I have seen two kinds of control schemes commonly used to start and stop motors from a control system such as a PLC or a DCS. One uses separate contacts for Start & Stop. The Start contact is pulsed Close to start the motor and the Stop contact is opened to stop the motor.
This uses two Discrete Outputs from the system.
The other uses a single N/O contact which is Closed to Start and Opened to Stop. This uses a single Discrete Output from the system. Obviously the second scheme halves the I/O required for operating motors.

I was hoping the illustrious members of the Automation List would be able to share their thoughts on which system is better.

Regards

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Sumeet Chimalkar

 

Your first example describes something similar to that discussed in NFPA Article 79, Electrical Standard for Industrial Machinery,
"www.nfpa.org":www.nfpa.org , which generally requires a stop signal, that is not controlled by software alone, with a separate start; in as much as
resetting the stop should not result in the startup of machinery. This should not be confused with your second example which describes more of a general fail-safe design.

[email protected]

 

Hi
We have a solid state latching control. Any dry contact will control it - like a reed, button, relay etc. It sends a very low voltage to the "control contacts" One contact to be N/C the other N/O.

The internal relay is rated at 240 V 8A so simply add a contactor if the motor is large.
go to page:
"http://www.almegcontrols.com/latching_relay.htm":http://www.almegcontrols.com/latching_relay.htm

Thanks Bob PS - the new model is orange with an LED on top.

 

>I have seen two kinds of control schemes commonly used to start and
>stop motors from a control system such as a PLC or a DCS. One uses
>separate contacts for Start & Stop. The Start contact is pulsed Close
>to start the motor and the Stop contact is opened to stop the motor.
>This uses two Discrete Outputs from the system. The other uses a single
>N/O contact which is Closed to Start and Opened to Stop. This uses a
>single Discrete Output from the system. Obviously the second scheme
>halves the I/O required for operating motors.

I prefer to have one input for connection to a PLC or computer. The two input system is convenient for manual pushbutton control. Many drives can be wired either way.


Bill