Motor cooling & VFD
Motor cooling & VFD
(OP)
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
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





RE: Motor cooling & VFD
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
RE: Motor cooling & VFD
RE: Motor cooling & VFD
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
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.
RE: Motor cooling & VFD
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.
RE: Motor cooling & VFD
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
RE: Motor cooling & VFD
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
RE: Motor cooling & VFD
RE: Motor cooling & VFD
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?
RE: Motor cooling & VFD
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.
RE: Motor cooling & VFD
Spoken like someone totally ignorant of how the magnetic coupling operates. Have a look at http://
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!
RE: Motor cooling & VFD
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.
RE: Motor cooling & VFD
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.
RE: Motor cooling & VFD
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
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"Why not the best?"
Jimmy Carter
RE: Motor cooling & VFD
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
www.gke.org
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: Motor cooling & VFD
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
RE: Motor cooling & VFD
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!
RE: Motor cooling & VFD
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.
RE: Motor cooling & VFD
RE: Motor cooling & VFD
LOSS(FRICTION + WINDAGE + CORE)=10*HP [Result is in Watts]
RE: Motor cooling & VFD
10*hp gives losses which equate to a motor motor efficiency of about 98.6%. Motors are usually around 90% efficient, give or take a few %.
I'd think a multiplier around 80 might be a better rule of thumb.
RE: Motor cooling & VFD
As itsmoked has posted in his first thread, I agree.
If you are going to run your motor at less than 70% FL speed, then with the use of an anemometer, calculate the wind speed at full speed that the motor fan 'sucks' and passes over the motor, times the area ( back of the fan cover) and calculate the l/s,gal/min. With that knowledge, purchase an additional motorised fan (yes, a squirrel cage motor) that has the correct output and mount that on your motor to compensate for the loss in air movement over the motor due to a loss in speed.
Hope that is of some help!