Torque measurement

[Guardiano]

Hi,

I am using the feedback values of power and speed from my VSD to calculate torque from the relationship P=Tw. I don't see the need to buy a strain gauge because the torque obtained mathematically from this equation is a direct measure of the output torque.
Am I missing something?

Guardiano

 

[Skogsgurra]

That is basically correct. But if you want to see torque transients in the millisecond and lower timespan, you usually cannot do that because the torque and speed information in a standard VFD are not updated very often. There may be 10 - 100 ms between updates. But if you do not need better temporal resulotion than that, you will probably be fine.

You should be aware of the losses in the motor. If P is input power to the motor, you will have an inaccuracy that you may need to compensate for.

Most VFDs have a calculated torque variable available in the system. Have you looked for that?

Gunnar Englund

http://www.gke.org

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Half full - Half empty? I don't mind. It's what in it that counts.

 

[Guardiano]

Thanks Skogsgurra for your comments. I understand the motor losses but what about the gearbox efficiency?

Guardiano

 

[Skogsgurra]

Depends a lot on what kind of gearbox you have. Worm gear are usually very ineffifient. Normal gears 5 - 10 % loss per stage. You have to ask the manufacturer.

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[Guardiano]

The gearbox is a planetary one. Is there a maximum speed input to a gearbox? Most gearboxes I have seen so far have a limit of 1800 rpm.

Guardiano

 

[BrianE22]

Yes, different gearboxes have different speed limits. Smaller units can usually go faster. Check with the manufacturer.

Planetaries are fairly efficient. You can count on at least 90% efficiency per mesh.

For your torque measurement be aware that during acceleration the amperage will be higher (your output torque will be less than calculated).