What I thought was a simple AC circuit with a Refrigerator & SSR

[nickmanc86]

I am half way through my Electrical Engineering program and have only just started to learn about AC circuits and inductive loads but I did a little reading and went off to try a simple project on my on. A company I work for wanted a switch to turn off their large refrigerator for certain periods of time. Most countdown timers you can buy will turn something ON for the set amount of time. I needed something that turned OFF for a set amount of time and ON the rest of the time. I created a timed switch using easy to obtain components:

A Normally Closed SSR (SNC-R2025-511)
http://www.alliedelec.com/images/products/datasheets/bm/NCC/70059698.pdf
An off the shelf timed switch (Sylvania SA 160)
http://www.smarthome.com/56217/Sylvania-SA-160-Dual-Mode-24-Hour-60-Minute-Digital-Auto-Shut-Off-Timer-20-Amp/p.aspx
The Refrigerator

I can't get much info on the Sylvania part other than the 120VAC, 20A rating for inductive and resistive loads and the fact that it is a SPST device. I assume its just essentially another SSR.

Included are two circuit diagrams of my box and of the refrigerator and one picture of the actual device. Sorry if they are messy and/or incorrect.

Anyway I put the timer and SSR into a box with a male and female plug. I made two of these for two different refrigerators. One a smaller 6A, 120VAC, 60Hz and the other a 9.6A, 120VAC, 60Hz model. They both work fine on the smaller fridge and other small appliances.However, on the larger fridge they will work for a little while and then something (perhaps the SSR) continuously cycles on and off. It will remain on for 15 seconds or less then off briefly then on again. The relay and the box are warm but I wouldn't say hot and the refrigerator is well within the acceptable load range of the device. Is there something I am missing here. Is there some kind of blowback voltage from the refrigerators inductive load causing the issue? Do I need to isolate the timer or install a protection circuit? Is it just heat? I know I must be overlooking something really simply hahaha, any help would be appreciated.

Also sorry if the formatting of my question is incorrect this is only my second time posting here.

 

[berkeman]

I'm not of any help on the operation questions, but how are you handling the safety/Earth ground? And to avoid a fire hazard, you probably should have an input fuse in the Hot lead right as it enters your junction box.

 

[nickmanc86]

Ya sorry the diagram was not explicit about the ground. Both ends are typical 3 prong Male/Female (respectively) plugs. The ground lead from the male plug, which plugs into the outlet, runs into and is attached to a grounding screw in the box. The same goes for the female plug which runs to the fridge. The ground wire from the fridge plug goes into the female plug which runs into the box and grounds on the same screw as the male side. Is that okay/safe? The fuse is an excellent idea, I will have to add that in.

 

[dlgoff]

I'd just attach and wire this to the side of the refrigerator. Cheap and safe.

http://www.foxelectricsupply.com/Content/Media/Images/Product/t103.jpg

http://www.foxelectricsupply.com/content/products/ProductCatalog.asp?qscategoryId=26328

 

[berkeman]

Ya sorry the diagram was not explicit about the ground. Both ends are typical 3 prong Male/Female (respectively) plugs. The ground lead from the male plug, which plugs into the outlet, runs into and is attached to a grounding screw in the box. The same goes for the female plug which runs to the fridge. The ground wire from the fridge plug goes into the female plug which runs into the box and grounds on the same screw as the male side. Is that okay/safe? The fuse is an excellent idea, I will have to add that in.

The grounding sounds reasonable. I'd add a switch in series with the Hot lead, right after the fuse. With the fuse and switch, you are coming close to being UL-approvable...

I like dlgoff's suggestion as well.

 

[wirenut]

Do you have any heatsink besides the 4x4 box? The specs for free air use are a maximum current of about 7.5 amps. For any current above that you should use a heatsink. I don't think you have enough contact with the box (and didn't use thermal compound). The specs call for a 1/8" 6" x 6" as the next size from free air (i think if you would use a 6"x6"x4" box with no knock outs where you are mounting the ssr, or as a test try putting a fan blowing on the 4"x4" box with the cover off to see if it works longer before cycling).
I would try the fan first before investing in any more parts.