Hand held variable speed drill motor and control system therefor

In a motor control system for a three phase induction motor having motor windings, the combination which comprises:

• a source of direct current,
• a pair of power supply conductors,
• an inverter, the inverter including a separate pair of switching devices connected across the power supply conductors for each phase winding of the motor, a junction intermediate each pair of switching devices connected to a respective phase winding, each switching device having a conducting and a nonconducting state,
• a variable frequency clock signal generator,
• switch control means coupled between the clock signal generator and the switching devices for alternately rendering one switching device of each pair conducting and the other switching device of each pair nonconducting in a predetermined phase sequence to provide three phase current for the motor windings to cause the motor to rotate in a first direction at a speed proportional to the frequency of the clock signal from said clock signal generator, and
• voltage regulating means connected between the direct current source and the power supply conductors and responsive to the clock signal from said clock signal generator for supplying a direct current voltage across the supply conductors which varies proportionately to the frequency of the clock signal from said clock signal generator over a pre-established range.

In a variable speed hand held dental drill the combination which comprises:

• a polyphase induction motor, including phase windings and a rotor;
• a chuck carried by the rotor of said motor for releasably securing a tool;
• a pair of power supply conductors;
• means for supplying direct current at a selected voltage across the supply conductors;
• an inverter, the inverter including a plurality of pairs of switching devices connected across the power supply conductors, a junction intermediate each pair of switching devices connected to respective phase windings of said motor, each switching device having a control electrode and being arranged to provide a closed circuit between the respective power supply conductor and the associated phase winding in response to the application of a first control signal to the control electrode thereof and to provide an open circuit between the respective power supply conductor and the associated motor winding in response to the application of a second control signal to the control electrode;
• a variable frequency clock signal generator;
• switch control means coupled between said clock signal generator and the inverter for alternately applying the first and second control signals to the control electrodes of each pair of switching devices in a predetermined phase sequence to provide polyphase current to the motor windings to cause the motor to rotate in a first direction at a speed proportional to the frequency of the clock signal from said clock signal generator, and
• stop control means coupled to the switch control means for selectively controlling the switch control means to continuously apply for a preset time the first control signal to the control electrode of one switching device of each pair and the second control signal to the control electrode of the other switching device of each pair to cause direct current to flow through the phase windings to thereby apply a braking torque to said rotor.