Inverter Phase Current Measurement Using Transistor Calibration
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Solution Overview
Problem
Existing methods for measuring phase currents in three-phase electric machines using a single common shunt resistor are imprecise due to the inability to simultaneously sense currents and the nonlinearity of transistor drain-to-source voltages, while still seeking cost savings.
Innovation Solution
An inverter system employing a bridge configuration with a shunt resistance and control circuitry that uses space vector pulse width modulation (SVPWM) to determine phase currents by calibrating drain-to-source resistances and voltages across transistors, allowing for accurate phase current calculation even with a single shunt resistor.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If a single common shunt resistor is used to measure phase currents, then cost is reduced, but measurement precision deteriorates due to inability to simultaneously sense currents and transistor nonlinearity
Solution Approach 1:
The patent replaces the traditional shunt resistor-based current sensing method with an alternative approach that uses voltage measurements across transistor elements combined with calibration data. This substitution eliminates the need for multiple shunt resistors while achieving accurate phase current measurements through mathematical calculation based on measured voltages and calibrated resistance values.
Solution Approach 2:
The patent introduces calibration transistors that are calibrated at different operating points to account for transistor nonlinearity. By measuring voltages across these calibrated transistors and using the calibration data to calculate drain-to-source resistances at various operating conditions, the system achieves accurate current measurements without requiring simultaneous sensing of all phases.
2Device complexity
If drain to source voltages of transistors are measured for current sensing, then device complexity is reduced, but measurement precision deteriorates due to transistor nonlinearity
Solution Approach 1:
The patent performs preliminary calibration of transistors at multiple operating points before actual current measurement. The calibration process establishes the relationship between drain-to-source voltage and resistance for each transistor under different conditions. This preliminary action creates a lookup table or calibration data that compensates for transistor nonlinearity during normal operation, enabling accurate current sensing from voltage measurements.
Solution Approach 2:
The system uses feedback from voltage measurements across calibrated transistors to determine phase currents. By continuously measuring the drain-to-source voltages and using the pre-established calibration relationships, the system calculates the actual current flowing through each phase, providing accurate feedback for control purposes while accounting for transistor nonlinearity.
3Ease of manufacture
If currents are sensed sequentially rather than simultaneously using a single shunt resistor, then cost is reduced, but measurement precision deteriorates
Solution Approach 1:
The patent employs periodic action by measuring voltages across different calibrated transistors at different periods within the PWM cycle. Instead of attempting to measure all phase currents simultaneously, the system sequentially measures voltages during specific time windows when particular transistors are conducting, then uses the calibration data to calculate the corresponding currents. This periodic measurement approach, combined with proper timing and calibration, achieves accurate current sensing without requiring simultaneous measurement capability.
Data Source
AI summary
A three-phase load is powered by a PWM (e.g., SVPWM) driven DC-AC inverter having a single shunt-topology. A shunt voltage and a branch voltage of the inverter (across a transistor to be calibrated) are measured during a second period of each SVPWM sector, and the drain-to-source resistance of the calibrated transistor is calculated. During the fourth period of each SVPWM sector, the branch voltage is measured again, and another branch voltage across another transistor is measured. Using the drain-to-source resistance of the calibrated transistor and the voltage across the calibrated transistor measured during the fourth period, the phase current through the calibrated transistor is calculated. Using the other branch voltage measured during the fourth period and the drain-to-source resistance of its corresponding transistor (known from a prior SVPWM sector), the phase current through that transistor is calculated. From the two calculated phase currents, the other phase current can be calculated.