I have an application that requires torque limiting and I need to determine if VFD torque limiting will provide sufficient protection. I have used VFDs for torque limiting through gearboxes before with success, but I am hoping to find a way to calculate the result as opposed to just gut feeling.

10HP motor
1775 rpm
69.5:1 gearbox reduction (im assuming 50% efficient)
25.5 rpm at driving sprocket

I have looked at some frequency drives to get an indication of their torque/amp sensitivity. Ive seen 0.1A, 0.01A, 0.1% of torque and 0.01% of torque as control resolutions, but have not found any literature specifically addressing if this resolution can be reasonably expected.

In this situation I am gearing down, so inertia, backlash and controller reaction time will not be an issue.

  • $\begingroup$ If you are driving a sprocket at 10 hp, the motor needs to supply 20 hp if the gear is 50% efficient. $\endgroup$
    – user6431
    Commented Jul 31, 2016 at 16:40
  • $\begingroup$ @CharlesCowie Good catch, I will fix my numbers. $\endgroup$
    – ericnutsch
    Commented Aug 1, 2016 at 19:13

2 Answers 2


Induction motors don't have a consistent current vs. torque relationship. In order accurately control or limit torque, VFDs must determine an equivalent circuit for the motor then monitor and control the electrical parameters that determine the torque. I believe that VFDs that have a good torque limiting capability will have a specification stating the accuracy with which they can do that. They will only be able to state what can be done at the motor shaft. The variations in torque lost in the gearbox will significantly degrade the torque limiting or controlling accuracy. I suspect that finding out what to expect from the gearbox will be the most important and most difficult task in this situation.

Can you state what VFD models you have experience with and/or are considering for use in this situation?

Re link in comment.

If you click the "Specifications" tab, you will see "PowerFlex 700 AC Drive Technical Data," a document that you should download. On page 4 of that document, you will find:

enter image description here

You might find another manufacturer that can do better, but I doubt any will do a lot better. When a speed-controlled drive goes into torque limit, the motor will slow down until it reaches an operating point where the limiting value of torque is sufficient to drive the load. If no such point is reached, the drive will be at a standstill. At that point, the torque should not exceed the setpoint, but it may be less than the setpoint.

Re Answer Posted by Asker

I went through your answer, made a diagram and added some notes as shown below. The way I interpreted your numbers, my calculations gave slightly different results, but I put yours on the diagram. If you you used "best estimate" losses, I would suggest that you also do the calculations using the highest and lowest losses that you think might be possible. I believe that the lowest losses will show the smallest margin between normal operating torque and the failure level torque.

It seems to me that you might get better performance by sizing the drive for the maximum desired operating torque. You calculations seem to indicate it is sized at the failure level torque.

enter image description here

  • $\begingroup$ Considering: ab.rockwellautomation.com/Drives/PowerFlex-700 $\endgroup$
    – ericnutsch
    Commented Jul 30, 2016 at 3:14
  • $\begingroup$ Thanks for your help Charles. I awarded you the bounty, but i think i may go ahead an answer my own question to see if i can get some feedback on my solution. $\endgroup$
    – ericnutsch
    Commented Aug 1, 2016 at 19:06
  • $\begingroup$ Thanks for the bounty. If you post a comment after you post your answer, I believe I will get a notice that will remind me to come back read it. $\endgroup$
    – user6431
    Commented Aug 1, 2016 at 20:40
  • $\begingroup$ My solution is posted, let me know your thoughts. $\endgroup$
    – ericnutsch
    Commented Aug 1, 2016 at 22:00
  • $\begingroup$ I couldn't see your answer while typing mine. I now see you used worst case gearbox efficiency. I would also suggest doing the calculations with best case efficiency. $\endgroup$
    – user6431
    Commented Aug 2, 2016 at 1:29

Lots of assumptions. Some gearbox data and empirical motor current data would go a long way to predicting the performance.

      Sensorless vector mode
      Startup torque will be ignored by the drive
10     hp Powerflex 700 VFD
10     hp motor
1775     rpm
29.6     ft lbs
69.5     gear box ratio
200     ft lbs assumed constant gearbox torque consumption, referenced on output
50%     worst case planetary gearbox efficency (in additon to constant)
26     rpm @ driving sprocket
928     ft lbs @ driving sprocket
95%     estimated chain efficency
19     rpm @ driven sproket
1176     ft lbs @ driven sprocket
      Full 10 hp has previously damaged mixing rotor
1176     ft lbs Estimated emperical damaging torque
700     ft lbs, desired torque limit
553     ft lbs, pre gearbox, desired torque limit
18.8     ft lbs seen by motor, desired torque limit
500     ft lbs Estimated Normal opperational torque, (motor needs amp'd for emperical data here)
395     ft lbs, operational torque
14.2     ft lbs seen by motor, operational torque
15.4%     Torque as range of total

0.1% percent of motor rated toque, torque resolution. It is not clear if this requires torque feedback or not. Page 22 of powerflex700 programming manual
No closed loop torque sensor will be used as they are prohibitively expensive

Worst published torque regulation on the drive is +/-5% in the powerflex700 technical data

15.4% > 5%, VFD torque limiting will be satisfactory in this application


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