Freight elevator drive control
Consider a simplified drive scheme of a freight elevator with an induction motor. Engine starting is controlled by a reversible magnetic starter and stopping by an ETM electromagnetic brake. A control station with a button is usually located on the ground floor near the mine. The number of trigger buttons corresponds to the number of floors. Pressing the button on a certain floor is done using floor switches EP and floor relays ÉР. Three position switches are activated by the cab itself when it moves up and down.
In the diagram of fig. 1, the two contacts of the floor switch are open on the floor where the car is currently located. On all floors below the car, the left contacts are closed, and on the floors above the car, the right contacts are closed. For an emergency stop of the cabin, press buttons C. In the control circuit, together with button C, the door limit switches of all floors and safety contact KL are connected in series.
Rice. 1. Schemes of the electrical drive of the freight elevator
Let's look at the principle of operation of the elevator control circuit (see Fig. 1). The car stopped on the second floor, which is why the EP2 contacts are open. When the BB input switch is on, it is possible, for example, to lower the cabin to the first floor.
To do this, press the start button P1 on the first floor and thus close the circuit of the coil of the contactor KN. In this case, the current path will be as follows: from the line wire L1 through the door limit switches BD1, BD2, BD3, BD4, block opening contacts KB, KN, start button P1, relay coil ER1, left contact EP1 on floors switch, block opening contact KB, contactor coil KN, cabin safety cut-off button KL, button C and line wire L3.
After the KH contactor is turned off, the contact of the KN block is turned off, but the supply circuit of the contactor coil is not interrupted, because the current in the KH coil will pass through the closing contact ER1 of the ER1 relay, in addition to the KN blocking contact and the P1 button .
Rice. 2. Freight elevator
The ETM electromagnetic brake will receive power simultaneously with the motor stator winding and release the brake pads. The motor will move the car down the first floor until it encounters floor switch EP1, which will turn off its contacts and thus break the supply circuit to the contactor coil KH. The brake magnet will immediately release its pads and stop the engine.
If it is necessary to move the car, for example with cargo, to the fourth floor, it is necessary to close the doors of the car first, and then press the button on the fourth floor P4.From the line wire L1, the current will pass through the mine door limit switches BD1, BD2, BD3, BD4, the opening auxiliary contacts KB and KN, the start button P4, the coil of the relay ER4, the right contact of the floor switch EP4 , the opening contact block KN, the coil of the contactor KB, the button of the safety device of the cabin KL, the button C «stop» and the line wire L3. Once energized, the KB contactor coil will close the KB power contacts.
The electromagnetic brake and the motor will be powered. The motor will start to rotate in the opposite direction and raise the cab up. At the same time, the auxiliary contact of KB opens, but the supply circuit of the coil of the contactor KB is not interrupted, because after actuation of the relay ER4, it self-locks with its closing contact ER4 and the current will flow through the KB and KH auxiliary contacts and button P4. When the car reaches the fourth floor, floor switch EP4 will break the supply circuit of the KB contactor coil and the motor will stop immediately.
If before starting the engine, any door is not closed or tightly closed, then the engine cannot be started, because all four door shaft limit switches are connected in series with the coils of the reversing magnetic starter. The motor is protected by an automatic BB switch.