Drive design identifies weakest point

The most obvious points of failure in any modern drive are the electrolytic capacitors and the fans that are used to cool the power section.

That’s why when Great Yarmouth-based Hitachi HID launched its new SJ300 AC variable speed drive late last year, the company went to great pains to design both the DC bus capacitor system and the fans as modular subsystems that could be easily removable for installation and maintenance purposes.

As most physicists will know, all electrolytic capacitors in all drives have a finite lifetime that is determined by the temperature at which they operate. Electrolytic capacitors in a drive, for example, are likely to last for around five years at ambient temperature before they fail. If the temperature is higher, they won’t last as long. On the fan front, the most obvious points of failure are the bearings inside the fans themselves.

In the cut-throat UK market, however, Hitachi have found that some competitive drive sales engineers do not seem to understand, or ignore the laws of physics to their advantage. In one instance, for example, Stuart Harvey, the National Sales Manager of HID Hitachi was told by one customer that a competitive drive did not have such problems with their capacitor subsystem and had dismissed the Hitachi product on that basis.

Only when Mr. Harvey requested that his customer ask for a guarantee in writing from the competitive drive manufacturer did the truth emerge. The competitor, of course was, at the last minute curiously unable to promise that his drive would not be subject to such a failure.

Aside from its unique modular design, the new Hitachi SJ300 drive itself is also higher in performance and smaller than its predecessors. It is claimed to deliver 150% torque at 0Hz and greater than 200% torque at 0.5Hz, and it uses Hitachi’s Trenchgate IGBT technology that has allowed a significant reduction in drive size – the SJ300 is typically about 30% to 50% smaller than its popular J300 counterpart.