Variable Speed Drive Input Current Estimation for Chiller Control
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Solution Overview
Problem
Existing chiller assemblies with variable speed drives (VSDs) face challenges in accurately estimating input current, especially at higher frequency ratings, leading to suboptimal performance due to assumptions about equal output and input current ratings, which is not accurate for systems with different ratings.
Innovation Solution
A chiller assembly with a variable speed drive that includes sensors and a processing circuit to estimate the total input RMS current by determining DC link power, line-to-line input voltage, input current with distortion, and transformer current values, allowing for responsive actions to prevent input current limits from being exceeded.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Speed
If variable speed drive operates at higher frequency ratings, then motor speed control capability is improved, but input current estimation accuracy deteriorates due to assumptions about equal output and input current ratings
Solution Approach 1:
The patent replaces direct measurement of input current with a computational estimation system that calculates input current based on DC link power, voltage, and power factor. This substitution allows accurate input current determination without relying on inaccurate assumptions about equal output and input current ratings, thereby resolving the measurement precision deterioration at higher frequency ratings.
Solution Approach 2:
The patent introduces DC link power and voltage as intermediary parameters to estimate input current. By using these intermediate measurements combined with power factor calculations, the system accurately determines input current without directly measuring it, thus maintaining measurement precision across all frequency ratings including higher frequencies.
2Device complexity
If variable speed drive uses assumed equal output and input current ratings, then device complexity is reduced, but operational reliability deteriorates due to inaccurate current management
Solution Approach 1:
The variable speed drive performs self-diagnosis and self-regulation by continuously monitoring DC link power and voltage, calculating power factor, and estimating input current. This self-service capability enables accurate current management and reliable operation without requiring additional external measurement devices, thus maintaining operational reliability while avoiding excessive complexity.
Solution Approach 2:
The patent implements a feedback mechanism where the estimated input current is continuously compared against rated values, and the system responds by adjusting operation or providing warnings. This feedback loop ensures reliable operation by preventing overloads and maintaining accurate current management, resolving the reliability deterioration caused by simplified assumptions.
3Ease of manufacture
If variable speed drive lacks accurate input current estimation, then manufacturing cost is reduced, but performance optimization capability deteriorates
Solution Approach 1:
The patent makes the existing DC link power and voltage sensors serve multiple functions: they not only monitor DC link conditions for basic operation but also enable accurate input current estimation for performance optimization. This multi-functionality achieves performance optimization capability without requiring additional expensive sensors or measurement equipment, thus maintaining ease of manufacture.
Solution Approach 2:
The patent changes the approach from directly measuring input current to calculating it through parameter relationships (DC link power, voltage, and power factor). This parameter transformation enables performance optimization through accurate current knowledge while avoiding the cost of additional measurement hardware, resolving the contradiction between manufacturing cost and performance optimization capability.
Data Source
AI summary
A chiller assembly is provided. The chiller assembly includes a compressor (102), a condenser (106), an expansion device and an evaporator (108) connected in a closed refrigerant circuit. The chiller assembly further includes a motor (104) connected to the compressor to power the compressor, and a variable speed drive (110) connected to the motor to power the motor. The variable speed drive is operable to provide a variable voltage to the motor and a variable frequency to power the motor. The variable speed drive includes multiple sensors and an input current estimator that determines an estimated RMS input current based on sensor data received from the sensors. The chiller assembly further includes a control panel to control operation of the variable speed drive.