Reverse start of the engine is necessary in order to cause rotation in both directions. The principle is found in many devices: drilling, turning, milling machines. What about overhead cranes? There, all drives operate in reverse mode to enable the bridge to move forward and backward, the hoist to the left and right, and the winch up and down. And this is not all where this mode of operation is applied. It is about the reverse engine start scheme that you can read in the article below.
What causes the reverse switching of a three-phase motor
To begin with, let's look superficially, what causes the reverse? It is caused by the change of 2 wires in places, as a rule, in the branded box of the engine.
On the photo: a sample of a branded box with a star connection.
In the figure above, we see that the beginning of the windings (C1, C3, C5) are free to be included in the network. winding ends(C2, C4, C6) connected together.
In the photo: connection with direct connection of the engine to the network.
In the figure, the colored circles indicate the contacts for connecting the phases. Phase A is indicated in yellow, and it is connected to contact C1, green - phase B (C3), yellow - phase C (C5).
Observing the above conditions, we will swap any 2 phases and connect as follows. Phase A remains in its place, contact C1, phase B is placed on contact C5, and phase C is placed on contact C3.
On the photo: star connection with reverse switching.
Thus, it turns out that we need 2 starters. One starter is required for direct switching, and the second for reverse switching.
Determining the operating mode
Now let's decide how the engine will work: constantly on and off when the stop button is pressed. As, for example, in drilling, turning, milling machines. Or we need it to work while holding the start-right or start-left button, as, for example, in winches, electric carts, crane beams.
For the first case, it is necessary to draw up a circuit for reversing the start of an asynchronous motor in such a way that the starter is self-bypassing, and also to protect against accidental switching on of the second starter.
Reversing circuit with blocking and protection
Description of the above workschemes
Let's analyze the operation of the circuit diagram of the reverse start of the engine. The current comes from phase C to the normally closed common button KnS, the stop button. Then it passes through a common current relay, which will protect the motor from overloads. Then, when you press KnP "right", the current passes through the normally closed contact of the KM2 starter. Entering the coil of the KM1 starter, the core is drawn in, closing the power contacts, breaking the power to the KM2 starter.
This must be done in order to break the power of the second starter and protect the circuits from short circuits. After all, the reverse is ensured by the fact that any 2 phases are reversed. Thus, if the KNP "left" button is pressed when KM1 is on, the start will not occur. Self-shunting is provided by an auxiliary contact, depicted under the “right” button. When the starter is on, this contact is also closed, providing power to the starter coil.
In order to stop the engine, it is necessary to press KNS (“stop”), as a result of which the starter coil will lose power and return to normal. Now that KM1 has returned to its normal state, it has closed the normally closed group of auxiliary contacts, thanks to which the KM2 starter coil can again receive power, and it has become possible to start rotation in the opposite direction. To do this, press the KnP "left", thereby including the KM2 starter. Receiving power, the coil draws in the core and closes the power contacts, including power to the motor, swapping 2 phases.
Analyzing the operation of this reverse engine start circuit, you can see thatshunting is provided by a normally open auxiliary contact, shown under the button KnP "left", and it breaks the power to the KM1 starter, making it impossible to turn it on.
The circuit for a three-phase drive was considered above. At the very beginning of the circuit, immediately after the KNS, you can see a normally closed contact from the current relay. In case of excessive current consumption by the motor, the relay is activated, interrupting the power to the entire control circuit. Everything that works in the control circuit will lose power, and this will save the engine from failure.
Details on deadlock
Induction motor reverse start circuit requires interlock. It should be understood that to change the direction of rotation of an asynchronous motor, you need to change any 2 phases in places. To do this, the inputs of the starters are connected directly, and the output is connected crosswise any 2 phases. If both starters are turned on at the same time, a short circuit will occur, which, most likely, will burn the power contact groups on the starters.
In order to avoid a short circuit when installing a reverse motor start, it is necessary to exclude the simultaneous operation of both starters. That is why it is necessary to apply a deadlock scheme. When the first starter is turned on, the power to the second starter is interrupted, which excludes its accidental activation, for example, both start buttons are pressed at the same time.
If it so happened that when you press the button that should turn on the “rotation to the right”, and the engine rotates to the left, and, conversely, when you press the “rotation to the left”, the enginerotates to the right, do not reassemble the entire circuit. Just swap 2 wires at the input - that's it, the problem is solved.
It may happen that it is impossible to do this on the input due to some circumstances. In this case, swap the 2 wires in the branded box on the motor. And again the problem is solved. The right button will rotate right, and the left button will rotate left.
Wiring diagram for reversing start of an asynchronous (single-phase) motor
The diagram above shows the reverse connection of a single-phase motor. This reverse engine start scheme is much simpler than the previous one. This uses a 3 position switch.
Description of the circuit for reversing the connection of a single-phase motor
In position 1, the mains voltage is transmitted to the left leg of the capacitor, due to which the motor rotates, relatively speaking, to the left. In position 2, power is supplied to the right leg of the capacitor, due to which the motor rotates, conventionally speaking, to the right. In the middle position, the engine is stopped.
PT is much simpler here. As you can see, here, too, simultaneous switching on by a 3-position switch is excluded. For those who are interested in the question, what, nevertheless, will happen when turned on at the same time, the answer is simple: the engine will fail.
Reversing circuit without self-shunting
We will tell you more about the starting control circuit for a reversible asynchronous motor as follows. When the KNP “right” button is pressed, power is supplied through the normally closed KNP “left” contact, and thanks to the mechanical connection, it breaks the power supply to the KM2 starter, excluding the possibility of turning on the KM2 when 2 buttons are pressed simultaneously. Further, the current flows to the normally closed contact of the KM2 starter to the coil of the KM1 starter, as a result of which it works, including power to the motor. The reverse is turned on by the KnP "left", which also breaks the power supply of the KM1 starter with its normally closed contacts, and normally open switches on the power supply of the KM2 starter. That, in turn, turns on the power to the engine, but with the change of 2 phases in places.
Let's pay attention to the control scheme. Or rather, deadlock. It's set up a little differently here. The power supply of one starter, not only is it blocked by the normally closed contact of the opposite starter, it is also blocked by pressing a button. This is done so that when 2 buttons are pressed simultaneously, in those fractions of a second, until the starter breaks the power of the second starter, they do not turn on at the same time.
Single-phase motor diagram
When you press and hold one button, the power is broken to the second button, the power comes to the 1st leg of the capacitor. When the second button is pressed, the power is cut off after the first button and goes to the 2nd leg of the capacitor. RT still protects the motor from overloads.
Conclusion
In conclusion, wherever you use these schemes, pay attention to deadlock. This is the necessary measure that will protect the equipment from damage. In addition, you need to correctly select starters for three-phase options, and buttons for single-phase options. After all, improperly selected equipment in terms of power, current and voltage will quickly become unusable, and can also damage the engine.
