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Novel inverter surge suppressor is 1/80 the size of a filter
Published:  15 April, 2009

The Japanese cable-maker OKI Electric Cable and researchers from Tokyo Metropolitan University have developed a new method for suppressing inverter switching surges which, they claim, is much smaller and uses much less power than conventional filters.

For a 15kW motor, the new “eco Surge µ” system (shown above) is less than 1/80th the size and consumes less than 1/30th of the power of a standard filter. The device – weighing just 300g and measuring 10 x 9 x 4cm – will suppress more than 90% of micro-surge noise, they add. Previously, a conventional filter measuring around 30 x 30 x 33cm would have been needed for this task.

The small size and high efficiency of the new device will allow it to be used in enclosures that would not have space for conventional filters, as well as in applications such as saving energy for inverter-fed motors.

The new technology is based on a combination of a surge-suppressing cable developed by OKI, and an energy regeneration module, which includes delay and rectifier circuits. By efficiently returning only the surge energy to the inverter input, the researchers say they have developed a surge suppression method with an extremely low power consumption, and a low heat output.

The new method does not require high-power capacitors or coils. By adjusting the delay-circuit constants of the energy regeneration module, more than 90% of the micro-surge noise can be suppressed and re-used as power.

“We are very excited about this revolutionary method that has enabled us to make the micro-surge suppressor ultra-small,” says Takashi Hattori, president of OKI Electric Cable. “We believe we can apply this method not only to surge suppressors, but also as to other noise countermeasures.”

The diagram above shows how surges occur when a motor is connected to directly to an inverter. The conventional approach is to insert an LC filter between the motor and inverter (as shown below) to slow down the rise time of the inverter drive waveform, and to dissipate the surge energy as heat. This reduces the energy in the waves and eliminates surges caused by the waves bouncing back. However, with this method, as the motor power increases, it is necessary to use large and costly high-power capacitors and coils.

In the new design (shown below), the surge current is returned to the DC terminal via a surge-suppressing cable and a compact, efficient energy regeneration module containing the delay and rectifier circuits. 

The new technology is expected to be especially useful for high-speed precision machine tools. As these become faster, the speed of inverter switching is also increasing. In addition, by turning to higher voltages to improve efficiency, micro-surges induced by impedance mismatches are also increasing. These can cause malfunctions in peripheral circuits and have adverse effects on inverter-fed motors, such as damaging the insulation in the motor coils.

OKI plans to start selling the new suppressor technology in July this year, and expects to generate sales worth ¥100m ($1m) this year and ¥500m ($5m) in 2010.

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