Magnetic starter

The motor protection ( for example, a motor circuit breaker ) protects electric motors ( usually asynchronous ) against thermal overload due to mechanical overload or failure of a single or two phase conductors.

• 3.1 Thermal triggering overload protection
• 3.2 Electronic Overload Protection

Basics

Electric motors are designed for a particular design operation in which remain the critical parts of the engine within the permissible temperature range and short-term overloads do not affect hazardous. The motor protection device must on the one hand the economic operation of the engine with full use of the characteristic values ​​allow, but on the other hand react sufficiently quickly to overloads. Due to typically 4 different insulation classes for the winding and specific interpretations of the motors for special applications, there are different maximum permissible period of temperatures and different demands on the motor protection device that must be met by proper planning and execution.

The motor protection more generally divided into the themes protection needs ( the engine and the system) and protection methods ( current dependent protection and / or temperature- dependent protection ).

Motor protection

There are two basic ways to protect a motor against overload in operation: firstly, monitoring of its power consumption, on the other hand, the direct monitoring of the temperature in the motor windings. Often, a combination of both methods is done (eg in difficult load curves ). The first category includes the motor circuit breaker and overload relay (also motor protection relays), the second self-resetting bimetallic switch and PTC.

The tripping time of the overload protection is determined by the current intensity or temperature and the setting of the tripping area. You must be set for all current values ​​below the danger time, the motor insulation. As a result, according to EN 60947 the requirement to specify maximum times for the overload. To avoid unnecessary trips, here limits for the boundary current and the motor standstill are fixed.

Motor protection switch

Motor circuit breakers are manufactured, for example, for three-phase AC motors. In this embodiment, the motor protection are OR -linked activation by monitoring the currents in the three leads ( current overload protection system ). The monitoring can be realized thermal- mechanical ( bi-metal), thermal- electronic ( PTC) or electronically ( current measurement).

Three-phase motors should be connected via suitable motor protection switch or motor protection relay to the power supply to prevent damage caused by overload or failure of an outer conductor. This type of motor protection is usually always set according to the rated motor current In. Restarting after triggering either automatically or manually by pressing a unlock key done.

The motor protection switch partly undervoltage release are integrated. Motor protection switch often protected by a short-circuit release and the supply network from short circuits, but this is not necessary to qualify as a motor protection switch. Motor protection switch, which serve the short-circuit and overload protection must be incorporated at the start of the 0113 motor cable according to DIN VDE. The short-circuit protection function can also be acquired by a coupled circuit breaker at the beginning of the supply line.

Overload relay

Overload and motor protection relay function on the same principle as the motor protection switch, but they themselves do not directly switch the motor off. If a motor protection relay trips, one or several smaller contacts operated ( auxiliary contacts ). Usually, a contactor is controlled through one of these contacts, which then turns off the consumer. Many motor protection and overload relays are designed so that they can be directly attached to a contactor and be connected without additional lines with this. In contrast to the motor protection switch, a motor protection relay has a short -circuit release, but only the thermal release (bimetal ), which would react may be too slow in case of short circuit. It is therefore necessary in the lead for one or more engines that are protected with overload relays, fuses are installed.

Overload relay is also available in higher voltage levels of the power supply network. You are connected there via suitably insulated current transformers and solve a power switch.

Triggering mechanism

Thermal triggering overload protection

The protection is accomplished by thermally activated bimetallic by heating coils (phase resistors) through which the motor current flows, to be heated. In this case, a separate bimetal is provided with corresponding heating winding for each current-carrying wire to the motor. Exceeds the power consumption of only one winding of the motor to the predetermined value over several seconds, triggers the deformed by the heat from the switching mechanism of the bimetal thermal relay and interrupts the circuit to the motor. Similarly, in case of failure of an outer conductor ( uneven heating of the bimetal strip ) is after a short time off ( OR operation). Upon thermal activation, the switch turns on again after cooling of the bimetallic strips. The trip currents thermally triggering motor protection switches are within certain limits (up to a factor of 1.6 ) adjustable and must be set to the rated current of the motor. It should be noted in particular the windings of three-phase motors, the connection type (star or delta). The resetting after tripping can take place only after the bimetal has cooled sufficiently.

Electronic overload protection

Electronic timer with current transformers measure the current of each phase conductor and mimic the behavior of a thermal motor protection switch after. Advantages of electronic equipment is a larger area of the adjustable rated motor current and a greater variety of functions. For example, in many electronic motor protection switches, the tripping characteristic can be set ( eg delayed tripping for motors with heavy starting ) and the switch can be controlled remotely after a fault reset.

Phase failure monitoring

To monitor the equality of all three voltages of the three phase -to-phase, there is phase control relay. Protect three-phase loads (motors, rectifiers, transformers) before the phase imbalances associated with increased loads. Such monitoring relays often have an adjustable tolerance threshold.

Single-phase motors and loads

Is a three-pole overload protection for single-phase motors is used, the three current paths must be connected in series. When using only one current path many switches would be premature shut down because they often also monitor the uniform load or voltage of the three phase conductors. Whether this circuit is permitted with electronic motor protection switches, must be obtained from the product data sheets of the manufacturer in the individual case.

Small single-phase motors (eg refrigerator compressors ) are often protected with a simple self-resetting bimetallic switch.

For equipment and control panels, there are also toggle switch with thermal overcurrent trip.

Engine monitoring with thermistors ( motor protection )

Today, many engines have incorporated thermistors with which the temperature of the windings is monitored. Exceeding the permissible temperature then leads to the message or to switch off the engine. The thermistors are thermistors which have a resistance increase in the nominal cut, for example, 1 k. They are standardized in DIN 44081. Since the temperature of the motor windings is monitored directly here, this protective device is suitable for all types of electric motors, regardless of power and connection. A restart is only possible after sufficient cooling of the engine, depending on the external circuit either manually or automatically.