Author: Mélanie Olivier

The electrical protection regulations require thermal protection for electric motors.

In direct motor supply (motor supply without inverter or electronic starter) the thermal protection function is provided by default via a bimetal thermal relay.

This is an economical solution at the time of purchase but has some disadvantages in terms of motor availability during operation compared to digital thermal protection.

What is a bimetallic thermal protection relay?

The bimetallic thermal relay has the current flowing through it to supply the motor. It protects the motor against abnormal heating of its windings. This occurs when the current drawn by the motor is too high for a certain period causing the windings to overheat.

If the phenomenon persists, it causes melting of the windings, which makes the motor unusable and requires replacement.

The bimetallic thermal relay is set to heat up faster than the motor windings, so it will cut off power to the motor to preserve it.

Bimetal thermal protection induces uncertainty in the process management

The bimetal thermal protection relay is a simple electro-mechanical device that does not allow its overheating status to be known. Therefore, it is not possible to know how soon it will open the motor supply circuit.

The available “thermal reserve” is therefore not known before the motor is switched on and off. In this case, this uncertainty must be allowed for in the operation of the manufacturing process including the motor.

If the bimetal thermal relay protects the motor, the motor supply circuit will remain open for a given time, which corresponds to the bimetal cooling down (and also to the motor since it is no longer supplied during this period).

Once the Bimetal has cooled down, it re-establishes the circuit and the engine is again available for the next start.

From the process control, the waiting time for cooling is not known, so it is necessary to wait, with the engine stopped, without being informed about the time of this situation.

When the thermal relay re-establishes the motor supply circuit, after a trip and a certain waiting time, the motor can be restarted to re-launch the process.

The motor’s peak power consumption on start-up may cause the thermal relay to trip again, as it did not have enough thermal reserve to support a new start.

This is a return to the fault point and it is again necessary to wait for the end of the cooling cycle before attempting a restart.

These three typical examples show that using a bimetallic thermal relay can protect the electric motor, but it does not provide a smooth operation of a manufacturing process that requires availability and continuity of service.

What is a digital protection ?

The digital thermal relay also measures current to the windings of the electric motor and calculates their temperature rise using the formula I²t. Unlike the bimetallic thermal relay, it does this via electronics and a microprocessor. A motor protection curve is selected to match the protected windings.

Digital relays ensures smooth process control

The temperature rise and thermal reserve of the engine are known at all times. This provides a valuable real-time indication of the engine’s condition and thus gives early warning that the engine’s operating limits may be exceeded.

This time can then be used to start a back-up motor or to adjust the motor load to avoid tripping and consequently stopping production.

If nothing was corrected, the thermal relay is triggered after its warning countdown when the engine needs to be protected. However, the time required for the engine to cool down is calculated and thus the waiting time before a possible restart is immediately announced.

This is useful information for the process control as the waiting time before engine restart is known. The situation’s critical nature in relation to the current production process can be judged immediately.

Once this waiting time has elapsed and been counted, the digital thermal relay starts the motor again.

As seen earlier, restarting the motor with a bimetallic thermal relay after a warm-up can be uncertain. This is not the case with a digital thermal protection.

Indeed, the digital protection relay knows the motor’s temperature rise on starting. This allows it to make the motor available for restarting when its windings have cooled down but also, and above all, when the motor has enough thermal reserve to withstand a start.

Thus, the operator can be sure that he will be able to restart his motor when he is allowed to do so!

These three situations show the advantages of digital thermal protection over bimetal thermal protection. Digital protection ensures smooth operation of a manufacturing process that requires availability and continuity of service.

Going further with electronics

As the digital thermal relay has electronics and a microprocessor, it is possible to go further than just motor protection. Many other useful protections are available from current and voltage information.

For example, phase unbalance protection, over or under current protection, over or under power protection, phase reversal, detection of too long a start or limitation of the number of starts in a time period… The list is long!

> Discover GemStart 5, our digital thermal protection relay to protect your hydrogen production motors.