Currently, the most common three-phase asynchronous motors with a squirrel-cage rotor. Starting and stopping of such motors when switched on at full mains voltage are carried out remotely using magnetic starters.

The most commonly used scheme is with one starter and "Start" and "Stop". In order to ensure the rotation of the motor shaft in both directions, a circuit with two starters (or with a reversing starter) and three buttons is used. This scheme allows you to change the direction of rotation of the motor shaft "on the fly" without first stopping it.

Engine starting diagrams

The electric motor M is powered by a three-phase alternating voltage network. The three-phase QF circuit breaker is designed to disconnect the circuit in the event of a short circuit. The SF single-phase circuit breaker protects the control circuits.

The main element of the magnetic starter is the KM contactor (powerful relay for switching high currents). Its power contacts switch three phases, suitable for the electric motor. Button SB1 ("Start") is for starting the engine, and button SB2 ("Stop") - for stopping. Thermal bimetallic relays KK1 and KK2 switch off the circuit when the current consumed by the electric motor is exceeded.

Rice. 1. Scheme of starting a three-phase asynchronous motor using a magnetic starter

When the SB1 button is pressed, the KM contactor is triggered and contacts KM.1, KM.2, KM.3 connect the electric motor to the network, and with the KM.4 contact blocks the button (self-locking).

To stop the electric motor, it is enough to press the SB2 button, while the KM contactor releases and turns off the electric motor.

An important property of the magnetic starter is that in case of an accidental loss of voltage in the network, the engine turns off, but the restoration of the voltage in the network does not lead to spontaneous start of the engine, since when the voltage is turned off, the KM contactor releases, and to turn it on again, press the SB1 button.

In the event of a malfunction of the installation, for example, when the motor rotor jams and stops, the current consumed by the motor increases several times, which leads to the operation of the thermal relays, the opening of the contacts KK1, KK2 and the shutdown of the installation. The return of the KK contacts to the closed state is done manually after the malfunction is eliminated.

A reversible magnetic starter allows not only starting and stopping an electric motor, but also changing the direction of rotation of the rotor. For this, the starter circuit (Fig. 2) contains two sets of contactors and start buttons.

Rice. 2. Scheme of starting the engine using a reversible magnetic starter

The KM1 contactor and the SB1 button with self-locking are designed to turn on the motor in the "forward" mode, and the KM2 contactor and the SB2 button turn on the "reverse" mode. To change the direction of rotation of the rotor of a three-phase motor, it is enough to swap any two of the three phases of the supply voltage, which is provided by the main contacts of the contactors.

The SB3 button is designed to stop the engine, the contacts KM 1.5 and KM2.5 interlock, and the thermal relays KK1 and KK2 provide overcurrent protection.

Turning on the motor at full mains voltage is accompanied by high starting currents, which may be unacceptable for a limited power network.

The circuit for starting an electric motor with a starting current limitation (Fig. 3) contains resistors R1, R2, R3, connected in series with the motor windings. These resistors limit the current at the moment of starting when the KM contactor is activated after pressing the SB1 button. Simultaneously with the KM, when the KM.5 contact is closed, the KT time relay is activated.

The delay carried out by the time relay must be sufficient to accelerate the motor. At the end of the holding time, the contact KT closes, the relay K is activated and by its contacts K.1, K.2, K.3 it shunts the starting resistors. The starting process is complete and the motor is at full voltage.

Rice. 3. Scheme of starting the motor with limiting the starting current

Next, we will consider two of the most popular braking schemes for three-phase asynchronous squirrel-cage motors: a dynamic braking scheme and an opposition braking scheme.

Engine braking circuits

After removing the voltage from the motor, its rotor continues to rotate for some time due to inertia. In a number of devices, for example, in lifting and transport mechanisms, it is required to carry out forced braking to reduce the amount of stick out. Dynamic braking consists in the fact that after the removal of the alternating voltage, a direct current is passed through the windings of the electric motor.

The dynamic braking circuit is shown in Fig. 4.

Rice. 4. Diagram of dynamic braking of the motor

In the circuit, in addition to the main contactor KM, there is a relay K, which turns on the braking mode. Since the relay and the contactor cannot be turned on at the same time, an interlocking scheme is used (contacts KM.5 and K.3).

When the SB1 button is pressed, the KM contactor is activated, supplies power to the engine (contacts KM.1 KM.2, KM.3), blocks the button (KM.4) and blocks the relay K (KM.5). Closing KM.6 triggers the KT time relay and closes the KT contact without time delay. Thus, the engine is started.

To stop the engine, press the SB2 button. The KM contactor releases, contacts KM.1 - KM.3 open, turning off the motor, closes the KM.5 contact, which triggers relay K. Contacts K.1 and K.2 close, supplying direct current to the windings. Rapid braking occurs.

When the contact KM.6 is opened, the KT time relay releases, the time delay begins. The holding time must be sufficient to stop the motor completely. At the end of the time delay, the KT contact opens, relay K releases and removes the constant voltage from the motor windings.

The most effective way of braking is to reverse the motor, when immediately after power is removed, a voltage is applied to the electric motor, causing the appearance of a counter torque. The circuit of opposing braking is shown in Fig. 5.

Rice. 5. Circuit of motor braking by opposition

The engine speed is monitored by a speed relay with contact SR. If the speed is greater than a certain value, the SR contact is closed. When the engine stops, contact SR opens. In addition to the direct-on-line contactor KM1, the circuit contains a contactor for reversing KM2.

When the engine is started, the KM1 contactor is activated and with the KM 1.5 contact breaks the KM2 coil circuit. When a certain speed is reached, the SR contact closes, preparing the circuit for switching on the reverse.

When the engine stops, the KM1 contactor releases and closes the KM1.5 contact. As a result of this, the KM2 contactor is activated and supplies a reversing voltage to the electric motor for braking. A decrease in the rotor speed causes SR to open, the KM2 contactor releases, braking stops.

The article discusses the scheme of starting an asynchronous motor with a squirrel-cage rotor using non-reversible and reversible magnetic starters.
Squirrel cage induction motors can be controlled using magnetic starters or contactors. When using low-power motors that do not require limitation of starting currents, starting is carried out by switching them on to full mains voltage. The simplest motor control circuit is shown in Fig. 1.

Rice. 1. Control circuit of an asynchronous squirrel-cage motor with a non-reversible magnetic starter
To start, the QF circuit breaker is turned on and thereby energizes the circuit power circuit and the control circuit. When the SB1 "Start" button is pressed, the power circuit of the KM contactor coil is closed, as a result of which its main contacts in the power circuit are also closed, connecting the stator of the electric motor M to the supply network. At the same time, the KM blocking contact closes in the control circuit, which creates a power supply circuit for the KM coil (regardless of the position of the button contact). The shutdown of the electric motor is carried out by pressing the SB2 "Stop" button. In this case, the power supply circuit of the KM contactor is broken, which leads to the opening of all its contacts, the motor is disconnected from the network, after which it is necessary to turn off the QF circuit breaker.
The scheme provides for the following types of protection:

From short circuits - by means of the QF circuit breaker and FU fuses;
from overloads of the electric motor - with the help of thermal relays KK (the opening contacts of these relays during overloads open the power supply circuit of the KM contactor, thereby disconnecting the motor from the network);
zero protection - using the KM contactor (when the voltage drops or disappears, the KM contactor loses power, opening its contacts, and the motor is disconnected from the network).
To turn on the engine, press the SB1 "Start" button again. If direct start-up of the motor is not possible and it is necessary to limit the starting current of an asynchronous squirrel-cage motor, an undervoltage start is used. To do this, an active resistance or a reactor is included in the stator circuit, or a start through an autotransformer is used.

Rice. 2 Control circuit of an asynchronous squirrel-cage motor with a reversible magnetic starter
In fig. 2 shows a control circuit for an asynchronous squirrel-cage motor with a reversible magnetic starter. The circuit allows you to directly start an asynchronous squirrel-cage motor, as well as change the direction of rotation of the motor, i.e. reverse. The engine is started by turning on the QF circuit breaker and pressing the SB1 button, as a result of which the KM1 contactor receives power, closes its power contacts and the motor stator is connected to the network. To reverse the engine, press the SB3 button. This will lead to the disconnection of the KM1 contactor, after which the SB2 button is pressed and the KM2 contactor turns on.
Thus, the motor is connected to the network with a change in the phase sequence, which leads to a change in the direction of its rotation. The circuit uses blocking from a possible erroneous simultaneous switching on of the KM2 and KM1 contactors using the KM2, KM1 break contacts. The motor is disconnected from the mains with the SB2 button and the QF circuit breaker. The circuit provides for all types of motor protection considered in the control circuit for an asynchronous motor with a non-reversible magnetic starter.

Asynchronous electric motors with squirrel-cage rotor are the main drive of most ship mechanisms that do not require wide speed control. They are easy to manufacture and operate, have high reliability and durability, and have a relatively low cost.

The starting properties of an induction motor are assessed by its starting characteristics:

the value of the starting current Iп or its multiplicity Iп / Inom;

the value of the starting Mp or its multiplicity Mp / Mp nom;

the duration and smoothness of starting the engine in motion;

the complexity of the launch operation;

the cost-effectiveness of the starting operation (the cost and reliability of the starting equipment), as well as energy losses in it.

Starting current value

,………………….(7)

where R 1 and X 1 are the active and inductive resistance of the stator, and R 2 and X 2 are the reduced active and inductive resistance of the rotor.

From the analysis (7) it follows that it is possible to improve the starting properties of the engine by increasing the active resistance of the rotor circuit R 2, since in this case the starting current decreases and the starting torque increases. A decrease in voltage U 1 has a favorable effect on Ip (decreasing its value), however, the starting torque Mp also decreases. The possibility of using one or another method for improving starting characteristics is determined by the type of engine, operating conditions, and requirements for it.

Drive control for non-reversing mechanisms most often consists in remote start and shutdown of the electric motor. A circuit of this kind can be easily automated by replacing the manual control buttons with a device that makes or breaks the contacts when the threshold value of the parameter is reached, when it is necessary to turn on or off the electric motor.

a) The simplest way to connect an asynchronous motor is direct start by means of a magnetic starter (Figure 5.18).

Here, when the SB2 (Start) button is pressed, the coil of the KM line contactor is powered, and the motor is switched on to the network. By pressing the SB1 (Stop) button, the KM coil loses power and the motor is disconnected from the mains. When the motor is overloaded, the contact of the KK thermal relay opens, which also de-energizes the KM contactor circuit. Starting current asynchronous motor with a squirrel-cage rotor with direct connection to the network reaches (6-7) I number. If, for example, the power of the starting engine is 30% of the power of the operating generator, then such a large starting current causes a sharp short-term decrease in the mains voltage, called a voltage dip of 15-20%. With a higher relative engine power, the voltage dip increases significantly, which can lead to the shutdown of the magnetic starter of operating electric drives, to a surge in the generator current and triggering of its protection, etc. Therefore, engines of comparable power to a generator on ships are launched according to special schemes that limit the starting current.

Figure 5.18. Scheme of direct start of blood pressure.

b) Start with the inclusion of resistors in the stator circuit (Figure 5.19). The engine accelerates in two stages. On the first, a resistance is included in the circuit of all three phases, which is shunted to the second degree by the contacts of the acceleration contactor KM2: 1. The operating time at the starting stage is controlled by an electromagnetic time relay KT1. The scheme works as follows. When the SB1 (Start) button is pressed, the relay KT1 is powered, which by its contacts KT1: 1 bypasses the SB1 button and turns on the acceleration contactor KM2, and it opens its contacts KM2: 1, bypassing the starting resistances and thereby prepares the circuit for starting. The KM2 contactor closes the circuit of the KM1 line contactor, which connects the motor to the network through the starting resistor R. The block contact KM1: 3 bypasses the button SB1 and the contact KM2: 2, providing power to the KM1 coil. The second block contact KM1: 2 breaks the power supply circuit of the KT1 relay, which, with a time delay, opens its contact in the KM2 contactor circuit. Contactor KM2 with its contacts KM2: 1 bypasses the starting resistance R.

Starting resistance R limits the starting current to the required value

;

where R .П - respectively the active resistance of the engine in the starting mode.

It should be borne in mind that these resistances during acceleration do not remain constant, since the reduced rotor resistances included in them depend on slip. The voltage loss U = I p R in the starting resistance reduces the voltage on the stator of the motor U d.

Figure 5.19. Scheme (a) and graph (b) of starting the blood pressure by introducing a starting resistance into the circuit

For an induction motor, the torque on the shaft is proportional to the square of the voltage. Therefore, the starting mechanical characteristic when the resistor is connected to the stator circuit (curve 1, Figure 5.19, b) has a significantly lower starting torque than at the rated voltage (M p1 M p2), which is characteristic of direct connection of the motor to the network (curve 2). It may happen that when choosing the starting resistance R P to reduce the starting current, it will turn out to be so large that the starting torque M p1 will be insufficient to overcome the moment of resistance and starting will become impossible.

c) Autotransformer start (Figure 5.20) provides for starting connection of the engine from a low voltage source - an autotransformer. Here, the starting current drawn from the mains due to voltage transformation is less than the current consumed by the motor during direct start. This leads to the fact that in the considered scheme, in contrast to the previous ones, a decrease in the starting current occurs to the same extent as when the starting torque on the motor decreases.

The autotransformer starting circuit has an increased cost and its use is justified when other cheaper circuits do not provide the necessary reduction in the starting current. The circuit works as follows. When the SB2 button is pressed, the KM2 contactor is switched on, which, with the KM2: 1 contact, connects the TV autotransformer and bypasses the SB2 button, and also supplies power to the KM1 line contactor. The engine is connected to the network via TV, the valve-type KT1 time relay is switched on by the KM1: 2 block contact. After a period of time t, the KT1: 1 contact will close the power supply circuit of the KM3 acceleration contactor, which by its KM3: 1 contact shunts the autotransformer and connects the motor directly to the network. The KM3: 2 block contact opens the power supply circuit of the KM2 contactor, which, in turn, will open the autotransformer circuit. The second block contact KM3: 3 will keep the power supply circuit of the KM1 contactor.

Figure 5.20. Scheme of autotransformer start of blood pressure

d) Starting by switching the stator winding from star to delta is carried out according to the scheme shown in Figure 5.21. When starting, the stator winding is star-connected, the starting voltage on the phase will be in

times less than the nominal, which will lead to a decrease in the starting current by 3 times. At the same time, the starting torque, proportional to the square of the voltage, will also decrease three times, which is not always acceptable, especially for mechanisms with a significant static moment of resistance.

The scheme works as follows. When the SB2 button is pressed, the electromagnetic time relay KT1 is powered, which connects the KM2 (star) contactor, which, with its main contacts KM2: 1, closes the three-phase stator winding according to the star circuit, and with the auxiliary contacts KM2: 3 turns on the KM1 line contactor and breaks the KM3 contactor circuit (triangle ). Contactor KM1 with its main contacts KM1: 1 connects the motor to the mains, and with auxiliary contacts KM1: 4 shunts the start button SB2. At the same time, the auxiliary contact KM1: 2 de-energizes the KT1 time relay, which releases with a time delay and with its KT1: 1 contact de-energizes the KM2 contactor, which opens the star connection. The KM2: 3 block contact closes the circuit of the KM3 contactor, which collects the delta wiring diagram. The operation of the contactors KM1, KM2, KM3 is electrically mutually interlocked by the corresponding auxiliary contacts, excluding an unintended or incorrect sequence of connections.

Figure 5.21. The scheme of starting the IM by switching the stator winding from star to

triangle

e) Soft start of AC motors. Currently, soft starters of AC motors based on thyristor switches and converters are widely used. Due to the smooth acceleration of the EM, it is possible to achieve a significant decrease in the value of the starting current and thereby limit its effect on the voltage of the ship's network.

A modern soft starter is a non-reversible three-phase thyristor switch (TC) with a multifunctional control system (CS) based on a microprocessor controller (MC) and a developed user interface hardware-provided by a digital input-output device (IO). The principle of operation and the device of the starter is explained by the functional diagram shown in Fig. 5.22.

Rice. 5.22 Soft starter

The main power element of the TC is a thyristor switch, which is a chain circuit consisting of a number of series-connected links, and each link consists of two thyristors connected in antiparallel. To equalize the voltage between the series-connected thyristors in static and dynamic modes, resistor and resistor-capacitive circuits are connected in parallel to each link, as well as a thyristor state sensor.

Information about the state of the thyristors is transmitted to the control system via a fiber-optic cable. Each of the key thyristors has its own pulse transformer control unit. To reduce the spread in the turn-on times of thyristors connected in series, the primary windings of their pulse transformers are connected in series. Potential separation between the high voltage power section and the low voltage control system is made using fiber optic cable and pulse transformers.

Triol AC15 has three thyristor switches described above according to the number of supply phases. By changing the angle of control (switching on) of the thyristors, it is possible to regulate the voltage supplied to the stator winding of the motor and, accordingly, the current. Reducing the voltage supplied to the stator winding of the motor allows you to reduce currents in dynamic modes (at start-up) and avoid shock loads on the mechanism. The presence of a current regulator ensures that the set current value is maintained practically during the entire acceleration time by increasing the voltage at the TC output. This is achieved by decreasing the control angle of the thyristors. Acceleration with a given starting current value continues as long as the current value of the thyristor control angle is greater than the changing angle of displacement between the first harmonics of voltage and current. When this ratio is not met, which occurs at the end of the start, the thyristors open fully. At this point, however, the current should no longer exceed the set value with the correctly set parameters of the starting device.

By changing the gain and the integration constant of the current regulator, as well as the initial value of the opening angle of the thyristors and the magnitude (multiplicity) of the starting current, the required dynamic characteristics can be obtained. It should be noted that the value of the starting current should not exceed the rated value of the current specified in the passport of a particular starting device. In Triol AC15, at loads significantly lower than the nominal value, an energy saving mode is provided, in which, due to a change in the thyristor control angle, the drive operates with a reduced voltage. The starter can brake the motor:

Run-out, by removing the control pulses from the TC thyristors;

With a slope, by reducing the voltage supplied to the stator winding of the electric motor (by smoothly increasing the control angles of the TC thyristors);

Dynamic braking, by supplying the stator winding of the motor with a constant voltage in the direction.

Current sensors DT1, DT2 on current transformers in the AC15 power channel are used to control, regulate and measure the starting or load current of an electric motor, incl. for protection against overload and short-circuit currents.

Voltage sensors DN1 and DN2 on high-voltage voltage transformers serve to synchronize the control system with the power supply network, control the presence of all phases of the power voltage and the correctness of their alternation.

The multichannel power supply MT converts the mains alternating voltage of 380 V into a system of DC voltages of the required levels and degree of stability, galvanically coupled and not coupled to each other, to power the control devices.

The MC microprocessor controller generates the operating modes of the device with the specified parameters using control signals: thyristor control signals, AC15 protection and emergency shutdown signals, reception and transmission

giving external control, setting and information signals.

The I / O device of the air-blast device is designed to receive and transmit external control signals.

UVV has a set of discrete inputs and outputs. Galvanic isolation devices are included in the input and output circuits of the air-blast devices for potential separation from external control circuits. PI pulse shapers (drivers) are designed to generate the required levels of thyristor control signals, galvanic separation of power circuits and control circuits of thyristors and MC. The device includes a built-in control panel PU, which contains a keyboard for controlling operating modes, setting and programming parameters, as well as indicating and signaling elements for displaying values ​​of pa_

parameters and diagnostics. By agreement with the Customer, the delivery set may include a remote control (RC), the functions of which are similar to those of the RC.

For the convenience of the operator, the programmable and informational parameters of the device are grouped into functional groups. Further in the text of the link and the corresponding parameters are given in the form [XX YY],

where XX is the group number, YY is the parameter number.

Below in Fig. 1.2 ... fig. 1.4 illustrates the implementation of individual technological procedures during starting and during stopping of the engine, respectively.

links, and each link is two thyristors connected in antiparallel. To equalize the voltage between series-connected thyristors in

in static and dynamic modes, resistor and resistor_capacitive circuits are connected in parallel to each link, as well as a thyristor state sensor. Information about the state of the thyristors is transmitted to the control system via a fiber-optic cable. Each of the key thyristors has its own pulse transformer control unit. To reduce the spread in the turn-on times of the thyristors connected in series, the primary windings of their pulse transformers are connected in series. Potential separation between the high voltage power section and the low voltage control system is made using fiber optic cable and pulse transformers. Triol AC15 has three thyristor switches described above according to the number of supply phases. By changing the angle of control (switching on) of the thyristors, it is possible to regulate the voltage supplied to the stator winding of the motor and, accordingly, the current. Reducing the voltage supplied to the stator winding of the motor allows reducing currents in dynamic modes (at start-up) and avoiding shock loads on the mechanism. The presence of a current regulator ensures that the set current value is maintained practically during the entire acceleration time by increasing the voltage at the TC output. This is achieved by decreasing the control angle of the thyristors. Acceleration with a given starting current value continues as long as the current value of the thyristor control angle is greater than the changing angle of displacement between the first harmonics of voltage and current. When this ratio is not met, which occurs at the end of the start, the thyristors open fully. At this point, however, the current should no longer exceed the set value with the correctly set parameters of the starting device.