In order to protect the electric motor from high load currents, in addition to the circuit breaker, it is necessary to install a thermal relay. The principle of operation of the thermal relay is outrageously simple. At that moment, when an excess load occurs on the electric motor, the thermal relay cuts off the power from the magnetic starter coil.

Phase cutting to the coil occurs due to the heating of the bimetallic plates, which diverge at high load. The manufacturer calculates the expansion of the plates, which heat up when current passes through them in excess of the allowable rate.

Simply put, when a load arose, the bimetallic plates expanded and cut off the power to the magnetic starter. The thermal relay must be selected based on the power of the electric motor. For finer tuning, all thermal relays have an adjustable range that can be set up to one ampere.

The thermal relay is connected between the magnetic starter and the electric motor. In some models, all three phases pass through the thermal relay, but basically two phases are passed through the heater, and the third goes directly from the magnetic starter.

With the power ends going to the electric motor, we figured it out, now let's look at how to make it so that at a high load, the magnetic starter cuts off the power to the electric motor.

In order to connect a thermal relay, you need to read the article connecting a magnetic starter. If you already know this, then move on. As you remember, the phase going to the stop button is taken from the top contacts of the starter.

The phase going to the buttons must be passed through special contacts on the thermal relay. The principle is simple, the phase has entered - the phase has left. If there is a load on the electric motor, the plates between these contacts will open and the starter will turn off. You will find the location of the contacts on the relay yourself. In total there are five clamp contacts, three power and two for control. As you can see, everything is simple and without unnecessary chatter.

In order to choose the right thermal relay, you need to look at the power of the electric motor and its current rating, which are indicated on the motor plate. It happens that the plate is missing, then take the pliers and measure the currents in each phase, preferably under load. If the electric motor is not hot, feel free to focus on the readings of the device. Let's say you showed 16 amperes, add 20% percent for starting currents and choose a thermal relay where you can set 20 amperes and safely connect it.

When triggered, a button pops up on the thermal relay, which can then be turned on. If the operation starts to occur frequently, and the load, in your opinion, does not increase, then it is quite possible that you have an interturn short circuit, which you can also read about on our website about electricity.