The article discusses the starting circuit for an asynchronous motor with a squirrel-cage rotor using non-reversible and reversible magnetic starters.
Squirrel-cage asynchronous motors can be controlled using magnetic starters or contactors. When using low-power motors that do not require starting current limitation, starting is carried out by turning them on at full mains voltage. The simplest engine control circuit is shown in Fig. 1.
Rice. 1. Control circuit for an asynchronous squirrel-cage motor with an irreversible magnetic starter
For starting, the QF circuit breaker is turned on and thereby supplies voltage to the power circuit of the circuit and the control circuit. When you press the SB1 “Start” button, the power circuit of the contactor coil KM 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 circuit for the KM coil (regardless of the position of the button contact). The electric motor is turned off 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 engine is disconnected from the network, after which it is necessary to turn off the QF circuit breaker.
The scheme provides the following types of protection:
From short circuits - using a QF circuit breaker and FU fuses;
from electric motor overloads - using thermal relays KK (the opening contacts of these relays during overloads open the power circuit of the KM contactor, thereby disconnecting the motor from the network);
zero protection - using a KM contactor (when the voltage decreases or disappears, the KM contactor loses power, opening its contacts, and the engine is disconnected from the network).
To turn on the engine, you must press the SB1 “Start” button again. If direct starting of the motor is impossible and it is necessary to limit the starting current of an asynchronous squirrel-cage motor, a low-voltage start is used. To do this, an active resistance or reactor is included in the stator circuit, or a start through an autotransformer is used. 
Rice. 2 Control circuit for an asynchronous motor with a squirrel-cage rotor with a reversible magnetic starter
In Fig. Figure 2 shows a control diagram for an asynchronous motor with a squirrel cage rotor and a reversible magnetic starter. The circuit allows direct starting of an asynchronous squirrel-cage motor, as well as changing 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, you must press the SB3 button. This will turn off 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 order, which leads to a change in the direction of its rotation. The circuit uses blocking from possible erroneous simultaneous activation of contactors KM2 and KM1 using normally open contacts KM2, KM1. The engine is disconnected from the network using the SB2 button and the QF circuit breaker. The circuit provides all types of electric motor protection considered in the control circuit of an asynchronous motor with an irreversible magnetic starter.
Electric motors are devices for converting electrical energy into mechanical energy and vice versa, but these are already generators. There is a huge variety of types of electric motors, and therefore there are a great variety of electric motor control circuits. Let's look at some of them
Where smooth and precise control of the speed and torque of an electric motor over a wide range is required, a DC motor control circuit is required
This amateur radio development is based on the operating principle of a servo drive with a single-circuit control system. The design diagram consists of the following main parts: - SIFU, Regulator, Protection
It can be used to control single-phase asynchronous motors, in particular, for starting and braking an asynchronous motor with a low-power squirrel-cage rotor, having a starting winding or starting capacitor that is switched off before the end of the start. It is possible to use the device for starting more powerful motors, as well as for starting three-phase motors operating in single-phase mode.
In another simple scheme, to control a single-phase asynchronous motor for starting and braking, an electromagnetic relay is used, a starting capacitor of the MBGO-2 or MBGCh type, which is turned on and off by the relay contacts
Asynchronous single-phase electric motors with a starting winding are widely used in electric drives of various household appliances (washing machines, refrigerator compressor units); they are used by radio amateurs for their needs.
Having known advantages, such electric motors require the use of an additional device that ensures automatic connection of the starting winding when turned on, as well as when stopping work in cases of excessive short-term increase in load.
Many radio amateurs often try to use a three-phase electric motor for various amateur radio homemade products. But the problem is that not everyone knows how to connect a three-phase motor to a single-phase network. Among the various starting methods, the simplest is by connecting the third winding through a phase-shifting capacitor, but not all motors work well from a single-phase network.
In amateur radio practice, all non-standard methods are good, and since our hands are free, low-power motors can be reversed using the TP1 switch from old second-class tube TVs
This amateur radio development is designed to regulate and maintain a stable rotation speed of a low-voltage motor with a power from a few watts to 1000 watts at U no more than 20V. The sensor of the electronic ignition system of a VAZ car is used as a rotation speed sensor
The DC motor speed controller circuit operates on the principles of pulse width modulation and is used to change the speed of a 12 volt DC motor.
Regulating the engine shaft speed using pulse-width modulation gives greater efficiency than simply changing the DC voltage supplied to the engine, although we will also consider these schemes
A simple stepper motor controller circuit is considered that controls a stepper motor using a parallel computer port.
The stepper motor is used for the manufacture of printed circuit boards, micro drills, automatic feeders and in the designs of robotic devices.
Typically, speed control for 220-volt motors is carried out using thyristors. A typical circuit is considered to be connecting an electric motor to a break in the anode circuit of a thyristor. But in all such schemes there must be reliable contact. And therefore they cannot be used in regulating the rotation speed of commutator motors, since the brush mechanism artificially creates small open circuits.
The asynchronous electric motor has found its application due to its reliability, simplicity and low cost. To extend its service life and improve its parameters, additional devices are needed that allow you to start, regulate and even protect the engine.
Vladimir Rentyuk, Zaporozhye, Ukraine
The article provides a brief overview and analysis of popular circuits designed to control brushed DC motors, and also proposes original and little-known circuit solutions
Electric motors are probably one of the most popular electrical engineering products. As the all-knowing Wikipedia tells us, an electric motor is an electrical machine (electromechanical converter) in which electrical energy is converted into mechanical energy. The beginning of its history can be considered the discovery made by Michael Faraday back in 1821, establishing the possibility of rotating a conductor in a magnetic field. But the first more or less practical electric motor with a rotating rotor waited until 1834 for its invention. While working in Königsberg, it was invented by Moritz Hermann von Jacobi, better known to us as Boris Semenovich. Electric motors are characterized by two main parameters - the speed of rotation of the shaft (rotor) and the torque developed on the shaft. In general terms, both of these parameters depend on the voltage supplied to the motor and the current in its windings. Currently, there are quite a lot of varieties of electric motors, and since, as our famous literary character Kozma Prutkov noted, it is impossible to grasp the immensity, we will dwell on the consideration of the features of controlling DC motors (hereinafter referred to as electric motors).
There are two types of DC motors - the brushed motors we are used to and brushless (stepper) motors. In the first, an alternating magnetic field, which ensures rotation of the motor shaft, is formed by the rotor windings, which are powered through a brush commutator - commutator. It interacts with the constant magnetic field of the stator, rotating the rotor. For the operation of such engines, external commutators are not required; their role is played by the collector. The stator can be made of either a system of permanent magnets or electromagnets. In the second type of electric motor, the windings form the stationary part of the motor (the stator), and the rotor is made of permanent magnets. Here, an alternating magnetic field is generated by switching the stator windings, which is performed by an external control circuit. Stepper motors (“stepper motor” in English) are much more expensive than commutator motors. These are quite complex devices with their own specific features. Their full description requires a separate publication and is beyond the scope of this article. For more complete information on engines of this type and their control circuits, you can refer, for example, to.
Brushed motors (Figure 1) are cheaper and generally do not require complex control systems. For their operation, it is enough to supply a supply voltage (rectified, constant!). Problems begin to arise when it becomes necessary to adjust the rotation speed of the shaft of such an engine or use a special torque control mode. There are three main disadvantages of such motors - low torque at low rotation speeds (therefore, a gearbox is often required, and this affects the cost of the design as a whole), generation of a high level of electromagnetic and radio interference (due to the sliding contact in the commutator) and low reliability (more precisely, low resource; the reason is in the same collector). When using commutator motors, it is necessary to take into account that the current consumption and the rotation speed of their rotor depend on the load on the shaft. Brushed motors are more versatile and more widely used, especially in low-cost applications where price is a determining factor.
Since the rotation speed of the rotor of a commutator motor depends, first of all, on the voltage supplied to the motor, it is natural to use circuits for its control that have the ability to set or adjust the output voltage. Such solutions that can be found on the Internet are circuits based on adjustable voltage stabilizers and, since the age of discrete stabilizers has long passed, it is advisable to use inexpensive integrated compensation stabilizers, for example. Possible options for such a scheme are presented in Figure 2.
The scheme is primitive, but it seems very successful and, most importantly, inexpensive. Let's look at it from an engineer's point of view. Firstly, is it possible to limit the torque or current of the motor? This can be solved by installing an additional resistor. In Figure 2 it is denoted as R LIM. Its calculation is included in the specification, but it worsens the characteristics of the circuit as a voltage stabilizer (more on this below). Secondly, which speed control option is better? The option in Figure 2a gives a convenient linear control characteristic, which is why it is more popular. The option in Figure 2b has a non-linear characteristic. But in the first case, when the contact in the variable resistor is broken, we get the maximum speed, and in the second case, the minimum. What to choose depends on the specific application. Now let's look at one example for a motor with typical parameters: operating voltage 12 V; maximum operating current is 1 A. The LM317 IC, depending on the suffixes, has a maximum output current from 0.5 A to 1.5 A (see specification; there are similar ICs with higher current) and developed protection (against overload and overheating). From this point of view, it is ideal for our task. Problems are hidden, as always, in the little things. If the engine is brought to maximum power, which is very realistic for our application, then the IC, even with the minimum permissible difference between the input voltage V IN and output V OUT equal to 3 V, will dissipate power of at least
P = (V IN - V OUT)×I = 3×1 = 3 W.
Thus, a radiator is needed. Again the question is: what is the power dissipation? At 3 W? But no. If you take the time to calculate the load graph of the IC depending on the output voltage (this is easy to do in Excel), then we get that, under our conditions, the maximum power on the IC will be dissipated not at the maximum output voltage of the regulator, but at an output voltage of 7.5 V ( see Figure 3), and it will be almost 5.0 W!
As you can see, the result is something that is no longer cheap, but very bulky. So this approach is only suitable for low-power motors with an operating current of no more than 0.25 A. In this case, the power on the control IC will be at the level of 1.2 W, which will already be acceptable.
The way out is to use the pulse width modulation (PWM) method for control. It is indeed the most common. Its essence is the supply of unipolar rectangular pulses modulated in duration to the engine. According to signal theory, the structure of such a sequence has a constant component proportional to the ratio τ/T, where: τ is the pulse duration, and T is the sequence period. It is she who controls the speed of the engine, which distinguishes her as an integrator in this system. Since the output stage of a PWM-based regulator operates in switching mode, it, as a rule, does not require large radiators to remove heat, even at relatively high engine powers, and the efficiency of such a regulator is incomparably higher than the previous one. In some cases, it is possible to use step-down or step-up DC/DC converters, but they have a number of limitations, for example, in terms of the depth of output voltage regulation and minimum load. Therefore, as a rule, other solutions are more common. The “classical” circuit design of such a regulator is presented in Figure 4. It is used as a throttle (regulator) in a professional model railway.
A generator is assembled on the first operational amplifier, and a comparator on the second. A signal from capacitor C1 is supplied to the input of the comparator, and by adjusting the response threshold, a rectangular signal with the desired ratio τ/T is generated (Figure 5).
The adjustment range is set by trimming resistors RV1 (faster) and RV3 (slower), and the speed adjustment itself is carried out by resistor RV2 (speed). I would like to draw the attention of readers to the fact that a similar circuit with errors in the values of the divider that sets the threshold of the comparator is circulating on the Internet in Russian-language forums. The motor is directly controlled through a switch using a powerful field-effect transistor. The features of this MOSFET type transistor are a high operating current (30 A constant, and up to 120 A pulsed), ultra-low open channel resistance (40 mOhm) and, therefore, minimal power losses in the open state.
What should you pay attention to first when using such schemes? The first is the execution of the control circuit. There is a small flaw here in the diagram (Figure 4). If over time problems arise with the moving contact of the variable resistor, we will get complete, almost instantaneous acceleration of the engine. This may damage our device. What's the antidote? Install an additional high-resistance resistor, for example, 300 kOhm, from pin 5 of the IC to the common wire. In this case, if the regulator fails, the engine will be stopped.
Another problem with such regulators is the output stage or motor driver. In such circuits, it can be made using both field-effect transistors and bipolar ones; the latter are incomparably cheaper. But in both the first and second options, it is necessary to take into account some important points. To control a MOSFET, it is necessary to charge and discharge its input capacitance, which can amount to thousands of picofarads. If the gate series resistor (R6 in Figure 4) is not used or its value is too small, the op amp may fail at relatively high drive frequencies. If you use R6 of a large value, then the transistor will remain in the active zone of its transfer characteristic longer and, therefore, we have an increase in losses and heating of the switch.
One more note about the circuit in Figure 4. The use of an additional diode D2 makes no sense, since the structure of the BUZ11 transistor already has its own internal high-speed protective diode with better characteristics than the one proposed. Diode D1 is also clearly superfluous, transistor BUZ11 allows a gate-source voltage of ± 20 V, and polarity reversal in the control circuit with a unipolar supply, as well as voltages above 12 V, are impossible.
If you use a bipolar transistor, then the problem of generating a sufficient base current arises. As is known, to saturate a switch on a bipolar transistor, its base current must be at least 0.06 of the load current. It is clear that the operational amplifier may not provide such a current. For this purpose, in an essentially similar regulator, which is used, for example, in the company’s popular PT-5201 mini-engraver, a transistor is used, which is a Darlington circuit. There's an interesting point here. These mini-engravers sometimes fail, but not due to overheating of the transistor, as one might assume, but due to overheating of the IC (maximum operating temperature +70 °C) by the output transistor (maximum permissible temperature +150 °C). In the products used by the author of the article, it was pressed closely to the IC body and placed on glue, which unacceptably heated the IC and almost blocked the heat sink. If you come across such a design, then it is better to “unstick” the transistor from the IC and bend it as far as possible. For this know-how, the author of the article was awarded by Pro’sKit with a set of tools. As you can see, everything needs to be solved in a comprehensive manner - look not only at the circuitry, but also pay close attention to the design of the regulator as a whole.
There are several more interesting circuits of simpler PWM regulators. For example, two single operational amplifier circuits with driver are published in [
Typical control circuits for electric drives garden
IMs with a squirrel-cage rotor of low and medium power are started by direct connection to the network without limiting the starting currents. Control circuits for IMs with a wound rotor of medium and high power must provide for current limitation during their starting, reversing and braking using additional resistors in the rotor circuit.
A reversible control circuit for an IM with a squirrel-cage rotor is shown in Figure 8.9.
Rice. 8.9. Reversible blood pressure control circuitwith squirrel-cage rotor
The main element This circuit is a reversible magnetic starter, which includes two linear contactors KM1 and KM2 and two thermal protection relays KK. The circuit provides direct starting and reversing of the engine, as well as back-on braking during manual (non-automatic) control.
The circuit provides protection against motor overloads (KK relay) and short circuits in the stator circuit (QF circuit breaker) and control circuit (FA fuses). In addition, the control circuit provides zero protection against loss (decrease) of the network voltage (contactors KM1 and KM2).
Starting the engine when the QF circuit breaker is turned on, in the conventional directions “Forward” or “Backward” is carried out by pressing the SB1 or SB2 buttons, respectively. This leads to the activation of the contactor KM1 or KM2, the connection of the motor to the network and its run-up.
For reversing or braking engine, the SB3 button is first pressed, which leads to the switching off of the contactor that was still turned on (for example, KM1), after which the SB2 button is pressed. This leads to the switching on of the KM2 contactor and the supply of voltage from a power source with a different phase sequence to the IM. The magnetic field of the engine changes the direction of rotation to the opposite, and the reverse process begins, consisting of two stages: counter-braking and take-off in the opposite direction.
Whenonly need to brake When the engine reaches zero speed, the SB3 button must be pressed again, which will disconnect the engine from the network and return the circuit to its original position. If the SB3 button is not pressed, this will lead to the engine running in the other direction, i.e. to its reverse.
To avoid short circuit in the stator circuit, which can occur as a result of the simultaneous erroneous pressing of buttons SB1 and SB2, reversible magnetic starters sometimes provide a special mechanical interlock. It is a lever system that prevents one contactor from retracting if another is energized. In addition to mechanical interlocking, the circuit uses typical electrical interlocking used in reversible control circuits. It provides for cross-connection of the breaking contacts of the KM1 device into the coil circuit of the KM2 device and vice versa.
Note that the use of a QF air circuit breaker in the circuit contributes to increasing reliability and ease of use. Its presence eliminates the possibility of the drive operating in the event of a break in one phase, in the event of a single-phase short circuit, as can occur when installing fuses, and it also does not require replacement of elements (as in fuses when their fuse-link burns out).
The IM control circuit, which provides direct start-up and dynamic braking as a function of time, is shown in Fig. 8.10.
Rice. 8.10. IM start-up and dynamic braking circuit
Starting the engine is carried out by pressing the SB1 button, after which the KM linear contactor is activated, connecting the motor to the power source. At the same time, closing the KM contact in the KT time relay circuit will cause it to operate and close its contact in the KM1 braking contactor circuit. However, the latter does not work, since the KM breaking contact in this circuit previously opened.
To stop the engine the SB3 button is pressed, the KM contactor is turned off, opening its contacts in the motor stator circuit and thereby disconnecting it from the AC mains. At the same time, the KM contact in the circuit of the KM1 device closes and the KM contact in the KT relay circuit opens. This leads to the activation of the braking contactor KM1, the supply of direct current to the stator windings from the rectifier V through the resistor Rt and the engine switching to dynamic braking mode.
The KT time relay, having lost power, begins counting the time delay. After a time interval corresponding to the time the engine is stopped, the KT relay opens its contact in the KM1 contactor circuit, which turns off, stopping the supply of direct current to the stator circuit. The circuit returns to its original position.
The intensity of dynamic braking is regulated by resistor Rt, with the help of which the required constant current is set in the motor stator.
To exclude the possibility of simultaneous connection of the stator to sources of alternating and direct current, the circuit uses a standard blocking using breaker contacts KM and KM1, connected crosswise in the coil circuits of these devices.
The control circuit for starting and braking a counter-connected motor with a wound rotor in the EMF function is shown in Figure 8.11.
Rice. 8.11. Control circuit for starting and braking by back-on IM
with wound rotor
After applying voltage, the KT time relay is turned on, which, with its opening contact, breaks the power supply circuit of the KM3 contactor, thereby preventing its activation and premature short-circuiting of the starting resistors in the rotor circuit.
Turning on the engine is done by pressing the SB1 button, after which the KM1 contactor is turned on. The motor stator is connected to the network, the electromagnetic brake YB is released, and the motor starts to run up. Switching on KM1 simultaneously triggers the contactor KM4, which with its contact bypasses the back-off resistor R, which is unnecessary at start-up D 2, and also breaks the coil circuit of the CT time relay. The latter, having lost power, begins counting the time delay, after which it closes its contact in the coil circuit of the KM3 contactor, which operates and bypasses the starting resistor R d1in the rotor circuit, and the engine returns to its natural characteristic.
Brake control provides a KV braking relay that controls the level of EMF (speed) of the rotor. Using resistor R Rit is adjusted in such a way that at start-up, when the motor slip is 0< S < 1, наводимая в роторе ЭДС будет недостаточна для включения, а в режиме противовключения, когда 1 < S < 2, уровень ЭДС достаточен для его включения.
To apply braking engine, the double button SB2 is pressed, the opening contact of which breaks the power circuit of the contactor coil KM1. After this, the engine is disconnected from the network and the power supply circuit of the KM4 contactor is broken, and the power supply circuit of the KT relay is closed. As a result of this, contactors KM3 and KM4 are turned off, and resistance R is introduced into the motor rotor circuit d1+ R D 2.
Pressing the SB2 button simultaneously closes the power circuit of the contactor coil KM2, which, when turned on, reconnects the motor to the network, but with a different phase rotation of the mains voltage on the stator. The engine goes into reverse braking mode. Relay RY is activated and, after releasing the SB2 button, will provide power to the KM2 contactor through its contact and the closing contact of this device.
At the end of braking, when the speed is close to zero and the rotor EMF decreases, the KV relay will turn off and, with its break contact, open the circuit of the KM2 contactor coil. The latter, having lost power, will disconnect the motor from the network, and the circuit will return to its original position. After turning off KM2, the HC brake, having lost power, will provide fixation (braking) of the motor shaft.
In Figure 8.12. The diagram of the panel type PDU 6220 is shown.
Panel type PDU 6220 is part of a standardized series of control panels for wound and squirrel cage motors and provides two-stage motor starting and time-based dynamic braking.
When a voltage of 220 V and an alternating current of 380 V is applied to the circuit (closing the QS switches 1 and QS 2 and the QF machine) the time relay KT1 is turned on, which prepares the engine for starting with a full starting resistor in the rotor circuit. At the same time, if the command controller handle is in the zero (middle) position and the maximum current relays FA1-FA3 are not turned on, the KV protection relay against a decrease in the supply voltage will turn on and prepare the circuit for operation.
Rice. 8.12. Diagram of panel type PDU 6220
Starting the engine is carried out according to any of the two artificial characteristics or a natural characteristic, for which the handle SA must be installed in position 1, 2 or 3, respectively. When the handle is moved to any of the indicated positions SA, the linear contactor KM2 is turned on, connecting the motor to the network, the brake control contactor KM5, connecting the coil YA of the electromagnetic brake to the network, which at the same time releases the engine and the KT3 time relay, which controls the dynamic braking process. When SA is moved to position 2 or 3, the acceleration contactors KM3 and KM4 are turned on, and the engine begins to accelerate.
Engine braking occurs when the SA handle is moved to the zero (middle) position. In this case, the contactors KM2 and KM5 will turn off and the dynamic braking contactor KM1 will turn on, which will connect the motor to a DC source. As a result of this, there will be an intensive process of combined (mechanical and dynamic) engine braking, which will end after relay KT3 has counted its time delay corresponding to the braking time.
The diagram of an asynchronous electric drive with a thyristor starting device is shown in Figure 8.13.
How
Rice. 8.13. Asynchronous electronic circuit diagram
with thyristor starting device
An effective method for generating the desired graphs of changes in motor current and torque in transient modes isvoltage regulation on its statorusing thyristor starting devices (TPU). Most often, this is done to limit the current and torque of the engine when starting (“soft” starting method), although with the help of these devices it is also possible to increase the engine torque when starting (“hard” starting method).
Thyristor starting device turns on between the power source (AC mains) with voltage U 1 and the motor stator. In a non-reversible TPU, its power part is formed by three pairs of back-to-back thyristors VS1-VS6, which are controlled by voltage pulses supplied to them from a pulse-phase control system (PPCS). Current and torque are limited by reducing the voltage supplied to the motor, which is achieved by a corresponding change in time of the thyristor control angle.The starting voltage can vary according to various laws – increase linearly from zero to mains voltage, be lowered throughout the entire starting time, or change according to the so-called booster option, in which, to facilitate starting the engine, a certain voltage is first applied to it abruptly, which then continues to increase according to a linear law. In a closed system, the stator current can also be maintained at a given level.
8.6. Adjusting the coordinates of an asynchronous motor
using resistors
This method of coordinate control, often called rheostatic, can be carried out by introducing additional active resistors into the stator or rotor circuits of the IM (see Fig. 8.14). It attracts primarily due to the simplicity of its implementation, while at the same time being distinguished by low indicators of quality of regulation and cost-effectiveness.
Rice. 8.14. Connection diagrams for IM with wound rotor (a)
and with a squirrel-cage rotor (b)
1d into the stator circuit It is used mainly for regulating (limiting) in transient processes the current and torque of an IM with a squirrel-cage rotor.
All artificial electromechanical characteristics are located in the first quadrant below and to the left of the natural one. Taking into account the fact that the ideal idle speed ω 0 when turning on R 1ddoes not change, the resulting artificial electromechanical characteristics can be represented by a family of curves (Fig. 8.15 a).
a) b)
Fig.8.15. Electromechanical (a) and mechanical (b) characteristics of IM
when adjusting coordinates using resistors in the stator circuit
Characteristics 2–4 are located below natural characteristic 1, built at R 1d= 0, with a larger value of R 1dcorresponds to a greater slope of artificial characteristics 2-4.
The mechanical characteristics of the IM are presented in Figure 8.15 b.
Coordinates of the extremum point M Toand S Tochange with varying R 1d, namely: in accordance with (8.15) and (8.16) with increasing R 1dcritical moment M Toand critical slip S Toare decreasing. The starting torque is also reduced.
At the same time, artificial mechanical characteristics (Fig. 8.15b) are of little use in regulating the speed of arterial pressure: they provide a small range of speed changes; the rigidity of the characteristics of blood pressure and its overload capacity, characterized by a critical moment, as it increasesR 1d decreases; The method is also characterized by low efficiency. Due to these shortcomings, regulation of the speed of IM using active resistors in its stator circuit is rarely used.
Turning on additional resistors R 2d into the rotor circuit It is used both for the purpose of regulating the current and torque of the IM, as well as its speed (Fig. 8.14a).
Artificial electromechanical characteristics at R 2d= var have the form shown in Figure 8.15a and can be used to regulate (limit) the starting current I short circuit= I P.
Idle speed of ideal idle speed ω 0 and maximum (critical) engine torque M Toin accordance with remain unchanged when adjusting R 2d, and the critical slip S To, as follows from , changes.
The analysis performed allows us to construct a natural 1 (R 2d= 0) and artificial 2–3 (R 2d3>R 2d2) characteristics (Fig. 8.16) and conclude that due to changes in R 2dit is possible to increase the starting torque of the blood pressure up to the critical moment M Towithout reducing the overload capacity of the engine, which is very important when regulating its speed.
Rice. 8.16. Mechanical characteristics at various resistances R 2dadditional resistor in the rotor circuit
Otherwise, the method under consideration is characterized by the same indicators as for DPT NV. The speed control range is small - about 2-3 - due to a decrease in the rigidity of the characteristics and an increase in losses as it increases. The smoothness of speed control, which changes only downward from the main one, is determined by the smoothness of the change in the additional resistor R 2d.
The costs associated with the creation of this ES system are low, since simple and cheap ones are usually used for regulation. resistors. At the same time, operating costs turn out to be significant, since losses in PD are high.
As slip S increases, losses in the rotor chain increase, so the implementation of a large range of speed control leads to significant energy losses and a decrease in the efficiency of the electric motor.
Speed control using this method is carried out with a small speed control range or short-term operation at reduced speeds. This method has found wide application, for example, in electronic control of lifting and transport machines and mechanisms.
Calculation of the resistance of the additional resistor R 2dcan be performed in several ways depending on the form of specifying the required artificial mechanical characteristic.
If the artificial characteristic is fully defined, then the resistance of the additional resistor (for example, R 2d1) can be determined by the expression:
, (8.30)
Where– resistance of the rotor phase of the IM.
If the artificial characteristic is specified by its working part, then the method of segments can be used, for which a vertical line corresponding to the nominal moment M is drawn in Figure 8.16 nom, and characteristic points are marked: a, b, c, d, e. Resistance of the desired resistor R 2d1defined as
R 2d1= R 2nomab/ac, (8.31)
Where
–
nominal blood pressure resistance;–
Rotor EMF at S = 1;–
rated rotor current.
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IMPORTANT! Before connecting the electric motor, you must ensure that it is correct in accordance with its specifications.
Symbols on diagrams
(hereinafter referred to as the starter) is a switching device designed to start and stop the engine. The starter is controlled through an electric coil, which acts as an electromagnet; when voltage is applied to the coil, it acts with an electromagnetic field on the movable contacts of the starter, which close and turn on the electrical circuit, and vice versa, when the voltage is removed from the starter coil, the electromagnetic field disappears and the starter contacts are under the action of the spring returns to its original position, breaking the circuit.
The magnetic starter has power contacts designed for switching circuits under load and block contacts which are used in control circuits.
Contacts are divided into normally open- contacts that are in their normal position, i.e. before applying voltage to the coil of the magnetic starter or before mechanical impact on them, are in an open state and normally closed- which in their normal position are in a closed state.
The new magnetic starters have three power contacts and one normally open block contact. If it is necessary to have a larger number of block contacts (for example, during assembly), an attachment with additional block contacts (contact block) is additionally installed on the magnetic starter on top, which, as a rule, has four additional block contacts (for example, two normally closed and two normally open).
Buttons for controlling an electric motor are included in push-button stations; push-button stations can be one-button, two-button, three-button, etc.
Each button of the push-button post has two contacts - one of them is normally open, and the second is normally closed, i.e. Each of the buttons can be used both as a “Start” button and as a “Stop” button.
Electric motor direct connection diagram
This diagram is the simplest diagram for connecting an electric motor; it does not have a control circuit, and the electric motor is turned on and off by an automatic switch.
The main advantages of this scheme are its low cost and ease of assembly, but the disadvantages of this scheme include the fact that circuit breakers are not designed for frequent switching of circuits; this, in combination with inrush currents, leads to a significant reduction in the service life of the machine; in addition, this scheme does not include Possibility of additional motor protection.
Connection diagram for an electric motor via a magnetic starter
This scheme is also often called simple motor starting circuit, in it, unlike the previous one, in addition to the power circuit, a control circuit also appears.
When you press the SB-2 button (the “START” button), voltage is applied to the coil of the magnetic starter KM-1, while the starter closes its power contacts KM-1 starting the electric motor, and also closes its block contact KM-1.1 when the button is released SB-2 its contact opens again, but the coil of the magnetic starter is not de-energized, because its power will now be provided through the KM-1.1 block contact (i.e. the KM-1.1 block contact bypasses the SB-2 button). Pressing the SB-1 button (the “STOP” button) leads to a break in the control circuit, de-energizing the magnetic starter coil, which leads to the opening of the magnetic starter contacts and, as a result, to stopping the electric motor.
Reversible motor connection diagram (How to change the direction of rotation of an electric motor?)
To change the direction of rotation of a three-phase electric motor, you need to swap any two phases supplying it:
If it is necessary to frequently change the direction of rotation of the electric motor, the following is used:
This circuit uses two magnetic starters (KM-1, KM-2) and a three-button post; the magnetic switches used in this circuit, in addition to a normally open block contact, must also have a normally closed contact.
When you press the SB-2 button (START 1 button), voltage is applied to the coil of the magnetic starter KM-1, while the starter closes its power contacts KM-1 starting the electric motor, and also closes its block contact KM-1.1 which bypasses the button SB-2 and opens its block contact KM-1.2 which protects the electric motor from turning on in the opposite direction (when the SB-3 button is pressed) until it stops first, because An attempt to start the electric motor in the opposite direction without first disconnecting the KM-1 starter will result in a short circuit. To start the electric motor in the opposite direction, you need to press the “STOP” button (SB-1), and then the “START 2” button (SB-3), which will power the coil of the KM-2 magnetic starter and start the electric motor in the opposite direction.
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