Hello Everyone,

My aim is to operate a 5HP, 3 phase motor through a microcontroller. The motor currently employees a DoL ( Direct On Line ) Starter something similar to this.
There are 2 ways I found this could be done.
1. Have two relays, NO for start button & NC for Stop button.
2. Replace the DOL starter with contactor with 24V coil, like this one.

In both the cases I couldn't find schematic for a good protection circuit. My most important concern is that if suppose microcontroller gets hung & relay fluctuates then how to protect the motor.

I would want to know, which is the most reliable way of doing it, also if there is any other better way of doing it please suggest.
Regards,
Chaitannya

 

Hello Everyone,

My aim is to operate a 5HP, 3 phase motor through a microcontroller. The motor currently employees a DoL ( Direct On Line ) Starter something similar to this.
There are 2 ways I found this could be done.
1. Have two relays, NO for start button & NC for Stop button.
2. Replace the DOL starter with contactor with 24V coil, like this one.

In both the cases I couldn't find schematic for a good protection circuit. My most important concern is that if suppose microcontroller gets hung & relay fluctuates then how to protect the motor.

I would want to know, which is the most reliable way of doing it, also if there is any other better way of doing it please suggest.
Regards,
Chaitannya

There are many ways to implement what you're trying to do. But first you need to clarify what you mean when the "microcontroller gets hung"

If your meaning is what I think it is, then you could implement an external 555 missing pulse detector circuit that would be fed from the MCU during normal operation. If the MCU somehow becomes unresponsive, then the routine in charge of sending pulses to the 555 circuit should stop working too. That's when a third circuit should cut power to the starter/contactor.

As for fluctuations in the relay, I'd consider using a triac to activate the contactor, not a relay.

 

That starter IS your motor protection. I don't know why you would need to install the starter and then add protection.

I would go with 2 relays. Install a NC relay in series with the stop button and a NO relay in parallel with the start button. Then your MCU controls the starter in the same way as a human would. Pulse the start relay to start, and it stays running. Pulse the stop relay to stop. And a human can still start/stop the motor if whatever strange failure you envision occurs.

 

In both the cases I couldn't find schematic for a good protection circuit. My most important concern is that if suppose microcontroller gets hung & relay fluctuates then how to protect the motor.
I would want to know, which is the most reliable way of doing it, also if there is any other better way of doing it please suggest.

When I control a motor contactor via a PLC I use the unused N.O. contact on the O/L that is normally fitted between contactor and motor to detect an over-current/overload.
If the status of the contactor itself requires monitoring I use one of the aux contacts typically found on the contactor itself.
Max.

 

There are many ways to implement what you're trying to do. But first you need to clarify what you mean when the "microcontroller gets hung"

This is what we had observed in case of PIC microcontroller, in case of a EMI surge the microcontroller used to get hung & output pins were in tri-state, so the relays use to switch on/off very quickly ( like 10 times in a sec)

 

If using an AC coil ensure a snubber is fitted to the contactor coil, if DC, a diode rectifier is used, this also applies to any control relays.
Max.

 

This is what we had observed in case of PIC microcontroller, in case of a EMI surge the microcontroller used to get hung & output pins were in tri-state, so the relays use to switch on/off very quickly ( like 10 times in a sec)

Oh! ... I've gone through that nightmare before... Max is spot on, first thing is add snubbers to all coils, then caps to in-circuit mcu's, and then emi filters to the lines feeding the troublesome contactors. Sometimes even an isolation transformer is needed to completely exorcise those demons!

 

You’ll absolutely want to opto isolate your micro outputs/inputs. You will also want to address appropriate shielding. Your opto can fire a small local relay, which in turn drives the separate control relay, which is now part of the motor circuit. Control voltage, interruptable by a safety switch, is passed through this control relay to drive the motor contractor. Feedback from aux contactors is more of a problem, in that transients must be cleansed before the input opto.

Motor protection is accomplished with OC and O/L devices. The latter interrupt the control voltage.