Variable Frequency Drive Gets Big and Fast

Siemens answers rising energy costs with a VFD that sets a new benchmark in size and speed

Joseph Ogando, Senior Editor -- Design News, August 19, 2008

With energy costs on the rise, it's no surprise users of large motors have started to ask for big, fast variable frequency drives that can help them cut their power costs. Siemens Energy & Automation has responded with a VFD that sets a new benchmark in combined size and speed.

The company has introduced an 8-MW Robicon Perfect Harmony VFD that operates with an output frequency of 500 Hz. According to Mark Harshman, engineering director for Siemens large drive applications, it's the world's largest high-speed VFD. "There are some big drives out there with outputs in the 180 to 240 Hz range, but we've eclipsed them," he says.

The leap to even faster VFD output frequencies is significant because it lets users deploy smaller, faster motors without giving up power. Harshman estimates some heavy-duty industrial applications could get by with motors that are about one-third the size of what they would be with lower-frequency VFDs. Significant energy savings would result from the motor downsizing.

Siemens had previously produced large drives with outputs in the 315 to 400 Hz range, but a control algorithm redesign bumped the output up to 500 Hz. "We won't be able to achieve the next step through software alone. The hardware will have to change too," Harshman says.

In testing, the new 8-MW drive was able to operate the motor in encoderless vector control with 920 Hz IGBT switching frequency, resulting in an effective switching frequency of 9.2 kHz and a sample frequency of 9.2 kHz.

This new breed of VFDs can drive the efficient permanent-magnet motors that are increasingly attractive in oil, gas and transportation applications – particularly those requiring high-speed compression.

Harshman predicts the output frequencies will continue to rise with subsequent generations of VFDs. "It's all being driven by the motor vendors and users," he says. "We're just keeping up."