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In order to reduce costs of inverters for home electrical appliances using the PMSM, the AC current sensor-less control is required as well as the position sensor-less control. This paper proposes a PWM technique suitable for calculating AC currents from sampled DC current data in the one-shunt system which is one of the AC current sensor-less methods. Figure 1 shows the configuration of the proposed system. Three phase currents are calculated from dc current data and PWM data. The PWM technique using a triangle carrier wave leads to the dc current sampling at near the top and bottom of the carrier wave, and AC current calculation within one cycle of the carrier under normal conditions. Figure 2 shows some differences between usual PWM method and the proposed PWM method. The proposed method changes modulation signals every half cycle of the carrier. To keep the pulse width for the dc current sampling larger than the minimum width required for stable sampling, modulation signals are modified in some cases with keeping average output voltages at their reference values.In one-shunt system, the current calculation error is caused by the current ripple in case of small inductance and low carrier frequency. To reduce errors in the AC current calculation, the current ripple waveforms are calculated from the instantaneous inverter output voltages and used for a compensation. Figure 3 shows an experimental result where inductive loads are used in stead of a PMSM. With the proposed compensation method, a waveform nearer with actual waveform is calculated.

A Method for Calculating AC Currents from Sampled DC Current Data in a Three-phase PWM Inverter