Turn the SAME motor On/Off from two seperate controls - Logic Help Needed?

[panic mode]

Nobody has mentioned what happens if a normally closed start button gets shorted.

i guess this was a typo, it should read 'normally open start button...'
i agree that use of latches is not good idea, sealed contact is way to go.

the off buttons don't need one shots (i prefer when they are used sparingly - just like latches).

holding stop button down should never be the only way to stop the machine, this is what E-Stop is for...

 

[danny.michael]

Thanks for catching the typo. It should have read "normally open start button".

Not all motors have E-Stops. If you're thinking "machine", then yes, there is usually an E-Stop. But in the process world, pumps, agitators, blowers, etc., do not usually have an E-Stop. In those cases, if a normally open start button shorts, the only way to stop the motor is to hold in the stop button (many start/stop stations have a way to lock the stop button in) or open the MCC breaker.

I keep going back to the original question: How to control a motor from two different locations, hard wired field buttons and an HMI station, and continue to use one location if the other fails in any manner. The original poster didn't ask if this was a good idea, just how to accomplish it. The logic in my post is one answer to that question.

To reiterate, I do not think controlling a motor in this way is a good idea, although I have no idea what the motor in question does. For all we know, it could be a small motor in an application that poses no safety hazard if it cannot be stopped at will.

 

[pouch]

Is is not a reassuring that a discussion on how to control something will soon revert to the safety aspects of that system.

On 99% of the installations I am involved in the motors have isolators with early break contacts in the stop circuit. This gives the oportunity of stopping the motor by turning the isolator off.

Regards

Pouch

 

[ToolGuyFred]

Is is not a reassuring that a discussion on how to control something will soon revert to the safety aspects of that system.

On 99% of the installations I am involved in the motors have isolators with early break contacts in the stop circuit. This gives the oportunity of stopping the motor by turning the isolator off.

Absolutely. Anything safety critical should be hard-wired into an emergency stop circuit which overrides the PLC control. I also understand that local motor isolators are a requirement of the latest wiring regs.

As stated in posts above, good programming and PLC practice also suggests that a N/C stop pushbutton is a good idea. I would also suggest that all safety critical outputs are unlatched on e-stop. I usually put mine inside an MC-MCR pair.

 

[rsdoran]

I apologize, the 2 stop buttons must be in series, they will not work as I have them in the picture.

The best thing, IMO, to do is to monitor both stops in separate rungs and trigger an alarm if either fails.

AS mentioned it is a "safety issue" even if it is just a simple motor ckt.

 

[Paul T]

danny.michael wrote:

But, the original question was how to continue to operate the motor from one location if the hardware in the other location failed.

Ha. You know, you're right. I was just thinking local/remote control.

If the question is "How do I arrange it so that if one side fails the other can override it" the answer is "You don't." If one side fails, you shift to the working side and fix the broken one, but you never ever set it up so that a Stop button has no effect when used. While it can be done (as demonstrated several times) it shouldn't be done.

If you can "override" the local stop button signal by simply pressing "Start" on the HMI, then you might as well save the money on the local Stop because it might as well not be there. I go out to maintain the machine, I swing in the latch to lock down the Stop button and somebody in the control room starts the motor anyway? No thanks. There are a bunch of ways to handle local/remote and lockout, but anything like what the orginal post had- two real inputs and two HMI inputs- is not one of them.

 

[danny.michael]

Since the focus of this thread has shifted to safety; a stop button should never be a lockout point. A motor lockout should remove the possibility of voltage being applied to the motor. The MCC breaker should be locked, or fuses removed if the circuit is so equipped.

To carry this further, should all devices which have a stop button on the HMI be forced to stop if communication is lost with the HMI? Is this not the same scenario as a field stop button that has no effect when used?