Powering a 240v fridge with a 2 channel relay

Hello all!

I've been working on an Arduino project for a while now. I have coded it and got the Arduino performing all the required tasks.

It has to switch a relay based on my program between one and the other. I have made a paint drawing of the way I am wiring it. Basically, one power cord will be bringing in the mains power which will be bridged across two connections. So far, so good.

Now, I haven't implemented any of this yet, but from what I can gather, running an inductive load through one of these relays can be problematic. The relays have shortened lifespan due to voltage spikes and in some cases can compromise safety.

I looked at a few solutions: Arduino relay controlling a bigger relay and SSR's. From what I can gather, however, the simplest solution would be to implement a snubber.

I understand the basic principle (I think). The snubber only allows current through over a certain threshold; cutting the power to the relay and dissipating this power as heat. Seems like a good way to prevent ware on the terminals.

So my problem is I cannot find any resources on how exactly to make one of these? Can anyone help me with this? I think it involved a capacitor and a resistor inline but I am unsure what they need to be.

Here are some details:

Relay: 2x High-current relays, AC250V 10A ; DC30V 10A.

Fridge(measured myself): 243.5v, 1.22amp(running), 4.6amp(starting), 184.1 watts
Fridge(on the sticker): 240v - 50hz, 1.2amp, 200w

Here is my dodgy illustration of what I am looking at doing:

Where would the snubber go here? I'm assuming bridging the positive and neutral wires at the back of the socket.

I understand this is probably convoluted so I am more than happy to elaborate or provide additional information.

Thanks!

Hi,
Snubber Design is pretty well understood. This is a good starting point:

And you'll find lots more HERE

The snubber would go across the relay contacts in most cases.

blackout182:
Fridge(measured myself): 243.5v, 1.22amp(running), 4.6amp(starting), 184.1 watts
Fridge(on the sticker): 240v - 50hz, 1.2amp, 200w

Expect a fridge motor to draw several hundred watts for a brief period at startup. Use a relay rated for 20 amps or so and rated for inductive loads.

...R

Several points.

1. I wouldn't assume you're switching a 100% inductive load. Only the compressor is inductive and if it's a recent refrigerator, the compressor is likely controlled by a variable speed drive. In those cases, there is no inductive load as seen by the supply side.

2. An RC snubber works best when in parallel with contact you're trying to protect. But, that creates a secondary problem of leakage current (and power) when the the relay is off. It also presents a safety hazard because there would be voltage present at the outlet even though the relay is off. The only safe place is in parallel with the appliance but the snubber is less effective.

3. if I was doing this "project", I'd just go with a 30 amp relay and forget snubbers or anything else to protect the contacts. You never mentioned it but how many times per day could this relay turn on and off? The lower the duty cycle, the less concern with contact wear.

4. Google "rc snubber for relay contacts" if you want more appropriate references, you're not designing a switch mode power supply;-)

5. An appropriate reference: How to design an RC snubber for a solenoid relay driving an inductive load? - Electrical Engineering Stack Exchange

Finally, just a curiosity question. Why do you need or want to turn off a refrigerator?

avr_fred:
Why do you need or want to turn off a refrigerator?

You beat me to it: that's my question too. Perhaps its own internal temp regulation is stuffed? Or it might be at their holiday home and turned off in their absence, so want to "call ahead" to turn it on so it's cool when they arrive.

OP mentions safety right at the top, which is laudable, but I hope s/he's also keeping the regulatory / insurance aspects in mind. If there's a fire the insurers may still be able to ditch the claim because it's not legal / certified etc even if it's made pretty much according to the same approach as a certified product.

Thanks for answers all.

Yeah, a larger relay was what I would prefer I'm just having such a hard time finding one that's suitable for some reason.

I am building a fermenter. The refrigerator has a small heater in it as well and the Arduino keeps the inside of the fridge in a temperature range suitable for fermentation.

In theory anyway. Need to hunt about for a better relay then. Would I use this one as a slave to the larger one? If so, how would I wire that?

I've been using two of these to control a 240 vac submersible well pump for three plus years running, zero issues. Ten to twenty cycles a day, average. The nice feature of the relay are the 6.3mm push-on terminals on the top for the power connections.

http://www.ebay.com/itm/2pcs-SLI-12VDC-SL-A-12V-30A-250VAC-SONGLE-Power-relay-/291662014862

We break both legs of 240 vac here in the US as both are live with respect to ground. in your case, only one relay would required.

Edit: You would need a transistor or mosfet to drive this relay. You could use the smaller relay as a pilot device if already in place.

Thanks man, that's super helpful.

With regards to driving it, is there a data sheet or something available? How many input volts? I'm not super great at reading these yet.

It's a 12 volt device. In retrospect, that probably just complicates things with another voltage level. This may be a more suitable, complete module:

http://www.ebay.com/itm/1-Channel-30A-5V-12V-24V-Relay-Module-Board-With-Optocoupler-H-L-Level-Triger-/361368219371

Select the 5 volt option if you order one. While the relay doesn't have the push-on terminals, it has the same ratings and a screw terminal strip to terminate.

Awesome, even better :).

So I've been copping a bit of flak for not getting relays rated to handle inductive loads. They're quite difficult to find. But I assume from what you've said, this can handle it. 30a X 250v is 7500 watts it should be able to handle and I don't know what kind of fridge is gonna draw that much on start up.

Would it pay to out a 30a fuse on the line just in case? Pretty cautious around AC power.

In the context of what's being discussed here, a fuse would be sized to protect the weakest current handling device. Your typical appliance cord set tops out at about 10 amps making that a more appropriate fuse size.

Ten amps sounds a lot more sensible. You should be aware that ordinary fuses don't fail the instant the load exceeds their rated value so they can deal with very brief surges.

...R

Ah yes, no worries.

blackout182:
Awesome, even better :).

So I've been copping a bit of flak for not getting relays rated to handle inductive loads. They're quite difficult to find. But I assume from what you've said, this can handle it. 30a X 250v is 7500 watts it should be able to handle and I don't know what kind of fridge is gonna draw that much on start up.

Would it pay to out a 30a fuse on the line just in case? Pretty cautious around AC power.

Relays rated for some inductive load or a motor of some size is because of the insulation needed to withstand the voltage when the circuit is opened. A snubber or diode can limit on DC, but can't be used on AC. there may be other devices to limit voltages for AC.

Paul

Relays rated for some inductive load or a motor of some size is because of the insulation needed to withstand the voltage when the circuit is opened

Relays with such derates are about withstanding the higher current inrush upon contact closure. Think motor locked rotor current. The voltage withstand is part of the base design and is generally not affected in the derate. The eBay relay mentioned is a good example, the voltage rating does not go down with an inductive load but there is a much lower current limit which is in reality a power limit.

DC usage is different discussion altogether.

The HP is related to the inductive capacity right? I did some measurements of the fridge and it comes under 1/2hp so it should be okay.

I agree that it should be just fine.

Inductive, not so much. The HP rating is more about the inrush current associated with a motor.

When full voltage is applied to a single phase motor at zero speed, the inrush can be five to ten times* higher than the running amperage. The resulting arc and heat created at the moment of contact closure causes contact pitting which of course reduces relay life. Derating the relay to keep the inrush current below the point of contact damage is the reason for the additional motor rating.

Looking at the eBay relay, it is rated 240vac, 30amps. That is resistive load only and represents a maximum load of 7200 watts. The relay is also shows ratings of 1hp at 120vac and 1/2hp at 240vac. This is not logical as the power is not constant, it appears IMO, backwards.

A good relay will have a datasheet with a contact load limit curve at different voltages. This is unfortunately not one of those relays and while it does have a datasheet, it is in direct conflict with the ratings printed on the relay. A quick read of the datasheet shows are more logical contact rating of 2hp at 240vac and 1hp at 120vac for the Form-C, NO contact set, the intended set of use in your application.

SLA-05VDC-SL-C_Datasheet

One horsepower is approximately 750 watts which says the motor maximum current rating is 750/120 or roughly 6 amps, roughly one fifth of maximum load. Two horsepower would be 1500 watts and at 240vac represents a load of 1500/240 or the same 6 amps in the 120vac rating.

This derate to 6 amps makes sense when the average current at contact closure can in fact be 5 times that, the ultimate rating of 30 amps. This is the basis for the derate. This is SWAG (scientific-wild-a$$-guess) on my part since the datasheet is rather limited in data. Imagine that (insert comment here on device country of origin).

Overall, there is the chance that a higher voltage will cause more arcing, especially when contacts open. But, we don't know that for sure, again because there is not hard data to examine. In the case of this relay, I don't think we know with a large degree of certainty that this is the absolutely best part to use but it should be better than a 10 amp device in the same application. Sure, you can spend $30 and get a relay with a pedigree in the form a complete datasheet. But, is that really necessary? I don't think so, given what you're doing. That's why these devices exist, there is market at this price point. The only price for device failure is a temperature alarm. You investigate and resolve. If it means replacing the relay two years from now, I think you still come out ahead.

• • while the inrush can be as high as ten times, your run-of-the-mill appliance motors are in the 4 to 6 range. This is highly variable and is meant as a point of reference, not an absolute specification.

Thanks mate, very helpful.

My only concerns really were producing something dangerous. ie. electrical fire or hot appliance.

But I'll insulate it in plastic housing. I don't mind a dead relay eventually, will have probably moved onto bigger and better things my then anyway

Thanks heaps for the explanation.

Another question guys, what should I do from a safety perspective?

Is there anything I can add to my design to 100% ensure I never end up with a live refrigerator?

Also, following the safety ideas, With regard to the ground, obviously I will pass the earth straight through the power point and to the fridge. Do I connect that ground with the Arduino ground? Some places are saying yes and others no so I was just wanting a definitive answer. I am thinking the metal case for this project with an earth wire attached to the frame which I assume would also be attached to the ground.

Basically, should I simply connect all grounds together?

Ground is an overused word especially when comparing electrical to electronic terms.

Electrically, the ground lead (yellow/green) in the power cord is for safety. With mains distribution, ground, or the correct term, Earth, is not meant to be a current carrying conductor. It is for human safety and connecting the Earth wire to anything metal is done so that should a live conductor ever come into contact with that metal, the fuse, circuit breaker, etc will trip thereby eliminating the shock hazard.

Electronically, ground is a general term meaning a common point of zero volts potential. That zero volts should be zero volts with respect to earth but there is no rule that says that it has to be.

The ground (or gnd) terminal on a Arduino is the common reference point for all voltages (+5 and +3.3) be they digital or analog.

When you have an Arduino powered by a desktop computer's USB connection, the GND of the Arduino is (99.9% chance) connected to earth. Laptops, even connected to a wall powered brick, maybe. Does it have a two wire or three wire AC mains plug? That's the only source for earth with a laptop, that plug.

With an externally powered Arduino, if you know your supplies are isolated from mains, and they absolutely should be, it is okay to connect GND to earth. If you don't know for sure, I would not attach the gnd terminal of the Arduino to earth.

If you have an Arduino in a metal box into which AC mains is routed, the box MUST be earthed. See paragraph two above!