Variable speed drive input current control

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Solution Overview

Problem

Conventional variable speed drives (VSDs) often assume equal output and input current ratings, which is inaccurate for higher frequency operations, leading to suboptimal performance in chiller assemblies, especially when output and input current ratings differ.

Innovation Solution

A system and method for estimating VSD input current using existing sensors and circuitry, incorporating a processing circuit with sensors to calculate input current based on output current, DC link power, and line-to-line input voltage, while considering the presence of an active harmonic filter to account for harmonic distortions.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Ease of operation

If conventional VSDs assume equal output and input current ratings, then the system is simpler to operate, but the current measurement precision deteriorates leading to inaccurate current ratings especially at higher frequencies

Engineering Contradiction:

Improvesimplicity of current rating assumptionVSAvoidaccuracy of input current rating

Core Design Contradiction:

Ease of operationVSMeasurement precision

Solution Approach 1:

The patent replaces direct physical current sensing on the input side with an electrical calculation method. The processor calculates input current by measuring output current and using the relationship between input and output power, substituting complex hardware-based input current measurement with a computational approach that uses existing sensors and mathematical models.

Inventive Principle:

Principle #28Mechanics substitution (Replace mechanical system)

2Productivity

If VSDs operate at higher frequency ratings, then the productivity increases, but the measurement precision of input current deteriorates due to the invalid assumption of equal current ratings

Engineering Contradiction:

Improveoperating frequency capabilityVSAvoidinput current rating accuracy

Core Design Contradiction:

ProductivityVSMeasurement precision

Solution Approach 1:

The patent changes the operational parameters by which current ratings are determined. Instead of assuming fixed relationships between input and output current, the system dynamically calculates input current based on actual operating conditions including frequency, voltage, and power measurements, allowing accurate operation across a wide frequency range.

Inventive Principle:

Principle #35Parameter changes

Solution Approach 2:

Inventive Principle:

Principle #28Mechanics substitution (Replace mechanical system)

3Reliability

If VSDs in chiller assemblies are designed with different output and input current ratings, then the reliability improves for optimal chiller performance, but the device complexity increases

Engineering Contradiction:

Improveoptimal chiller assembly performanceVSAvoidcurrent rating differentiation

Core Design Contradiction:

ReliabilityVSDevice complexity

Solution Approach 1:

The system uses its own existing sensors and processing capabilities to determine input current ratings. By leveraging the output current sensor and processor already present in the VSD, the system self-determines input current without requiring additional dedicated input current sensing hardware, thus maintaining reliability while limiting complexity increase.

Inventive Principle:

Principle #25Self-service

4Measurement precision

If additional sensors are added to directly measure input current, then the measurement precision improves, but the device complexity and cost increase

Engineering Contradiction:

Improvedirect input current measurementVSAvoidnumber of sensors and circuitry

Core Design Contradiction:

Measurement precisionVSDevice complexity

Solution Approach 1:

The patent makes the existing output current sensor and processor serve multiple functions. The same sensor and processor used for output current measurement are also used to determine input current through calculation, eliminating the need for separate dedicated input current sensing components and achieving multi-functionality with existing hardware.

Inventive Principle:

Principle #6Universality (Multi-functionality)

Solution Approach 2:

Inventive Principle:

Principle #25Self-service

Data Source

PatentEP3688387B1Variable speed drive input current control

Publication Date: 2023.11.15 JOHNSON CONTROLS TYCO IP HLDG LLP

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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.