First of all, I'm not a expert on this topic so.
so instead of interfacing the PI with the motor, can i interface it with PLC for turning motor on/Off. We currently have a PLC but its not too intelligent.
That is also not correct. The PLC need to control everything. As an example what happend if you don't. You set the temperature to 250C. And all the sensors are connected to the Rpi. And the Rpi drive the PLC (motor). The Rpi decide to turn on/off* the motor. And an x amount of time the Rpi crashes, dies, restart, ect. And your motor stays in that last state forever. Because the PLC sees that his input is still high/low*, and doesn't know the other imported parameters to drive the motor.
If a PLC controls everything, you can program it in such a way, or buy specials parts, that it is always in a safe state is something goes wrong. Here is a nice document
. And also here
And yes, this is not a easy task if you are not familiar with it. However, like I said before. If you don't have the knowledge about this subject**, hire someone that does have the right knowledge about his topic and also knows how it is legally structured in your country. I can not give you all the information, because I don't have all the information about your project and the topic.
*what ever is the worst case for safety
** I think you don't have the correct knowledge about this topic, based on the questions you are asking.