Protection of three-phase asynchronous electric motors
Three-phase electric motors at accidental shutdown of one of phases quickly overheat and fail if in time not to disconnect them from network. Therefore the different systems of the automatic protection disconnecting devices were developed, however they either are difficult, or are insufficiently sensitive.
Devices of protection can be separated conditionally on relay and the diode-transistor.
Relay, unlike the diode-transistor, are simpler in production. Let's consider several relay circuits of automatic protection of the three-phase engine at accidental shutdown of one of phases of the power supply of the network circuit.
It is the most widespread way checked by time. Protection of the engine against shutdown of one phase is provided with use of the TZ thermal relay. The sense of this protection consists that the constant of heating of the thermal relay is selected in the same way, as the constant of heating of the electric motor. That is, in other words, the relay heats up as well as the engine. And when exceeding temperature the relay is higher admissible switches-off the engine. At shutdown of one phase current through other phases sharply increases, the engine and the thermal relay begin to heat up quickly that causes operation of the thermal relay. The way is good also the fact that it provides also protection of the engine against the overload and breakdown of one phase on the body. But for reliable protection against breakdown on the body the engine shall be grounded or занулен. The lack of this way is that it needs to be selected and adjusted rather precisely. Ideally its rated current has to be same, as well as at the engine.
The additional relay P with normally opened contacts of P1 is entered into the normal system of start of the three-phase engine. In the presence of tension in three-phase network the winding of the additional relay P resides energized and contacts of P1 are closed. When pressing the Start-up button through the winding of the electromagnet of the MT magnetic actuator there passes current, and the electric motor is connected by MP1 contact combination to three-phase network. At accidental disconnection from network of the wire And the relay P it will be deenergized, contacts of P1 will be opened, having disconnected the winding of the magnetic actuator which contact combination of MP1 will disconnect the engine from network from network. At disconnection from network of wires In and With the winding of the magnetic actuator is deenergized directly. As the additional relay P the MKU-48 alternating current relay is used.
The scheme of the protection device is similar to the scheme considered in the second way. When pressing the Start-up button the relay P turns on, contacts of P1 closing the feed circuit of the coil of the MT magnetic actuator. The magnetic actuator works and contacts of MP1 turns on the electric motor. At break of line conductors In or With the relay P is switched-off, at wire break And or With - the MT magnetic actuator. In both cases the electric motor is switched off by contacts of the magnetic MP1 actuator. In comparison with the scheme of the protection device of the three-phase engine considered in the first way, this device has advantage: the additional relay P at the switched-off engine is deenergized.Top