Eddy Current Brake - Vertical Load

[bernie_carlton]

I have been working with servo motors power a vertical axis by way of a linear screw actuator. When the load is lowered there are controls to deal with overvoltage generated by the load. In some the DC voltage generated is shared among multiple modules. In others the excess is routed to a dissipation resistor.

Could the excess instead be routed to a, normally non-powered, eddy-current brake on the vertical axis. Has anyone seen an application like this? It seems a logical use of the overvoltage generated by the load - to help slow that very load.

There is a holding brake within the drive motor but that is only for holding the load after it is stopped.

 

[DickDV]

That's an interesting proposal, bc. I don't see why that wouldn't work in principal.

I can see where, if the eddy current brake is too highly energized, you could get instability--lock up and release, lock up and release, etc.

The other issue would be voltage. Most EC brake exciters I've seen are 90VDC. It is likely that the brake resistor voltage would be higher than that.

Actually, that higher voltage might work out ok tho because, with a series resistor you could divide the voltage and as a result the intensity of braking on the EC brake.

Wish I was there to watch how this unfolds. I think you are on to something useful there.

 

[bernie_carlton]

I see the eddy-current brake action quickly reaching a steady state middle action.

I started on this thinking of the control on the take up capstan of inexpensive cassette tape drives. This is controlled by a little friction brake controlled by the tension of the tape. Too little tension releases the take up reel somewhat to take up the tape faster, too much tension brakes the take up reel slightly. This was also used in VCR's to control tension of the tape around the scan head. (I worked on both before getting into industrial control.)

This could also be used as a safety concept. If a contactor is released then the leads of the motor are routed through an appropriate bridge generating the DC to the EC brake causing a slow let down.

 

[DickDV]

Mmmm, bernie, that last about a de-energized motor won't work. An AC induction motor needs to be externally magnetized to get it to regenerate. An open contactor will result in the motor free-wheeling.

 

[bernie_carlton]

Actually I'm talking about servo motors which have permanent magnent rotors. So they would generate while being backdriven.