VFD instead of Servo Amplifier


Thread Starter


I’m thinking about updating the controls on a manual hoist and converting it to an automated hoist. Currently, the hoist uses Variable Frequency Drives (VFD’s) for the lift and travel motors. I was wondering if it would be possible to control the VFD’s with a +/- 10V signal from a servo controller? (I like the thought of having a servo controller worry about the positioning of the hoist rather than me trying to code it into a PLC using a high speed counter card) I don’t need the extreme accuracy of a full servo system. Positioning the hoist to within + or - 1/4 inch would be fine. I was told that it could easily be done, but I always thought that a VFD doesn’t have the response needed to work in such a setup. Does anyone have any thoughts on the matter? Thanks.
Guess it depend on the capabilities of the VFD. I wouldn't have any qualms doing it using an ABB 'DTC', Control Tech Unidrive, or other similar drive. On the other hand, don't think I'd try it using a cheap 'pop-corn' VFD.


Steve Myres, PE

This is commonly done in the metal finishing industry for batch lines. You'll want a good quality vector type drive, possible one which accepts direct speed feedback. I've done this a number of times with very satisfactory results.

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Mike Molesky

I completed a 4 axes crane VFD system with 2 of the axes using encoders and the other 2 using LASER feedback. The system has been running for approximately 9 years. The PLC controls all 4 axes. You should have absolutely no problems using some of the new vector drives. 3 of the 4 axes are accurate to within +/- .1 inches and the other is +/- .3 inches and this is mainly because of mechanical limitations.

Yes, it can work just fine. In fact, you may be surprised how well it works. Especially since you should be in full vector mode, since by design, you should have an encoder feedback on the motor.

Obviously, by my prior statements, I would recommend a vector drive. I am not sure of the performance with a standard V/F drive.

Bill Sturm