Arching of a Triac

[gauravkothari23]

Hi All,
I am trying to drive a 120W AC gear reduction motor 220V single - phase speed control motor forward and reverse motor (Image Attached) using MicroController. Have attached a circuit diagram of Motor Driver where i am using a Traic to switch the motor ON and OFF and Relay (OMRON G5LE-14-dc12) to make the motor move in forward and reverse direction.
The problem what i am facing is many a times when making the relay switch ON to OFF or OFF to ON, the relay contact sticks to its position itself and does not goes OFF or ON. it goes ON or OFF only after tapping on the relay. As i am aware that the problem is due to Arching when switching the Relay ON/OFF. but what i have done is
For Example.
if Relay is in ON Position. and when i need to switch the relay in OFF position. i initially switch the Triac OFF, wait for atleast 1 second to get the motor completely OFF, so now there is no current present in the relay and then Make the relay in OFF Position and then Switch the Triac ON again. so i guess in this case the Arching will not take place. so why does the relay sticks to its pole. The motor current i have measured was around 2 Amps at full Load.
I guess i cannot even use snubber for Relay contacts in Both C-NO and C-NC pins. because when any relay contact is open, the snubber would still provide a small amount of current to Motor Capacitor which might make the motor malfunction.
So can anybody please provide me the solution for the same.

 

[dick_freebird]

A proper snubber should not pass any DC current.

 

[gauravkothari23]

A proper snubber should not pass any DC current.

but i am working on AC current.

 

[betwixt]

Break the problem down:

either -
1. the relay is still energized
2. the contacts are welding.
3. the relay is mechanically 'stiff' and it's spring isn't strong enough to pull the contact apart.

If the contacts had welded I would doubt tapping it would release them. Therefore I would look first at whether something is holding the relay in the 'on' state when it should be off. First try shorting the base and emitter of the transistor together, this will prove whether any bias current is making it partially conduct. If that doesn't make the contacts release, try shorting out the relay coil itself.

Brian.

 

[gauravkothari23]

Break the problem down:

either -
1. the relay is still energized
2. the contacts are welding.
3. the relay is mechanically 'stiff' and it's spring isn't strong enough to pull the contact apart.

If the contacts had welded I would doubt tapping it would release them. Therefore I would look first at whether something is holding the relay in the 'on' state when it should be off. First try shorting the base and emitter of the transistor together, this will prove whether any bias current is making it partially conduct. If that doesn't make the contacts release, try shorting out the relay coil itself.

Brian.

Yes, i have tried this but still the same problem.
what i have done is, i have replaced the transistor with ON/OFF switch, and kept on trigerring the switch after approx 5 to 10 seconds, but after several attempts, the relay got stuck. As soon as the relay got stuck as you said i shorted both the relay coil terminals, but no changes has been observed. and then finally i have to tap on the relay.

--- Updated Jun 17, 2022 ---

Break the problem down:

either -
1. the relay is still energized
2. the contacts are welding.
3. the relay is mechanically 'stiff' and it's spring isn't strong enough to pull the contact apart.

If the contacts had welded I would doubt tapping it would release them. Therefore I would look first at whether something is holding the relay in the 'on' state when it should be off. First try shorting the base and emitter of the transistor together, this will prove whether any bias current is making it partially conduct. If that doesn't make the contacts release, try shorting out the relay coil itself.

Brian.

As i am using a Inductive load to drive through relay, there are chances of transient voltage, surges or spikes, As i am not able to use Snubber across relay can i use MOV 10D471K across Relay Common Pin and Neutral OR it has to be across Triac T1 and T2 pins.

 

[FvM]

Contact arcing or even welding can only occur if the triac is not switched off while the relay is changing its state. May be your control circuit is behaving differently than expected.

Contact welding or reversible sticking, as apparently observed in your circuit, doesn't occur due to inductive load. It can e.g. happen due to discharging the motor capacitor when reversing an AC motor in operation.

 

[gauravkothari23]

Contact arcing or even welding can only occur if the triac is not switched off while the relay is changing its state. May be your control circuit is behaving differently than expected.

Contact welding or reversible sticking, as apparently observed in your circuit, doesn't occur due to inductive load. It can e.g. happen due to discharging the motor capacitor when reversing an AC motor in operation.

Yes, i agree with you in this point.
But a mention earlier in Post 1, i am switching the relay only after the motor is switched OFF. i have a feedback pin which comes from motor which provides me a voltage from 1 to 24V AC when motor is rotation as per the speed defined. which is further converted to DC and using a voltage divider circuit, i am monitoring the Voltage using ADC pin on controller.
so i wait approx 1 second after the voltage from the feedback pin or ADC pin goes to 0 which indicates that the motor is OFF. and then i switch the relay.

 

[gauravkothari23]

Contact arcing or even welding can only occur if the triac is not switched off while the relay is changing its state. May be your control circuit is behaving differently than expected.

Contact welding or reversible sticking, as apparently observed in your circuit, doesn't occur due to inductive load. It can e.g. happen due to discharging the motor capacitor when reversing an AC motor in operation.

And anyhow its a gear motor, when any load is connected, it stops immediately as soon as the TRIAC is switched OFF. but then too for safety reasons, i am waiting for 1 seconds before switching the relay