OT: Motor / VFD question

naegely

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Join Date
May 2008
Location
Florida
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I have a VFD controlling a 2 HP motor that is not inverter duty. I have been told that this is not cause for too much concern in low HP motors. Can anyone confirm that? Is there any rule of thumb for what size motor must be rated for inverter duty if running on a VFD?

Our quality group is requesting that I replace the motor, and I don't want to do that if there is no benefit.

Thanks for any help.

Rob
 
A motor that does not have the correct insulation class to withstand the peak to peak voltages of the carrier frequency will break down sooner than a motor with the sufficient insulation rating.

Also, the cooling of the motor may be a factor if it is running at less than its base frequency.

Depending on these factors and the difficulty of/caused by replacing it, you may just want to have an inverty duty motor on hand for the future.

Just last week we replaced a motor with insulation class B running on a VFD that lasted almost a year and a half. it was a 1HP running at 50Hz.

There are some very good experts on this forum with respect to motors and drives. They can probably offer more detailed information.

Paul
 
Ron
You can run a non inverter rated motor on a VFD. The question is for how long? The reason is that because of voltage spikes caused by VFD on motor line the insulation can break down in non inverter rated motors. Inverter rated motors have a "better" insulation in them.

If you have only the one I would leave well enough alone. If you have a bunch of 2 HP all identical I would buy a spare inverter rated motor.
Dan Bentler
 
A little more information is needed to properly answer this question.

First, what voltage is this system running at? If 230V and the motor does not run below 15hz at full load, there is no problem. If 460V, go on to the following questions.

Second, how long are the motor leads. That's the wire length between the drive output and the motor. If less than 10 feet, the motor will be ok if not fully loaded below 15hz. If more than 10 feet and the motor is Insulation Class B, replace it immediately or install 5% motor lead reactors to protect it. If Insulation Class F but not inverter duty, it will be ok out to 40 feet. If more than 40 feet, replace it immediately with a motor with Insulation Class F and an MG1 Part 31 endorsement (that would make it an inverter duty motor) or add the motor lead reactor. If more than 60 feet, even an inverter duty motor is in trouble. Add a motor lead reactor if the leads are between 60 and 120 feet. Use a dv/dt filter instead if the leads are between 120 and 180 feet. Beyond that, you need a sine filter in the motor leads (expensive!).

Third, if the normal operation of the motor is fully loaded below 15hz, then an additional issue is motor cooling capacity at these slow speeds. Within the above rules, you will also need to choose a TENV motor or one with an auxiliary cooling fan (that would be a fan NOT operated by the motors shaft but by another motor running constant speed).

The above is based on the following assumptions: The voltage is not more than 500VAC and the drive PWM or carrier frequency is not higher than 8khz.

Sorry but this is not a simple question to answer. Hope this helps you.
 
We also found that there is pitting of the bearings in non-inverter motors on VFDs (in our application). Changed to inverter rated motors and the problem went away.
 
Carrier Frequency is the rate which the output switches in the VFD chop the DC bus to creat the PWM pulse pattern that goes to the motor. The frequency for this varies from 1khz to 8khz with some drives capable of going as high as 18khz.

The higher carrier frequencies make the motor less noisy and the shaft rotation at very low speeds a little smoother but make for a lot of additional stress and deterioration in motor insulation. A good rule to follow: Use the lowest carrier frequency you can get away with.

As to shaft currents and premature bearing failure, an inverter-duty motor is as vulnerable to this as a commodity motor. To prevent this kind of bearing failure, I recommend the shaft grounding ring made by Electrostatic Technologies in Mechanics Falls, Maine or a similar device made by Parker Inc.
 
dickdv
can you explain the effect of cabel lenth,what about type and cross section area of the cabel,i always use non invertor motor in upgrading asphalt plants , the distance about 70 m
it work good from three years till now?
 
DICKDV

What are the steps that i should take when trying to determine what carrier frequency i can get away with ? When you say higher carrier frequency provides less noise do you mean audible or electrical noise ? Sorry if these seem like dumb questions but i have not been able to find mech detailed info on this anywhere on the internet or other sources.
 
DICKDV

What are the steps that i should take when trying to determine what carrier frequency i can get away with ? When you say higher carrier frequency provides less noise do you mean audible or electrical noise ? Sorry if these seem like dumb questions but i have not been able to find mech detailed info on this anywhere on the internet or other sources.

Well I am not Dick just Dan.

If I recall correctly the main consideration on choosing a higher carrier freq is the audible noise emitted by motor.

18 KHz is at the upper bound of human hearing. Especially so if there is any noise induced hearing loss (upper freqs generally the first to deteriorate -- especially so in industry)

So if you do not want to have employee complaints about motors whining then go to a high freq. If that is not a problem then go to lower freq.


Dan Bentler
 
As to carrier frequency, the motor noise I'm refering to is audible noise. On PWM power, motors tend to squeal and sing and if that motor is in an air duct in a hospital, the noise will be most unacceptable. If it is on a factory floor next to a stamping press, the motor will not be heard regardless of carrier frequency. Thus, I say, use the lowest carrier frequency the application will tolerate. Higher carrier frequencies shorten motor life somewhat, sometimes force a derate on the drive output capacity, and definitely increase radiated electrical noise (EMI and RFI). I have never found any benefit at all with carrier frequencies above 8Khz.

Drive/motor lead lengths are another rather complicated subject. As a general rule, longer motor leads, smaller hp ratings, and higher power supply voltages make the insulation stress worse.

In a 460VAC environment, motor leads on a 10hp motor of 60 feet or more and on a 100hp motor of 250 feet or more result in voltage pulses often approaching 1400 volts. I don't have specific data on other voltages but I think it would be logical to assume the same proportional increases occur. If your motor is built to withstand these levels of pulses, you will be ok. If not, well........

In view of the above, a NEMA motor rated 460V has to pass a test with high frequency pulses of 1600V to get an MG1 Part 31 endorsement. A conventionally rated motor is tested at 1200V sine wave.
 
Dick

I have looked for a definition for
Inverter NEMA MG 1 Part 31:
100:1 Variable and 12:1 Constant Torque

The part I do not understand or better yet understand what it really means and how to apply it regarding motor selection is
100:1 variable torque and 12:1 constant torque.

Can you refer me to a good source that defines these?
Does NEMA MG 1 part 31 define and explain it well and should I just break down and spend the bucks to get a copy of MG 1??

Since I am teaching a class on implementing VFD and 3 phase motor to electric vehicle I better understand this to give better info to students.

Thanks
Dan Bentler
 
Dan, MG1 P31 is primarily a testing specification for motor insulation. Of course, actually reading the spec would be the best way to get all the painful details but, to summarize:

1. The test is to be done at 1600V on a 460VAC motor
2. The power is to be rectangular pulses, not sine wave. These pulses have to be with a very fast rise and fall time so they simulate the rapid switching behavior of IGBT's. These steep rise and fall times define the high frequency components of the pulse, as well.
3. The spec also requires a higher temperature grease for the bearings since motors on PWM power tend to run hotter.

I must confess that I have never read the spec either but, based on what I've been told by people I trust at Reliance and ABB, the above is pretty much the "core" of MG1 P31.

Good luck teaching this material. I do the same in three-day seminars around the country on a part-time basis for NTT in Centennial CO. Aside from making a living, I find it really satisfying to help others (especially young people) learn this stuff. Best wishes!
 
Dan, as to the 100:1 VT and 10:1 CT, this is really the motor manufacturer defining the cooling capacity as the motor slows down. The terminology is terribly imprecise.

VT or variable torque in this context actually means decreasing torque as the load speed decreases. In practice, this means centrifugal loads such as centrifugal pumps and centrifugal fans. Sometimes you will see the statement that these loads follow "affinity laws" which simply means that the torque falls by the square of the speed and the hp falls by the cube of the speed. A motor connected to this kind of load can be run down very slow because its load torque drops off to nearly nothing below about half speed (.5 x .5 x .5 = .125).

CT or Constant Torque is a load that requires the same torque as it slows down. Examples would be conveyors, hoists, and many industrial machines. With these kinds of loads, the motor has to develop its full torque as it slows down generating essentially the same amount of waste heat regardless of speed. If the motor depends on its own shaft fan (as in TEFC or ODP motors), you rapidly get to a point where the motor cannot cool itself anymore. That would be the slowest speed at which constant torque loads could be operated continuously.

These speed limits are usually expressed as a ratio (100:1 or 10:1 or 4:1). At 100:1 a 60hz motor could operate down to 60/100 or .6hz, at 10:1 it would be 60/10 or 6hz, and at 4:1 it would be 60/4 or 15hz.

Does that do it for you?
 

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