Can you use a Solid State Relay in a 240v 2-wire circuit?

Thread Starter

Etienne145

Joined Mar 2, 2019
7
I searched the forums and Google but didn't find any hits.

I'm trying to set up a control circuit for a 240v woodshop dust collector using a SSR and a simple 12vdc control circuit run from each blast gate. The dust collector uses a 2-wire 240v circuit (hot-hot-ground). As a proof of concept, I built a test unit using a 120vac light bulb and a 12vdc control. Everything worked fine at 120v, so I scaled up to the 240v dust collector (9A rated motor and a 25A rated SSR). It functioned correctly and the load side of the relay closed when the DC control voltage was applied, the dust collector started and everything worked fine. Unfortunately, the load side of relay failed to re-open when the DC control voltage was removed. Further testing showed that the relay had failed closed.

So... Is there something about a 2-wire 240v circuit that wouldn't play nice with a SSR or did I maybe just get a lemon (Chinese made...)?

The SSR that I used was rated for; 25A, 24-380vac load side, 3-32vdc control side

Thanks
 

MaxHeadRoom

Joined Jul 18, 2013
19,067
I would use reliable makes such as Opto22, there should be a snubber internally, possibly this was lacking with the one you used.
What is the type of motor?
Max.
 

Thread Starter

Etienne145

Joined Mar 2, 2019
7
Thanks for the responses!

It's a 9A (supposed 2hp) Capacitor-Start Induction motor

Not using a snubber (actually I had to look up what that is just now...).

If I understand it correctly, the 2-wire 240v alternates back and fourth between hot and common for each leg. I was wondering if that that maybe was causing problems?
 

Thread Starter

Etienne145

Joined Mar 2, 2019
7
Thanks,

I don't have a schematic, but what I've got is pretty basic. I'm trying to use the relay to turn the motor on/off based on a 12vdc input (see initial post for a longer description).
 

MaxHeadRoom

Joined Jul 18, 2013
19,067
If I understand it correctly, the 2-wire 240v alternates back and fourth between hot and common for each leg. I was wondering if that that maybe was causing problems?
Normally reputable makes have a snubber internal, but an external one may help.
The 240v current just alternates direction on each (L1-L2) the SSR normally has a Triac output so it can handle this.
The load for a Induction motor is quite high initially, so it is imperative the motor gets up to speed ASAP.
Any reason you are using a SSR for just on/off control?
Max.
 

sghioto

Joined Dec 31, 2017
1,039
I don't have a schematic, but what I've got is pretty basic. I'm trying to use the relay to turn the motor on/off based on a 12vdc input (see initial post for a longer description).
Then why do you need the SSR? Just use a 12 volt DC relay with contacts that can handle the load.
SG
 
Last edited:

MaxHeadRoom

Joined Jul 18, 2013
19,067
If going the relay route, you have to switch both poles on 240v, if frequent on/off, a small contactor would be ideal.
One draw back with the SSR method with no neutral that both poles should be switched.
Max.
 

Thread Starter

Etienne145

Joined Mar 2, 2019
7
MaxHeadRoom; Thanks for clearing that one up, one less item to consider.

I'm just using it for on/off, but controlled via a 12vdc passed through micro switches embedded in the blast gates at each tool (about a half dozen of them). If I open any gate it closes the micro switch which feeds the 12vdc to the SSR to (hopefully) energize the 240v dust collector.

I've got a spare Crydom D2425 from a previous project, but I can't find any mention of a snubber in the literature...

Also, I figured that a 25A SSR would handle the starting current...? Should I switch to a higher rating?

Thanks for any info
 

Thread Starter

Etienne145

Joined Mar 2, 2019
7
No, just simplicity and my knowledge/comfort level... and I've got a couple SSRs laying around from another project.

Would one of those (D.P. Relay or contactor) be a better solution?

Thanks
 

sghioto

Joined Dec 31, 2017
1,039
From the data sheet the D2425 has a snubber if there is a "S" in the part number. What is the complete part number? It may be designed for resistive loads only.
SG
 

Thread Starter

Etienne145

Joined Mar 2, 2019
7
Thanks, I'll look into pursuing it that way... not the answer that I'd hoped for, but better to do it right.

You know what they say, when you've got a hammer (or in this case a SSR) everything looks like a nail...
 

mvas

Joined Jun 19, 2017
538
You need to select a Crydom SSR that is rated for a 2 HP 240 AC Motor.
The Crydom D2425 is not rated by Crydom for any motor.
The output is rated at only 600 V Transient Over-Voltage

For a panel mount 240 VAC 2HP Motor Control SSR,
you must select either the "CW" series or the "HD16W" series SSR ...

A CWD4825 series SSR = supports a 2 HP 240 VAC motor with 1,200 Peak Inverse Voltage
A HD16WD6050 series SSR = supports a 2 HP 240 VAC motor with 1,600 Peak Inverse Voltage
 
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