I would like to control a 1-phase 3HP motor using a SSR ( the reason being, I need to switch it on/off using a signal (0-5VDC).

Are normal SSRs (ex:Fotek 40A model) capable of handling the starting current of a 3HP motor?

What are the protection devices that I need to use in the circuit.

I guess a Motor MCB is required. Is anything else required?

All opinions/suggestions are welcome. Thanks in advance


2 Answers 2


Yes, but...

There's not great data available on the Fokotek products, but in general switching HP sized motor loads with solid state relays is a fairly expensive proposition. Because the voltage and current values don't cross zero at the same time, motor loads (inductive loads) are harder to switch than resistive loads. 3HP is just on the cusp where SSRs are still practical if you don't mind spending a little money, but you have to be careful about product selection. Here are some things that can help you figure out what a given device is rated for when you read the datasheet:

  • Products that are intended to switch voltage to motors will be rated with a horsepower capacity in addition to an amperage. For examply, Cryodon does make SSRs that are rated for a 3HP motor load at 120VAC (I'm assuming you're in the US using residential power, switching is slightly more efficient at higher voltages,) but note that the nominal current rating of the contactor is 125 Amps!
  • On the more technical end, switching capacities will actually be rated according to a specific standard. The relay needs to handle a different load at the time of switching for a light bulb, a transformer, and a motor all of the same nominal amperage. For equipment marketed to the US, you'll often find the rated switching current defined for each IEC utilization category that the device is intended for. Your motor probably falls under AC-3 so you would look for a specific rating in that category and compare it to the HP or Full Load Amperage (FLA) of your motor.
  • Sometimes listings will be shown with respect to a specific standard as well, eg UL508 or IEC 60947. In this case the datasheet should indicate specifically the type of motor and the voltage that the HP rating applies to.
  • In general, if it doesn't have an HP rating or a specific utilization category, and nothing on the data sheet specifically says it can be used on motor loads, you should assume that it cannot be. Depending on the size it may work for a while, but you are likely to damage the switching element over time and it will eventually fail. In the world of motors, many failure modes can be dangerous, including an inability to stop the motor and a resistive short causing a fire in the switching element.

As far as other protective devices, you are correct that you should use a dedicated circuit breaker to protect from a short in the motor. In addition to this you should* have a thermal overload device. This device is meant to protect the motor in case of a stall where there's no short circuit, but the motor is drawing it's full amperage for too long without being able to cool itself. The trip current is much lower than the circuit breaker, but the time over which this device averages data is much longer. Motors are available with thermal overloads built into them, and separate thermal overload devices are also available (usually as an accessory for a contactor.

If your application involves a safety concern when it operates, then even if you use an SSR for routine operations, your lockout/tagout and/or emergency stop functions should be accomplished using a more positive means of power disconnection (probably a disconnect switch for the LOTO and a mechanical contactor for E-stop.)

If the main thing driving you to the SSR is the control voltage available from your top-level controller (perhaps an arduino?) there are a few options that may be better choices for you:

  • A conventional contactor with a thermal overload, and a smaller relay to step up your 5VDC signal. Your controller would control a small relay with a 5V or less coil voltage. The load side of the small relay would be connected to your Line Voltage at 120VAC (or whatever your motor uses) but instead of powering the motor, it would power the coil on a traditional motor contactor (this is just one example). In this configuration the small relay (often called a pilot relay or a PLC relay) on has to be rated for the coil consumption current of the relay so 1.1 amps AC-14 or AC-140 in the example I linked to. This solution also allows you to use a conventional thermal overload accessory without having to mount it separately in your enclosure. Note that since most thermal overloads and contactors are designed for 3 phase motors, and include a mechanism to detect loss of a single phase, you'll need to feed the same current through all 3 paths on the contactor. In practice, this is easy, you just run one leg of your power in through two paths in series.
  • Alternately, you could use a small variable frequency drive. A VFD is basically 3 SSRs and a top-level controller, and it would give you lots of options for control, and typically includes monitoring that counts as your thermal overload detection. THe biggest down side is that VFDs are pretty much only available for 3-phse motors, so if you already have a single phase motor you might not be able to use a VFD. VFDs are available that accept single phase inputs, but still output three phase. VFDs are designed to be controlled my outside sources, so communicating over a 5 volt interface or even something more sophisticated will be straightforward. Additionally, VFDs will reduce the amount of inrush power that your motor pulls off of the line voltage supply, which can start to be a problem in motors of this size.

*(Whether this is strictly required or not depends on your jurisdiction and some other details, but it's definitely a good practice if you don't like fire.)


You can also just use a VFD similar to this Schneider Electric one. Looks to be only 2HP but there should be 3HP available.


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