Soft starter for power tools. Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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Soft starter for power tools. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Electric motors

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Failures of hand-held power tools that sometimes occur - grinders, electric drills and jigsaws - are often associated with their high starting current and significant dynamic loads on gearbox parts that occur when the engine starts abruptly.

The soft start device for a commutator electric motor, described in [1], is complex in design, it contains several precision resistors and requires painstaking setup. By using the KR1182PM1 phase regulator microcircuit [2], it was possible to produce a much simpler device for a similar purpose that does not require setup. You can connect to it without any modification any hand-held power tool powered by a single-phase 220 V, 50 Hz network. The engine is started and stopped by the power tool switch, and when it is turned off, the device does not consume current and can remain connected to the network indefinitely.

The scheme of the proposed device is shown in the figure.

Power tool soft starter

The XP1 plug is plugged into the power socket, and the power tool's power plug is inserted into the XS1 socket. You can install and connect in parallel several sockets for tools that work alternately.

When the power tool motor circuit is closed by its own switch, voltage is supplied to the phase regulator DA1. Capacitor C2 begins to charge, and the voltage across it gradually increases. As a result, the turn-on delay of the internal thyristors of the regulator, and with them the triac VS1, in each subsequent half-cycle of the mains voltage decreases, which leads to a smooth increase in the current flowing through the motor and, as a result, an increase in its speed. With the capacitance of capacitor C2 indicated in the diagram, acceleration of the electric motor to maximum speed takes 2...2,5 s, which practically does not create a delay in operation, but completely eliminates thermal and dynamic shocks in the tool mechanism.

After the engine is turned off, capacitor C2 is discharged through resistor R1. and after 2...1 seconds. everything is ready to start again. By replacing the constant resistor RXNUMX with a variable one, you can smoothly regulate the power supplied to the load. It decreases with decreasing resistance.

Resistor R2 limits the current of the control electrode of the triac, and capacitors C1 and C3 are elements of a typical circuit for switching on the phase regulator DA1.

All resistors and capacitors are soldered directly to the terminals of the DA1 chip. Together with them, it is placed in an aluminum case from a fluorescent lamp starter and filled with epoxy compound. Only two wires are brought out, connected to the triac terminals. Before pouring, a hole was drilled in the lower part of the body into which an M1 screw was inserted with an outward thread. This screw secures the unit to the heat sink of the VS100 triac with an area of XNUMX cm." This design has proven to be quite reliable when used in conditions of high humidity and dust.

The device does not require any setup. Any triac can be used, voltage class of at least 4 (that is, with a maximum operating voltage of at least 400 V) and with a maximum current of 25-50 A. Thanks to the smooth start of the engine, the starting current does not exceed the rated one. The reserve is needed only in case the tool jams.

The device has been tested with power tools up to 2,2 kW. Since the DA1 regulator ensures the flow of current in the control electrode circuit of the triac VS1 during the entire active part of the half-cycle, there is no restriction on the minimum load power. The author even connected a Kharkov electric razor to the manufactured device.

Literature

Biryukov S. Automatic soft start of commutator electric motors - Radio 1997, No. 8. pp. 40, 42
Nemich A. Microcircuit KR1182PM1 - phase power regulator - Radio 1999, No. 7, p. 44-46.

Author: K.Moroz, Nadym, YNAO

See other articles Section Electric motors.

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