Sudden heat death excluded!
Reliable protection of electric motors
Unnecessary premature tripping must be prevented under all circumstances. This is the only way to ensure customer satisfaction - in our example, for the user of the garden shredder. This means that the protective element must be precisely matched to the limit temperature characteristic of the motor.
Motor heat can be monitored directly or indirectly. For direct temperature monitoring we use either PTC thermistors or bimetal temperature limiters. They directly measure the temperature in or on the motor winding. However, indirect temperature monitoring using circuit breakers is done by measuring the current flow in the supply line. The motor's power consumption serves as a heat indicator. The higher the overload current, the faster the acceptable motor temperature limit is reached. The permissible overload time is correspondingly shorter.
The circuit breakers most used in motor protection operate with thermal bimetals. As the load on the motor increases, the current flow through the bimetal also increases, causing the temperature of the bimetal to rise. The metal with the higher thermal expansion coefficient expands more than the other, causing the bimetal to bend. If the load is too high, it will finally trip the switching mechanism by overcoming the pressure of the spring.
Advantages of indirect temperature monitoring with circuit breakers
Benefit 1: Flexible installation location
Thermistors and temperature limiters measure the temperature of the motor directly, which is why they must be installed in or in the direct vicinity of the motor, whereas circuit breakers can only measure the temperature indirectly via the current flow, and can therefore be installed almost anywhere within a device or machine. The circuit breaker can easily be installed in a way that the reset button is clearly visible and accessible.
Benefit 2: Various additional functions
The main function of a circuit breaker is to disconnect harmful overcurrents. But circuit breakers can do much more than that. Circuit breaker/switch combinations also serve as ON/OFF-switches for equipment and machinery. If additional protection against automatic re-start after a power failure is required, the circuit breaker can be fitted with an undervoltage release module.
Benefit 3: Customer-specific adjustment
Circuit breakers with strip bimetals as tripping elements can be individually adjusted using an adjustment screw that changes the pre-tension of the bimetal. This means the time/current characteristics of the circuit breakers used can be adjusted to the temperature limit curve of the motors used. This ensures that the motor power is utilised to its full extent. The goal of circuit breaker selection is to avoid unnecessary and annoying premature tripping and to switch off harmful overcurrents without compromise.
