DC motor control with feedback. DC motor control units. A few words about inductive loads
When I started developing a control unit for a brushless motor (wheel motor), there were many questions about how to compare a real motor with an abstract circuit of three windings and magnets, which, as a rule, explains the principle of controlling brushless motors.
When I implemented control using Hall sensors, I still didn’t really understand what was happening in the engine beyond the abstract three windings and two poles: why 120 degrees and why the control algorithm was exactly that way.
Everything fell into place when I began to understand the idea of sensorless control of a brushless motor - understanding the process occurring in a real piece of hardware helped to develop the hardware and understand the control algorithm.
Below I will try to describe my path to understanding the principle of controlling a brushless DC motor.
For a brushless motor to operate, it is necessary that the constant magnetic field of the rotor be entrained by the rotating electromagnetic field of the stator, as in a conventional DC motor.
The rotation of the stator magnetic field is carried out by switching the windings using an electronic control unit.
The design of a brushless motor is similar to that of a synchronous motor if you connect the brushless motor to a three-phase AC network that satisfies electrical parameters engine, it will work.
A certain switching of the windings of a brushless motor allows it to be controlled from a DC source. To understand how to create a commutation table for a brushless motor, it is necessary to consider the control of an AC synchronous machine.
Synchronous machine
The synchronous machine is controlled from a three-phase alternating current network. The motor has 3 electrical windings, offset by 120 electrical degrees.
Having started a three-phase motor in generator mode, a constant magnetic field will induce an EMF on each of the motor windings, the motor windings are distributed evenly, a sinusoidal voltage will be induced on each of the phases and these signals will be shifted among themselves by 1/3 of the period (Figure 1). The shape of the EMF changes according to a sinusoidal law, the period of the sinusoid is 2P (360), since we are dealing with electrical quantities (EMF, voltage, current), let's call it electrical degrees and measure the period in them.
When three-phase voltage is supplied to the motor, at each moment of time there will be a certain current value on each winding.
Figure 1. Waveform of a three-phase AC source.
Each winding generates a magnetic field vector proportional to the current in the winding. By adding 3 vectors you can get the resulting magnetic field vector. Since over time the current in the motor windings changes according to a sinusoidal law, the magnitude of the magnetic field vector of each winding changes, and the resulting total vector changes the angle of rotation, while the magnitude of this vector remains constant.
Figure 2. One electrical period of a three-phase motor.
Figure 2 shows one electrical period of a three-phase motor; this period is marked with 3 arbitrary moments; to construct a magnetic field vector at each of these moments, we plot this period, 360 electrical degrees, on a circle. Let's place 3 motor windings shifted by 120 electrical degrees relative to each other (Figure 3).
Figure 3. Moment 1. Magnetic field vectors of each winding (left) and the resulting magnetic field vector (right).
Along each phase, a magnetic field vector created by the motor winding is constructed. The direction of the vector is determined by the direction of the direct current in the winding; if the voltage applied to the winding is positive, then the vector is directed in the opposite direction from the winding, if negative, then along the winding. The magnitude of the vector is proportional to the magnitude of the voltage on the phase in this moment.
To obtain the resulting magnetic field vector, it is necessary to add the vector data according to the law of vector addition.
The construction is similar for the second and third moments of time.
Figure 4. Moment 2. Magnetic field vectors of each winding (left) and the resulting magnetic field vector (right).
So, over time, the resulting vector smoothly changes its direction; Figure 5 shows the resulting vectors and shows the complete rotation of the stator magnetic field in one electrical period.
Figure 5. View of the rotating magnetic field generated by the windings on the motor stator.
This electric magnetic field vector is followed by the magnetic field of the permanent magnets of the rotor at each moment of time (Figure 6).
Figure 6. A permanent magnet (rotor) follows the direction of the magnetic field generated by the stator.
This is how an AC synchronous machine works.
Having a direct current source, it is necessary to independently form one electrical period with a change in current directions on three motor windings. Since a brushless motor is the same in design as a synchronous motor and has identical parameters in generator mode, it is necessary to build on Figure 5, which shows the generated rotating magnetic field.
Constant pressure
The DC source has only 2 wires “plus power” and “minus power”, which means that it is possible to supply voltage to only two of the three windings. It is necessary to approximate Figure 5 and highlight all the moments at which it is possible to connect 2 phases out of three.
The number of permutations from set 3 is 6, therefore, there are 6 options for connecting the windings.
Let's depict possible options commutations and select a sequence in which the vector will rotate further step by step until it reaches the end of the period and starts over.
We will count the electric period from the first vector.
Figure 7. View of the six magnetic field vectors that can be created from a direct current source by switching two of the three windings.
Figure 5 shows that when controlling a three-phase sinusoidal voltage, there are many vectors that rotate smoothly over time, and when switching with direct current, it is possible to obtain a rotating field of only 6 vectors, that is, switching to the next step must occur every 60 electrical degrees.
The results from Figure 7 are summarized in Table 1.
Table 1. The resulting sequence of switching of the motor windings.
The appearance of the resulting control signal in accordance with Table 1 is shown in Figure 8. Where -V is the switching to the minus of the power supply (GND), and +V is the switching to the plus of the power source.
Figure 8. View of control signals from a DC source for a brushless motor. Yellow – W phase, blue – U, red – V.
However, the real picture from the motor phases will be similar to the sinusoidal signal from Figure 1. The signal forms a trapezoidal shape, since at moments when the motor winding is not connected, the permanent magnets of the rotor induce an EMF on it (Figure 9).
Figure 9. View of the signal from the windings of a brushless motor in operating mode.
On an oscilloscope it looks like this:
Figure 10. View of the oscilloscope window when measuring one motor phase.
Design features
As was said earlier, for 6 switchings of the windings, one electrical period of 360 electrical degrees is formed.
It is necessary to relate this period to the actual angle of rotation of the rotor. Motors with one pair of poles and a three-tooth stator are used extremely rarely; motors have N pairs of poles.
Figure 11 shows motor models with one pair of poles and two pairs of poles.
A. b.
Figure 11. Model of a motor with one (a) and two (b) pairs of poles.
A motor with two pairs of poles has 6 windings, each winding is a pair, each group of 3 windings is offset by 120 electrical degrees. In Figure 12b. One period is delayed for 6 windings. Windings U1-U2, V1-V2, W1-W2 are connected to each other and in the design they represent 3 phase output wires. To simplify the figure, the connections are not shown, but remember that U1-U2, V1-V2, W1-W2 are the same.
Figure 12, based on the data in Table 1, shows the vectors for one and two pairs of poles.
A. b.
Figure 12. Diagram of magnetic field vectors for a motor with one (a) and two (b) pairs of poles.
Figure 13 shows the vectors created by 6 commutations of motor windings with one pair of poles. The rotor consists of permanent magnets, in 6 steps the rotor will rotate 360 mechanical degrees.
The figure shows the final positions of the rotor; in the intervals between two adjacent positions, the rotor rotates from the previous to the next switched state. When the rotor reaches this final position, the next switching should occur and the rotor will tend to the new set position, so that its magnetic field vector becomes aligned with the electromagnetic field vector of the stator.
Figure 13. End positions of the rotor during six-speed commutation of a brushless motor with one pair of poles.
In motors with N pairs of poles, N electrical periods are required to complete a mechanical revolution.
A motor with two pairs of poles will have two magnets with poles S and N, and 6 windings (Figure 14). Each group of 3 windings are offset from each other by 120 electrical degrees.
Figure 14. Final rotor positions during six-speed commutation of a brushless motor with two pairs of poles.
Determining the rotor position of a brushless motor
As was said earlier, for the engine to operate, it is necessary to connect voltage to the required stator windings at the right times. It is necessary to apply voltage to the motor windings depending on the position of the rotor, so that the magnetic field of the stator always leads the magnetic field of the rotor. An electronic control unit is used to determine the position of the motor rotor and winding switching.
Tracking the rotor position is possible in several ways:
1. By Hall sensors
2. By back EMF
As a rule, manufacturers equip the engine with Hall sensors at exhaust, so this is the most common control method.
Switching the windings in accordance with back EMF signals allows you to abandon the sensors built into the motor and use as a sensor the analysis of the free phase of the motor, to which the back EMF will be induced by the magnetic field.
Brushless motor control with Hall sensors
In order to switch the windings at the right times, it is necessary to monitor the position of the rotor in electrical degrees. Hall sensors are used for this.
Since there are 6 states of the magnetic field vector, 3 Hall sensors are needed, which will represent one absolute position sensor with a three-bit output. Hall sensors are installed in the same way as windings, offset from each other by 120 electrical degrees. This allows the rotor magnets to be used as an actuating element of the sensor.
Figure 15. Signals from Hall sensors for one electrical revolution of the engine.
To rotate the engine, it is necessary that the stator magnetic field is ahead of the rotor magnetic field, the position when the rotor magnetic field vector is co-directed with the stator magnetic field vector is final for this commutation, it is at this moment that the switch to the next combination should occur in order to prevent the rotor from hanging in a stationary position
Let's compare the signals from the Hall sensors with the combination of phases that need to be switched (Table 2)
Table 2. Comparison of Hall sensor signals with motor phase switching.
| Engine position | HU(1) | HV(2) | HW(3) | U | V | W |
| 0 | 0 | 0 | 1 | 0 | - | + |
| 1 | 0 | 1 | + | - | 0 | |
| 1 | 0 | 0 | + | 0 | - | |
| 1 | 1 | 0 | 0 | + | - | |
| 0 | 1 | 0 | - | + | 0 | |
| 360/N | 0 | 1 | 1 | - | 0 | + |
When the engine rotates uniformly, the sensors receive a signal shifted by 1/6 of the period, 60 electrical degrees (Figure 16).
Figure 16. View of the signal from Hall sensors.
Control using a back EMF signal
There are brushless motors without position sensors. The rotor position is determined by analyzing the EMF signal in the free phase of the motor. At each moment of time, “+” is connected to one of the phases to the other “-” power supply, one of the phases remains free. While rotating, the magnetic field of the rotor induces an EMF in the free winding. As rotation occurs, the voltage on the free phase changes (Figure 17).
Figure 17. Voltage change on the motor phase.
The signal from the motor winding is divided into 4 moments:
1. Winding connected to 0
2. Winding not connected (free phase)
3. The winding is connected to the supply voltage
4. Winding not connected (free phase)
By comparing the signal from the phases with the control signal, it is clear that the moment of transition to the next state can be detected by the intersection of the midpoint (half of the supply voltage) with the phase that is not currently connected (Figure 18).
Figure 18. Comparison of the control signal with the signal on the motor phases.
After detecting an intersection, you must pause and turn on the next state. Based on this figure, an algorithm for switching winding states was compiled (Table 3).
Table 3. Algorithm for switching motor windings
| Current state | U | V | W | Next state |
| 1 | - | + | 2 | |
| 2 | - | + | 3 | |
| 3 | + | - | Waiting for the midpoint to cross from + to - | 4 |
| 4 | + | Waiting for the midpoint to cross from - to + | - | 5 |
| 5 | Waiting for the midpoint to cross from + to - | + | - | 6 |
| 6 | - | + | Waiting for the midpoint to cross from - to + | 1 |
The intersection of the midpoint is easiest to detect with a comparator; the midpoint voltage is supplied to one input of the comparator, and the current phase voltage is supplied to the second.
Figure 19. Midpoint detection by comparator.
The comparator is triggered when the voltage passes through the midpoint and generates a signal for the microcontroller.
Signal processing from motor phases
However, the signal from the phases when regulating the PWM speed differs in appearance and has a pulsed nature (Figure 21), in such a signal it is impossible to detect intersection with the midpoint.
Figure 20. View of the phase signal when controlling the PWM speed.
That's why this signal should be filtered with an RC filter to obtain an envelope, and also divided according to the requirements of the comparator. As the duty cycle increases, the PWM signal will increase in amplitude (Figure 22).
Figure 21. Circuit of a signal divider and filter from the motor phase.
Figure 22. Signal envelope when changing the PWM duty cycle.
Midpoint diagram
Figure 23. View of the virtual midpoint. Picture taken from avislab.com/
Signals are removed from the phases through current-limiting resistors and combined, and this is the picture we get:
Figure 24. View of the virtual midpoint voltage oscillogram.
Due to PWM, the midpoint voltage is not constant, the signal also needs to be filtered. The midpoint voltage after smoothing will be quite large (in the region of the motor supply voltage), it must be divided by a voltage divider to half the supply voltage.
After the signal passes through the filter, the oscillations are smoothed out and a flat voltage is obtained relative to which the intersection of the back EMF can be detected.
Figure 26. Voltage after the divider and low-pass filter.
The midpoint will change its value depending on the voltage (PWM duty cycle), as well as the signal envelope.
The received signals from the comparators are sent to the microcontroller, which processes them according to the algorithm above.
That's all for now.
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 you to automatic protection 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 SB2 button, which closes (by the lever) only at the maximum resistance value (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 current pulses, and through 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 printed circuit board.
Fixed resistors are used type S2-23, variable R1 - type PPB-15T, R7 - SP-196, R3 - type PEV-25. Capacitors C1 and C2 of any type, for an operating voltage of at least 100 V. Rectifier diodes VD1 ... VD4 for a current of 10 A and reverse voltage 300 V, for example D231 D231A D232, D232A, D245, D246.
The T1 pulse transformer is made on a ferrite ring M2000NM of 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, according to 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 sets the maximum voltage across the load at the minimum value of resistance R1).
If, with proper installation, the thyristors do not open, then it is necessary to swap the terminals in secondary windings 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.
Eliminate the possibility of filing maximum voltage on the armature winding at the moment of switching on, you can 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
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, ranging from small motors that rotate a drum washing machine, and ending with huge colossuses that drive factory conveyors and mine elevators.
Traditionally, electric motors are divided into DC motors And AC motors. 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 (DCMs) also have their advantages, and they will continue to spin their shafts for a long time in merciless operation mode in various technical fields, so today we will talk about DMCs, or more precisely about the control of brushed DC motors.
Such units were the first engines to find widespread use in industrial equipment, and they are still used where a low cost 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. Motor armature voltage D in G-D system 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 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 PSD output we will get 0 V. The advantage of the symmetrical method is 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 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 is short-circuited either to power or to ground. When the sign of the reference changes, the transistors are controlled in the opposite way.
DC motors are not used as often as AC motors. Below are their advantages and disadvantages.
In everyday life, DC motors are used in children's toys, since they are powered by batteries. They are used in transport: in the subway, trams and trolleybuses, and cars. In industrial enterprises, DC electric motors are used to drive units that use batteries for uninterrupted power supply.
DC Motor Design and Maintenance
The main winding of a DC motor is anchor, connected to the power source via brush apparatus. The armature rotates in the magnetic field created by stator poles (field windings). The end parts of the stator are covered with shields with bearings in which the motor armature shaft rotates. On one side, mounted on the same shaft fan cooling, driving a flow of air through the internal cavities of the engine during operation.
The brush apparatus is a vulnerable element in the engine design. The brushes are ground to the commutator in order to repeat its shape as accurately as possible, and are pressed against it with constant force. During operation, the brushes wear out, conductive dust from them settles on the stationary parts, and must be removed periodically. The brushes themselves must sometimes be moved in the grooves, otherwise they get stuck in them under the influence of the same dust and “hang” above the commutator. The characteristics of the motor also depend on the position of the brushes in space in the plane of rotation of the armature.
Over time, brushes wear out and need to be replaced. The commutator at the points of contact with the brushes also wears out. Periodically, the armature is dismantled and the commutator is turned on a lathe. After grinding, the insulation between the commutator lamellas is cut to a certain depth, since it is stronger than the commutator material and will destroy the brushes during further processing.
DC motor connection circuits
Presence of field windings – distinctive feature DC machines. The electrical and mechanical properties of the electric motor depend on the way they are connected to the network.
Independent excitation
The excitation winding is connected to an independent source. The characteristics of the motor are the same as those of a permanent magnet motor. The rotation speed is controlled by the resistance in the armature circuit. It is also regulated by a rheostat (adjusting resistance) in the excitation winding circuit, but if its value decreases excessively or if it breaks, the armature current increases to dangerous values. Motors with independent excitation cannot be started at idle speed or with a low load on the shaft. The rotation speed will increase sharply and the motor will be damaged.
The remaining circuits are called self-excited circuits.
Parallel excitation
The rotor and excitation windings are connected in parallel to one power source. With this connection, the current through the excitation winding is several times less than through the rotor. The characteristics of electric motors are rigid, allowing them to be used to drive machines and fans.
Regulation of the rotation speed is ensured by the inclusion of rheostats in the rotor circuit or in series with the excitation winding.
Sequential excitation
The field winding is connected in series with the armature winding, and the same current flows through them. The speed of such an engine depends on its load; it cannot be turned on at idle. But it has good starting characteristics, so a series excitation circuit is used in electrified vehicles.
Mixed excitement
With this scheme, two excitation windings are used, located in pairs on each of the poles of the electric motor. They can be connected so that their flows are either added or subtracted. As a result, the motor can have characteristics similar to a series or parallel excitation circuit.
Currently, DC motors are widely used in various industries. DC motors are used where smooth and precise control of speed and torque over a wide range is required. In this article I will talk about creating a control unit for a DC motor that would allow changing the speed of the motor shaft and stabilize the speed at a certain level, regardless of the load on the motor shaft.
The development is based on the operating principle of a servo drive with a single-circuit control system.
The control unit consists of the following components:
- SIFU (Pulse-Phase Control System)
- Regulator
- Protection
The schematic diagram of the drive is shown below.
Larger
Let's take a closer look at the diagram.
So, SIFU (Pulse-Phase Control System) - converts the sinusoidal network voltage into a series of rectangular pulses going to the control electrodes of power thyristors. When turning on the control unit AC voltage 14-16V is supplied to bridge rectifier D1, where it is converted into a pulsating voltage, which serves not only to power the circuit, but also to synchronize the operation of the drive. Diode D2 prevents the smoothing of pulses by capacitor C1. Next, the pulses arrive at the “zero detector” - DA1.1, assembled on one op-amp of the LM324 chip, operating in comparator mode. While there is no pulse, the voltages at the direct and inverse inputs are approximately equal and the comparator is in a balanced state. When the phase passes through “0”, pulses appear at the inverse input of the comparator DA1.1, which plays the role of a “zero detector”, switching the comparator, as a result of which rectangular synchronizing pulses are generated at the output of DA1.1, the repetition period of which is strictly tied to the phase transition through “0” "
Below are oscillograms that explain the operating principle. 
From top to bottom: KT1, KT2, KT3.
The circuit was simulated in Multisim 11. Here is the project file. You can download, run and see how this node works.
Next, the clock pulses are sent to an integrator with a transistor switch (C4, Q1), where a sawtooth voltage is generated. At the moment the phase passes through “0,” the clock pulse opens transistor Q1, which discharges capacitor C4. After the pulse decays, the transistor closes and the capacitor is charged until the next clock pulse arrives, resulting in Q1 on the collector (oscillator KT4). a linearly increasing sawtooth voltage is formed, stabilized by a stable current generator made on field-effect transistor T1. The amplitude of the “saw” equal to 9V is set by trimming resistor RP1. The “saw” voltage is supplied to the direct input of the comparator DA1.2.
The reference voltage is supplied to the inverse input of the comparator DA1.2 and at the moment when the sawtooth voltage exceeds the voltage at the inverse input of the comparator, the comparator switches and a pulse is formed at the output of the comparator (oscillation KT4). The pulse is differentiated through the chain R14, C6 and goes to the base of transistor Q2. The transistor opens and pulse transformer Tr1 generates opening pulses for power thyristors. By increasing (decreasing) the reference voltage, the duty cycle of the pulses in CT5 changes.
Here are the oscillograms. 
But we will not see any impulses in KT5 until we press the “Start” button - S1. When the button is not pressed, the +12V supply voltage through the normally closed contacts S1 along the chain R12, D3 is supplied to the inverse input DA1.2 and is equal to about 11V. Since this voltage exceeds the “saw” voltage of 9V, the comparator is locked and control pulses for opening the thyristors are not generated. To prevent an accident and failure of the engine, if the operator does not set the speed controller to “0,” the circuit provides an acceleration unit C5, R13, which serves for smooth acceleration of the engine. In the “Start” mode, the circuit works as follows: when you press the “Start” button, the normally closed contacts open and capacitor C5 along the chain - “ground”, R13, - C5 begins to smoothly charge and the voltage on the negative plate of the capacitor smoothly tends to zero. At the same time, the voltage at the inverting input DA1.2 smoothly increases to a value determined by the reference voltage, and the comparator begins to generate control pulses for the power thyristors. The charging time is determined by the ratings C5, R13. If during engine operation it is necessary to change its speed in order to avoid sudden surges in speed, the circuit provides an “acceleration-braking” unit R21, C8, R22. When the reference voltage increases (decreases), capacitor C8 is smoothly charged (discharged), which prevents a sharp “surge” of voltage at the inverse input of the amplifier and, as a result, prevents a sharp increase in engine speed.
Now let's look at the principle of operation speed controller.
The regulator is designed to maintain constant engine speed in the control zone. The regulator is differential amplifier with the summation of two voltages: the reference voltage and the feedback voltage. The reference voltage is set by resistor RP1 and is supplied through filter R20, C8, R21, which simultaneously performs the functions of an “acceleration-deceleration” unit, and is supplied to the inverse input of the op-amp regulator DA1.3. As the reference voltage at the output of op-amp DA1.3 increases, the output voltage decreases linearly.
The output voltage of the regulator is supplied to the inverse input of the comparator SIFU DA1.2 where, summed with sawtooth voltage pulses, it is converted into a series of rectangular pulses going to the control electrodes of the thyristors. As the reference voltage increases (decreases), the output voltage at the output of the power unit also increases (decreases).
This graph shows the dependence of engine speed on the reference voltage. 
Engine speed values are given as an example.
The voltage divider R22, R23 connected to the direct input of the DA1.3 regulator serves to prevent engine failure when the feedback is broken (if the feedback is broken, the engine goes into overdrive).
When the drive is turned on, the tachogenerator begins to generate voltage proportional to engine speed. This voltage is supplied to the input of a precision detector DA1.4, DA2.1 assembled using a full-wave circuit. The voltage taken from the output of the precision detector DA1.4, DA2.1 is supplied through filter C10, R30, R33 to the scaling feedback amplifier DA2.2. The amplifier is used to adjust the feedback voltage coming from the tachogenerator. Voltage from the output of op-amp DA2.2. is supplied both to the input of the regulator DA1.3 and to the protection circuit DA2.3.
Resistor RP1 sets the engine speed. When the engine is running without load, the voltage at the output of the scaling amplifier is lower than the voltage at pin 6 of op-amp DA1.3. ≈ +5v, so the drive works as a regulator. As the load on the motor shaft increases, the voltage received from the tachogenerator decreases and, as a consequence, the voltage from the output of the scaling amplifier decreases.
When this voltage is less than the voltage at pin 5 of op-amp DA1.3, the drive enters the current stabilization zone. A decrease in the voltage at the non-inverting input of op-amp DA1.3 leads to a decrease in the voltage at its output, and since it operates on the inverting amplifier DA1.2, this leads to a larger opening angle of the thyristors and, consequently, to an increase in the voltage at the motor armature.
PROTECTION CIRCUIT
Overspeed protection is designed to protect the engine from an accident if the set engine speed is suddenly exceeded. The circuit is assembled using op-amp DA2.3, connected according to the comparator circuit. The reference voltage from the divider R36, R37, RP3 is supplied to the inverse input of the comparator. Resistor RP3 sets the protection threshold. The voltage from the output of the scaling amplifier DA2.2 is supplied to the direct input of the protection comparator DA2.3. When the engine speed exceeds the rated speed, the voltage at the direct input of the comparator exceeds the threshold of the protection setting determined by RP3 - the comparator switches. Due to the presence of positive feedback in the circuit, R38 causes the comparator to “click,” and the presence of diode VD12 prevents the comparator from resetting. When the protection is triggered, the voltage from the output of the protection comparator (≈ +11v) through the VD14 diode is supplied to the inverse input 13 DA1.2 SIFU, and since the protection voltage exceeds the “saw” voltage (= 9v) - the issuance of control pulses to the control ones is instantly prohibited thyristor electrodes. The voltage from the output of the protection comparator DA2.3 opens the transistor VT4, which causes relay P1.1 to operate and the VL1 LED to light, signaling an emergency. You can remove the protection only by completely de-energizing the drive, and after pausing for 5 - 10 seconds, turning it on again.
Power part of the control unit.
The power section diagram is shown below 
Transformer Tr1 is designed to power the control unit circuit. The controlled rectifier is assembled using a half-bridge symmetrical circuit and contains two power diodes D1, D2
and two power thyristors T1, T2, and 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 feedback to control speed can be performed as follows:
1. Use a current transformer connected to the power circuit of the controlled rectifier 
If a current transformer is used, then place jumper P1 on the control unit diagram
to position 1-3, this is necessary because as the load increases, the armature current will increase, therefore the voltage removed from the current transformer will also increase, so the feedback voltage must be applied to the inverting
output of the DA1.3 chip. You can also install a standard current shunt, but only in the motor armature circuit, after the rectifier, and remove the feedback signal from it.
2. Use an armature voltage sensor. The diagram is shown below. 
The armature voltage sensor is a filter-divider and is connected directly to the armature terminals of the electric motor. The drive is configured as follows. Resistors “Task” and “Scaling Uoc” are set to the middle position. Resistor R5 of the armature voltage sensor is placed in the lower “ground” position. We turn on the drive and set the voltage at the motor armature to approximately 110 volts. By controlling the voltage at the motor armature, we begin to rotate resistor R5. At a certain point of regulation, the voltage on the armature will begin to decrease, this indicates that the feedback has begun to work.
Now let's move on to the design and adjustment of the control unit.
The control unit was made on a printed circuit board (PCB file) 
The board is connected by MGTF wire to the connector for easy dismantling during repairs.
Settings
During setup, the power part was assembled using a wall-mounted installation, and a regular incandescent lamp was used as a load. 
We begin the setup by checking the supply voltages and the supply voltage at operational amplifiers DA1, DA2. It is advisable to install microcircuits in sockets. Then we monitor the oscillograms at control points KT1, KT2, KT3 (oscillograms at these points are given at the beginning of the description of the SIFU). Now, we place the oscilloscope at the control point KT4. There should be sawtooth pulses, as in the osillogram above (the “Start” button should be open at this moment). Using the trimmer resistor RP1, it is necessary to set the swing of the “saw” to 9 volts; this is a very important point, since it determines further work scheme. Since the spread of parameters field effect transistors can be quite significant, perhaps the adjustment range of RP1 may not be enough, then by selecting the value of resistor R10, achieve the desired range. At the control point KT3, the pulse duration should be 1.5 - 1.8ms; if not, then select resistor R4 (towards a decrease) to achieve the required duration.
By rotating the RR1 regulator at control point KT5, check the change in the duty cycle of the pulses from maximum to their complete disappearance when the RR1 slider is in the lower position. In this case, the brightness of the light bulb connected to the power unit should change.
Next, we connect the control unit to the engine and tachogenerator. We set it with the RR1 regulator
armature voltage is about 40-50 volts. Resistor RP3 should be set to the middle position. By controlling the voltage on the motor armature, we begin to rotate resistor RP3. At a certain point of regulation, the voltage on the armature will begin to decrease, this indicates that the feedback has begun to work. For those who want to experiment: to increase the rigidity of the drive, you can also increase the resistance R24, thereby increasing the gain of the regulator, or increase the resistor R32.
If motor armature current feedback is used.
For this, as mentioned above, you need a current transformer included in the power circuit
controlled rectifier. The current transformer calibration diagram is given below. By selecting a resistor, obtain an alternating voltage of ≈ 2 ÷ 2.5v at the transformer output. Load power RN1 must match the engine power. 
Attention! Do not turn on the current transformer without a load resistor.
We connect the current transformer to the feedback circuit P1 and P2. While setting up the “Regulator”, it is advisable to unsolder the D12 diode to prevent false triggering of the protection.
Oscillograms at control points KT8, KT9, KT10 should be as in the figure below. 
Further settings are the same as in the case of using a tachogenerator.
If motor armature voltage feedback is used.
As noted above, you can apply armature voltage feedback; for this, an armature voltage sensor is assembled. The control unit is configured as follows. Resistors “Task” and “Scaling Uoc” are set to the middle position. Resistor R5 of the armature voltage sensor is placed in the lower “ground” position. We turn on the drive and set the voltage at the motor armature to approximately 110 volts. By controlling the voltage at the motor armature, we begin to rotate resistor R5. At a certain point of regulation, the voltage on the armature will begin to decrease, this indicates that the feedback has begun to work.
This control unit was manufactured for a boring machine. Here is a photo of this monster 
On this machine, the electric machine amplifier, which controlled the DC motor for moving the table, failed.
Here's an electric machine amplifier. 
This control unit was made instead.
Here is a photo of the DC motor itself. 
The control unit was assembled on an insulating base, where all the main elements are located. 
Power diodes and thyristors are installed on heat sinks. A panel with connectors was also made, where signals from the control points of the circuit were output. This was done for ease of setup and repair directly on the machine.
Here is the mounted control unit in the power cabinet of the machine 
A small control panel was installed on the other side of the power cabinet. 
It contains:
-toggle switch for turning on the unit
-operating mode toggle switch. Since for the installation movements of the machine table, precise control and stabilization of revolutions is not needed, the feedback circuit is bypassed during this time.
- knobs for adjusting the number of revolutions. Two were delivered resistor variables, one for rough adjustment, the second - multi-turn - for precise setting of the required speed when roughing and finishing boring a part.
For those interested, below is a video of the machine in operation. First, the boring of a hole in a 20mm thick steel plate is shown. Then it is shown at what frequency the machine table feed screw rotates. At this speed, the part is fed to the cutter, and this speed of rotation of the feed screw is provided by the DC motor, for which, in fact, all this was done.
The control unit performed well, there were no failures or accidents.