switch off motor with limit switches

[geekEE]

Hi all,
I have a low-speed motorized turntable that I would like to modify by adding a couple of limit switches (maybe reed switches?) that will kill power (120VAC) to the turntable if it hits either switch. Additionally, I would like to have a push button that will let me override the system and restore power to the motor, even though it is actuating one of the limit switches. Maybe a buzzer for the fault condition would be helpful too. This is to prevent damage to the equipment. The operator is not endangered by a failure.

This seems like a very simple application and I’m only building a couple of them. Would a smart relay be appropriate for this? Or is there something else that I should be looking at? Do you have any recommendations on specific manufacturers and models?

Thanks in advance.
Glenn

 

[jraef]

Not sure what size motor you are using, but things like limit / reed switches do not have much capacity for direct motor power switching. I suggest using a relay or contractor with the appropriate HP rating for your motor to switch the motor on and off, then let the limit switches turn the relay on and off. That then also makes it easy to use another manual switch to override the limit switches.


"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington

 

[zeusfaber]

Adding to what Jeff said:

The fact that you mention "a couple" of limit switches suggests that you have a motor which can move in either direction.

With that in mind, I'd consider using relays to do the main switching (including direction-control, not just limit sensing). The beauty of this approach is that you can design so each limit switch inhibits travel in one direction only, making it possible to get your turntable off the stops without having to invoke any over-ride systems.

Have a look at the way automotive electric window controls are designed.

I would have thought a smart relay was overkill for a job like this.

A.

 

[geekEE]

Unfortunately I don't have such direct control over the motor. It's already installed, and it's a closed system where I can only send serial commands to it to tell it the direction and speed. I just want to add a fail-safe way to kill power in case of computer bugs, crashes, etc. I'll have to see if I can do it with simple relays. Maybe I was just interested in the smart relay because it sounded like it'd be fun to learn something new.

 

[Skogsgurra]

That's one of the best reasons! Just do it.

Gunnar Englund

http://www.gke.org

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

 

[squeeky]

If your access to the circuit is via the power side, you can still run your relay into the power circuit and limit the power relay via your limit switches. You will have to watch the overshoot from the inertia. The activation section may have to be extended to cover the overshoot or it may start up again. A normally open push button wired in parallel with the "reed switch" will allow it to by-pass the stopping device.Again this should be to the power relay control side.