Almost any electric motor can be made to rotate in one direction or the other. This is often necessary, especially when designing various mechanisms, such as closing and opening systems for gates. Usually, the factory direction of movement of the shaft is indicated on the motor housing, which is considered straight. Torsion in the other direction in this case will be reverse.
What is reverse
Simply put, reverse is a change in the direction of movement of any mechanism in the opposite direction from the selected main one. The reverse scheme can be obtained in several ways:
- Mechanical
- Electric.
In the first case, by switching the gear links connecting the drive shaft with the driven one, the latter is rotated in the opposite direction. All gearboxes work this way.
The electric method implies a direct impact on the engine itself, where electromagnetic forces take part in changing the movement of the rotor. This method has the advantage of not requiring complex mechanical transformations.
In order to get the reverse of the electric motor, it is necessary to assemble a special electrical circuit, which is called the motor reverse circuit. It will be different for different types of electrical machines and supply voltage.
Where does the reverse apply
It's easier to list cases where reverse is not used. Almost all mechanics are built on the transmission of torque clockwise and vice versa. These include:
- Household appliances: washing machines, audio players.
- Power tools: reversible drills, screwdrivers, wrenches.
- Machines: boring, turning, milling.
- Vehicles.
- Special equipment: crane equipment, winches.
- Automation elements.
- Robotics.
The situation that an ordinary person most often encounters in practice is the need to assemble a circuit for connecting a reverse AC induction motor or a DC collector motor.
Connecting an asynchronous motor 380 V to a three-phase network in reverse
The asynchronous connection diagram in the forward direction has a certain sequence of supplying phases A, B, C to the motor contacts. It can be improved, for example, by adding a switch that would swap any two phases. In this way, you can get a reverse motor circuit. In practical schemes, B and A are considered to be such phases.
Optional equipment:
- Magnetic type starters (KM1 and KM2).
- Station for three buttons, where twocontacts have a normally open position (in the initial state, the contact does not conduct current, when the button is pressed, the circuit closes), one is normally closed.
The scheme works as follows:
- By turning on the automatic fuses AB1 (power line), AB2 (control circuit), the current is supplied to the three-button switch and the terminals of the magnetic contactors, which are initially open.
- By pressing the "Forward" button, the current passes to the coil of the electromagnet of the contactor 1, which attracts the armature with power contacts. At the same time, the control circuit of contactor 2 is interrupted; now it cannot be turned on with the Reverse button.
- The motor shaft starts to rotate in the main direction.
- By pressing the "Stop" button, the current in the control winding circuit is interrupted, the electromagnet releases the armature, the power contacts open, the blocking contact of the "Reverse" button closes, and it can now be pressed.
- When the "Reverse" button is pressed, similar processes occur only in the circuit of contactor 2. The motor shaft will rotate in the opposite direction from the main direction.
Connecting a 220V motor to a single-phase network in reverse
It is possible to achieve a reverse movement of the motor shaft in this case, if there is access to the outputs of its starting and working windings. These motors have 4 outputs: two for the starting winding connected with a capacitor, two for the working winding.
If there is no information about the purpose of the windings, it can be obtained by dialing. The resistance of the starting winding will always be greater than the working winding due to the smaller section of the wire with which it is wound.
In a simplified version of the motor connection diagram, 220 V is supplied to the working winding, one end of the starting winding to the phase or zero of the network (no difference). The motor will begin to rotate in a certain direction. To get the reverse circuit, you need to disconnect the end of the starting winding from the contact and connect the other end of the same winding there.
To get a complete working circuit, you need equipment:
- Protection machine.
- Post button.
- Electromagnetic contactors.
The reverse and forward stroke scheme in this case is very similar to the three-phase motor connection scheme, but the switching here is not phases, but the starting winding in one direction or the other.
Scheme of reversing a three-phase motor in a single-phase network
Since a three-phase asynchronous motor will lack two phases, they need to be compensated by capacitors - starting and working, to which both windings are switched. The torsion of the shaft in one direction or another depends on where to attach the third one.
The diagram below shows that winding number 3 is connected through a working capacitor to a three-position toggle switch, which is responsible for the forward / reverse engine operation modes. Its other two contacts are combined with windings 2 and 1.
When turned onengine, you must adhere to the following algorithm of actions:
- Submit power to the circuit through a plug or a switch.
- Toggle switch for switching operating modes to move forward or backward (reverse).
- Put the power switch to the ON position.
- Press the "Start" button for a time not exceeding three seconds to start the engine.
Wiring diagram for DC reverse motor
DC motors are somewhat more difficult to connect than AC machines. The difficulty lies in the fact that the designs of such devices can be different, or rather, the method of excitation of the winding is different. On this basis, engines are distinguished:
- Independent way of excitation.
- Excitation independent (there are serial, parallel and mixed connections).
As for the first type of devices, here the armature is not connected to the stator winding, they are each powered from their own source. This achieves the enormous power of the engines used in production.
In machine tools and fans, parallel excitation motors are used, where the source energy is the same for all windings. Electric vehicles are built on the basis of series excitation of windings. Less common is mixed arousal.
In all types of motor designs described, it is possible to start the rotor in the opposite direction from the main stroke, that isReverse:
- With a series excitation circuit, it does not matter where to change the direction of the current in the armature or stator - in both cases, the motor will work stably.
- In other options for excitation of machines, it is recommended to use only the armature winding for reversal purposes. This is due to the danger of a break in the stator, a jump in electromotive force (EMF) and, as a result, damage to the insulation.
Starting the motor with a star-delta circuit
When powerful three-phase electric motors are directly started using a reverse control circuit, voltage drops occur in the network. This is due to the large starting currents flowing at this moment. To reduce the current value, a gradual start of the motor is used according to the star-delta scheme.
The bottom line is that the beginning and end of each stator winding is brought into a box with terminals. The circuit is controlled by three contactors. They gradually turn on the windings in a star, and then, when the engine accelerates, they bring the system to working condition when connected by a triangle.
How to tell a reversing starter from a direct starter
A reversing starter is a more complex device. In fact, it consists of two conventional direct starters, the latter combined in one housing. The internal circuitry of the reversing device is characterized by the fact that it is impossible to run two modes at the same time - direct and reverse. The interlock circuit is responsible for this process, which can be electrical or mechanical.
In conclusion
Necessaryremember that only qualified specialists authorized to work with high-voltage equipment are allowed to connect three-phase voltage motors to a 380V network. Makeshift electrical circuits can cause electrical injury!
