Back-EMF Sensing Circuit for Stable Sine-Wave Motor Control
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Solution Overview
Problem
Existing methods for controlling three-phase permanent magnet motors without position sensors face challenges, particularly when using sine wave control, as they can result in significant errors and fluctuations in back electromotive force observation.
Innovation Solution
A back electromotive force calculation method is introduced, which involves sampling the phase current of one phase and the DC bus voltage, and then using digital filtering to reduce the fluctuation of the calculated back electromotive force, as expressed in the formula Ea[kTs] = Udc[kTs] × (PWMa[kTs] - PWMa[kTs] + PWMb[kTs] + PWMc[kTs]3) - Ra × Ia[kTs] - La × Didtfilter[kTs].
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If back electromotive force observation is performed using single-phase current detection without position sensors, then control costs and installation space are reduced, but significant errors and fluctuations occur in back electromotive force observation especially under sine wave control
Solution Approach 1:
The patent changes the observation parameters by introducing a compensation parameter based on the differential of phase current. Instead of directly observing back electromotive force through simple voltage measurements, the system calculates a compensated back electromotive force by adding the product of phase inductor and current differential to the original back electromotive force calculation. This parameter transformation resolves the contradiction by improving measurement precision while maintaining the simplicity of single-phase current detection without position sensors
Solution Approach 2:
The patent introduces an intermediary element - the current differential term (La × Didtfilter) - that acts as a mediator to correct the back electromotive force observation errors. This intermediary compensation term bridges the gap between the simplified single-phase detection method and accurate back electromotive force measurement, particularly under sine wave control conditions where direct observation fails
2Measurement precision
If digital filtering is applied to reduce back electromotive force fluctuation, then measurement accuracy improves, but computational complexity increases
Solution Approach 1:
The patent applies partial filtering action by selectively filtering only the current differential term (Didtfilter) rather than filtering the entire back electromotive force signal. This partial application of filtering reduces computational complexity while still achieving the goal of reducing back electromotive force fluctuation and improving measurement precision. The filtering is applied excessively to the differential term to compensate for the noise introduced by differentiation
Data Source
AI summary
A back electromotive force sensing circuit can include: a sampling circuit configured to acquire a sampling signal representing a phase current of one of three phases of a three-phase permanent magnet motor, where the three-phase permanent magnet motor adopts sine wave control; and a signal processing circuit configured to receive the sampling signal, and to obtain a back electromotive force of the one phase according to a difference between a phase voltage of the one phase and a sum of a voltage across a phase resistor and a voltage across a phase inductor of the one phase.