I installed a circuit to automatically start a dust extractor when I turn on one of my woodworking machines.

I have one 230V circuit dedicated to powering the machines and another circuit powering the dust extractor. The live wire for the machine circuit goes through a current sensing relay, which then turns on a 40A solid state relay, which switches the extractor live on and off. The motor on the extractor is a single phase 1500W ac motor with a single capacitor.

I also wired in the motor thermal cutout, so that it switches off the 40A SS relay. Also a manual switch that I could use to switch off the 40A relay if I wanted/needed to.

My initial calcs were that the motor draws 6A at full load, so I thought that a 6 times starting current would be enough, hence the 40A solid state relay. Both circuits are protected by Type C 32A RCBOs (Residual current circuit breaker with Overcurrent Protection). I thought that the type C would cope with the starting surge better.

The setup ran fine for about 6 months, then stopped working. I checked the motor and thought that the starter winding was open circuit. I fitted a replacement motor, but the extractor wouldn't start via my circuit. I plugged the extractor directly into a normal supply and it started.

Further investigation showed that the solid state relay was not working - the input is still switching an indicator light on, but the output is staying open circuit.

I tried to start the extractor through the relay a few times while running diagnostics. The motor will now not start even when plugged into a simple mains circuit. It just hums loudly, which seems to indicate that the starter circuit is not functioning.

However, the single, 50uF capacitor tests OK on a multimeter; there is no short to ground (using a multimeter) on the coils or the thermal cutout; the thermal cut out is 0 ohms; and the resistance on the start and run coils are both 3.1 ohms. (Interestingly, the orginal motor tests with similar values - I was mistaken when I thought that the starter circuit was open circuit)

  1. Any fundamental problems with my simple circuit? Should there be additional protection other than the RCBOs which have never tripped?

  2. Both motors seems to test out OK, but won't start. Any ideas what to try next?

  3. I think that the relay has burnt out because it is under size. Would a 60A chinese replacement be good enough or should I get a motor specific brand name one or a bigger one?

  4. Am I correct to include the thermal cutout in my relay circuit?

40A SSR: https://www.amazon.co.uk/gp/product/B07RLQZ85Z/

Current Sensing Switch: https://www.amazon.co.uk/gp/product/B07RWCMWCK


  • \$\begingroup\$ Did you measure C by RC = T response time ? or just open circuit? or use a diode test mode and compare a know good cap, like a low voltage type with similar behaviour vs time. If it is a high efficiency motor, except 8x surge current \$\endgroup\$ Oct 29, 2021 at 15:39
  • \$\begingroup\$ I really liked my 5000 CFM 3 phase uC AC BLDC controlled blower I got with current sensing and 1x surge current. smooth quiet and efficient. (cheap $80 but lucky, half price because the cardboard box was damaged.) All the noise is from the blade air interference but for you white noise is good with a mill. \$\endgroup\$ Oct 29, 2021 at 15:49
  • \$\begingroup\$ Everything's look fine. Do the SSR has a heat sink? Usual triac drop voltage is 1V, and heat radiator need dissipate 6W. Body of SSR not enough. \$\endgroup\$
    – user263983
    Oct 29, 2021 at 19:50

4 Answers 4


The triac would remain in conducting mode, if damaged due overcurrent. A capacitor would loose the capacitance, as it is probably self healing. With an overvoltage, the dielectric is being damaged and those punctures make a short circuit and the surrounding conductive foil melts due high curent, thus isolating the puncture. More punctures cause the loss of the capacitance.

If the triac gor damaged in non-conductive state, then the cause could be overvoltage.

There could be several causes for overvoltage:

  • The motor has a run capacitor (only one), but it runs unloaded. If your load isn't always present, then you do need a motor with a centrifugal switch that at certain RPM disconnects one capacitor (start cap) and continues only with run cap.
  • If there is no RC snubber inside the SSR, then it could misfire and cause large dump.
  • A suitable SSR for inductive load (motor) has to be random phase and not zero-cross type, but there is no data for your product. So you'd better avoid products without clear specifications.

IMO: you should use a contactor instead, triac isn't suitable for switching capacitive loads.


Solid state relay has inside optocoupler and triac. In "on" condition triac usually has drop voltage approx 1V. Average power is 6W and if there is no heatsink, the device overheated. Overcurrent is not an issue because triac can stand current 10 times from maximum for one halfwave. The body of your SSR designed to work with heatsink. And there is some probability the triac is defective.


OK That's all interesting stuff. I don't have a heat sink on the SSR, so that is probably why it failed. I think that I might take Marko's advice and install a contactor instead.

Interestingly, both motors now run on the bench under no load - if I simply hook them up to a 230V supply without any connections on the thermal switch. I've mounted one back onto my dust extractor and connected up to the thermal switch circuit and it runs OK under load from a 230V ring main.

I don't know why the motor wouldn't run before, but I think that this topic can be closed. Thank you for your help.


As a gross generalization, using an SSR on a single phase motor with a capacitor start is a race to see which one fails first. To the SSR, the charging current of the capacitor looks like a short circuit, so too much dI/dt (rapid change in current) which can cause the SSR to short itself out (become a full time conductor). To the capacitor, the SSR firing control can result in a harmonic heating effect that damages the capacitor. If the capacitor fails, the start windings are damaged.

Use a contactor, not an SSR on a capacitor start motor.


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