Change in motor control method
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Our company currently controls the motor in the following way: the normally open start contact from the DCS’s DO output is connected in parallel with the normally open start button located on the factory floor, to the auxiliary normally open contact of the contactor; whereas the normally closed stop contact from the DCS’s DO output and the normally closed stop button on site are connected in series within the contactor’s control circuit. Thereby enabling remote and local control of the motor. Now I want to use the normally open DO contact of DCS in series with the control circuit of the contactor, in order to achieve control over the start and stop of the motor. The start/stop button at the site is connected to the DCS as a digital input signal. The control logic is programmed into the program. As long as the start variable on the DCS interface or the on-site start button is activated, the motor will start and engage in self-locking via its operation feedback; the motor stops when the on-site stop button or the stop variable on the DCS interface is activated. If the motor load is overloaded, the thermal relay activates, the contactor trips, and the operation feedback turns OFF; this results in the output being turned off. Even after the thermal relay is reset, the motor will not start. The advantage of such an improvement is that it simplifies the control circuit of the contactor; the buttons on site are not exposed to high voltage, and there are no three-wire control cables coming from the power distribution room as before. Of course, the button input signal for the two signals has been added. Currently, the workshop is starting to install 24-core distribution boxes that come from the control cabinets; one 24-core cable can supply 12 connection points, and one distribution box can provide 32 connection points. This makes it very convenient to connect buttons from the on-site distribution box. At the same time, my workshop is a pilot plant, and the utilization rate of the equipment there is very low, usually around 20% at most. In this way, by making changes in the program, the same button can be re-associated with motors on other devices, and then a new label can be attached to it. This allows for the full utilization of the DCS DI points and buttons. For frequency converters, contactors are not required; simply connect the DO output to the control terminal of the frequency converter.
Personal opinion, for reference only: 1. It is not recommended to use the on-site manual start/stop buttons to control the motor via the DCS system. The start/stop buttons on the on-site control panel are connected in series or parallel to the control circuit of the motor contactor; operating the motor independently using these buttons without relying on the DCS system constitutes direct control of the motor’s control circuit. This approach ensures that the motor can be stopped by pressing the stop button in emergency situations； Theoretically, it is possible to connect the start and stop buttons on the control column to the DCS as the conditions for starting and stopping. But in the event of failures in the DCS’ IO cards or relays, can it be ensured that the motor will stop? There are things where more automation isn’t necessarily better. 2. For thermal relays, it is generally recommended to connect their normally closed contacts to the control circuit of the motor’s contactor coil in order to provide overload protection; they can also be connected to a DCS system for the same purpose. It is advisable to use manual reset for thermal relays (it is safest to perform a manual reset after checking on-site for the cause of the overload and ensuring that there are no issues with the wiring), as automatic reset is not recommended.