Switching of hand power tools. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Home, household, hobby
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Usually, but hand tools are installed with brushed commutator motors. Such engines are convenient because they have a high specific power (kW/kg). Switching hand tools often break down due to high inrush currents, electric arc transients during shutdown, complexity of the mechanism, and unreliability of shutdown button parts.
In the Parma electric chain saw, produced in Perm (Russia), the switch button broke. The power of the Parma electric motor is 2 kW, and it is not easy to find a sufficiently reliable small-sized instantaneous switch for it. For switching the Parma, I decided to use a circuit similar to the one already used in the EPC-3 electric saw (3 kW, 400 Hz, 220 V, 3-phase asynchronous motor). In this electric saw, the control button lasts a very long time, since it switches very low powers.
Direct switching of the engine is carried out by a magnetic starter (MP), and the control button supplies the voltage necessary to operate this powerful relay (Fig. 1). The voltage is removed from the secondary winding of the transformer and is safe for humans (12...36 V).
Using a similar circuit (Fig. 2), you can power the power tool through a 4-wire cable. Considering that a 3-core cable is quite common and cheaper than a 4-core cable, amateurs can also use it, but they must have a correct connection to the network - this circuit is only suitable for competent users who are well aware of circuit design and safety rules.
Details. The MP contacts must withstand the power of the switched electric motor. The transformer must provide the operating current of the MP and not heat up when holding the contacts of the MP. Any button will do, as long as it is reliable and has sufficient power. A cable with flexible (for example, rubber) insulation and multi-conductor copper cores. It is better to take three-phase plugs, as they should completely prevent incorrect connection, so as not to confuse phases and ground. This is especially important when using a 3-core cable (Fig. 3).
Author: V.V. Dubrovny
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