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Variable Freq. Drive. 3

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nolacagsr

nolacagsr

Aerospace
Dec 7, 2009
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I am trying to put together a drive system that will be able to operate at 400 to 35000 rpm. I was wondering if this can be accomplished with a VFD and if I will be able to control the lower speeds that are needed.
 
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Generally, yes.
But there are a lot of details between what you ask and what you need. If you FULLY describe what you are trying to do we can offer suggestions.


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I think that can be accomplished with a VFD. You will probably need a vector drive with encoder feedback. There may be a manufacturer that would provide the motor with encoder and the VFD as a package. That would be the best way to buy it even if the supplier doesn't actually manufacture both the VFD and motor.

A brushless DC drive package might be the best or only alternative below some power level.

CJC
 
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Yes, it can be done. I have used permanent magnet synchronous motors for that. The highest speed I have ran was 120 kRPM and power was then around 80 kW. Others have done better than that.

35 kRPM is typical compressor speed. There are companies specializing in compressors for trucks that build such drives. These speeds need some experience and knowledge to turn out right.

Gunnar Englund
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
 
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I am trying to construct an alternator test stand for different stator/rotor combination that I test. I was looking into VFD just because they seemed to be the easiest to control as far as rpms go. The VFD and motor set-up needs to be able to operate at rpms of 400 to 35000 and I have had some trouble trying to locate a good combination for this. The motor does not need to have a lot of torque I would just like to find something or configure something that will fit into my parameters and give me the needed stability of a good control device.
 
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You will need a VFD with capability for 590Hz output if the speeds are to be directly on the motor shaft. That will limit your selection of VFD's considerably.

If you can locate a speed increaser gearbox with 2/1 ratio, the motor speed drops to 17500 to 200rpm which is well within most VFD capabilities assuming a two pole special-built motor.

That won't be an easy gearbox to locate either.

I've got test cell motors running at 22,500rpm and 500hp but that's still a long way from 35000rpm.

If the kw or hp is small, you might want to check out manufacturers of woodworking spindle motors. Many of these work to 100,000rpm. There used to be a German company named Spindel that made these motors and drives.
 
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What usually happens is that the motor follows and runs with the new speed. I do not quite understand this question. Do you see a complication in changing the speed?

Gunnar Englund
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
 
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Depends on what you mean by "actual motor RPM". If you mean the nameplate value, you and the motor manufacturer can determine the motor's suitability for operation at higher than nominal speeds. I have several that we run at 105% of nominal.

If you mean it's running at X RPM and you move the speed setting to "Y" RPM, the motor will attempt to match the drive output, subject of course to a number of limitations.

You can, of course, juggle ramp times, etc., to lessen and shocks to the system.

old field guy
 
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lets say I have a 4 pole motor 1730 RPM, and on the Drive setting I put in 3480 RPM .....is that a problem??? is the drive actually sent out a different type of pulse to control the motor ?? or its just a parameter for the drive to make calculation for the display purposes ?
 
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Doubling the speed of the motor may cause mechanical problems. You need to check with the manufacturer. The drive will try to output twice the nominal frequency to achieve this speed. The motor will exhibit high current draw as it accelerates to the higher speed. Ramp times can soften this effect.

Assuming that he motor is able to stand the mechanical stresses, doubling RPM is going to radically increase the load. A centrifugal pump will require around four times the horsepower at twice the RPM.

old field guy
 
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Yes wedoca, the drive changes its output to make the motor run at the speed you set it to run. I do not understand your thinking at all. Do you think that it is all a simulator?

There is actual hardware. And the speed changes.

Running a standard motor at 3+ kRPM is usually not a problem. But you have to check balancing and bearings to make sure.

Noticed the use of capital letters? It makes reading easier. It isn't hard to master. Just needs some training. Adding full stops and apostrophes takes a little more effort, but it's worth it in the long run.

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

I am thinking that the VFD put out 0 ~60 HZ regardless of the VFD frequency setting and the frequency setting requaird, so that the VFD can calculate the motor operating RPM based on the output frequency ( 0 ~60 HZ ) and make such information avliable to the user.

please let me know if I am wrong

Thanks
 
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Wedoca-

No, in induction motors, the RPM is directly proportional to the input frequency. Therefore we have term VFD - variable FREQUENCY drive. The INPUT frequency doesn't change.

The VFD rectifies the AC input to make DC, then by means of clever electronics it chops up the DC to make the output look like AC, and it can control this "chopping" to make AC of varying frequencies.

If the drive puts out 60 HZ (US standard) then the motor will run at its 60Hz RPM. To get a different RPM, you instruct the drive to change its output frequency. The motor then runs at the RPM proportional to that frequency.

old field guy
 
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also, below the 'base frequency' of the motor, as the frequency changes the voltage changes too. This ensures optimum performance of the motor. Above the base frequency, the frequnecy will increase to whatever the max frequency of the VFD is, but the voltage will remain constant. Therefore constant voltage variable frequency but your torque capability will drop off. Depending on the application and the torque requirments, this may or may not be a problem.
Mechanically, the motor needs to be able to withstand higher speeds too. If you are doubling a 4 pole motor then mechanically the motor tends to be ok as the bearing strucutres are designed for 2 pole motor speeds.
 
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Wedoca said:
Lets say I have a 4 pole motor 1730 RPM, and on the Drive setting I put in 3480 RPM .....is that a problem??? is the drive actually sent out a different type of pulse to control the motor ?? or its just a parameter for the drive to make calculation for the display purposes ?
Click to expand...
As much as I don't like encouraging hijacking of threads, I didn't start it, so I'll chime in on my opinion.

I interpreted that you (Wedoca) are meaning that if you PROGRAM THE MOTOR PARAMETERS in a VFD for a higher speed than what the motor is actually designed for, is the output V/Hz pattern going to change?

If that's the question, the answer is; it depends on the drive. An older V/Hz (non-vector) drive will probably not change anything, but if the VFD display offers a scaled readout, i.e. RPM or FPM or GPM as some do, it will be incorrect. However if it is a Vector drive, open loop or closed lop, the vector algorithms are tied to a motor model and if you set the motor base speed (number of poles) incorrectly in the setup, most of the motor models will end up incorrect.

If that is not what you meant, then never mind.

And next time, please start a new thread and reference an existing thread if it is pertinent.


"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
For the best use of Eng-Tips, please click here -> faq731-376
 
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