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Using VFD with standard duty motor

Using VFD with standard duty motor

Using VFD with standard duty motor

What are the draw-backs of using a standard duty motor with a Variable Frequency Drive as opposed to installing an Inverter Duty Motor?

Does the motor service factor get de-rated under this service.

The application is a 200HP I-R Rotary Screw Air Compressor.

RE: Using VFD with standard duty motor

The standard duty motor is designed for its standard speed(s). If the frequency drive lowers speed there is a cooling problem which may be solved by adding a blower motor. See
for example how that is done.
Another aspects are harmonics generated by the variable frequency drives. They tend to generate heat and vibration. Common mode currents and their impacts, e.g. on bearings. See
for more information on application of motor drives with some useful solutions to motors, feeders, grounding, etc.

RE: Using VFD with standard duty motor

This has been very helpful, but with increased cooling for the motor can the speed be lowered below 50% of nameplate speed. Also would I be better off using an Inverter Duty motor.

Also, nothing was mentioned about de-rating the standard motor service factor?

Thank you again, as I said your information has been very HELPFUL

RE: Using VFD with standard duty motor

1. The Lincoln Motors state in their posted (second http:) ADR9 - How to Build Superior Motors for PWM Drive Applications "Derating (using a larger motor) and restricting operation to certain speed ranges may prolong the life of standard motor. However, these means are inadequate in the face of increased demands for adjustable-speed drive performance." This means that you may run your standard motor below 50% of its rated speed; however, its life will shorten. It is much better to have a motor design for the inverter duty. This prolongs its life.
2. To derate the service factor is contrary to its definition, since the service factor is supposed to be used at the rated motor terminal voltage and at the rated frequency (NEMA MG-1 Standard). This is contrary to what the variable frequency drive does since it varies the frequency and motor speed.
Suggestion: The motor manufacturer should have its recommendations ready in this case since there are various industry standards, the motor manufacturers adhere to.

RE: Using VFD with standard duty motor

  jbartos is pointing you in the right direction. At 200 hp load talk direct to the motor maker. I am curious why you  would want to run an IR screw comp at a varying speed. I would suggest (and this is electrical speaking in mech field) that the compressor would be better run at fixed speed with some starting gear if amps are an issue. If the motor speed is wrong for the compressor talk to your favourite motor rewinder he may be able to help. It would keep your system simpler.

regards Don

RE: Using VFD with standard duty motor


Thanks for your input. The purpose of using a VFD on a rotary screw is to match KW consumption to actual air usage.
There are a couple of manufacturers who already make VFD designed units. Typical energy savings is in the 30% range. The problem is they use an inverter duty motor. My question involves taking a standard unit and converting it to VFD technology. In Europe 50% of all new compressor applications are of the VSD type control. It is the wave of the future!

Thanks again for responding.


RE: Using VFD with standard duty motor

Suggestion: The standard motor could be replaced by the inverter duty motor. If one is lucky, perhaps one could trade the standard motor for the inverter duty motor and pay the difference. Else, the standard motor needs to have effective cooling added, which may be more expensive than to pay the difference for the inverter duty motor.

RE: Using VFD with standard duty motor


On the surface, that would seem like the logical thing to do. The problem is that I-R uses a special extended shaft motor upon which the main drive gear is mounted on the end. I have search many motor catalogs for an inverter duty motor of this type with no sucess.

Next question is, how to determine how much cooling is enough to prevent premature failure? If one can save 20-30% of the total operating cost per year, it certainly is worth pursuing?

RE: Using VFD with standard duty motor

Suggestion: Let me put it this way: If this was my business, I would pursue the inverter motor choice. I would try to procure a coupling that would produce an exact replica of the motor shaft arrangement that you have now. The second choice will always have some noticeable risk since the motor was not designed for that duty. The harmonic effects inside the rotating machinery are fairly complex, and not fully predictable for the standard duty motor.

RE: Using VFD with standard duty motor


Thank you so much for your help on this matter. I have been researching this for one of my customer. After your help and the research you have provided, I am convinced that a standard duty simply won't work and would not be cost effective.



RE: Using VFD with standard duty motor

back again,
   but not with advice but questions - you say up to 30% cost savings on energy.
OK that's really got my interest our Atlas Copco is only 8 years old and around 125KW motor and costs a packet to run. (24hr operation)

  It uses the good old load / unload with shutdown after x minutes unload type control plan. According to its magic computer panel it has about 60 % duty cycle,
ie 60% load 30 % unload off time. Star delta start.

  Does this sound like a candidate for big savings.
What opinions guys? I,ve already put my foot in it so now I learn

Thanks Don

RE: Using VFD with standard duty motor


Funny you should ask? Atlas makes the first true vsd compressor, designed from the ground up. The savings are there. First off, you need to determine what it is costing you to operate your current machine. There is is a standard formula for calculating that. Next, you need to develop a load profile for your system. This is a little more involved, Atlas Copco distributors have access to a device called a motor box, which will monitor your operation and determine your patterns of usage. With this information in hand, that can be downloaded into a software program (also provided from Atlas) to develop a stratagey for savings.
You may add tanks, use a smaller machine as base load and trim with a VSD or simply run a VSD. It will handle alot of different scenarios to determine if ther are savings to be had.

By the way, if you haven't figured it out yet, I work for an Atlas Copco Distributor.

Please feel free to contact me anytime.


RE: Using VFD with standard duty motor

  Armed with this inside knowledge I will pester our local agency
  ? does it matter if the compressor is upside down.?
   we are in Australia and we think some stuff from up there isn't designed to run at the bottom of the world.
all the best for the new year  to all


RE: Using VFD with standard duty motor


Always loved that Aussie humor!

Good Luck, if I can help you in any way, let me know.

And a Happy New Year to you as well.


RE: Using VFD with standard duty motor

Although I do not claim to be an expert in motor issues, one of the principal challenges in using a std drive as a VFD is the possibility of a torsional resonance problem, and within the stated range (less than 50% speed), it sounds like a distinct probability of occurrence.

The torsional resonance may be encountered upon start-up of the drive, but the transient nature of start-up makes it a relatively innocuous item. Operating in its range (if it does exist for your motor) could wreck the motor, the shaft, the bearings and lose production. As a reliability expert, I think that the savings are worth pursuing BUT retrofitting a design to do so may yield a Frankeinstein machine.

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