Motor PWM Timing for Stable Single-Shunt Current Detection
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Solution Overview
Problem
Conventional motor control devices experience torque variations and increased noise due to stepwise changes in motor current when detecting three-phase currents, as the phases of PWM signals are sequentially shifted, leading to inefficient detection of phase currents.
Innovation Solution
A motor control device with a current sensing element and PWM signal generation unit that generates three-phase PWM signals, allowing for the detection of two-phase currents at fixed time-points within a carrier period, by adjusting duty cycles relative to the carrier period and phase positions, thereby stabilizing current detection and reducing noise.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If the phases of PWM signal are sequentially shifted to detect three-phase currents, then all three phase currents can be detected, but stepwise changes occur in motor current causing torque variations and increased noise
Solution Approach 1:
The patent applies periodic action by using a triangular wave carrier that periodically alternates between increasing and decreasing phases. The PWM signals are generated by comparing duty ratios with this periodic carrier wave, allowing current detection at specific periodic intervals (when the carrier passes through zero) without requiring sequential phase shifting. This periodic detection method eliminates the stepwise current changes caused by conventional phase-shifting approaches.
2Device complexity
If a single shunt resistance is used in the direct-current part of the inverter circuit, then device complexity is reduced, but detection of all three phase currents becomes difficult without sequential phase shifting
Solution Approach 1:
The patent makes the single shunt resistance serve multiple functions by detecting currents from all three phases through different PWM signal patterns. The same shunt resistance is used to detect U-phase, V-phase, and W-phase currents by appropriately selecting which switching elements are ON and which PWM phases are shifted, eliminating the need for separate sensing elements for each phase while maintaining detection capability for all three phases.
Solution Approach 2:
The patent segments the detection process into different time intervals within each carrier period. By dividing the carrier period into sections where different phase combinations are detectable, the system can sequentially detect all three phase currents through time-division multiplexing using the single shunt resistance, avoiding the need for simultaneous detection that would require multiple sensing elements.
3Measurement precision
If two or more phase currents are made detectable by shifting PWM signal phases, then current detection coverage is improved, but motor current changes in a stepwise manner
Solution Approach 1:
The patent uses the periodic triangular carrier wave to smoothly transition between different detection states. Instead of abrupt phase shifts, the carrier's natural periodic oscillation allows continuous current detection by selecting appropriate detection points during the carrier's increasing and decreasing phases, maintaining current continuity while achieving comprehensive phase detection coverage.
Data Source
AI summary
A motor control device driving a motor via an inverter circuit includes a current sensing element, a PWM signal generation unit and a current detection unit. The PWM signal generation unit is configured to generate three-phase PWM signals in a manner such that the current detection unit is capable of detecting two-phase currents at two fixed time-points within a carrier period of the PWM signal. A duty of one of the three-phase PWM signals is increased/decreased to both phase lag side and phase lead side with reference to any phase. A duty of another phase PWM signal is increased/decreased to both phase lag side and phase lead side with reference to any phase away one half of the carrier period from the reference phase. A duty of the other phase PWM signal is increased/decreased to either phase lag side or phase lead side with reference to any phase.