by Jack Rosebro


Above: conventional pulse-width modulated (PWM) motor control. Below: Hitachi’s pulse harmonic modulated (PHM) motor control. Click to enlarge.

Hitachi has announced the development of a high-speed motor control strategy which improves overall motor and inverter efficiency by as much as 3%, while reducing inverter cooling requirements by as much as 40%.

Conventional motor control strategies utilize pulse width modulation (PWM) of switched direct current to produce voltage signals that drive an alternating current. However, the switching required to produce such signals reduces available peak voltage and produces heat which much be transported away from the switching devices. Alternate switching strategies are typically employed at higher motor speeds to minimize such losses.

Hitachi has developed Pulse Harmonic Modulation (PHM), a drive strategy which reduces switching by as much as 80%, leading to reduced heat losses at the inverter.

The harmonic content of the waveform has been minimized, and the waveform has been refined to minimize copper losses as well as iron losses. At higher speeds, motor output can be as much as 10% higher using pulse harmonic modulation.

However, significant copper losses remain at speeds below 60 km/h (37 mph), and Hitachi envisions a two-part motor control strategy that would utilize PWM at lower speeds and switch to PHM at higher speeds. Although pulse harmonic modulation was developed for 50-100 kW motors, it can be adapted to larger motors, as well.

Hitachi forecasts that the market for hybrid, plug-in hybrid, and electric vehicles will expand to 13 million units by 2020, with demand for hybrid construction machinery and hybrid-electric locomotives increasing by the same year to 12,000 units and 2,000 units, respectively.

The company is presently supplying electric motors to US-based manufacturers of hybrid and plug-in hybrid vehicles, and expects to see its PHM strategy adopted in production vehicles within the next two to three years.