Relay Shield & Motor question

I am planning on using the Relay Shield and a Photon to control the motor of a ventilation fan. However, I am now questioning whether I can control the motor from the relays, or use the relays to control contactors which in turn control the motor.

The motor is single phase, 120V AC, 60Hz, 1/20Hp with operating current of 1.7A.
Digging deeper, it is a “shaded pole” design. I was not able to find any information about the inrush current for start-up.

The Panasonic JS relays on the Particle Relay Shield are rated for 10A at 125V AC. On first glance, that looks good. However, the Panasonic General Application Guidelines caution that motor inrush current could be 5x to 10x the steady state current. That means an estimated inrush current of 8.5A to 17A. That is a very high current and probably exceeds the relay rating. However, the caution indicates inrush could last up to 0.5 seconds.

Does anyone have real-world experience with these relays for controlling inductive loads?
I searched for inrush current information for my motor, but could not find anything.
Searching in general for inrush current for shaded pole motors also turned up no information. Is there any experience if the Panasonic estimates are realistic for a motor such as this?

Also, the general application guidelines seem to indicate a CR circuit is the best choice to protect the relay contacts for an inductive load(motor). Would this still be recommended to protect the contactor contacts, in case using contactors to control the motor is recommended. Again, would this same protection be used to protect the relay when driving the contactor, as I would assume a contactor is also an inductive load?

@peekay123 & @IOTrav,

I think you two are quite familiar with hardware work. Am I at least thinking along the correct lines? I know I have to find my own solution, but without a lab I am trying to minimize trial-and-error which could get costly. Should I be looking into test equipment to get a better handle on things (which is sometimes hard to justify the cost for personal use)?

Another option might be to use a solid state relay like the SSR-40 DA instead of the relay shield. No contacts to worry about, and that relay can handle 40 Amps.

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@cyclin_al, the relays on the relay shield may arc or weld due to the inductive inrush current so I don’t recommend using them. Shaded pole motors typically use single windings and have low starting torque making them well suited for triac-based (SSR) speed control though in your case I suspect it is just on/off control.

@Ric’s suggestion of using an SSR is good. A Fotek SSR-40 DA includes a snubber (RC) circuit to soften the inrush current peak though it does come at the cost of a small leakage current (2-5ma) at the 125VAC motor voltage. This requires caution as the AC side of the SSR will still be “live” even when turned off.

The SSR-40 can handle the 10x inrush and also gives total electrical isolation (opto-isolated). You will want to use a heatsink for the SSR and you could add an MOV accross the SSR AC terminals to protect against high voltage transients.


I have been selling relay controllers for about as long as I can remember now. It’s pretty much our bread and butter. @Ric and @peekay123 are absolutely right. The ideal choice for inductive load switching is a good Solid State Relay.

In my opinion Crydom makes the best solid state relays in the industry which is why we use them. We have relay controllers here with Crydom Solid State Relays which optically isolate the load from the control electronics. There is no better solution for inductive load switching in my opinion:

That said if your application is highly cost sensitive it is possible to use mechanical relays such as the Particle Relay Shield you were looking at. However if you are going to do that you need a really good snubber circuit to deal with the high voltage discharge coming back from the inductive load. I generally recommand a .56uf polyester film capacitor for this(physically the bigger the better).

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@Ric, Thank you very much for suggesting a SSR. I was not familiar with these, but they appear to be ideal for my intended use.

@peekay123, I was concerned about arcing or welding, hence I mentioned a CR circuit per the Panasonic guidance. This is so much easier now that I realize SSR is the appropriate device.

My motor is in fact 2 speed. There are 2 separate leads for power and the speed is determined by which one is turned on. There is no information about the motor state if both are turned on at the same time, so I plan to ensure both are not turned on at the same time, to avoid any unknown risk. Besides avoiding GPIO pins used for JTAG, are there any other things to look out for?
(If I had been using relays, the speed control would have been SPDT.)

Thanks for the mention that the AC side could still be live. Canadian electrical code requires a manual or emergency switch near a ventilation fan in this type of application, so this reinforces that requirement.

Thanks also for mention of the heatsink, I noticed that it was in the datasheet. However, I am not familiar with an MOV?

@IOTrav, Thanks for mentioning Crydom; I will take a closer look. Cost sensitive means not having to re-do the project, but the relay shield was my starting point and was on hand.

I knew you carried I2C devices including I2C extenders, but just did not realize an SSR was a suitable device.

In summary, it looks like a SSR is what I will use. Thank you all for the direction and advice!

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