Variable Speed Drive for a Chiller System with a Switched Reluctance Motor

Abstract

A variable speed drive with a boost converter is provided for a chiller system driven by a switched reluctance motor. The boost converter can be a diode or thyristor rectifier followed by a boost DC / DC converter or a three-phase pulse width modulated boost converter. The boost converter provides a boosted voltage to the DC link, which results in a boosted voltage being applied to the switched reluctance motor by the inverter of the variable speed drive.

Description

BACKGROUND OF THE INVENTION [0001] The present invention relates generally to a variable speed drive for a chiller system. More specifically, the present invention relates to a variable speed drive with boost capability for a variable speed chiller system with a switched reluctance motor. [0002] In the past, the induction motors for driving compressors in chiller systems were designed to operate from standard line (main) voltages and frequencies that were available from the power distribution system of the facility where the motor was being operated. The use of line voltages and frequencies typically required the compressors to use some inefficient mechanical means (such as inlet guide vanes for centrifugal compressors and a slide valve for screw compressors) for modulating capacity as a result of the motor being limited to one operating speed that was based on the input frequency to the motor. In addition, if the operating speed of the motor was not equal to the desired operating s...

AI Technical Summary

Benefits of technology

[0009] One advantage of the present invention is increased system efficiency and potential cost reduction by eliminating gears between the motor and the compressor.

[0010] Another advantage of the present invention is a reduction in motor and inverter losses.

[0011] A further advantage of the present invention is improved reliability of the chiller system.

[0012] Another advantage of the present invention is improved input power quality in the variable speed drive.

[0013] Still another advantage of the present invention is that the variable speed drive has improved ride-through capabilities during input voltage sags.

[0014] A further advantage of the present invention is a higher maximum operating speed and a faster dynamic response of the motor.

Problems solved by technology

The use of line voltages and frequencies typically required the compressors to use some inefficient mechanical means (such as inlet guide vanes for centrifugal compressors and a slide valve for screw compressors) for modulating capacity as a result of the motor being limited to one operating speed that was based on the input frequency to the motor.

Furthermore, motors that required their own controller or electronic drive, e.g., switched reluctance motors, could not be used for these chiller systems, as such motors could not operate directly from standard (main) voltages and frequencies.

One limitation of prior induction motor style VSDs is that the magnitude of the output voltage from the VSD can be no larger than the magnitude of the input, or utility, line voltage to the VSD.

This limitation on the output voltage of the variable speed drive limits the maximum speed of the conventional induction motor to a speed that corresponds to the speed of the motor operated at line voltage (because of the constant volts / hertz ratio required by a conventional induction motor).

In addition, this limitation on the output voltage from the VSD limits the operating speed range of high speed motors, including high speed switched reluctance motors, in the chiller system.

The high speed motors, used to obtain faster compressor speeds without the “step up” gearing arrangement, are limited because it is more difficult to design an efficient and cost-effective motor when only a limited voltage range is available.

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