Re: dc motor control help please

Jul 14, 2003 1 Replies
D

Howdy,



Well I have the same motor and I think you may encounter a probably in trying to stop it at specific steps. The motor controller I use changes the speed and allows me to alter the speed range but no matter what speed it is running at the motor will coast for a short period of time until it reaches a lower speed. I think if you want it to stop at several specific points you may have to use some kind of electric clutch.



Somehow you have to have the lift disengage from the motor drive to overcome the



coasting effect of the DC motor.



Is this any help?


snipped-for-privacy@execul> Hi - I am the proud new owner of a 90V Leeson DC 1/4hp motor and KB



electronics control for fun and hobby. I'm really green about this stuff
> and I need some info for safety reasons.
>
> The Project: I want to use the motor to make a lift that will go up and down
> and stop at any point in between.
>
> The controller manual says I can perform rapid switching on the AC inputs
> without damaging anything. Although, It says I cannot perform this with the
> DC outputs - else I can expect catastrophic results - whatever those would
> be. OK, then without having to purchase a different controller how do I
> make this controller work? I need the motor to go CW and CCW using
> hopefully a rocker switch of some sort. Sounds like I might have to have 2
> switches, one for the AC on/off and one to toggle the DC polarity when the
> AC is off. Sure would be nice to only use one rocker switch though.
>
> Regarding safety, do I need to mount the switches in a special box or have
> specially rated switches, etc.? I don't want to get bit.
>
> Any help is appreciated
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F

It looks like what you have is what is called a non-regenerative, two quadrant controller, probably using armature voltage feedback. You'll not get the kind of close control you need out of one of those, they're for controlling conveyor belts, and things that need to run at constant speed in one direction.

What you need is a four-quadrant, regenerative controller, which will actually apply reverse torque for braking. you'll also get reversability with one of those, without switching. You really nead proper speed feedback (tachogenerator) to get best performance, though it can be done without.

With the controller you've got, you're stuck with coast stops, the more inertia you have, the worse it gets.

You *could* try this (maybe):

In sequence:

Shut off the AC supply

Using a changeover contactor, disconnect the controller from the motor, then slug a damn' great power resistor of a few ohms across the motor armature terminals. The motor is now a generator, and its sored energy (flywheel effect), is being dissipated in the resistor (It'll get hot). The motor will slow down at a rate determined by the value of the resistor, the fewer ohms, the faster it slows down, but don't exceed the maximum rated armature current.

When you've reached the state you want (lower speed or stop), reverse the above steps.

I didn't tell you that! If you kill yourself or burn the building down, it's YOUR fault!

Anyway, the switchgear will cost more than a proper controller.

Bring back Ward-Leonard sets, that's what I say :-)

Then there's duct tape ... (Garrison Keillor) nofr@sbhevre.pbzchyvax.pb.hx
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