Variable speed drive with pulse-width modulated speed control

1. A drive system for a compressor of a chiller system, the drive system comprising:

a variable speed drive, the variable speed drive being configured to receive an input AC voltage at a fixed AC input voltage and provide an output AC power at a variable voltage and variable frequency, the variable speed drive comprising:

a converter stage connected to an AC power source providing the input AC voltage, the converter stage being configured to convert the input AC voltage to a DC voltage;

a DC link connected to the converter stage, the DC link being configured to filter and store the DC voltage from the converter stage; and

at least one inverter stage connected to the DC link;

a motor connectable to the compressor for powering the compressor; and

a controller configured to control switching in the converter stage and the inverter stage, the controller configured to apply randomized pulse width modulation to vary the switching frequency of transistors in the converter stage and the inverter stage at each switching cycle.

2. The drive system of claim 1 , wherein the motor is a permanent magnet synchronous motor.

3. The drive system of claim 1 , wherein the controller is further configured to apply randomized pulse width modulation to reduce a magnitude of harmonic components present in a line current and a line voltage of the motor.

4. The drive system of claim 1 , wherein the controller is further configured to apply randomized pulse width modulation to reduce the level of the electromagnetic interference and the level of acoustic noise generated by the drive system.

5. The drive system of claim 1 , wherein the controller is further configured to apply randomized pulse width modulation to reduce the magnitude of the electromagnetic noise through reduction of the harmonic currents in high frequency range.

6. The drive system of claim 2 , wherein the permanent magnet synchronous motor further comprises a plurality of permanent magnets arranged in multiple pole arrangements in a rotor portion.

7. The drive system of claim 6 , wherein the plurality of permanent magnets being adhesively affixed to a rotor cage, the plurality of permanent magnets enveloped by a sleeve portion to maintain the plurality of permanent magnets on the rotor cage when centrifugal forces act on rotor portion during rotation.

8. The drive system of claim 7 , wherein the rotor cage is a cylindrical steel rotor cage.

9. The drive system of claim 7 , wherein the sleeve portion comprises a flexible, high strength, magnetically non-permeable material.

10. The drive system of claim 9 , wherein the material is carbon fiber tubular sheet material or stainless steel.

11. The drive system of claim 1 , wherein the controller is further configured to apply a randomized sampling of sampling feedback current and voltage values for a closed loop control operation for randomized pulse width modulation.

12. A chiller system comprising:

a compressor, a condenser, and an evaporator connected in a closed refrigerant loop;

a motor connected to the compressor to power the compressor; and

a variable speed drive connected to the motor, the variable speed drive being configured to receive an input AC power at a fixed input AC voltage and a fixed input frequency and provide an output power at a variable voltage and variable frequency to the motor, the variable voltage having a maximum voltage greater in magnitude than the fixed input AC voltage and the variable frequency having a maximum frequency greater than the fixed input frequency, the variable speed drive comprising:

a converter stage connected to an AC power source providing the input AC voltage, the converter stage being configured to convert the input AC voltage to a DC voltage;

a DC link connected to the converter stage, the DC link being configured to filter and store the DC voltage from the converter stage; and

at least one inverter stage connected to the DC link; and

a controller configured to control switching in the converter stage and the inverter stage, the controller configured to apply randomized pulse width modulation to vary the switching frequency of transistors in the converter stage and the inverter stage at each switching cycle.

13. The chiller system of claim 12 , wherein the motor is a permanent magnet synchronous motor.

14. The chiller system of claim 12 , wherein the controller is further configured to apply randomized pulse width modulation to reduce a magnitude of harmonic components present in a line current and a line voltage of the motor.

15. The chiller system of claim 12 , wherein the controller is further configured to apply randomized pulse width modulation to reduce the level of the electromagnetic interference and the level of acoustic noise generated by the drive system.

16. The chiller system of claim 12 , wherein the controller is further configured to apply randomized pulse width modulation to reduce the magnitude of the electromagnetic noise through reduction of the harmonic currents in high frequency range.

17. A method of controlling a variable speed drive having a converter stage, a DC link stage, and an inverter stage, the method comprising:

providing an inverter stage connected to a DC link stage, the inverter configured to power a corresponding load; and

generating a randomized switching signal for the inverter, the randomized switching signal being operable to vary the switching frequency of a pulse width modulated signal to activate and deactivate the inverter to obtain a preselected output power and a preselected output frequency from the inverter.