Method and apparatus for quiet fan speed control

A load control device for controlling a speed of an AC motor to be driven from an AC supply voltage of an AC power source, the load control device comprising:

• a first capacitor and a second capacitor, each of the first and second capacitors adapted to be coupled in series electrical connection between the AC power source and the AC motor;
• a first controllably conductive switch coupled in series electrical connection with the first capacitor and a second controllably conductive switch coupled in series electrical connection with the second capacitor;
• and
• a control circuit operable to control the first and second controllably conductive switches to provide a plurality of discrete speeds of the AC motor;
• wherein the control circuit is operable to change the speed of the AC motor by
• rendering the first switch conductive at a first zero-crossing of the AC supply voltage to allow the first capacitor to substantially charge to a first predetermined voltage;
• rendering the second switch conductive at a second zero-crossing of the AC supply voltage to allow the second capacitor to substantially charge to a second predetermined voltage;
• and
• rendering the first and second switches conductive at a predetermined time after a third zero-crossing of the AC supply voltage;
• wherein the control circuit is operable to render the first switch non-conductive when the first capacitor has substantially charged to the first predetermined voltage and to render the second switch non-conductive when the second capacitor has substantially charged to the second predetermined voltage;
• and
• wherein the control circuit is operable to monitor the voltages across the first and second capacitors and to compare the voltages across the first and second capacitors to the first and second predetermined voltages, respectively, in order to determine when to render the first and second switches non-conductive.

A load control device for controlling a speed of an AC motor to be driven from an AC supply voltage of an AC power source, the load control device comprising:

• a first capacitor and a second capacitor, each of the first and second capacitors adapted to be coupled in series electrical connection between the AC power source and the AC motor;
• a first controllably conductive switch coupled in series electrical connection with the first capacitor and a second controllably conductive switch coupled in series electrical connection with the second capacitor;
• and
• a control circuit operable to control the first and second controllably conductive switches to provide a plurality of discrete speeds of the AC motor;
• wherein the control circuit is operable to change the speed of the AC motor by
• rendering the first switch conductive at a first zero-crossing of the AC supply voltage to allow the first capacitor to substantially charge to a first predetermined voltage;
• rendering the second switch conductive at a second zero-crossing of the AC supply voltage to allow the second capacitor to substantially charge to a second predetermined voltage;
• and
• rendering the first and second switches conductive at a predetermined time after a third zero-crossing of the AC supply voltage;
• wherein the control circuit is operable to render the first switch non-conductive when the first capacitor has substantially charged to the first predetermined voltage and to render the second switch non-conductive when the second capacitor has substantially charged to the second predetermined voltage;
• the load control device further comprising;
• a voltage compare circuit operable to compare the voltages across the first and second capacitors to the first and second predetermined voltages, respectively;
• wherein the control circuit is operable to render the first and second switches non-conductive in response to the comparisons of the voltage compare circuit.

A load control device for controlling a speed of an AC motor to be driven from an AC supply voltage of an AC power source, the load control device comprising:

• a first capacitor and a second capacitor, each of the first and second capacitors adapted to be coupled in series electrical connection between the AC power source and the AC motor;
• a first controllably conductive switch coupled in series electrical connection with the first capacitor and a second controllably conductive switch coupled in series electrical connection with the second capacitor;
• and
• a control circuit operable to control the first and second controllably conductive switches to provide a plurality of discrete speeds of the AC motor;
• wherein the control circuit is operable to change the speed of the AC motor by
• rendering the first switch conductive at a first zero-crossing of the AC supply voltage to allow the first capacitor to substantially charge to a first predetermined voltage;
• rendering the second switch conductive at a second zero-crossing of the AC supply voltage to allow the second capacitor to substantially charge to a second predetermined voltage;
• and
• rendering the first and second switches conductive at a predetermined time after a third zero-crossing of the AC supply voltage,
• wherein the control circuit is operable to render the controllably conductive switches non-conductive when a current through the AC motor becomes substantially zero amps, the load control device further comprising;
• first and second limiting resistors in series electrical connection with the first and second capacitors, respectively;
• wherein the control circuit is operable to monitor the voltages across the limiting resistors to determine when the current through the AC motor becomes substantially zero amps.

A method for controlling the speed of an AC motor to be driven from an AC supply voltage of an AC power source, the method comprising the steps of:

• coupling a first capacitor in series electrical connection between the AC power source and the AC motor;
• coupling a second capacitor in series electrical connection between the AC power source and the AC motor;
• coupling a first controllably conductive switch and a second controllably conductive switch in series electrical connection with the first capacitor and the second capacitor, respectively;
• detecting zero crossings of the AC supply voltage using a zero cross detect circuit coupled to the AC supply voltage;
• changing the speed of the AC motor by:
• rendering the first switch conductive at a first zero-crossing of the AC supply voltage to allow the first capacitor to substantially charge to a predetermined voltage;
• rendering the second switch conductive at a second zero-crossing of the AC supply voltage to allow the second capacitor to substantially charge to the predetermined voltage;
• and
• rendering the first and second switches conductive at a predetermined time after a third zero-crossing of the AC supply voltage.

A method for changing the speed of an AC motor in a load control device comprising a first capacitor and a second capacitor, each of the first and second capacitors adapted to be coupled in series electrical connection between an AC power source and the AC motor, a first controllably conductive switch coupled in series electrical connection with the first capacitor, and a second controllably conductive switch coupled in series electrical connection with the second capacitor, the method comprising the steps of:

• detecting zero crossings of the AC supply voltage using a zero cross detect circuit coupled to the AC supply voltage;
• rendering only the first switch conductive at a first zero-crossing of the AC supply voltage to allow the first capacitor to substantially charge to a predetermined voltage;
• rendering only the second switch conductive at a second zero-crossing of the AC supply voltage to allow the second capacitor to substantially charge to the predetermined voltage;
• and
• rendering both the first and second switches conductive at a predetermined time after a third zero-crossing of the AC supply voltage.

A load control device for controlling the speed of an AC motor to be driven from an AC supply voltage of an AC power source, the load control device comprising:

• a first capacitor in series electrical connection with a first controllably conductive switch, the first capacitor and the first switch adapted to be coupled in series electrical connection with the AC power source and the AC motor;
• a second capacitor in series electrical connection with a second controllably conductive switch, the second capacitor and the second switch adapted to be coupled in series electrical connection with the AC power source and the AC motor;
• a control circuit operatively coupled to control the first and second switches, the control circuit operative to selectively render the first and second switches conductive in response to detected voltage zero crossings of the AC supply voltage;
• and
• a zero-cross detect circuit adapted to be coupled to detect voltage zero-crossings of the AC supply voltage, the voltage zero-crossing detector coupled to provide indications of detected AC supply voltage zero-crossings to the control circuit;
• wherein the control circuit is operative to render the first switch conductive in response to a first AC supply voltage zero-crossing;
• the control circuit is operative to render the second switch conductive in response to a second AC supply voltage zero-crossing subsequent to the first AC supply voltage zero-crossing;
• and the control circuit is operative to render both the first and the second switches conductive substantially simultaneously at a predetermined time after a third AC supply voltage zero-crossing subsequent to the second AC supply voltage zero-crossing.