Method and circuitry arrangement for operating a brushless electric motor

• US 7,839,107 B2
• Filed: 10/30/2007
• Issued: 11/23/2010
• Est. Priority Date: 10/31/2006
• Status: Active Grant

• Current Assignee: Kavo Dental GMBH (Envista Holdings Corporation)
• Original Assignee: Kaltenbach & Voigt GmbH (Danaher Corporation)
• Inventors: Sauter, Johannes, Buerk, Richard
• Primary Examiner(s): Ro; Bentsu
• Application Number:
  US11/929,607
• Publication Number:
  US 20080101777A1
• Time in Patent Office: 1,120 Days
• Field of Search: 318/400.32, 318/400.34, 318/400.35, 318/400.36, 318/700, 318720-724
• US Class Current: 318/400.35
• CPC Class Codes: G01P 3/44 for measuring angular speed G01P3/56 takes precedence
• CPC Class Codes: G01P 3/48 by measuring frequency of generated current or voltage in general G01R23/00
• CPC Class Codes: H02P 6/182 using back-emf in windings

Abstract

In a method of operating a brushless electric motor, having a permanent magnet rotor and a stator with three windings electrically offset by 120°, provided for example for driving a dental treatment instrument, the zero crossings of a voltage induced by rotation of the rotor in the stator windings is detected by a comparison of a voltage at an inactive stator winding in a monitoring phase with a comparison voltage, and on the basis of the detected zero crossings there is determined the speed of rotation of the motor and/or a suitable commutation point for an intermediate circuit voltage delivered to the stator windings. The comparison voltage is formed by the voltages at the two further, active stator windings in the monitoring phase.

Claims

1. A method of operating a brushless electric motor having a permanent magnet rotor and a stator with three windings electrically offset by 120°

, said method comprising;

detecting zero crossings of a voltage induced by rotation of the rotor in the stator windings by comparing a voltage at an inactive stator winding in a monitoring phase with a comparison voltage formed by the voltages at the two further, active stator windings in the monitoring phase, and

determining on the basis of the detected zero crossings at least one of the speed of rotation of the motor and a suitable commutation point for an intermediate circuit voltage delivered to the stator windings,

wherein each of the two further active stator winding voltages is passed through a respective resistor, each resistor having the same resistance value, the resistance value being twice as large as a resistance value of a resistor through which the inactive stator winding voltage passes.

2. Method of claim 1, comprising forming the comparison voltage with the aid of a voltage divider, which is formed by two resistances which are connected to the two active stator windings.

3. Method of claim 1, comprising detecting a zero crossing as soon as the difference between the voltage induced in the inactive stator winding and the comparison voltage exceeds a predetermined hysteresis value.

4. Method of claim 1, comprising effecting the comparison between the voltage induced in the inactive stator winding and the comparison voltage by a comparator.

5. Method of claim 4, comprising bridging the inputs of the comparator by a capacitor.

6. Method of claim 4, comprising reducing the voltages delivered to the comparator to values corresponding to the working range of the comparator by a voltage divider or by clamping diodes.

7. Circuitry arrangement for detecting the zero crossings of a voltage induced in stator windings, by rotation of a rotor, of a brushless electric motor, having a permanent magnet rotor and a stator with three windings electrically offset by 120°

, said circuitry arrangement comprising a comparison device for comparing a voltage at an inactive stator winding in a monitoring phase with a comparison voltage wherein the comparison voltage is derived from the two voltages at the two further, active stator windings in the monitoring phase, each of the two further active stator winding voltages being passed through a respective resistor before entering the comparison device, each resistor having the same resistance value, the resistance value being twice as large as a resistance value of a resistor through which the inactive stator winding voltage passes before entering the comparison device.

8. Circuitry arrangement of claim 7, comprising, for forming the comparison voltage, a voltage divider formed by two resistances which are connected to the two active stator windings.

9. Circuitry arrangement of claim 7, wherein the comparison device detects a zero crossing as soon as the difference between the voltage induced in the inactive stator winding and the comparison voltage exceeds a predetermined hysteresis value.

10. Circuitry arrangement of claim 7, wherein the comparison device is formed by a comparator.

11. Circuitry arrangement of claim 10, wherein the comparator comprises inputs bridged by a capacitor.

12. Circuitry arrangement of claim 10, comprising a voltage divider or clamping diodes for reducing the voltages delivered to the comparator to values corresponding to a working range of the comparator.

13. Circuitry arrangement of claim 7, wherein the monitoring of the zero crossings of the voltages induced in the stator windings is effected in each case by separate comparison devices.

14. Brushless electric motor, which is provided for example for driving a dental treatment instrument or a dental-technical instrument, said electric motor comprising:

a) a permanent magnet rotor,

b) a stator with three windings electrically offset by 120°

,

c) a circuitry arrangement for detecting zero crossings of a voltage induced by rotation of the rotor in the stator windings, and

d) a control and regulation unit, which on the basis of the detected zero crossings detects at least one of the speed of rotation of the motor and a suitable commutation time point for an intermediate circuit voltage delivered to the stator windings, wherein the circuitry arrangement for detecting the zero crossings is the circuitry arrangement of claim 7.

15. Dental treatment instrument or dental-technical instrument comprising an electric motor of claim 14.