Variable-speed drive for single-phase motors

• US 6,121,749 A
• Filed: 05/10/1999
• Issued: 09/19/2000
• Est. Priority Date: 05/11/1998
• Status: Expired due to Fees

• Current Assignee: Work Smart Energy Enterprises, Inc.
• Original Assignee: Work Smart Energy Enterprises, Inc.
• Inventors: Wills, Frank E., Kopko, William Leslie
• Primary Examiner(s): Ip, Paul
• Assistant Examiner(s): Duda, Rina I.
• Application Number:
  US09/307,701
• Time in Patent Office: 498 Days
• Field of Search: 318/727, 318/794-795, 318/797, 318/747, 318/748, 318/749, 318/774, 318/778, 318/779, 318/803, 318/805, 318/811, 318/812
• US Class Current: 318/811
• CPC Class Codes: H02P 1/44 by phase-splitting with a capacitor
• CPC Class Codes: H02P 1/445 by using additional capacitors switched at start up
• CPC Class Codes: H02P 27/047 V/F converter, wherein the voltage is controlled proportionally with the frequency
• CPC Class Codes: H02P 27/08 with pulse width modulation

Abstract

A single-phase variable-frequency power supply with two output voltages drives a single-phase AC motor that has a start circuit and main winding. For permanent split-capacitor motors, the power supply maintains a constant voltage to the start winding while it reduces the voltage to the main winding at lower frequencies. The power supply can similarly control the speed of capacitor-start and split-phase motors. For fans and pumps and other variable-torque applications, the power supply maintains approximately a constant ratio of main-winding voltage to the square of frequency. The power supply is preferably an electronic pulse-width modulated inverter.

Claims

1. An apparatus for controlling the speed of a single-phase AC motor that has a main winding and a start circuit that has an impedance that is different from that of said main winding and that includes a second motor winding, said apparatus comprising:

a variable-frequency AC power supply with at least one common terminal connected to a first end of said main winding and a first end of said start circuit,

a first terminal for supplying a variable voltage that is connected to a second end of said main motor winding,

and a second voltage terminal whose voltage output is substantially in phase with the output of said first voltage terminal and is connected to a second end of said start circuit whereby said power supply reduces the ratio of the voltage across the main winding to that across the start circuit at lower output frequency.

2. The apparatus of claim 1 where said second terminal of said power supply provides a fixed voltage to said start circuit.

3. The apparatus of claim 1 wherein said AC power supply comprises a converter with a variable frequency output that can accept input power from a utility power line.

4. The apparatus of claim 3 further comprising a bypass switch that connects said main winding and said start circuit to said utility power line so that said motor may run at a fixed speed without said converter.

5. The apparatus of claim 3 wherein said converter comprises:

four controllable switching devices that each have a control input, a power input, and a power output;

a first rectifier for supplying a positive DC voltage relative to said common terminal that accepts power from said power line and is connected to the power inputs of the first and second controllable switching devices;

a second rectifier for supplying a negative DC voltage relative to said common terminal that accepts power from said power line and is connected to the power inputs for the third and fourth controllable switching devices; and

a pulse-width modulating controller connected to the control inputs for said first, second, third, and fourth controllable switching devices whereby the power outputs of said first and third controllable switching devices are connected to said first terminal of said power supply and the power output of a said second and fourth switching devices are connected to said second terminal of said power supply.

6. The apparatus of claim 3 wherein said converter comprises:

six controllable switching devices that each have a control input, a power input, and a power output,

a rectifier that is connected to said power line and which supplies a positive voltage to the power inputs for the first, second, and third controllable switching devices and supplies a negative voltage to power inputs for the fourth, fifth, and sixth controllable switching devices,

the power outputs of said first and fourth controllable switching devices are connected to said common terminal,

the power outputs for said second and fifth controllable switching devices are connected to said first voltage terminal,

the power outputs for said third and sixth controllable switching devices are connected to said second voltage terminal, and

a pulse-width-modulated controller connected to the control inputs of said first, second, third, fourth, fifth, and sixth controllable switching devices.

7. The apparatus of claim 1 wherein said power supply comprises:

a first alternator for supplying variable AC voltage with a variable-speed shaft input and a first output terminal that is connect to said common terminal of said power supply and a second output terminal that is connect to said first voltage terminal of said power supply,

a second alternator that shares said variable-speed shaft input and that has a first output terminal that is connected to said common terminal of said power supply and a

second output terminal is connect to said second that supplies an AC voltage to said second voltage terminal of said power supply.

8. The apparatus of claim 7 wherein said variable-speed shaft input comprises a variable-speed electric motor that receives power from a utility power line.

9. A method of controlling a single-phase AC motor having a first winding and a start circuit that has an impedance that differs from that of the first winding and includes a second motor winding, said method comprising:

supplying a first variable-frequency AC voltage across the main winding,

supplying a second variable-frequency AC voltage across said start circuit that is substantially in phase with said first variable-frequency AC voltage, and

reducing the ratio of the voltage across the first winding to that across the start circuit at lower output frequencies.