Electric motors are widely used in industry and in everyday life. When operating some mechanisms, it is necessary to ensure the rotation of the motor shaft in different directions, that is, it is necessary to reverse. For this, a certain control scheme is used and an additional magnetic starter (contactor) or reversing starter is used.
Content
- Theoretical basis
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The principle of operation of an asynchronous motor
- Three-phase network
- Single phase mode
- DC machines
- Pros of using magnetic starters
- Safety engineering
Theoretical basis
The type of reverse engine start circuit depends on the following factors:
- type of electric motor;
- supply voltage;
- the purpose of electrical equipment.
Therefore, reverse circuits can be very different, but once you understand the principles of their construction, you can assemble or repair any such circuit.
Before disassembling the engine reverse circuits, you need to decide on the concepts, which will be used when describing the work:
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A normally open (open) contact is a contact that is in an open state without external influence. An external influence, first of all, is understood as the supply of voltage to the control coil of a relay or magnetic starter. In the case of buttons, the contacts are switched mechanically.
- A normally closed (closed) contact is a contact that is in a closed state without external forces.
- A magnetic starter is an electromagnetic device that has three normally open power contacts and several auxiliary contacts. When the supply voltage is applied to the solenoid coil, the armature is attracted and all contacts are switched simultaneously. Power contacts are used to connect the electric motor to the network, and auxiliary contacts are needed to build a control circuit, so they can be normally open or closed. After removing the control voltage, the device returns to its original state under the action of the springs.
- A reversing starter is two identical magnetic starters mounted on one base, with a common housing. The device is intended for reversing three-phase motors, therefore the power contacts are connected to each other in a certain way.
- A thermal relay is a device for protecting the motor from overheating caused by increased currents in the windings.
- A contactor is a switching device in many ways similar to a magnetic starter. But unlike it, it can have from two to four normally open power contacts with arc chutes and is designed to switch large currents.
- A circuit breaker is a device for protection against short-circuit currents.
In order for the electric motor to change its rotation, it is necessary to change its magnetic field. To do this, you need to make some switches, which depend on the type of electrical machine.
The principle of operation of an asynchronous motor
The operation of the electric motor can be carried out in both three-phase and single-phase modes. The principle of operation of the circuits changes slightly, however, there are some additions to the power supply from a single-phase network.
Three-phase network
The electrical schematic diagram of the reversible start of a three-phase electric motor with a squirrel-cage rotor is as follows way (the diagram is shown in Fig. 1) The power supply of the entire circuit is carried out from a three-phase AC network with a voltage of 380 V through automatic machine AB.
In order to reverse such an electric machine (M), you need to change the alternation of any two phases connected to the stator. In the diagram, the magnetic starter Mn1 is responsible for forward rotation, and Mn2 is responsible for the opposite. The figure shows that when Mn1 is turned on, the phases alternate on the stator A, B, C, and when Mn2 is turned on - C, B, A, that is, phases A and C change places, which is what we need.
When voltage is applied to the circuit, the coils Mn1 and Mn2 are de-energized. Their power contacts Mp1.3 and Mp2.3 are open. The electric motor does not rotate.
When you press the Start1 button, power is supplied to the Mp1 coil, the starter is triggered and the following happens:
- The power contacts Mp1.3 are closed, the supply voltage is applied to the stator windings, the motor starts to rotate.
- The normally open auxiliary contact Mp1.1 closes. This contact provides self-locking of the Mp1 starter. That is, when the Start1 button is released, the Mn1 coil will remain energized due to the Mn1.1 contact and the starter will not turn off.
- The normally closed auxiliary contact Mp1.2 opens. This contact breaks the control circuit of the Mn2 coil, thus providing protection against the simultaneous activation of both contactors.
If it becomes necessary to stop the engine or reverse, you need to press
Stop button. At the same time, the power supply circuit Mp1 is opened, the contactor is disconnected, its contacts return to the original state shown in the figure, the electric motor stops.
In order for the engine to start rotating in the opposite direction, you need to press the Start button 2. By analogy with Mp1, the contacts Mp2.3, Mp2.1, Mp2.2 will work, the phases will switch on the stator winding and the motor will start to rotate in the opposite direction.
The control circuit is powered from two phase wires. With this connection, contactors with 380 V coils must be used. Fuses Pr1 and Pr2 provide protection against short-circuit currents. In addition, removing these fuses completely de-energizes all controls and avoids the risk of electrical injury during maintenance and repairs.
The protection of the electrical machine against overloads is provided by the thermal relay RT. When an increased current flows in any of the three stator windings, the bimetallic plate RT is heated, as a result of which it bends. At a certain current, the plate heats up so much that its bending causes the thermal relay to operate, due to which it opens its normally closed contact PT in the control circuit of the coils Mp1 and Mp2 and the motor is disconnected from networks.
The response time depends on the magnitude of the current: the higher the current, the shorter the response time. Due to the fact that the RT acts with a certain delay, the starting currents, which can be 7-10 times higher than the rated ones, do not have time to provoke the protection operation.
Depending on the type of device and settings, after the thermal relay is triggered, there are two options for returning the circuit to an operating state:
- Automatic - after the sensing element has cooled down, the relay returns to its normal state and the engine can be started with the Start button.
- Manual - you need to press a special flag on the RT body, after which the contact will close and the circuit will be ready to start.
The considered reverse circuit of a three-phase motor can be modified depending on conditions and needs. For example, the control circuit can be powered from a 12 V network, in this case all control elements will be under safe voltage and such an installation can be safely used at high humidity.
The motor can be reversed only when the motor is completely stationary, otherwise the starting currents will increase several times, which will trigger the protection. In order to control the fulfillment of this condition, a time relay can be added to the control circuit, the contacts of which are connected in series to MP2.2 and Mp1.2. Thanks to this, after pressing the Stop button, the engine can only be started in the opposite direction after a few seconds, which are necessary to completely stop the mechanism.
Single phase mode
In order for a three-phase asynchronous motor with a squirrel-cage rotor to work from a single-phase 220 V network, a connection diagram with starting and working capacitors is used.
Three wires leave the stator winding of the electric motor. Two wires are connected directly to the phase and neutral wires, and the third is connected to one of the supply wires through a capacitor. In this case, the direction of rotation depends on which of the supply conductors the capacitor is connected to.
If it is required to turn such a connection scheme into a reversible one, it must be supplemented with a toggle switch that will switch the capacity from one power wire to another.
DC machines
Reverse starting of a DC motor can be done by changing the polarity of the connection of the armature winding or the field winding. Depending on how these two windings are connected to each other, DC motors have the following types of excitation:
- independent - the excitation windings and armatures are powered from various sources;
- consistent;
- parallel;
- mixed.
DC motors can run out of gear - a mode of operation in which the speed increases so much that it leads to mechanical damage.
In the case of a collector motor with parallel or independent excitation, this mode may occur when the excitation winding is broken. Therefore, the connection diagram of the reversible motor in this case is built in such a way that the armature winding was switched, and the excitation winding must be directly connected to power supply. That is, it is unacceptable to connect the excitation circuit through any contacts or fuses.
Otherwise, the control circuit differs from the reversible connection of a three-phase motor only in that two DC supply wires are switched, instead of three AC phases.
Pros of using magnetic starters
The main element in reversible electric motor connection schemes is a magnetic starter. The use of these devices allows you to solve a number of problems:
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Simultaneous connection of three phases.
- Implementation of commutation of large currents with small signals. Some devices can switch currents of the order of hundreds of amperes, and the current required to power the coil rarely exceeds one ampere.
- Remote start. Due to the design of the starter and low operating currents, the control buttons can be located at a distance of several hundreds of meters from the electric motor, which, in turn, provides not only ease of use, but also safety operator.
- Zero protection. If the voltage is cut off during operation, for example, due to the operation of the current protection, then after the resumption power supply, the mechanism will start to work spontaneously, which can lead not only to damage to equipment, but also to human sacrifice. The use of a contactor eliminates this possibility, since after de-energizing it will turn off and will remain in its state until the operator presses the start button.
- Versatility. Coils for a certain type of starter have the same characteristics and design, but the response voltage may be different. Thanks to this, by installing a suitable coil, the contactor can be used in various networks. This feature should be remembered when replacing one starter with another, since outwardly completely identical devices may have different operating voltages.
Safety engineering
During installation, commissioning and repair, it is necessary to strictly observe safety rules.
In the case of working with an electric motor control circuit, for a complete shutdown, it is necessary to de-energize the power section and control circuits. Some electric motors can be powered by two independent power sources, so be sure to study the connection diagram. Make the necessary disconnections and check with the indicator that there is no voltage not only on the power contacts, but also on the auxiliary contacts.
If capacitors are installed in the circuit, after turning off the power, give them time to discharge, before touching live parts.
