Arduino controlled SSR and single phase AC blower motor issue

Hi everyone. I have been having some trouble with and Arduino project I built to control by wood burner plenum blower and draft fan. I use the Arduino to turn on solid state relays (Fotek SSR-25 DA). One relay turns on the draft fan (to stoke the fire) and the other one turns on the plenum blower to carry the heat to the house. A simple K type thermocouple measures the plenum temperature. Everything works as expected except the plenum blower motor does not start properly. (Its a 1/2 HP single phase AC motor 910 RPM, 8A that uses 7uF cap as starter). When it first tries to start, it shakes and hums and is nowhere near the rpm it should be. Eventually (if I power cycle my control circuit a few times) it starts and runs correctly and continues to run fine even when the Arduino is shutting it on and off. If sits a while without running the problem starts all over again.

Please take a look at the output (load) side of the SSR image (first image) I have uploaded from my oscilloscope. The sine wave clearly has an issue. When the motor finally starts running smoothly the sine wave also smooths out and looks almost “normal”. As seen in the second image. I have tried a snubber on both the load and relay input to no avail. Any input would be appreciated

Also, I would like to add if I take the SSR out of the circuit, connect the AC motor to the SSR independent of my Arduino project and activate the low voltage side of the SSR with a 9V battery, it works 100% correctly all the time.

Also the draft fan works fine.

Look at the signal from the output_pin. Voltage OK ?

Try start/stop motor by gignaling directly from arduio 5V.

I have uses same type of SSR - feeding from 4 I/O pins and used "PORTx=" for control

Good thought. Voltage is about 4.75V from output pin. I was on the same page the other day. I took an Uno, plugged into a wall wart, then connect the SSR to the +5V and Ground of the Uno, nothing else connected. It acts the same way unfortunately

On the input side, the rating is 3-32VDC to turn it on (probably for a resistive load). However, I think the inductive load with capacitor starter is requiring the use of greater turn-on current by using for example 9-32VDC for complete turn-on.

Perhaps something like this would work:

dlloyd: On the input side, the rating is 3-32VDC to turn it on (probably for a resistive load). However, I think the inductive load with capacitor starter is requiring the use of greater turn-on current by using for example 9-32VDC for complete turn-on.

Perhaps something like this would work:

Good idea. I'll give this a try and report back. This my explain why the smaller draft blower works and the larger plenum blower does not.

You have a snubber network across the thing?

MarkT: You have a snubber network across the thing?

Yes I have. I used a Red Lion SNUB0000 and it actually made the problem worse. Gonna do the transistor trick mentioned here to switch 12V or so tomorrow.

Snubbers have to be matched to the load, alas, or it would be simple. That waveform looks like the SSR is toast, which may be due to voltage spikes caused by lack of snubber.

ericleejoe: Good idea. I'll give this a try and report back. This my explain why the smaller draft blower works and the larger plenum blower does not.

dlloyd: On the input side, the rating is 3-32VDC to turn it on (probably for a resistive load). However, I think the inductive load with capacitor starter is requiring the use of greater turn-on current by using for example 9-32VDC for complete turn-on.

Perhaps something like this would work:

Good call dlloyd. I put an NPN transistor on the thing with a +12VDC supply to the + side of the SSR and the blower kicks on and off as smooth as can be. Thanks a lot for the help. Thanks to everyone that replied too :grin: