If you correctly connect the reversing starter according to the scheme , then you can start any electric motor and make it rotate not only forward, but also backward. In fact, the reverse is provided by the presence of another contact group on the starter. But it needs to be connected correctly. For example, there are three phases A, B and C that are connected to the terminal block of an electric motor. In this case, the shaft rotates clockwise. To make it rotate in the opposite direction, it is enough to swap any two phases. For example, connect in this order - B, A, C.
Features of reversing starters
Such connection schemes are used in the construction of elevators, cranes, drilling machines. If you do not go into details much, then it may seem that the circuit for turning on the motor using reverse is more complicated. But in reality it turns out that there is nothing complicated - one more power unit and control was added to the design.
The cost of such devices is slightly higher due to the use of more elements. In fact, these are two electromagnetic starters combined in one housing. The principle of operation of the scheme is specific, you will need to carefully consider all the nuances.
The initial position of the elements
The circuit of the reversing magnetic starter in the initial state is open - the voltage is supplied only to the upper contacts and is “on duty” until the control system starts working. Phases are arranged as follows:
- From phase “A”, the control circuit is powered.
- The wire from phase "A" goes to the stop button.
- The phase also goes to the contacts of the SB2 and SB3 buttons.
- Be sure to protect the circuits - power and control.
In this form, the circuit is ready to start work, it remains only to click on the "Left" or "Right" button to start the electric motor. And it is necessary to study in more detail the processes taking place in the reversing starter circuit with control buttons during rotation of the motor rotor.
Rotor rotates counterclockwise
As soon as the SB2 button is pressed, phase “A” passes through the normally closed group of contacts KM2.2 to the starter coil. In this case, the winding is triggered, the contacts that were open are closed. A closed open.
As soon as the KM1.1 contacts are closed, the magnetic starter switches to self-pickup mode.
Therefore, as soon as the group of power contacts is closed, all three phases are supplied to the windings of the electric motor. And the rotor begins to accelerate, moving in a counterclockwise direction. A normally-closed group of contacts KM1.2, which is located in the circuit supplying the KM2 starter coil, opens and counteracts the voltage supply to the KM2 coil (KM1 thus works). The people call this scheme "protection against the fool."
Motor rotates clockwise
As mentioned earlier, to rotate the motor in the opposite direction, it is enough to simply swap the two phases. This is exactly what the element designated KM2 does in the circuit of the reversing motor starter. But, before changing the direction of movement, it is necessary to stop the motor. To do this, use the "Stop" button. Usually it is red. As soon as the operator presses the button, the power supply circuit of the coil of the magnetic starter KM1 will break.
In this case, the spring acts on the contacts and returns them to their original state. The electric motor is de-energized, the voltage disappears on the windings and the rotor stops. When the SB3 button is pressed, phase “A” is transferred via the normally-closed contact KM1.2 to the coil of the electromagnet KM2. The starter enters the self-pickup mode using the power contact KM2.1.
Two phases are transferred to them - for example, “A” and “B”. The KM2.2 contact group, which is located in the power supply circuit of the KM1 magnetic starter, opens and does not allow KM1 to turn on. The KM2 magnetic starter is working at this time.
Power circuit diagram
In general, a reversing starter wiring diagram in a three-phase network can be implemented in several ways. Most importantly, you can use two starters, if there is no way to put one.
It is important to correctly phase transfer in order to reverse. The phases in the KM1 magnetic starter are distributed as follows:
- "A" is fed to the winding "1".
- "B" goes to the motor winding "2"
- "C" is fed to the winding "3".
In this case, the rotor rotates counterclockwise. On the KM2 starter, the phases are distributed as follows:
- "A" to the winding "1".
- "C" goes to the winding "2".
- "B" is fed to the motor winding "3".
Consequently, the only difference is that two phases have switched places - “B” and “C”. The phase under the letter “A” remains all the same on the first contact. But the rotor will rotate in the opposite direction - a phase shift occurs in the windings.
Practical reverse starter circuit
The connection diagram of a reversing starter of a three-phase type is made in this way:
- The first phase A is connected to the contacts. It approaches the KM1 magnetic starter, as well as using a jumper with the same contact number on KM2.
- The outputs of both starters are connected in parallel with a jumper.
- The phase with the designation “B” is connected to the middle contact of KM1, as well as using a jumper with the rightmost KM2.
- Phase "C" is connected to the extreme right contact on KM1 and the middle on KM2.
It is in this way that a change in the direction of movement of the rotor occurs.
The reversing starter connection diagram is realized only by connecting the power contacts and changing their order. But it is imperative that the design of the drive must have protection against accidental activation of two magnetic starters at the same time.
How is protection
Before making a change in the direction of movement of the rotor, it is imperative to completely insure yourself against various errors. Suppose a design does not contain elements that can protect the circuit. Then, when the motor rotates counterclockwise, the magnetic starter KM1 is in working condition. All phases go to the corresponding motor windings.
If you immediately turn on the magnetic starter KM2, then the phases "B" and "C" will be closed. Consequently, a common interphase fault will occur, which can lead to fire or failure of various components. To prevent this phenomenon, normally-closed type contacts are used.
They are mounted directly in the power supply circuit of the starter coils. It is with their help that it becomes possible to turn on only one magnetic starter and completely eliminate the possibility of including one starter in the power circuit until the second is completely disconnected. Otherwise, the circuit breakers will constantly knock out, the operator will have to turn them on.
Conclusion
"Foolproof" is available in any circuitry. If this type of protection is not used in the reversing starter circuit, there will be many problems during operation. Operators who turn on the electric drive usually do not have knowledge of circuitry. Therefore, in order to exclude the possibility of an error, a circuit is used that does not allow two magnetic starters to be put into operation simultaneously.
It is advisable to use lamps in the circuits that will show the direction of rotation of the engine. To connect them, you need to correctly connect the groups of auxiliary contacts. You can use 220 volt lamps or, if there is a separate power supply, 12 volts. The feasibility of using these types of structures is doubtful, since it is much simpler to use one of the working phases as a voltage source. Usually they do, in rare cases, additional power sources are used.
It is advisable to power the control circuit from a low-voltage circuit, but there is a need for a constant voltage source - you will have to use special devices. To do this, just install a transformer and a simple rectifier, or use a ready-made power supply. Be sure to apply the protection circuit of the power circuit of the low-voltage part.