Electric drill speed controller. Encyclopedia of radio electronics and electrical engineering

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For high-quality drilling of holes in printed circuit boards, an electric drill with speed and torque control is required. Transistor regulators have, as a rule, low efficiency, which leads to an increase in the size and weight of the power transformer and heat sink. In this regard, trinistor devices are more advantageous, since energy losses in a trinistor operating in the key mode are insignificant. For this reason, there is no need to remove heat from it.

The scheme of a trinistor speed controller with torque stabilization, designed for an electric drill with a DC commutator motor, is shown in fig. 1. The electric motor and all components of the regulator are fed by an unsmoothed rectified voltage coming from the diode bridge VD1 -VD1 connected to the winding II of the transformer T4.

Fig. 1

The voltage limiter, consisting of a resistor R7 and a zener diode VD5, reduces the effect of changes in the mains voltage and load on the operation of the relaxation generator on the unijunction transistor VT1. The generated pulses are fed to the control electrode of a low-power SCR VS1, which serves as a preamplifier, and then through resistors R8 and R10 to the control electrode of a powerful SCR VS2, opening it.

The voltage at the anode of the trinistor VS2, while it is closed, is equal to the difference in the supply voltage coming from the rectifier on the diode bridge VD1 - VD4, and the back-EMF created by the rotating armature of the motor M1 (it is proportional to the rotational speed). The differential voltage is fed through the R12C5 filter to the time-setting circuit of the relaxation oscillator, changing the delay of the pulses generated by it relative to the beginning of each half-cycle of the mains voltage. Diode VD6 prevents the discharge of capacitor C4 in the time intervals when the trinistor VS2 is open. With an increase in the rotational speed, the delay of the pulses increases, which leads to a decrease in the effective value of the voltage applied to the electric motor M1. A decrease in the speed (for example, under the influence of a mechanical load) leads, in turn, to an increase in the voltage applied to the motor. Thus, the frequency of rotation of its shaft is stabilized. The stabilized frequency value can be adjusted by changing the parameters of the generator timing circuit with a variable resistor R4.

The current flowing through the open trinistor VS1 is limited by resistor R5. It is not recommended to reduce its value, since this may violate the conditions for the timely closing of the trinistor. SCR VS2 also closes at the end of each half cycle of the supply voltage. Thanks to the VD7 diode, the anode current of the trinistor is interrupted for a sufficient time for this.

Fig. 2

On the one shown in Fig. 2 printed circuit board of the regulator contains almost all of its parts, except for the capacitor C2, transformer T1 and diodes VD1 - VD4. Variable resistor R4 and LED HL1 are installed on the top cover of the device. A fuse holder FU1 and a switch SA1 are fixed on one of the side walls of the case, and a power cord is inserted through it.

The single-junction transistor KT117B, together with the trinistor KU101E, can be replaced with an assembly KU106V or KU106G containing both of these devices. The choice of trinistor VS2 and transformer T1 is determined by the power and rated supply voltage of the electric motor M1. The author used a transformer TN54-127 / 220-50, connecting in series its four secondary windings of 6,3 V each. The D304 germanium diodes used in the regulator have a small forward voltage drop, which eliminates the need for heat sinks.

When establishing, first of all, the resistance of the variable resistor R4 is set to a minimum and the stable switching on of the trinistor VS2 is achieved by rotating the tuning resistor R10 for this. Further, by increasing the resistance of the resistor R4, the frequency of rotation of the shaft of the electric drill is brought to the required one.

Practical tests of the regulator and the selection of the optimal values ​​of its elements were carried out with an electric drill equipped with a collector DC motor DPR72-F6-06 (case length - 80 mm, diameter - 40 mm). The idle speed of the drill was 600 min. With the feedback disabled in the regulator, it decreased under load to 260 min. When the feedback was turned on, the frequency increased to 520 min (with the same mechanical load). At the same time, the torque has increased markedly.

Author: V. Konovalov, Irkutsk; Publication: radioradar.net

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