stopping voltage spikes in an ac inductive load

Thread Starter

denison

Joined Oct 13, 2018
132
With a dc source inductive load you simply put a diode pointing back to the battery which clamps the spike voltage at the diode voltage.
However this cannot be done with an ac source.
Does anyone know how to stop voltage spikes with an ac source inductive load such as a 240vac electric motor? This must be for turning on and off the motor. I blew up a zero crossing triac optocoupler which had a resistive load when I connected a 240vac electric motor to the L and N terminals and switched on the motor.
At the moment I see no other way than to use a separate power lead for the motor and the power lead to the resistive load which is controlled by the optocoupler. I hope that the spike would not travel down the power lead of the motor to the power lead of the opto.
My application requires the motor to be on at full mains power while the opto controls the resistive load.
 

gerty

Joined Aug 30, 2007
1,291
Are you saying you put the motor in parallel with the resistive load? If so are you exceeding the current limit of Triac? For AC a snubber is used for spikes like that
 

Thread Starter

denison

Joined Oct 13, 2018
132
the motor load was not connected through the triac. It was simply connected to the L and N terminal of the 240vac mains supply. The only load on the triac and opto was a resistive load of 77 ohms. This was working ok when the motor was not connected. When the motor was connected and the circuit switched on the moc 3043 blew up.
Because there is no inductive load via the opto and triac I don't believe an ac snubber is appropriate in this case.
I have previously run a successful circuit with the motor and resistive load on different mains leads.
I would like to be able to run both on the one mains lead but have this spike problem from the motor. If there is a way to stop the motor spikes please advise. With a dc source and motor all you need is a diode to stop the spike.
 

MisterBill2

Joined Jan 23, 2018
7,040
Presuming that the system is powered by he 240 volt mains, not a square wave from some inverter, there is not an inductive spike from the motor running. There can be some spiking when switching off, but not from running. There IS a current inrush with a motor starting and that is able to cause a lot of problems. So there is more to this problem than is obvious.
 

Thread Starter

denison

Joined Oct 13, 2018
132
Presuming that the system is powered by he 240 volt mains, not a square wave from some inverter, there is not an inductive spike from the motor running. There can be some spiking when switching off, but not from running. There IS a current inrush with a motor starting and that is able to cause a lot of problems. So there is more to this problem than is obvious.
 

Thread Starter

denison

Joined Oct 13, 2018
132
The system is powered by the 240 volt mains. I wasn't sure if you got inductive spikes from the motor running but thanks for reassuring me that you don't.
The opto coupler blew when I switched on the power to the motor and the circuit. So it sounds like it may be caused by the current inrush at motor start. Then if you solved this problem you still might get a destroying voltage spike at switch off.
It appears that the only solution to the problem at the moment is to have separate mains leads to the motor and to my opto controlled resistive load. It is then a possibility that the spike could still be transmitted down the mains leads but I think this is unlikely. Some sort of arrester could be put on the mains lead to my opto triac circuit if this was to happen. Can you suggest a good commercial model if I still have this problem. Otherwise a discrete component or 2 which would be in the commercial model anyway and would be much cheaper.
 

ebeowulf17

Joined Aug 12, 2014
3,274
An RC snubber device sure sounds promising to me. I wouldn't rule it out without giving it a shot. Putting one in parallel with the motor terminals can greatly reduce noise generated by the motor getting into the rest of the system. Putting one in parallel with your TRIAC can reduce the severity of transients it experiences as well, regardless of where they came from.

The devil's in the details, and I'm not totally confident which, if either, of these placements would solve your problems, but I wouldn't be so quick to dismiss the possibility that it could help.

The other thing that might come in handy is a TVS diode which will limit the maximum voltages for transient events.
 
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