Three-Phase Current Sensing With Two Sensors and Inverse Clarke Transform
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Solution Overview
Problem
Existing current measurement systems for electric machines powered by three-phase inverters often rely on three current sensors, which can be costly. Systems with only two current sensors cannot provide adequate protection against ground faults and overcurrent conditions for all phases, as no sensor is dedicated to measuring the third phase current.
Innovation Solution
A current measurement circuit utilizing two current sensors and current computation circuitry to measure and compute the three individual phase currents of a three-phase current. The first current sensor measures a sum of the first, second, and third phase currents with a 2:1:1 turns ratio, while the second current sensor measures a sum of the second and third phase currents with a 1:1 turns ratio. The current computation circuitry applies an inverse Clarke transform to determine the individual phase currents.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If three current sensors are used to measure all three phase currents, then measurement precision and protection reliability are improved, but device complexity and cost increase
Solution Approach 1:
The patent introduces a computation circuit as an intermediary that processes outputs from two current sensors (measuring phase A and phase B currents) to calculate the third phase current using the principle that the sum of three-phase currents is zero. This mediator enables indirect measurement of phase C current without a dedicated sensor, resolving the contradiction between measurement completeness and device complexity.
Solution Approach 2:
The computation circuit creates a virtual copy of the current measurement capability for phase C by mathematically deriving it from measurements of phases A and B. Instead of physically measuring all three phases with separate sensors, the system copies the measurement information through calculation, reducing sensor requirements while maintaining measurement precision.
2Device complexity
If only two current sensors are used, then device complexity and cost are reduced, but protection reliability against ground faults and overcurrent conditions deteriorates
Solution Approach 1:
The computation circuit continuously calculates phase C current based on real-time measurements from phases A and B, providing ongoing feedback about the third phase's current status. This enables the protection system to monitor all three phases for ground faults and overcurrent conditions even though only two physical sensors are used, maintaining reliability while reducing complexity.
Solution Approach 2:
The computation circuit acts as a mediator that translates partial measurements (from two sensors) into complete protection information (for all three phases). By mathematically deriving the third phase current and feeding it into protection logic, the system achieves comprehensive protection coverage without requiring three physical sensors.
3Reliability
If three current sensors are deployed, then protection coverage for all phases is ensured, but cost increases
Solution Approach 1:
The system creates a virtual measurement channel for phase C current through computational copying of information from phases A and B measurements. This virtual copy provides sufficient protection coverage for phase C without requiring a physical sensor, reducing the quantity of sensors needed while maintaining comprehensive protection.
Solution Approach 2:
The computation circuit enables the two-phase measurement system to serve the function of a three-phase system by self-calculating the missing phase information. The system uses its own measurements from phases A and B to generate the necessary protection data for phase C, eliminating the need for additional sensors.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This solution enables accurate measurement and protection of all three phase currents, providing effective ground fault and overcurrent protection for power devices connected to the electric machine, while reducing the number of required current sensors and associated costs.
Implementation Method 1
The first current sensor is coupled to a first conductor, a second conductor, and a third conductor... The first current sensor comprises a turns ratio of 2:1:1 with respect to the first conductor, the second conductor, and the third conductor
Implementation Method 2
The second current sensor is coupled to the second conductor and the third conductor... the second current sensor comprises a turns ratio of 1:1 with respect to the second conductor and the third conductor
Data Source
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AI summary
A current measurement circuit includes first, second and third conductors, a first current sensor, a second current sensor, and current computation circuitry. The first conductor is configured to conduct a first phase current of a three-phase current. The second conductor is configured to conduct a second phase current of the three-phase current. The third conductor is configured to conduct a third phase current of the three-phase current. The first current sensor is coupled to the first, the second, and the third conductors. The second current sensor is coupled to the second conductor and the third conductor. The current computation circuitry is coupled to the first current sensor and the second current sensor, and is configured to determine the first current, the second current, and the third current by applying an inverse Clarke transform to the output of the first current sensor and the output of the second current sensor.