Which of inv-duty motors w/VFD is prefarable - increasing or reducing
Which of inv-duty motors w/VFD is prefarable - increasing or reducing
(OP)
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?
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?





RE: Which of inv-duty motors w/VFD is prefarable - increasing or reducing
If someone has a diffent point of view, I welcome it.
RE: Which of inv-duty motors w/VFD is prefarable - increasing or reducing
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
RE: Which of inv-duty motors w/VFD is prefarable - increasing or reducing
RE: Which of inv-duty motors w/VFD is prefarable - increasing or reducing
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.
RE: Which of inv-duty motors w/VFD is prefarable - increasing or reducing
RE: Which of inv-duty motors w/VFD is prefarable - increasing or reducing
Why would 30hp motor be permitted to run continuously up to 2700RPM and 40hp motor be permitted to run continuously up to 32,300RPM?
RE: Which of inv-duty motors w/VFD is prefarable - increasing or reducing
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.
RE: Which of inv-duty motors w/VFD is prefarable - increasing or reducing
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?
RE: Which of inv-duty motors w/VFD is prefarable - increasing or reducing
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. "Optimum Pole Configuration of AC Induction Motors Used on Adjustable Frequency Power Supplies"
At first glance I'm not sure I understand the whole point of the article.... what are the contraints and what are they optimizing.
RE: Which of inv-duty motors w/VFD is prefarable - increasing or reducing
Thanks.
RE: Which of inv-duty motors w/VFD is prefarable - increasing or reducing
http://www.reliance.com/prodserv/motgen/b7100_1.htm
(I'm still not sure exactly what the main point is).
RE: Which of inv-duty motors w/VFD is prefarable - increasing or reducing
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.
RE: Which of inv-duty motors w/VFD is prefarable - increasing or reducing
#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?
RE: Which of inv-duty motors w/VFD is prefarable - increasing or reducing
RE: Which of inv-duty motors w/VFD is prefarable - increasing or reducing
RE: Which of inv-duty motors w/VFD is prefarable - increasing or reducing
RE: Which of inv-duty motors w/VFD is prefarable - increasing or reducing
I have to compromise compressor air delivery to keep pressure on same level, or oversize the motor... Am I right?
RE: Which of inv-duty motors w/VFD is prefarable - increasing or reducing
#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 "derated".
RE: Which of inv-duty motors w/VFD is prefarable - increasing or reducing
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.
RE: Which of inv-duty motors w/VFD is prefarable - increasing or reducing
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
RE: Which of inv-duty motors w/VFD is prefarable - increasing or reducing
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
RE: Which of inv-duty motors w/VFD is prefarable - increasing or reducing
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).
RE: Which of inv-duty motors w/VFD is prefarable - increasing or reducing
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)
RE: Which of inv-duty motors w/VFD is prefarable - increasing or reducing
RE: Which of inv-duty motors w/VFD is prefarable - increasing or reducing
RE: Which of inv-duty motors w/VFD is prefarable - increasing or reducing
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?
RE: Which of inv-duty motors w/VFD is prefarable - increasing or reducing
RE: Which of inv-duty motors w/VFD is prefarable - increasing or reducing
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.
RE: Which of inv-duty motors w/VFD is prefarable - increasing or reducing
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.
RE: Which of inv-duty motors w/VFD is prefarable - increasing or reducing
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.
RE: Which of inv-duty motors w/VFD is prefarable - increasing or reducing
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.
RE: Which of inv-duty motors w/VFD is prefarable - increasing or reducing
RE: Which of inv-duty motors w/VFD is prefarable - increasing or reducing
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.
RE: Which of inv-duty motors w/VFD is prefarable - increasing or reducing
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.
RE: Which of inv-duty motors w/VFD is prefarable - increasing or reducing
RE: Which of inv-duty motors w/VFD is prefarable - increasing or reducing
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.
RE: Which of inv-duty motors w/VFD is prefarable - increasing or reducing
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?
RE: Which of inv-duty motors w/VFD is prefarable - increasing or reducing
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
RE: Which of inv-duty motors w/VFD is prefarable - increasing or reducing
40+ posts to this thread later, and the answer appears to be:
Use whichever one ends up cheapest!
Quando Omni Flunkus Moritati
RE: Which of inv-duty motors w/VFD is prefarable - increasing or reducing
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?
RE: Which of inv-duty motors w/VFD is prefarable - increasing or reducing
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
RE: Which of inv-duty motors w/VFD is prefarable - increasing or reducing
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)
RE: Which of inv-duty motors w/VFD is prefarable - increasing or reducing
"... 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
RE: Which of inv-duty motors w/VFD is prefarable - increasing or reducing
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.
RE: Which of inv-duty motors w/VFD is prefarable - increasing or reducing
Speed Reducer or Increaser, which is better ?
thread 237-40604
RE: Which of inv-duty motors w/VFD is prefarable - increasing or reducing
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.
RE: Which of inv-duty motors w/VFD is prefarable - increasing or reducing
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.
RE: Which of inv-duty motors w/VFD is prefarable - increasing or reducing
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