Reverse engine starting is necessary in order to cause rotation in both directions. The principle is found in many devices: drilling, turning, milling machines. And the crane beams? There, all drives operate in reverse mode to enable the bridge to move back and forth, hoist left and right, winches up and down. And this is not all where this mode of operation is applied. It is about the reverse engine starting circuit that can be found in the article below.
What causes the reverse inclusion of a three-phase motor
To begin with, we will examine superficially, what is the reason for the reverse? It is due to the change of 2 wires in places, usually in the branding box of the engine.
In the photo: a sample of the branding 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. The ends of the windings (C2, C4, C6) are connected together.
In the photo: connection with direct connection of the engine to the network.
In the figure, colored circles indicate the contacts for connecting the phases. Yellow indicates phase A, and it is connected to contact C1, green indicates phase B (C3), yellow indicates phase C (C5).
Observing the above conditions, we will change any 2 phases in places and connect as follows. Phase A remains in place, contact C1, phase B is placed on contact C5, and phase C is placed on contact C3.
In the photo: star connection with reverse switching.
Thus, it turns out that we need 2 starters. One starter is necessary for direct switching, and the second for reverse switching.
Definition of an operating mode
Now we’ll determine how the engine will work: it is 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” buttons, 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 the induction motor in such a way that the starter is self-shunted, and also protect against accidental switching on of the second starter.
Reverse switching circuit with interlock and protection
Description of the above scheme
Let us analyze the work of the concept of reverse engine starting. The current flows from phase C to a normally closed common button Kns, the button "stop". Then it passes through a common current relay, which will protect the motor from overloads. Then, when the “right” button is pressed, the current passes through the normally closed contact of the KM2 starter. Entering the coil of the KM1 starter, the core is retracted, closing the power contacts, breaking the power to the KM2 starter.
This must be done in order to disconnect the power supply of the second starter and protect the circuit from short circuit. After all, the reverse is ensured by the fact that 2 any phases are reversed. Thus, if the KN1 button is “left” when KM1 is on, the start will not occur. Self-shunting is provided by an auxiliary contact depicted under the KNP “right”. 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 will return to normal. Now that KM1 has returned to its normal state, it has closed a normally closed group of auxiliary contacts, due 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 retracts the core and closes the power contacts, including power to the motor, replacing 2 phases.
While analyzing the operation of this reverse engine start-up circuit, it can be noted that the bypass is provided with a normally open auxiliary contact, shown under the button "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 KnC, you can see a normally closed contact from the current relay. In case of excessive current consumption by the motor, the relay operates, breaking the power supply to the entire control circuit. Everything that works in the control circuit will lose power, this will save the engine from failure.
Learn more about deadlock.
The electrical circuit for reversing the start of an induction motor requires a deadlock. It should be understood that to change the direction of rotation of the induction motor, you need to change any 2 phases in places. To do this, the starter inputs are connected directly, and the output is connected crosswise any 2 phases. If both starters are turned on, a short circuit will occur at the same time, which is likely to burn the power contact groups on the starters.
In order to avoid a short circuit when installing a reverse engine 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 switched on, the power to the second starter is cut off, which excludes its accidental switching on, for example, both “start” buttons are pressed at the same time.
If it so happened that when you press a button that should turn on "rotation to the right," and the engine rotates to the left, and, conversely, when you click "rotation to the left," the engine rotates to the right, you should not reassemble the entire circuit. Just swap 2 wires at the input - that's all, the problem is solved.
It may happen that at the input it is impossible to do this for some reason. In this case, swap the 2 wires in the marking box on the engine. And again the problem is solved. The button that controls the rotation to the right will start the rotation to the right, and the button that controls the rotation of the left will start the rotation to the left.
Wiring diagram for reversing the asynchronous (single-phase) motor
Above is a diagram of the reversible connection of a single-phase motor. This reverse engine starting circuit is much simpler than the previous one. A 3-position switch is used here.
Description of the reversible connection circuit of a single-phase motor
In position 1, the mains voltage is transmitted to the left leg of the capacitor, so that the motor rotates, relatively speaking, to the left. In position 2, power is supplied to the right leg of the capacitor, so that the engine rotates, conventionally speaking, to the right. In the middle position, the engine is stopped.
RT here is much simpler. As you can see, the simultaneous inclusion of a 3-position switch is also excluded here. For those who are interested in the question, but what, nevertheless, will happen when turned on simultaneously, we answer simply: the engine will fail.
Reverse switching circuit without self-shunting
We will tell you more about the control scheme for starting a reversible induction motor. When the “right” button is pressed, the power is supplied through the normally closed “left” contact of the , and due to the mechanical connection it breaks the power supply of the KM2 starter, excluding the possibility of turning on the KM2 while pressing 2 buttons. Further, the current flows to the normally closed contact of the KM2 starter to the KM1 starter coil, as a result of which it is triggered, 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 turns on the power supply of the KM2 starter normally open. That, in turn, includes power to the engine, but with a change of 2 phases in places.
Pay attention to the control scheme. More precisely, for deadlock. It is arranged a little differently here. The power supply of one starter, not only is blocked by a normally closed contact of the opposite starter, it is also blocked by pressing a button. This is done so that while pressing 2 buttons simultaneously for those fractions of a second, until the starter cuts off the power to the second starter, they turn on simultaneously.
For single phase motor circuit
When you press and hold one button, the power breaks to the second button, the power comes to the 1st leg of the capacitor. When you press the second button, the power breaks after the first button and enters the 2nd leg of the capacitor. RT still protects the engine from overloads.
Conclusion
In conclusion, it can be noted that, wherever you apply such schemes, pay attention to deadlock. This is the necessary measure that will protect equipment from damage. In addition, you need to choose the right 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 it may also damage the engine.