Motor cooling & VFD

[Mwater]

I am interested in motor cooling.

1) What's the key component in motor (induction motor) heat generation? is it the current (amps) or the input horse power and efficiency at a specific point. I read different articles and they all mention current losses in the motor (stator and Rotor), but they don't mention if all these losses are converted to heat.

2) Also if a 50 hp motor is 90% efficient, what's the heat generated? Is it the 10% loss?

3) In addition with VFD there is a change in voltage, frequency and speed but the input current could remain the same so how that affect the motor cooling. This related to the first question; if the voltage/frequency is constant but the current change with load does this affect the fan if is speed independent.

4) In case of speed dependent or independent ventilation, what's the best setup to get the appropriate cooling?

Thanks in advance for your explanation

 

[itsmoked]

1) Virtually anything that doesn't go out the shaft results in heat loss in the motor. Some loss is the fan which is helping to remove the heat from within the motor. So it's contribution does not appear in the motor.

2) 10% of the loss minus the fan load. ~ 1~2hp

3) If you slow down a motor with a VFD the fan moves less air by the square of the speed reduction. You are also reducing the HP out of the motor so you can get away with this. However down below about 50% motor speed the fans function diminishes rapidly. VFDs drive the motor with a complex waveform that adds some losses in the motor.

4) Often I see a small blower,(squirrel cage), with a hose ducted to any opening on one end or in the case of TEFC, fans blowing on the outer housing fins.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[Mwater]

I have a question about part 3; the data that I have shows that at specific speed, voltage, and frequency the VFD horse power and current increases as the water horse power (output) decrease. this true for low speed (50, 40, 30% reduction)but for higher speed (10%, 20% reduction) the VFD's hp and amps output increase as water horse power increases. so I don't understand why the different to the change in valve opening , I expected the WHP to change always in the same direction as VFD output HP.

 

[gepman]

Mwater, I don't quite follow your last comment but I think that you are confusing the driven load (fan or pump) with the motor cooling fan which itsmoked referred to in his post.

There have been several recent discussions regarding energy savings and VFD's if that is what you are referring to. I recommend the threads

http://www.eng-tips.com/viewthread.cfm?qid=205555&page=2

and

http://www.eng-tips.com/viewthread.cfm?qid=203677&page=5

I do have a comment for itsmoked which I believe that I have also made previously to a similar statement by him. He states that the motor cooling fan varies with the square of the speed while the centrifugal and axial fans that I am aware of vary the flow relatively linearly with speed.

 

[DickDV]

I would add that, as far as heat buildup in the motor is concerned, current is by far the largest contributor. hp and speed are not particularly useful for predicting motor heating.

So, for illustration purposes, a motor drawing 200amps at 60hz will generate about the same heat as the same motor drawing 200amps at 20hz. The difference in resulting temperature is almost entirely due to the inability to get rid of the heat. And that is due to the slower speed that the shaft fan is operating at. If your motor has an auxiliary constant speed fan, I would expect the motor to be very close to the same speed at both shaft speeds.

 

[Mwater]

first I want to thank you for your assistance.

DickDV, how we can evaluate how much heat or efficiency loss we will get by looking at the amps. when the problem that we are having is how much ventilation we need. we use VFDs to control pumps in canals and we don't want to have motor failure so how we know if we need extra fan that has constant speed as itsmoked suggested.

also I find this quote in NEMA Motors and Generators part 30 " Efficiency will be reduced when a motor is operated on a bus with harmonic content. The harmonics present will increase the electrical losses which, in turn, decrease efficiency. This increase in losses will also result in an increase in motor temperature, which further reduces efficiency." first thought when I read this statement is that the loss of efficiency is not 100% converted in temperature increase. if someone could explain the meaning of this statement.


Thanks again

 

[itsmoked]

Hello Mwater.

All the losses will go to heat - laws of nature.

You should use max temperature stickers or buy a small battery powered logger and mount it(or its sensor) near the bottom of your VFD where it draws its cooling air. Let it run and see what the temperature reaches. Once a minute readings is probably adequate and will allow a week of readings. The temperature being drawn into the bottom of the drive is the Ambient Temperature as far as the drive is considered.

Another point to be made is how are your drives mounted? Are they mounted in an industrial box outside? Are they ALWAYS shaded? Direct sun on an enclosure will roast just about anything.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[valvedude]

You can avoid motor heat entirely by looking at

http://www.magnadrive.com

these guys are making VFDs look pretty much obsolete.

 

[dpc]

valvedude,

I'm familiar with the Magnadrive and I wasn't aware that it was capable of operating outside of the laws of physics.

Please explain in detail how the magnadrive eliminates motor losses?

 

[burnt2x]

valvedude,
Magnadrive works fine with smooth load on motors (fans, centifugal pumps, etc.). It Doesn't work well with block loads/uneven and sudden loading! The drive just slips when sudden load is applied.
respectfully.

 

[ScottyUK]

You can avoid motor heat entirely by looking at

http://www.magnadrive.com

these guys are making VFDs look pretty much obsolete.

Spoken like someone totally ignorant of how the magnetic coupling operates. Have a look at

http://www.lubrication.com.au/pdf/ASD_Brochure_FINALc.pdf

- the Magnadrive site is down at the moment - and read it. Do you think they have to install liquid cooling on drives above 500HP because there are no losses, or because they are lossy as hell? Magnadrive's literature doesn't hide the fact that they are lossy which makes your statement even more peculiar.

They are good in certain applications, but variable speed isn't one of their strong areas.



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If we learn from our mistakes I'm getting a great education!

 

[valvedude]

Wow I suppose I should check in here more often to see who is taking shots at my threads.

dpc, funny reply about the laws of physics thing but perhaps you should read what you are replying to.
What I said was that by using MagnaDrive in these applications the MOTOR heat is eliminated. The (ASD) coupling allows the motor to always operate at its full RPMs thereby, keeping it cool. Make sure that you have read this fully before replying again.
Not sure where you came up with me talking motor losses from that.

And ScottyUK,
I FULLY understand how the MagnaDrive couplers work. You ALSO need to read what you are replying to. Yes there are losses and applications where the MagnaDrive is not appropriate. But I suppose that same statement could be made for virtually everything on earth.
The ASD Couplers have a range where they are more efficent than other technologies, and also a range where they are not. Perhaps you should do a bit more hoemwork before you lob out statements like you do.
And I repeat, my thread said NOTHING about losses, just motor heat. Please read fully

It seems Burnt2X is the only one who read my statement, and he is right, the ASD is ONLY for centrifugal applications. MagnaDrive does however make couplers other than the ASD for constant speed applications, soft starts and other types of couplers.

 

[dpc]

Valvedude,

I read the entire thread and your comments. You have some serious misconceptions regarding motor losses and heat.

Motor heat comes from motor losses. And motor loss is heat.

So your statement that the Magnadrive eliminates motor heat but not losses is basically without any meaning.

 

[waross]

It is sometimes misleading to talk about increased losses without quantifying those losses. I saw the owner of an electrical contracting company arguing with the owner of a heating and ventilating company over who was responsible for paying for 2 blown fuses. While the argument raged, I found a catalog and looked up the fuses and the price. I circled the price and slid the catalog in front of the argument. The argument stopped. The fuses cost $0.20 each.
Sometimes the increased losses are not worth fretting about.
Cooling, The same motor frames are often used for 1200RPM, 1800 RPM and 3500RPM.
Given the characteristics of centrifugal pumps at reduced speed, I don't worry about cooling the motors at speeds above about 1000 RPM.
There are always exceptions where a motor and pump is pushed to the performance limit, either intentionally or accidentally, but for most centrifugal pump installations, motor cooling at reduced speeds is not much of an issue.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[Skogsgurra]

The Dude probably meant that you can avoid the overheating that results from running an ASM at low speeds. That normally reduces cooling so that motor temperature goes up. He is correct there. But it is a very bad advice when it comes to power efficiency.

What is really - and very much so - wrong is the statement that "these guys are making VFDs look pretty much obsolete".

In fact, "these guys" have been used a lot longer than VFDs have existed. There used to be quite a lot of manufacturers and they were typically used for center driven winders, soft starting and some fan speed control.

The VFDs made the "magnetic slip coupling" which is the generic name for the Magnadrive "look pretty obsolete" and most of the manufacturers stopped making these electric heaters. Magnadrive is one of the remaining and there seems to be some applications where it can still be used. But there are not many.

Then again, some applications may exist where simplicity and misconceptions about efficiency are deciding factors. Price isn't a deciding factor any more since Magnadrives and the installation typically cost more than a standard drive. Not to speak of LCC. I think that the Dude is referring to those applications.

Gunnar Englund

http://www.gke.org

--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[Marke]

The big problem with the magnamaster type system, is that it is a variable torque coupling where the output torque is equal to the input torque, but the rotational speed varies. This wastes a lot of energy due to the slip.
If you have an application operating at half torque and half speed, then the energy in the load is one quarter of the full load rated power of the motor. The slip speed equals the load speed and the torque also equals the load torque, so the slip energy equals the load energy. The system is 50% efficient.At full speed and full power, the slip losses are in the order of 3%.
A variable speed motor, (AC VFD system) does not have these slip losses and so it is inherently more efficient. This is one reason that the VFD has replaced this technology.

Best regards,

Mark Empson

http://www.lmphotonics.com

 

[ScottyUK]

And ScottyUK,
I FULLY understand how the MagnaDrive couplers work. You ALSO need to read what you are replying to.

The motor will dissipate heat with either a VFD or a magnetic coupling, which is somewhat at odds with your statement "You can avoid motor heat entirely...". A lossless motor? And all you have to do is add a magnetic coupling to it? You should patent that idea...


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If we learn from our mistakes I'm getting a great education!

 

[valvedude]

For the last time and then this "issue" is over.

All that I said was that in using the magnetic coupler you eliminate motor heat because the motor runs at it full RPMs (and the Coupler does do this).... that is all...nothing else. My comments had nothing to do with losses, nothing to do with losss, nothing to do with losses.
There I hope that settles it.

And Finally my comment had nothing to do with losses.

 

[davidbeach]

Last I checked motors put out a lot of heat at "full RPMs", at least at full load (less at lower loads), and that heat is all due to losses. Sure the motor has a fan that moves that heat away from the motor, but that doesn't change the fact that there is a lot of heat produced. And, yes, your statement had everything to do with losses, even if you can't recognize that fact as losses are the [red]ONLY[/red] source of the heat in the motor.

 

[whycliffrussell]

As a general rule of thumb (I'm not sure exactly where I got this but I think it was from a much older senior engineer):

LOSS(FRICTION + WINDAGE + CORE)=10*HP [Result is in Watts]