Stabilized speed controller for power tools. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Electric motors

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Everyone who has ever used a power tool for material processing (grinder, drill, etc.) knows that as soon as the workload is increased, the speed of the tool starts to drop. The device, the scheme of which is shown in Fig. 1, allows, within certain limits, to change the frequency of rotation of the armature of the electric motor connected to it. In addition, it can significantly reduce the dependence of the armature speed on the mechanical load on the tool. When the resistor R7 of the feedback circuit (OS) is closed with an additional toggle switch, the device can also be used as a power regulator up to 500 W with an active load.

Ris.1

The principle of operation of the device is based on full-wave phase control of the triac VS1 (see diagram), which provides the electric drill motor with full power consumption. But a collector electric motor is installed in the drill, so the current in the triac circuit is interrupted due to the inductive load, an EMF of self-induction occurs, which leads to unstable operation of the triac. To eliminate this phenomenon, a circuit R8, C3 is connected in parallel with the triac.

The rectifier bridge VD1-VD4 and the zener diode VD5 provide power to the triac control unit VS1 with a pulsating voltage. Resistor R1 dampens excess mains voltage. The triac opening delay in phase determines the charging time of the capacitor C1 through resistors R2 and R3 from a voltage source, the level of which is determined by the zener diode VD5 and the transfer coefficient of the unijunction transistor VT1.

At a certain threshold voltage on the capacitor C1, the unijunction transistor opens, and a voltage pulse appears on its load resistor R5, which the transistor VT2 amplifies to the level necessary to turn on the triac. The triac remains open until the current flowing through it decreases to its turn-off threshold. In this case, the capacitor C1 is discharged to the closing voltage of the unijunction transistor VT1. After turning off the triac, the capacitor C1 charges again - the next cycle of the triac control unit begins.

Resistor R7 is an element of the OS circuit for current in the load. The action of the OS is illustrated by curves taken at a constant position of the engine of the variable resistor R2 and the operation of the electric drill at idle (Fig. 2) and under load (Fig. 3). Here t1 is the charging time of the capacitor C1, t2 is the time during which the triac is in the open state.

Ris.2

Ris.3

With an increase in the load on the motor shaft, the frequency of rotation of its armature decreases, which leads to an increase in the consumed current and a voltage drop (with the triac on) across the resistor R7. When the total voltage drop across the semistor and resistor R7 exceeds the closing voltage of the unijunction transistor VT1, the capacitor C1 begins to charge, as a result of which, in the new cycle of the device, the time it takes to charge to the opening voltage of the transistor VT1 becomes less. Therefore, the triac at each half-cycle will be in the open state longer, the power on the motor shaft will increase accordingly and the previous speed will be restored.

The regulator was tested in conjunction with an IE 1032-1 electric drill. For the regulator to work with another similar tool, you may need to select a resistor R7.

The regulator uses fixed resistors - MLT, variable resistor R3 - SP4-1, capacitor C1 - KM-6 (you can MBM), C3-MBGP, oxide C2-K50-6. Resistor R7 is wound with a nichrome wire with a diameter of 0,3 mm on an MLT-2 resistor with a resistance of at least 100 ohms. Unijunction transistor VT1 can be KT117A. Transistor VT2 - KT603A or any of the KT312, KT315 series. Diodes D223A can be replaced with D220 or KD521A, triac KU208G - with two trinistors of the KU202 series, by turning them on in anti-parallel, as shown in Fig. 4.

Transformer T1 - MIT-4 or self-made, made on an annular magnetic circuit of size K16x10x4,5 from ferrite 2000NM. The windings of a homemade transformer each contain 100 turns of PELSHO 0,12 wire. When replacing a triac with two trinistors, the pulse transformer must have two secondary windings (Fig. 4).

Ris.4

Establishing a regulator assembled from known good parts comes down only to selecting the resistance of the resistor R7, achieving stable operation of the device. In the case of using the regulator to work with electric drills of outdated models, it may be necessary to increase the capacitance of the capacitor C3 to 0,47 microfarads.

The regulator has direct contact with the mains. Therefore, when setting it up, you must be especially careful and follow the safety requirements when working with electrical installations.

Author: A.Titov

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