VLT Drives – Since 1968

By Clarence Klassen 

IEEE Industry Applications magazine has an interesting article in the July/August 2013 volume by Peter Stewart regarding the first variable-speed ac drive. This was the VLT5 by Danfoss, an industrial manufacturer of valves at the time.

The technology was Pulse Amplitude Modulation, which converted dc to 3 phase ac. The first VLT drives were shipped in 1968. Up to that point and for years to come, variable speed was accomplished with dc motors using either Motor-Generator sets or dc variable-speed drives. The first VLT’s were used for non-web-handling applications such as pump controls.

VLT was based on the words Velocity, Load, and Torque. Since that time there have been a lot of improvements in AC ASDs.