speed range.

The 2-pole would need to be oversized/derated based on torque considerations.

Does everyone agree?

 

[jraef]

Electricpete, the way you asked your last question made me go back and read everything in order to see where you went wrong! Not that you did, but I THOUGHT you did.

Actually I have been swayed by Radaray's thoroughness, albeit depending on how Vlad05 has determinied his motor requirement. As I look at the original post, I realized that I ASSUMED (and we all know what that means) that Vlad had already selected his motor based on a 2 pole torque/speed curve since his full air delivery was based on 2500RPM. Radaray made me realize that he said nothing of the sort. Shame on me. Given that, Radaray is absolutely right, the 4 pole motor will give more torque at 2500RPM than the 2 pole, even after accounting for the overspeed, making it the better choice with an unknown torque requirement. Bravo Radaray. Years ago in school I once made an application mistake on a test by applying the HP/Tq/Spd formulae to the motor instead of the load, and I continue to make that same mistake to this day on occasion. MOTOR torque output is derived from BASE speed which is fixed, whereas LOAD torque is determined from operating speed, which can be infinitely variable.

Of course, if Vlad had picked his motor based on a 2 pole 3600RMP torque/speed curve, it would not make a difference since the extra torque capability of the 4 pole would then be unnecessary. At least at the high end of his requirements, and herein lies the rub. I realized after reading all these posts that I missed a basic issue which was brought up by jbartos, that being the he wants to operate at 15Hz! This makes the 2 pole motor unacceptable no matter what the torque output at 120Hz is unless Vlad uses a blown motor.

Advantage: 4 pole

Quando Omni Flunkus Moritati

 

[jraef]

Come to think of it, even a 4 pole motor might have trouble cooling itself at 15HZ. Vlad05, you might want to consider blowing the motor either way if a significant amount of your duty cycle might be at low speeds. Given that, we are back to electricpete's basic price/performance statement regarding which one to choose. I would add to that a consideration of replaceability; if the application is critical and should the motor go down, 4 pole motors are slightly easier to find in an emergency than 2 pole.

40+ posts to this thread later, and the answer appears to be:

Use whichever one ends up cheapest!
Quando Omni Flunkus Moritati