Control circuit for electric motor of dental drill - uses semiconductors to produce three=phase current from DC source

摘要

The dental drill is driven by a three phase stator wound electric motor. The motor operates on a dc supply with an inverter consisting of several semi-conductors connected to certain of the motor phase windings and to a frequency variable signal pulse generator by a switching circuit. This circuit enables one pair of semi-conductors to be blocked whilst another pair becomes conducting. This in turn supplies a three-phase current to the motor to allow it to rotate in one direction at a speed proportional to the frequency of the signal pulse. A voltage regulator is also connected to the circuit and enables the dc voltage across the motor to be varied over a pre-determined range depending on the frequency of the signal pulse. The max. turning moment is obtained at the relatively low speed of 5000 r.p.m.