Schemes for manual control of DC motors. DC motor control. Regulating the speed of DC motors
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
Today, two main control schemes for an electric motor of this type have become widespread: a converter-motor (thyristor TP-D and transistor TrP-D options) and a generator-motor (G-D).
In both cases, torque control and angular velocity in direction and absolute value occurs by regulating the applied potential difference to the armature of the electric motor. Motor armature voltage G-D system adjusted by changing the current strength in the excitation winding of the generator Iвг. For this purpose, power magnetic amplifiers, thyristor or transistor converters are used as generator exciters. In TP-D systems, U armatures are changed by the method of phase control of thyristor switching, and in TrP-D systems, the duty cycle of the supply U is regulated, that is, using the pulse-width modulation (PWM) method.
The basis of transistor circuits is a pulse-width converter (PWC), consisting of four IGBT transistors. A load, that is, a motor armature, is connected to the diagonal of such an IGBT bridge. The SPID is powered by a DC source.
There are several ways to control a PWB converter using the armature circuit. The simplest of them is the symmetric method. With this control, all four IGBTs are in the switching state, and at the output of the PWB we observe alternating pulses, the duration of which is adjusted by the input signal. The switching principle itself is shown in the following figure. The advantage of the symmetrical method is its simplicity, but the bipolar U on the motor, which causes current pulsations in the armature, is a serious disadvantage. In practice, such symmetrical control circuits are mainly used to control low-power motors.
The asymmetrical control method is more advanced. It provides unipolar U out at the converter output. Therefore, in accordance with the diagram above, two transistors T3 and T4 are switched, while T1 is constantly open, and T2, on the contrary, is closed. In order for U average at the output of the converter to be zero, it is necessary that the lower switching transistor be closed. This approach is also not entirely correct, because the upper keys are loaded much more than the lower ones. Under heavy loads, this can lead to overheating and damage to the circuit.
But they also dealt with this drawback by coming up with a method for alternately controlling a DC motor. Here, as if moving in any direction, all the keys will switch. A prerequisite for the operation of the circuit is that the control voltages IGBT T1 and T2 for one group and T3 and T4 for the other are in antiphase.
This amateur radio development is based on the operating principle of a servo drive with a single-circuit control system. A DC motor control circuit consists of the following main parts:- SIFU - Regulator - Protection
SIFU - The Pulse-Phase Control System carries out a sinusoidal conversion of the network voltage into a sequence of rectangular pulses that flow to the control terminals of the power thyristors. When you turn on the circuit alternating voltage with a nominal value of 14 - 16 volts, it passes to the bridge rectifier and is converted into a pulsating voltage, which serves not only to power the structure, but also to synchronize the operation of the device. Diode D2 does not smooth out the pulses of capacitor C1. Then the pulses follow to the “zero detector” made on the LM324 operational amplifier, element DA1.1, turned on in comparator mode. While there are no pulses, the voltages at the direct and inverse inputs of the op-amp are approximately the same and the comparator is balanced.
When a sinusoid passes through the zero point, pulses appear at the inverse input of the comparator, switching the comparator, as a result of which rectangular synchronizing pulses are generated at the output of DA1.1, the repetition period of which depends on the zero point. Look at the oscillograms to understand the operating principle. From top to bottom: KT1, KT2, KT3.
The DC motor control circuit was simulated in the program. The archive with the full version of the design in question contains a project file for this program. You can open it and visually see how this unit works, and accordingly draw final conclusions about the control of a DC motor, before starting to assemble the amateur radio homemade product.
Let's get back to work - the clock pulses go to the integrator with a transistor switch (C4, Q1), where a sawtooth U is generated. At the moment the phase passes through the zero point, the clock pulse unlocks the first transistor, which discharges capacitance C4. After the pulse decays, the transistor is turned off and the capacitance is charged until the next clock pulse arrives, as a result of which a linearly increasing sawtooth voltage is formed at the transistor collector (oscillogram KT4), stabilized by a stable current generator on the unipolar transistor T1.
The amplitude of the sawtooth voltage of about 9 volts is set by trimming resistance RP1. This voltage is applied to the direct input of comparator DA1.2. The reference voltage follows the inverse input of comparator DA1.2 and at the moment when the amplitude of the sawtooth voltage exceeds the voltage at the inverse input, the comparator is switched to the opposite state and a pulse is generated at its output (oscillogram KT4).
The pulse is differentiated through a chain of passive radio components R14, C6 and follows to the base of the second bipolar transistor, which thanks to this opens and opening pulses of power thyristors are formed on the pulse transformer. By increasing or decreasing U of the task, you can adjust the duty cycle of the pulses in CT5.
But we will not see any pulses on the KT5 oscillogram until we press the S1 toggle switch. When it is not pressed, the +12V supply voltage through the front contacts S1 through R12, D3 goes to the inverse input DA1.2. Since this U is higher than U saw, the comparator closes and thyristor opening pulses are not generated.
To prevent emergency situations and damage to the electric motor, if the speed controller is not set to “0,” the circuit has an acceleration unit on elements C5, R13, designed for smooth acceleration of the engine.
When the toggle switch S1 is pressed, the contacts open and capacitance C5 begins to smoothly charge, and the voltage on the negative plate of the capacitor approaches zero. The voltage at the inverting input DA1.2 increases to the value of the reference voltage, and the comparator begins to generate pulses to open the power thyristors. The charging time is determined by radio components C5, R13.
If during engine operation it is necessary to adjust its speed, an acceleration and braking unit R21, C8, R22 has been added to the circuit. When the target voltage increases or decreases, capacitance C8 is smoothly charged or discharged, which eliminates a sharp “surge” of voltage at the inverse input and, as a result, eliminates a sharp increase in engine speed.
The regulator is used to maintain constant speed in the regulation zone. The regulator is made on the basis of a differential amplifier with the summation of two voltages: reference and feedback. The reference voltage is formed by resistance RP1 and follows through a filter on components R20, C8, R21, which acts as an acceleration and deceleration unit, and is supplied to the inverse input DA1.3. As the reference voltage at output DA1.3 increases, Uout decreases linearly.
The output voltage of the regulator follows to the inverse input of the comparator SIFU DA1.2 where, summed with the “saw” pulses, it turns into a series of rectangular pulses that travel to the electrodes of the thyristors. When the voltage increases or decreases, the reference increases or decreases and output voltage at the output of the power unit. The graph shows the dependence of engine speed on the reference voltage.
The voltage divider on resistors R22, R23 connected to the direct input of the DA1.3 regulator is designed to eliminate an emergency situation when the feedback is broken.
When the drive is turned on, the tachogenerator generates a voltage proportional to the speed of the electric motor. This voltage goes to the input of the precision detector DA1.4, DA2.1, built according to the classic full-wave circuit. From its output, the voltage follows through a filter on passive components C10, R30, R33 to the OS scaling amplifier DA2.2. The amplifier is used to adjust the OS voltage coming from the tachogenerator. The voltage from the DA2.2 output goes to the DA1.3 input and to the DA2.3 protection circuit.
Resistance RP1 generates motor speed. When operating without load, U out of the scaling amplifier is less than the voltage at the sixth pin of DA1.3, so the drive works as a regulator.
As the load on the shaft increases, the voltage removed from the tachogenerator decreases and, as a result, the voltage from the output of the scaling amplifier decreases. When this level is less than leg 5 of op-amp DA1.3, the drive will enter the current stabilization zone. Reducing the voltage at the non-inverting input DA1.3 will reduce the voltage at its output, and since it works on the inverting amplifier DA1.2, this will increase the opening angle of the thyristors and, therefore, increase the level at the armature of the electric motor.
Overspeed protection is assembled on an operational amplifier DA2.3, connected as a comparator. Its inverse input receives the reference voltage from the divider R36, R37, RP3. Resistance RP3 regulates the level of protection operation. The voltage from the output of the DA2.2 amplifier goes to the direct input of DA2.3.
When the speed is exceeded above the nominal value, the threshold of the protection setting, determined by resistance RP3, is exceeded at the direct input of the comparator and the comparator switches.
Due to the presence of positive feedback in the circuit, R38 causes the comparator to “latch”, and the diode VD12 does not allow the comparator to reset. When the protection is triggered, the comparator output goes through the VD14 diode to the inverse input 13 DA1.2 SIFU, and since the protection voltage is higher than the “saw” level, the issuance of control pulses to the electrodes of the power thyristors will be instantly prohibited.
The voltage from the output of the protection comparator DA2.3 unlocks the transistor VT4, which causes relay P1.1 to turn on and the LED to signal an accident lights up. You can remove the protection if you completely turn off the drive and, after a pause of 5 - 10 seconds, reapply power to it.
The control circuit, or rather the power part of the control unit, is shown in the figure below:
Transformer Tr1 is used to power the control unit circuit. The rectifier is assembled using a half-bridge circuit and includes two power diodes D1, D2 and two power thyristors T1, T2, as well as a protective diode D3. The field winding is powered by its own separate transformer and rectifier. If the engine does not have a tachogenerator, then the OS for speed control can be implemented as follows:
If a current transformer is used, then jumper P1 on the DC motor control unit diagram must be set to position 1-3.
You can also use an armature voltage sensor:
The armature voltage sensor is a filter-divider connected directly to the armature terminals. The drive is configured as follows. The “Task” and “Scaling Uoc” resistances are turned to the middle position. Resistance R5 of the armature voltage sensor is turned to a minimum. We turn on the drive and set the voltage at the armature to about 110 volts. Measuring the voltage on the armature, we begin to rotate resistance R5. IN certain moment changes, the voltage on the armature will begin to drop, this indicates that the OS has worked.
The drawing of a printed circuit board for controlling a DC motor is made in the program and you can easily make a printed circuit board with your own hands using the
Motor control design tuning: let's start by checking the supply voltages on the operational amplifier DA1, DA2. It is recommended to install microcircuits in sockets. Then we check the oscillograms at the control points KT1, KT2, KT3. At point CT4. we should see sawtooth pulses when the button is open.
Using the tuning resistance RP1 we set the “saw” swing to about 9 volts. At the control point KT3, the pulse duration is about 1.5 - 1.8ms; if we do not see this, then by decreasing the resistance R4 we achieve the required duration.
By rotating lever RR1 of the engine control circuit at control point KT5, we control the change in the duty cycle of the pulses from maximum to their complete disappearance with minimal resistance RR1. In this case, the brightness of the light bulb connected to the power unit that we connected as a load should change.
Then we connect the control unit to the engine and tachogenerator. We set the RR1 regulator to 40-50 volts at the armature. Resistance RP3 should be in the middle position. Measuring the voltage on the motor armature, we rotate the resistance RP3. At a certain point in the setting, U at the anchor will begin to fall, this indicates that the feedback.
If feedback is used in the motor control circuit based on the armature current, a current transformer is required, connected to the rectifier power circuit. The current transformer calibration circuit is discussed below. By selecting the resistance, obtain an alternating voltage of 2 ÷ 2.5v at the transformer output. The load power RN1 must be equal to the motor power
Remember that it is not recommended to turn on a current transformer without a load resistor.
We connect the current transformer to the OS circuit P1 and P2. During the adjustment, it is recommended to unsolder the D12 diode to avoid false triggering of the protection. Oscillograms at control points KT8, KT9, KT10 are shown in the figure below.
Further adjustment is the same as in the case of using a tachogenerator.
This DC motor control unit was made by hand for a boring machine. See the photos in the archive at the green link above.
The circuit shown in the figure below is capable of turning the "L" in both directions, both forward and backward. With the switch contacts open, the voltage on both terminals is the same, so it will not rotate, the same will happen if the buttons are pressed at the same time.
Electric motors are a very common control object in various devices and technical complexes. Without them, our modern life would not be so modern. They are used in many areas of consumer technology and industrial automation, from the small motors that spin the washing machine drum to the huge machines that drive factory conveyors and mine hoists.
Traditionally, electric motors are divided into DC motors And engines AC . The latter, due to the rapid development of scientific and technical thought, which offers more advanced vector control algorithms and fairly cheap and easy-to-use frequency converters, are becoming increasingly popular. But direct current motors (DC motors) also have their advantages, and they are also for a long time will spin their shafts in merciless operation mode in various technical fields, so today we will talk specifically about DFCs, or more precisely about the control of commutator DC motors.
Such units were the first engines to find wide application in industrial equipment, and they are still used where low cost of the final device is required, easy installation and management. On the rotor of these engines is located winding(1 in Figure 1), and on the stator - electromagnets(2 in Figure 1). Brush contacts(3 in Figure 1), which are installed around the circumference of the rotor shaft, are used to switch the polarity of the voltage applied to the rotor winding. They also create the main problem in the operation of a collector DPT - unreliability, since they undergo severe wear and require periodic replacement. Also, sparks occur between the brushes and switch contacts during operation, which can lead to strong electromagnetic interference. In addition, if used incorrectly, there is always a risk of creating an electric arc in the collector or, as it is also called, a circular fire. In this case, the engine armature is guaranteed to outlive its useful life.
Figure 1 - DC motor
Today, two engine control schemes of this type have become widespread: generator-motor(G-D) and converter-motor(thyristor TP-D and transistor TrP-D).
Figure 2 - power circuits of DC electric drives a) G-D, b) TP-D or TrP-D
Figure 2 shows two independently excited DC motor control circuits. In both cases, control of angular velocity and torque in absolute value and direction is carried out by regulating the voltage on the motor armature. The voltage at the armature of the motor D in the G-D system is regulated by changing the current strength in the excitation winding of the generator (VG). For this purpose, the exciter of the VG generator is used, which is used as power magnetic amplifiers(MU-G-D systems, although this is the last century, and in modern systems you won't see anything like this) thyristor(TV-G-D) or transistor(TrV-G-D) converters. In TP-D systems, the voltage at the motor armature is regulated by phase control of thyristor switching, and in TP-D systems by changing the duty cycle of the pulsating supply voltage, that is, using pulse-width modulation (PWM).
The popularity of G-D, as well as TP-D, is falling every year due to their bulkiness, hardware redundancy and complexity of control; in fact, they are mainly used in industry to control large engines. And TrP-D is increasingly used in various technical systems due to its simplicity, low cost and ease of use. Also due to the abundance on the market various models MOSFET and IGBT transistors and drivers for controlling their gates of the TrP-D system are used to control both low-power and large motors. I think it's worth getting to know such systems better.
So, the heart of TrP-D is a pulse-width converter (PWC), which consists of four transistors (Figure 3). The diagonal of such a transistor bridge includes a load, that is, the motor armature. The SPID is powered by a DC source.
Figure 3 - transistor PWB circuit
There are several ways to control the SPB via the armature circuit. The simplest one is symmetrical method. With this control, all four transistors are in the switching state, and the PSD output voltage is alternating pulses, the duration of which is regulated by the input signal. The switching principle itself is shown in Figure 4. It is logical to assume that if the relative switching duration is equal to 50%, then at the PSG output we will get 0 V. The advantage of the symmetrical method is the ease of implementation, but the bipolar voltage at the load, which causes current ripples in the armature, is its disadvantage . Essentially, it is used to control low-power DC motors.
Figure 4 - symmetrical method of controlling DPT
More perfect is . As we see in Figure 5, it provides a unipolar voltage at the output of the PSD. IN in this case only two transistors T3 and T4 switch, with T1 constantly open and T2 constantly closed. In order for the average voltage at the output of the PWB to be zero, it is enough that the lower switching transistor remains in the closed state. This approach is also not very good because the upper switches are loaded with more current than the lower ones. Under heavy loads, this can lead to overheating and failure of the transistors.
Figure 5 - asymmetrical method of controlling DPT
But they also coped with this drawback by inventing alternate control method(Figure 6). Here, both when moving in one direction and the other, all four transistors will switch. A prerequisite is that the control voltages of transistors T1 and T2 for one group and T3 and T4 for the other are in antiphase.
Figure 6 - alternate method of controlling DPT
From the figure we see that at a certain sign of the speed command signal, long pulses with a half-cycle difference are applied to diagonally opposite switches (in this case, T1 and T4). Accordingly, also with a half-cycle shift, short pulses are applied to the switches of the opposite diagonal. Thus, the load is connected to the source during the absence of short pulses, and during their presence it is short-circuited either to power or to ground. When the sign of the reference changes, the transistors are controlled in the opposite way.
Many machines use DC electric motors (EM). They easily allow you to smoothly control the rotation speed, changing the constant voltage component on the armature winding, at a constant voltage of the field winding (0V).
The scheme proposed below allows control an electric motor power up to 5 kW.
Powerful DC EMs have several features that must be taken into account:
a) it is impossible to apply voltage to the EM armature without supplying the rated voltage (usually 180...220 V) to the field winding;
b) in order not to damage the motor, it is unacceptable to immediately apply the rated voltage to the armature winding when turning it on, due to the large starting current, which exceeds the rated operating current by tens of times.
The above diagram allows you to ensure the required operating mode - smooth start and manual installation the required engine rotation speed.
The direction of rotation will change if you change the polarity of connecting the wires on the field winding or armature (this must be done only when the EM is turned off).
The circuit uses two relays, which allows automatic protection of circuit elements from overload. Relay K1 is a powerful starter; it eliminates the possibility of turning on the EV when the initial speed set by resistor R1 is not zero. To do this, a lever is attached to the axis of the variable resistor R1, connected to the button SB2, which closes (by the lever) only when maximum value resistance (R1) - this corresponds to zero speed.
When the contacts SB2 are closed, relay K1, when the START button (SB1) is pressed, will turn on and its contacts K1.1 will self-block, and contacts K1.2 will turn on the electric drive.
Relay K2 provides overload protection in the absence of current in the EM excitation winding circuit. In this case, contacts K2.1 will turn off the power to the circuit.
The control circuit is powered without a transformer, directly from the network through resistor R3.
The value of the effective voltage value on the armature winding is set by changing the opening angle of thyristors VS1 and VS2 with resistor R1. Thyristors are included in the bridge arms, which reduces the number of power elements in the circuit.
A pulse generator synchronized with the ripple period of the mains voltage is assembled on a unijunction transistor VT2. Transistor VT1 amplifies the current pulses, and through the isolation transformer T1 they are supplied to the control terminals of the thyristors.
When performing the design, thyristors VS1, VS2 and diodes VD5, VD6 must be installed on a heat sink plate (radiator).
Part of the control circuit, highlighted in the figure with a dotted line, is placed on the printed circuit board.
Fixed resistors type S2-23 is used, variable R1 is type PPB-15T, R7 is type SP-196, R3 is type PEV-25. Capacitors C1 and C2 of any type, on operating voltage not less than 100 V. Rectifier diodes VD1 ... VD4 for a current of 10 A and a reverse voltage of 300 V, for example D231 D231A D232, D232A, D245, D246.
Pulse transformer T1 is made on ferrite ring M2000NM standard size K20x12x6 mm and wound with PELSHO wire with a diameter of 0.18 mm. Winding 1 and 2 contain 50 turns, and 3 - 80 turns.
Before winding, the sharp edges of the core must be rounded off with a file to prevent punching and shorting of the turns.
When the circuit is initially turned on, we measure the current in the excitation winding circuit (0V) and, using Ohm’s law, calculate the value of resistor R2 so that relay K2 is activated. Relay K2 can be any low voltage (6...9 V) - the lower the operating voltage, the better. When choosing resistor R2, it is also necessary to take into account the power dissipated on it. - current in the circuit is 0V and voltage across the resistor, it can be easily calculated using the formula P=UI. Instead of K2 and R2, it is better to use special current relays produced by industry, but due to their narrow scope of application, they are not available to everyone. It is easy to make a current relay yourself by winding about 20 turns of PEL wire with a diameter of 0.7...1 mm on a larger reed switch.
To set up the control circuit, instead of the armature circuit of the motor, we connect a lamp with a power of 300...500 W and a voltmeter. It is necessary to make sure that the voltage on the lamp with resistor R1 changes smoothly from zero to maximum,
Sometimes, due to the variation in the parameters of a unijunction transistor, it may be necessary to select the value of capacitor C2 (from 0.1 to 0.68 μF) and resistor R7 (R7 is set at minimum value resistance R1 maximum voltage across the load).
If, with proper installation, the thyristors do not open, then it is necessary to swap the leads in the secondary windings of T1. Incorrect phasing of the control voltage coming to thyristors VS1 and VS2 cannot damage them. For the convenience of monitoring the operation of thyristors, the control voltage can be applied first to one thyristor, and then to the other - if the voltage on the load (lamp) is regulated by resistor R1, the phase of connection of the control pulses is correct. With both thyristors operating and the circuit configured, the voltage across the load should vary from 0 to 190 V.
You can eliminate the possibility of applying maximum voltage to the armature winding at the moment of switching on by electronically, using a scheme similar to that shown in Fig. 6.17. (Capacitor C2 ensures a smooth increase in the output voltage at the moment of switching on, and subsequently does not affect the operation of the circuit.) In this case, switch SB2 is not needed
Controlling a DC motor in an ACS implies either changing the rotation speed in proportion to a certain control signal, or maintaining this speed unchanged when exposed to external destabilizing factors.
4 main management methods are used that implement the principles listed above:
rheostat-contactor control;
control via the generator-engine (G-E) system;
control using the “controlled rectifier-D” (UV-D) system;
impulse control.
A detailed study of these methods is the subject of TAU and the course “Fundamentals of Electric Drive”. We will consider only the basic provisions that are directly related to electromechanics.
Typically 3 schemes are used:
when adjusting the speed n from 0 to nnom, a rheostat is included in the armature circuit (armature control);
if it is necessary to obtain n > nnom, the rheostat is included in the OB circuit (pole control);
for speed control n< nном и n >Normal rheostats are included both in the armature circuit and in the OF circuit.
The listed schemes are used for manual control. For automatic control use step switching R pa and R rv using contactors (relays, electronic switches).
If precise and smooth speed control is required, the number of switched resistors and switching elements must be large, which increases the system size, costs and reduces reliability.
Regulating the rotation speed from 0 to according to the diagram in Fig. is done by adjusting R in (U g changes from 0 to n nom). To obtain a motor speed greater than nnom, change R ind (a decrease in the motor OB current reduces its main flow F, which leads to an increase in speed n).
Switch S1 is designed to reverse the engine (change the direction of rotation of its rotor).
Since D control is carried out by regulating relatively small excitation currents G and D, it is easily adapted to the tasks of the automatic control system.
The disadvantage of such a scheme is the large dimensions of the system, weight, low efficiency, since there is a threefold conversion of energy (electrical to mechanical and vice versa, and at each stage there are energy losses).
Control using the “controlled rectifier – motor” system
The “controlled rectifier - motor” system (see figure) is similar to the previous one, but instead of an electrical machine source of regulated voltage, consisting of, for example, a three-phase AC motor and G=T, a controlled, for example, also three-phase thyristor electronic rectifier is used.
Control signals are generated by a separate control unit and provide the required opening angle of the thyristors, proportional to the control signal Uу.
The advantages of such a system are high efficiency, small dimensions and weight.
The disadvantage compared to the previous circuit (G-D) is the deterioration of switching conditions D due to ripples of its armature current, especially when powered from a single-phase network.
Using a pulse chopper, voltage pulses are supplied to the motor, modulated (PWM, VIM) in accordance with the control voltage.
Thus, changing the armature rotation speed is achieved not by changing the control voltage, but by changing the time during which the rated voltage is applied to the motor. It is obvious that engine operation consists of alternating periods of acceleration and deceleration (see figure).
If these periods are small compared to the total time of acceleration and stopping of the armature, then the speed n does not have time to reach the steady values of nnom during acceleration or n = 0 during braking by the end of each period, and a certain average speed nav is established, the value of which is determined by the relative duration of activation.
Therefore, the ACS requires a control circuit, the purpose of which is to convert a constant or changing control signal into a sequence of control pulses with a relative switching duration, which is a given function of the magnitude of this signal. Power elements are used as switching elements semiconductor devices – .
