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Preventing SSR chatter

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#1 dwalker

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Posted 07 April 2008 - 09:48 AM

I'm not sure if this is the place to post this but.....

I have an automation system that controls exhaust fans / supply fans in my facility. At the MCC in the motor starter bucket we have an Hand / Off / Auto switch installed. In Hand, the motor starter is in a manual mode. When in auto, the automation system can switch the starter on/off.

In order to control the starter in Auto, the HVAC contractor installed SSR's to switch on/off the motor starter. I've noticed that on some of the starters, when they go to switch off, the SSR starts to chatter and will eventually cause the breaker to trip because the starter is chattering too.

What is the best way to prevent chattering? Resistors, capicitors, diodes? How do you size them etc. I'm sure this is basic electronics, but it's been a while.

The SSR is switching a 120VAC load.

TIA.
- Dave

#2 BobLfoot

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Posted 07 April 2008 - 11:49 AM

I'm not sure if this is the place to post this but.....

I have an automation system that controls exhaust fans / supply fans in my facility. At the MCC in the motor starter bucket we have an Hand / Off / Auto switch installed. In Hand, the motor starter is in a manual mode. When in auto, the automation system can switch the starter on/off.

In order to control the starter in Auto, the HVAC contractor installed SSR's to switch on/off the motor starter. I've noticed that on some of the starters, when they go to switch off, the SSR starts to chatter and will eventually cause the breaker to trip because the starter is chattering too.

What is the best way to prevent chattering? Resistors, capicitors, diodes? How do you size them etc. I'm sure this is basic electronics, but it's been a while.

The SSR is switching a 120VAC load.

TIA.
- Dave

I am no Electrical Engineer but I'd check the current draw of your Motor Contactor Coil and the amperage rating of the SSRs your HVAC contractor supplied. It sound like the COntactor Coil Field collapse is swamping the SSR and forcing a "false" turn on. Nothing to do for this I know of but get the right sized components.
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#3 P Daniil

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Posted 07 April 2008 - 11:56 AM

Most likely an external (SSRs rarely have them inside) RC snubber is needed to allow the SSR thyristor/triac to turn off graciously.

The snubber RC values must be so calculated to limit the rate of voltage rising (dv/dt) at SSR turn-off for the given load (ie the relay coil) inductive current.

SSR manufacturers usually provide typical values for typical loads and some of them offer ready, off-the-shelf snubber networks.

Also keep in mind that an oversized snubber circuit will leak unnecessary current to the load which may prevent the relay/contactor to open.

#4 TConnolly

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Posted 07 April 2008 - 03:29 PM

The AC SSR is two back to back parallel SCRs, aka, a triac. SCRs almost always fail in the shorted, ie, ON, condition. That means that if one half of the triac has failed then half of the AC sinewave is getting through the SSR. Its not enough to pull in a completely open conctactor, but combined with the building/collapsing field around the starter coil as each half sine wave energizes the coil it might just be enough to keep an already energized contactor engaged enough for it to start chattering.

With your SSR disconnected, check it with a multimeter set on diode check. You need to check both forward and reverse polarity. A good triac will not show conductivity in either direction. If it conducts in either direction then the triac is bad and that is your problem.

If you do find bad SSRs then you need to address the reason why one side of the triac is blowing out. Chances are the collapsing coil field is taking out the triac when the contactor is de-energized. An RC snubber across the contactor coil should take care of that. Or you replace the SSR with a properly sized ice-cube relay.



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