In reply to Matthias Mulder:

There are many ways to wire up a Motor or VFD, and some depend on the capabilities of the device itself. I'd say the main driver to a given solution is whether the Motor requires permissives to be met that come from the DCS. If the motor can be started locally at any time, you might see a local start that bypasses the DCS commands. The DCS would then reflect this capability in its configuration. If the motor has interlocks or permissives from the Process that should inhibit starting it in the field, then bypassing the DCS Stop command might not be wise. Field start could be wired to DCS to allow a field start based on additional conditions, such as local visual supervision to jog the motor.

Running the field start command through the DCS allows the system to record how it was started and to apply safeguards to the request where appropriate. Those interlocks can still come into play if the DCS stop command is not bypassed at the device. If the DCS closes its Stop Bit when interlocks and permissives allows allow for a start, then a field start would be possible only when conditions allow. A local Jog button, which must be held for the motor to run might bypass the DCS stop bit, but that would be a case by case feature.

An E-stop contact might be used to independently prevent running a motor, and allow normal operation from the DCS and local buttons. This would not be easily bypassed and would have to cleared if activated (usually normally open contact).

Add to this the ability to command motors and VFD's over a bus or Ethernet and the Field starts/stop functions have to be considered. Wiring to the device means it needs to handle your use cases for local start commands. Wiring to the DCS is still an option so the DCS can apply needed permissive and interlock conditions. Loss of communication events likely trigger time out based trips and conditions to restart once communications are restored. Lots of ways to solve this based on your specific use case.

Andre Dicaire