Redundant overspeed protection for power tools

1. A power tool powered by an alternating current (AC) input signal, comprising:

a motor switch coupled in series with an electric the motor;

a motor control module interfaced with the motor switch and controls switching operation of the motor switch, wherein the motor control module is implemented by computer executable instructions executed by a computer processor;

an overspeed detection module that is separate from the computer processor and operates independently from the computer processor to determine rotational speed of the motor and issues a disabling signal to the motor control module when the rotational speed of the motor exceeds a threshold, wherein the motor control module opens the motor switch in response to the disabling signal from the overspeed detection module, thereby discontinuing motor operation; and

a protective switch connected across an AC line carrying the AC input signal and a fuse disposed in the AC line, where overspeed detection module closes the protective switch when the rotational speed of the motor exceeds the threshold, thereby shorting the AC line and opening the fuse.

2. The power tool of claim 1 wherein the overspeed detection module includes a tachometer disposed proximate to the motor and configured to determine the rotational speed of the motor.

3. The power tool of claim 1 wherein the overspeed detection module measures current through the motor and determines the rotational speed of the motor based on the measured current.

4. The power tool of claim 1 wherein the power tool is powered by an alternating current (AC) input signal and the switching operation of the motor switch occurs at a given phase of the AC input signal.

5. The power tool of claim 4 wherein the overspeed detection module receives the given phase from the motor control module, measures current through the motor, and determine the rotational speed of the motor as a function of the given phase and the measured current.

6. The power tool of claim 4 further comprises a protective switch connected across the motor and a fuse disposed in the AC line, where the overspeed detection module closes the switch when the rotational speed of the motor exceeds the threshold, thereby shorting the AC line and opening the fuse.

7. A power tool powered by an alternating current (AC) input signal, comprising:

a motor switch coupled in series with an electric the motor;

a motor control module interfaced with the motor switch and controls switching operation of the motor switch, wherein the motor control module is implemented by computer executable instructions executed by a computer processor;

an overspeed detection module that is separate from the computer processor and operates independently from the computer processor to determine rotational speed of the motor and operates cooperatively with the motor control module to reduce the rotational speed of the motor when the rotational speed of the motor approaches but is less than a maximum speed threshold; and

a protective switch connected across an AC line carrying the AC input signal and a fuse disposed in the AC line, where overspeed detection circuit closes the switch when the rotational speed of the motor exceeds the threshold, thereby shorting the AC line and opening the fuse.

8. The power tool of claim 7 wherein the overspeed detection module includes a tachometer disposed proximate to the motor and configured to determine the rotational speed of the motor.

9. The power tool of claim 7 wherein the overspeed detection module measures current through the motor and determines the rotational speed of the motor based on the measured current.

10. The power tool of claim 7 wherein the power tool is powered by an alternating current (AC) input signal and the switching operation of the motor switch occurs at a given phase of the AC input signal.

11. The power tool of claim 10 wherein the overspeed detection module receives the given phase from the motor control module, measures current through the motor, and determine the rotational speed of the motor as a function of the given phase and the measured current.

12. The power tool of claim 10 wherein the motor control module reduces the rotational speed of the motor by varying the phase at which the motor switch is closed.

13. A power tool configured to receive an alternating current (AC) input signal, comprising:

an electric motor;

a phase-controlled switch coupled in series with the motor;

a motor control module interfaced with the phase-controlled switch and modulates the conductive state of the phase-controlled switch at a given phase of the AC input signal, wherein the motor control module is implemented by computer executable instructions executed by a computer processor; and

an overspeed detection module that is separate from the computer processor and configured to receive the given phase from the motor control module, the overspeed detection module operates to measure current through the motor and to determine rotational speed of the motor as a function of the given phase and the measured current; and

a protective switch connected across an AC line carrying the AC input signal and a fuse disposed in the AC line, where overspeed detection module closes the switch when the rotational speed of the motor exceeds a threshold, thereby shorting the AC line and opening the fuse.

14. The power tool of claim 13 wherein the overspeed detection module determines rotational speed using a function defined as a linear combination of the predetermined phase and the measured current.

15. The power tool of claim 13 wherein the overspeed detection module samples current through the motor during a single period of the AC input signal and averages the current samples to derive the current measure.

16. The power tool of claim 13 wherein the overspeed detection module samples current through the motor during multiple periods of the AC input signal and averages the current samples to derive the current measure.

17. The power tool of claim 13 further comprises a protective switch connected across an AC line carrying the AC input signal and a fuse disposed in the AC line, where overspeed detection module closes the switch when the rotational speed of the motor exceeds the threshold, thereby shorting the AC line and opening the fuse.

18. The power tool of claim 13 further comprises a protective switch connected across the motor and a fuse disposed in the AC line, where the overspeed detection module closes the switch when the rotational speed of the motor exceeds the threshold, thereby shorting the AC line and opening the fuse.