Arduino + SSR for 120v 20Amp ERV (it's a big fan)

So I have an ERV (energy recovery ventilator) in my shop that provides fresh air and removes the stale air from the workspace. It's essentially two squirrel cage fans, and a small motor turning an entropy wheel. The thing is on a timer, and comes on at preset times (which can be changed, of course).

The problem is that we'd like it to come on in response to certain conditions. First, we still want it to turn on at the preset times. Second, we'd like it to run whenever the humidity in the shop rises above some value. Third, we'd like it to run for awhile whenever we ask it to.

Currently, functions 2 and 3 are not possible. So I got to thinking, I can have an arduino do all this for me. Replace the timer with an SSR, and the arduino can be configured to handle all three functions.

Well, at this point I have the arduino side all figured out, and have tested it with an LED as the output. Humidity was a rough one to get the settings right, but was able to do so (used adafruit's DHT22, fwiw).

Anyway, now I'm trying to spec out the SSR, and I ran up against some knowledge barriers. The ERV has 2 0.5HP motors. It appears to spin them up at the same time, but I can't be sure without taking it apart and running it. (The wheel motor is only 0.1HP, and it is on a delayed start, so I'm not worried about it). So I'm assuming a 1HP inductive load. Based on the inrush current, I should be looking at a 75-90 Amp rated SSR. Also, based on what I've read, for motor loads, random start is better than zero-crossing.

Of the myriad options available, SSRs which are roughly equivalent in spec will vary wildly in price, often an order of magnitude. (See this FOTEK-SSR-75DA-H (~$9) vs this Crydom CWD2490-10 (~$95)). And that is before the heatsink.

Is the Crydom worth the extra $86? What am I missing here? How important is random-start?

Thanks for any help.

They are both worth what the price is. That is the difference between poor and good quality. I think the quality of the Crydom is indeed 10 times better. When you select a cheap one from Ebay, buy one that can handle more current than needed.

I don't know if zero-crossing switch is better or not. It is hard to say something meaningful about it in this situation. There could even be a filter or a capacitor in the motor.


Interesting site.

Tom... :)

The Fotek is zero-cross only, while the Crydom is random. Lots of folks use the Foteks, and it seems like I recently saw a tear down that said the PCB traces were decent. It may be a good way for you to get your feet wet with controls.

The thing about (purely) inductive loads is that the best time to turn them on is at the AC peaks, not the zero crossing. Turning it on at the zero cross gives the inductor a complete half cycle to ramp up the current, with dramatic results. The surge current starts to drop only after the negative half cycle begins to counteract it.

In the steady state, the zero-current points are where it should be turned off... and those are at the voltage peaks for a purely inductive load. As was mentioned, the actual characteristics of your motor may shift that time.

If you want to use the random turn-on SSR, have you thought about how you're going to sense the AC phase so that you know when you should turn it on? It usually involves a transformer to step the voltage down to something acceptable to an analog input, and/or a voltage divider into an optoisolator that switches at the zero voltage crossing (almost :) ). You could go all-out and use a current sensor or transformer (CT) to sense the zero-current turn-off points.

Or, you can avoid the whole messing-with-dangerous-things and get an over-spec'ed ZC relay... 1/2 HP is not a huge motor, really: maybe 4A at 120V? Times two is 8A, but let's say 10A.

The inrush current doesn't last very long (a few cycles), so you might consider the "surge" rating, which is x10 to x40 times the normal load current. The Fotek 75DA has an 820A surge rating, and the 25DA has a 275A surge rating. The 25DA is probably ok for this application, because it can withstand 3 seconds of your surge current. Heat sink it for good measure. You won't need a big 'un to run it at 1/6 - 1/3 its rating.

BTW, there's also a "locked rotor" current that depends on the motor. You might want to figure that out. Then you can decide if you want the SSR to blow, or just count on the thermal shutdown or the breaker to trip. I lean towards frying a $7 SSR instead of the motor. D@#$ squirrels caught in the cage again!

Honestly, the ZC is ok because mechanical relays have been used for decades, and they connect whenever they get around to it. :D What's a little arcing between friends? :) At higher currents it gets more exciting, but you're not there yet. Could be a fun project, though...

YMMV, my $0.02, etc. /dev

I just noticed the post subject says "20A ERV"... Is there something else besides the three motors, or is that the circuit breaker size? If it's really 20A, you should probably go to the SSR-40DA instead of the 25DA.

Cheers, /dev

While zero crossing might be less than ideal, the existing (mechanical?) timer probably makes no attempt to do ideal contact timing so I wouldn't worry too much.