Universal motor speed controller

Abstract

Moderately accurate closed loop speed control of a universal motor is attained without the need for any type of speed sensor. Motor armature (across the brushes) voltage is sensed and supplied to a control circuit for processing along with sensed motor current and zero-crossing information. Integration of the motor armature voltage provides a value which is related to current motor speed. By adjusting the gating angle for triac actuation, the armature voltage integral can be maintained at a desired value associated with a desired motor speed. The sensed motor current is also integrated to provide a speed droop compensation value that is added to the desired value, and the gating angle for triac actuation is adjusted to move the armature voltage integral value to approach the summed value of the speed droop compensation value and desired value.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete understanding of the method and apparatus of the present invention may be acquired by reference to the following Detailed Description when taken in conjunction with the accompanying Drawings wherein:

FIG. 1 is a block diagram of a speed controller system in accordance with an embodiment of the present invention; and

FIGS. 2 and 3 illustrate alternatives for obtaining a motor current value.

Claims

1. A speed control circuit for a universal motor having an armature, comprising: a voltage sensor for sensing a voltage across the armature of the universal motor;a switch for controlling application of current to the universal motor in response to a control signal; anda circuit that compares the sensed voltage with a reference voltage associated with a desired motor speed, and adjusts the control signal to minimize a difference between the sensed voltage and reference voltage to control operation of the universal motor at about the desired speed.

2. The speed control circuit of claim 1 wherein the switch is a triac and the control signal is a triac gating signal having a gating angle, and wherein the gating angle is adjusted by the circuit so as to minimize the difference between the sensed voltage and reference voltage.

3. The speed control circuit of claim 1 wherein the circuit integrates the sensed voltage over a time period within which the switch is being controlled to apply current to the universal motor.

4. The circuit of claim 1, further comprising: a current sensor for sensing a current passing through the universal motor;wherein the circuit calculates a compensation voltage from the sensed current and compares the sensed voltage with the sum of the reference voltage and the compensation voltage, and adjusts the control signal to minimize a difference between the sensed voltage and the sum of the reference voltage and compensation voltage to control operation of the universal motor at about the desired speed.

5. The speed control circuit of claim 4 wherein the switch is a triac and the control signal is a triac gating signal having a gating angle, and wherein the gating angle is adjusted by the circuit so as to minimize the difference between the sensed voltage and the sum of the reference voltage and compensation voltage.

6. The speed control circuit of claim 4 wherein the circuit integrates the sensed current over a time period within which the switch is being controlled to apply current to the universal motor.

7. A method for controlling the speed of a universal motor having an armature, comprising: sensing a voltage across the armature of the universal motor;controlling application of current to the universal motor in response to a control signal;comparing the sensed voltage with a reference voltage associated with a desired motor speed; andadjusting the control signal to minimize a difference between the sensed voltage and reference voltage to control operation of the universal motor at about the desired speed.

8. The method of claim 7 wherein adjusting comprises adjusting a gating angle of a triac switch so as to minimize the difference between the sensed voltage and reference voltage.

9. The method of claim 7 wherein sensing the voltage further comprises integrating the sensed voltage over a time period within which current is being applied to the universal motor.

10. The method of claim 7, further comprising: sensing a current passing through the universal motor; andcalculating a compensation voltage from the sensed current;wherein comparing comprises comparing the sensed voltage with the sum of the reference voltage and the compensation voltage, andwherein adjusting comprises adjusting the control signal to minimize a difference between the sensed voltage and the sum of the reference voltage and compensation voltage to control operation of the universal motor at about the desired speed.

11. The method of claim 10 wherein adjusting comprises adjusting a gating angle of a triac switch so as to minimize the difference between the sensed voltage and the sum of the reference voltage and compensation voltage.

12. The method of claim 10 wherein sensing the current comprises integrating the sensed current over a time period within which current is being applied to the universal motor.

13. A method for controlling the speed of a universal motor having an armature, comprising: measuring a voltage value across the armature which represents a current speed of the motor;measuring a current through the motor which represents a presence of torque load on the motor;calculating a compensation voltage from the measured current;comparing the measured voltage value to a compensated armature voltage target which comprises the sum of the compensation voltage and a reference voltage associated with a desired motor speed; andadjusting the measured voltage to approach the compensated armature voltage target.

14. The method of claim 13, further comprising: controlling application of current to the universal motor in response to a control signal;wherein adjusting comprises adjusting the control signal to minimize a difference between the measured voltage and the compensated armature voltage target to control operation of the universal motor at about the desired motor speed.

15. A method for controlling the speed of a universal motor having an armature, comprising: measuring a voltage value across the armature which represents a current speed of the motor;comparing the measured voltage value to a reference armature voltage associated with a desired motor speed; andadjusting the measured voltage to approach the reference armature voltage.

16. The method of claim 15, further comprising: controlling application of current to the universal motor in response to a control signal;wherein adjusting comprises adjusting the control signal to minimize a difference between the measured voltage and the reference armature voltage to control operation of the universal motor at about the desired motor speed.

17. A motor system, comprising: a universal motor having an armature;a voltage sensor coupled to the motor to measure voltage across the armature;a switch that selectively applies voltage to the universal motor;a control circuit connected to control the switch and receive an output of the voltage sensor, the control circuit comparing the measured voltage across the armature to an armature reference voltage associated with a desired motor speed and controlling the switch in dependence on the caparison to adjust the measured voltage across the armature to approach the armature reference voltage.

18. The system of claim 17 wherein the switch is a triac responsive to a triac gating signal having a gating angle, and wherein control circuit adjusts the gating angle so as to minimize a difference between the measured voltage across the armature and the armature reference voltage.

19. The motor system of claim 17 further comprising: a current sensor coupled to measure current passing through the motor;the control circuit comparing the measured voltage across the armature to a target armature voltage comprising the sum of a compensation voltage derived from the measured current and the armature reference voltage and controlling the switch in dependence on the caparison to adjust the measured voltage across the armature to approach the target armature voltage.

20. The system of claim 19 wherein the switch is a triac responsive to a triac gating signal having a gating angle, and wherein control circuit adjusts the gating angle so as to minimize a difference between the measured voltage across the armature and the target armature voltage.