Newly introduced low-harmonic drive technology integrates an active filter with the drive rather than using an active front end to deal with system harmonics. New VLT(R) low-harmonic drive technology from Danfoss uses active filtering to eliminate disturbances on the supply grid by actively imposing currents to restore and insure the highest possible quality sine waves.

"In essence, the filter portion of VLT low-harmonic drives has the same working principle as a set of noise-canceling headphones, where the noise or distortion is measured and a computer phase signal is imposed to compensate for that noise," says Gregers Geilager, a product manager for Danfoss. He says that as a matter of scale, noise-canceling headphones can inject 100 mW at 50 to 1,000 Hz, while the drives can inject several hundred watts at 250 to 2,000 Hz.

Geilager says active filters and active front ends are often falsely recognized as one and the same, but the difference is significant and the optimal mitigation solution requires in-depth technical knowledge.

He says the Danfoss product is unique because it combines an active filter and standard variable-speed drive in one box. A paralleled filter that is only used to compensate the drive harmonics (40 percent of full load current) also offers a programmable value to go into sleep mode when mitigation is not needed to increase efficiency.

Because motor current has unhindered passage to the drive and is inherently not producing heat in the LCL coils, this energy is saved. And because the dc-link is not boosted, the technique used to deal with harmonics in active front-end (AFE) designs, motor voltage is not affected. Plus, the dc-voltage in the parallel filter can be boosted to provide better harmonic mitigation performance under non-ideal grid conditions.

Geilager says the trade-off with an active front-end design is that, to mitigate harmonics in the system, the voltage on the dc-link needs to be bigger than the grid voltage. This "boost voltage," which is typically 15 to 20 percent higher, creates better harmonic performance under non-ideal conditions such as pre-distorted grids or phase imbalance. But the bigger the boost, the higher the motor winding stress, motor bearing currents and EMC noise on the motor wire. Passing the full motor current through two IGBT modules and a LCL magnetic circuit also leads to high IGBT switching losses and LCL coil losses (efficiency typically 93 to 94 percent).

A unique aspect of the design is that it uses a ducted back-channel to pass cooling air over the heat sinks with minimal air passing through the electronics area. This allows 85 percent of the heat losses to be exhausted directly outside of the enclosure, improving reliability and prolonging life by dramatically reducing temperature rise and contamination of the electronic components. The unit has an IP54 seal between the back-channel cooling duct and the electronics area of the low-harmonic drive.

A key advantage of the new low-harmonic drives is that they share the same modular technology platform as Danfoss VLT high-power drives. By sharing common system components compared to active filters that are typically supplied in low volume, Danfoss claims it has created the world's first mass-produced low-harmonic drive. The active filter is also provided as a stand-alone product, which creates an ability to retrofit drives if system harmonics prove to be an application issue after the initial installation.

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Eliminating Drive Harmonics_A

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