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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING Electric drive controller. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Electric motors The engine speed controller, which stabilizes its speed when the load changes, significantly increases the operational capabilities of such household appliances as an electric drill, an electric saw, a food processor, etc. A simple and effective half-wave controller of a series excitation collector electric motor is known, which stabilizes the rotation speed due to feedback by the magnitude of the back-EMF that occurs on the motor rotor and depends on its load. Unfortunately, such a regulator has a significant drawback - it uses a highly sensitive thyristor with an opening current of less than 100 μA. It's almost impossible to find a replacement for him. In the published article, the author offers his own version of the circuit design solution for the regulator, in which the restrictions on the parameters of the thyristor are removed. Before proceeding to the description of the modernized electric drive controller, let us dwell briefly on the principle of operation of a simple control device [1]. Its schematic diagram is shown in fig. 1. This is a bridge, the left shoulder of which is formed by the mains voltage divider R1 - R2C1 - VD1, and the right - by the VS1 thyristor and the M1 motor. The control transition of the thyristor is included in the diagonal of the bridge. The signal that opens the thyristor is the sum of the signals that are added in antiphase: the mains voltage, set by the resistor R2 engine, and the back-EMF from the motor rotor. When the voltage remains unchanged, the bridge is balanced and the engine speed also does not change. An increase in the load on the motor shaft reduces its speed and, accordingly, reduces the value of the back-EMF, which leads to an unbalance of the bridge. As a result, the signal to the control transition of the thyristor increases, and in the next positive half-cycle it opens with less delay, thus increasing the power supplied to the motor. As a result, the decrease in the engine speed due to the increase in load turns out to be significantly less than it would be in the absence of the regulator.
In this case, the regulation is very stable, since the mismatch is eliminated in each positive half-cycle of the mains voltage. Most of all, the stabilization effect is expressed at low and medium engine speeds. With an increase in the regulation voltage across the resistor R2 and an increase in the engine speed, the degree of maintaining a constant engine speed deteriorates. Thyristor VS1 in the regulator performs two functions: threshold - according to the bridge mismatch signal and power - according to the switched current through the motor. Diodes VD1, VD2 provide a half-wave operation of the device, since the comparison of voltages from resistor R2 and back-emf is possible only in the absence of current through the motor. Capacitor C1 in the mains voltage divider expands the regulation zone towards low speeds, and capacitor C2 in the thyristor control electrode circuit reduces the regulator's sensitivity to motor brush sparking. The half-wave mode of the engine leads to a decrease in the output power. To achieve maximum power and speed, shunt the thyristor by pressing the SA1 button. In this case, both half-waves of the mains voltage will be supplied to the motor. As already mentioned, the main drawback of the considered regulator is the need to use a highly sensitive thyristor with an opening current of less than 100 μA, which is practically nothing to replace. The introduction of a transistor analog of the thyristor allows you to remove restrictions on the opening parameters of VS1 while maintaining the same control characteristics. Installing a zener diode in the mains voltage divider reduces changes in motor speed with fluctuations in the supply voltage. The scheme of the upgraded regulator is shown in fig. 2. Like the device discussed above, the regulator works only with a positive half-wave of the mains voltage.
The mismatch voltage of the bridge through the diode VD2 and the resistor R10 is supplied to the base-emitter junction of the transistor VT2. The sensitivity of this device and the quality of its regulation are higher, since the opening voltage of transistors is significantly less than that of thyristors. The control current, by analogy with the regulator shown in fig. 1 is chosen to be 0,1mA by shunting the transistor junction with resistor R7. If the voltage coming from the engine of the resistor R2 is higher than the voltage on the motor rotor, then the transistor VT2 opens and opens VT1. These devices form an analogue of a thyristor and, when turned on, form a powerful pulse of the discharge current of the capacitor C3, which, through the current-limiting resistor R9, is fed to the control electrode of the triac VS1. The triac turns on, voltage is applied to the engine, and the number of revolutions increases. If the voltage on the resistor R2 is lower than on the motor rotor, the triac will not turn on, the number of revolutions will decrease. The storage capacitor C3 is charged from the network through the resistor R5. The Zener diode VD4 limits the voltage across the capacitor to a level somewhat higher than the possible opening voltage of triacs or thyristors. In addition, the zener diode eliminates the appearance of reverse voltage on the amplifier transistors. Capacitor C4, in addition to reducing interference from the sparking of the motor brushes, performs the function of integration in the feedback circuit. An increase in its capacity increases the stability of the regulator, which is necessary in case of poor contact of the brushes, accompanied by their strong sparking, or when setting extremely low speeds, when the so-called "swing" of revolutions can occur. However, it should be remembered that with an increase in the capacitance of the capacitor C4, the dynamic characteristics of the drive deteriorate and the quality of speed stabilization decreases. The circuit constant R5C3 is such that the capacitor C3 is charged faster than the charge of the capacitor C4. This is done so that at any possible moment of opening the transistor VT2, the operating voltage necessary for generating a starting pulse is already present on the capacitor C3. Sometimes such a condition can be violated with a sharp unbalance of the bridge - when the engine is stopped (low rotor resistance) and the maximum voltage on the engine of the resistor R2 (large opening current from the divider). As a result, the transistors open before the charging of the capacitor C3 is completed, there is no voltage on it, and the discharge current pulse is not formed. The current flowing through the resistor R5 is sufficient to keep the transistors on, but small to turn on the triac, and therefore the motor does not rotate. Such a possibility can be regarded as positive, since in this case, even when the drive is jammed, the engine is switched off. If it is undesirable, it is eliminated by some decrease in the resistance of resistors R5 - R7 and (or) an increase in the resistance of resistor R1. The magnitude and shape of the voltage across the resistor R2 are practically independent of the change in the mains voltage due to the presence of the limiter R4 - VD1. As a result, fluctuations in the supply voltage do not lead to instability of the set phase angle of the triac opening. The instability of the network voltage of the set motor speed is also significantly reduced. With a constant phase angle, the speed changes only due to a change in the amplitude of the voltage on the motor. A feature of the described regulator is the use of a triac. The fact is that the switching of the maximum speed by closing the "anode-cathode" circuit assumes the presence of instantaneous contacts SA1 with sufficient breaking power. If the contacts are made differently, sparking or an electric arc may occur in them. The latter is highly undesirable, as it leads to burnout of the contacts and the printed circuit board and, therefore, is a fire hazard. The triac allows you to transfer switching to the control electrode circuit, which completely eliminates sparking in the contacts, simplifies their design and binding to the control resistor R2. When regulating, the triac works like a thyristor, and when the contacts are closed, it passes alternating current to the motor. Transistors during the open state of the triac are blocked and do not function. The inclusion of the stator and rotor windings shown in the regulator diagram is optimal for motors with separately output winding ends. When using motors with an internal connection of the rotor and stator windings, they are connected in place of the rotor winding shown in the diagram, and the stator winding circuit is replaced with a jumper. However, due to the presence of a stator winding in the feedback circuit, the latest version of the regulator has slightly worse speed control characteristics. Capacitors C2, C6 eliminate interference, and the R11C5 circuit suppresses brush sparking. Resistor R1 limits the limits of regulation of the open state of the triac by the beginning of a positive half-cycle. With an increase in the load on the shaft, the back-emf of the motor additionally shifts the moment of unlocking the triac to the beginning of the half-cycle relative to the position set by the adjusting resistor R2 at idle. If the resistor R1 was selected at idle, then under load, the counter-EMF, as it were, transfers the moment of opening the triac for the beginning of the half-cycle. As a result, it opens after a period and a “dip” (decrease) in speed occurs in the upper position of the resistor R2 slider. This phenomenon is eliminated by increasing the resistance of the resistor R1. During development, the regulator was tested with various collector motors: DK77 (for household electrical appliances and power tools), MSH-2 (for sewing machines) and even with a parallel excitation motor SL261M. The control of such significantly different engines did not require any changes to the regulator. When using a motor with parallel excitation, it should be borne in mind that its stator winding must be powered from a separate external source and, moreover, before voltage is applied through the regulator to the armature. The capabilities of the regulator illustrate the load characteristics (solid line without VD1, dashed line with VD1), taken with the DK77-280-12 engine at an idle speed of 1500 rpm and various mains voltages (Fig. 3). This 400 W motor at 1200 rpm is easily braked with a hand placed on its shaft until it comes to a complete stop if it is powered through an autotransformer, setting the same speed at 1500 rpm at idle.
With a slight complication relative to the prototype, the controller is completely uncritical to the spread of the parameters of the elements. As triacs, TS, TS2, 2TS112 and TS106 are applicable for currents of 6,3-10-16 A, as well as KU208G or 2U208G for 5 A. You can also use thyristors KU201L, 2U201L, KU202N-M, 2U202N-M, KU228I and others provided that a contactor is installed along the "anode-cathode" circuit. The need for heat removal is determined by the magnitude of the load current. Transistors must allow a current of at least 250 mA and a voltage of at least 15 V. VT1 functions can be performed by KT350A, KT209 (A-M), KT501A, KT502A (B-E), KT661 A, KT681A and others, and VT2 - KT503A (B -E), KT645A, KT660A (B), KT684A (B) and others with similar characteristics. Diodes can be for a current of at least 10 mA and a voltage of at least 400 V - KD105 (B-G), KD209 (A-V), KD221 (V-G), KD226 (V-D), D209, D210, D211, D226, D237 (B-C). The VD1 zener diode is suitable for a stabilization voltage of 120 ... 180 V (KS630A, KS650A, KS680A, 2S920A, 2S950A, 2S980A) and can be replaced by a chain of series-connected low-power zener diodes for a total voltage of 150 V. Zener diode VD4 - any low-power with a stabilization voltage of 9 ... 11 V, except for thermally compensated ones. Capacitors C1-C4 - ceramic KM, KM-6, K10-17 or film K73-17. Capacitors C5, C6 - K73-17 with a rated voltage of 630 V (capacitors of other types and K73-17 for a lower rated voltage cannot be used). Fixed resistors - MLT or any others. Resistor R2 - RP1-64A, it can be replaced by any non-wire variable resistor with a linear characteristic (SPZ-4M, SPZ-6, SPZ-9, etc.). The choice of a resistor with an inverse-logarithmic characteristic (B) will expand the smoothness of regulation in the zone of low engine speeds. Trimmer resistor R3 - SPZ-27, SPZ-38. It can be replaced with a selected constant resistor. The maximum speed switch SA1 is made in the form of a movable spring plate contact and a fixed stand on the regulator board. Between the resistor R2 and the movable contact there is an adapter plastic sleeve with a cam, which ensures the closure of the movable contact with the rack in the upper position of the variable resistor R2 according to the scheme. When adjusting the regulator, the slider of the resistor R2 should be set to the lower position according to the diagram and the desired minimum engine speed should be set with the tuning resistor R3. Further, by changing the position of the slider of the resistor R2, you should check the change in speed from minimum to maximum, the absence of a "swing" of revolutions at the minimum speed without load, the absence of a "dip" in revolutions at the maximum speed of the half-wave mode under load, as well as the operation of the maximum speed contacts. The swing is eliminated by increasing the capacitance of the capacitor C4, and the dip is eliminated by an increase in the resistance of the resistor R1, after which the position of the resistor R3 slider is again specified. In conclusion, it should be noted that in regulators of this type, the tachometer is the executive electric motor and the feedback voltage is determined by the residual magnetization of the motor magnetic circuit and the stability of the brush contact. For this reason, the quality of regulation directly depends on the specified characteristics of the motor used. However, the extreme simplicity of the control device and good load characteristics fully compensate for this shortcoming. Literature
Author: V. Zhgulev, Serpukhov, Moscow Region
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