I want to remote control (WiFi/Zigbee) a water pump AC motor, 1 phase or 3 phase depending on the case, with the smallest board size possible.

I have used large motor AC contactor in past, it works fine but it is also very heavy, bulky, and it has significant "bump" when activating, it also need a relay to control the coil which is in 240V AC, I cannot use that here for these reasons.

The load is a water pump AC motor, 240V AC 1 phase or 3 phase (so 240V between a phase and neutral or 400V between phases), the maximum power of the pump is 3.5HP, but if I can handle 3HP it would be OK too.

As of now there are two options I can think of, either a circuit based on three triacs (for example BTA41-800B) with MOC3083 to drive them, or a circuit based on 30A relays (model SLA-05VDC-SL-A).

The triac solution looks good but I have two concerns with it, first is heat, I don't know how much heat to expect from the triacs, I can add a small heatsink for the 3 triacs tabs (tab is insulated), for example 70*20*10mm but not very much bigger. The second concern is what will append when the capacitor on the pump motor fails and the triac will see pretty much a short circuit, I plan to add some high power (130A) hall current sensors in the circuit for monitoring so I could use that to stop the triac within an half wave in case of short but sadly I also know the triac cannot be turned off elsewhere than zero current cross, which will not append with a short, I also think a fuse will not help since the triac will likely blow before the fuse has a chance to melt.

The second option is 30A relays, 1 per phase, they are given for 30A @ 240V AC. They are rated 1-1/2HP, which to my understanding mean 1.5HP, now I wonder if I use one relay per phase on a 3 phase motor what is the total power it can handle? is it still 1.5HP total or 4.5HP total?

Does it look like a good plan to use such relays? Any advice would be very welcome.


After more input it looks like triac or back-to-back SCR is not really an option since for 20A each phase will have to dissipate 30W, which is too much for the heatsink surface I can provide here.

Now one question remains about the 30A relay, with one relay per phase, each relay rated 1.5HP / 30A@240V, should I consider the system capable of driving 3.5HP or only 1.5HP?

  • 1
    \$\begingroup\$ FYI, a triac is a economy equivalent (though not identical) of two SCRs anti-parallel to each other. Triacs conduct in both directions, whereas SCRs conduct in only one hence the anti-parallel to conduct AC. But, for technical reasons, SCRs are more rugged, give better performance, and behave symmetrically (triacs aren't quite symmetrical in operation). \$\endgroup\$
    – DKNguyen
    Commented Aug 8, 2019 at 21:48
  • \$\begingroup\$ Related/recent: electronics.stackexchange.com/q/452075/2028 \$\endgroup\$
    – JYelton
    Commented Aug 8, 2019 at 22:22
  • \$\begingroup\$ If you take a look at professional SSRs for that power, you will see they are the size and weight of a contactor. \$\endgroup\$
    – Janka
    Commented Aug 8, 2019 at 22:24
  • \$\begingroup\$ @DKNguyen yes thank you for pointing this, indeed i notices the back-to-back SCR configuration is often mentionned. \$\endgroup\$
    – Owayl
    Commented Aug 8, 2019 at 22:30
  • \$\begingroup\$ @JYelton this post is empty of useful answer and it doesnt mention inductive load nor high power. \$\endgroup\$
    – Owayl
    Commented Aug 8, 2019 at 22:31

1 Answer 1


It is not a good idea to control a motor using either electromechanical or solid-state relays that are not rated for motor control. Equipment that is so rated usually have a rating plate or catalog listing that indicates motor power rating in either horsepower or kW along with the related voltage and number of phases. Some manufacturers may have more compact equipment than others for a particular power rating. One reason for that is that the same basic design may be used for a range of motor power ratings. One manufacture may use a single design for 3.5, 5 and 7.5 Hp. While another may have a design for 1.5, 2 and 3.5 Hp. It pays to put some effort into shopping.

One 1.5 Hp rated relay per phase is not a substitute for a properly rated 3-phase contactor. You can probably use either a 2-pole relay or a 1-pole relay for a single-phase motor, but not two relays with lower ratings.

  • \$\begingroup\$ ok. Just to be clear, the SLA-05VDC is actually rated for 1.5Hp motor, it has a marking. If a single phase pump is used then the max rating is 1.5Hp and i dont intended to use 3 relay in parrallel to increase the single phase rating. However the idea was to use 3 relay with 3 phase motor, where each relay handles 1.5 Hp on it phase. \$\endgroup\$
    – Owayl
    Commented Aug 9, 2019 at 0:45
  • \$\begingroup\$ Three individual relays for a three phase motor is not proper motor control regardless of the relay rating. For a 3-phase motor, the line-to-line voltage needs to be used for the relay. If the motor is 400 V L-L, I would expect immediate and catastrophic failure of SLA relays. If the voltage is 240 V L-L, the relays might not fail, but it still be a bad design. \$\endgroup\$
    – user80875
    Commented Aug 9, 2019 at 1:24
  • \$\begingroup\$ right, the voltage here for 3 phase is 400V L-L and the relay is indeed rated 240V, thanks for clarification. \$\endgroup\$
    – Owayl
    Commented Aug 9, 2019 at 6:41

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