Motion Control Resources
Universal Drives Simplify Integration - Sidebar
Motion Control & Motor Association Posted 09/24/2007
There was a time that adjustable speed drives meant induction motors that could be directed to run a little faster or slower, but were still basically dumb units. Not any more. Today’s adjustable speed drives combine leverage feedback from encoders to turn simple induction motors into vector motors capable of sophisticated positioning while providing full torque down to zero RPM.
“Over the years they have gotten smarter,” says Jeff Lovelace, drives production manager at Baldor Electric (Fort Smith, Arkansas). Motors using adjustable speed drives can basically be classed as inverters or vector motors. An inverter motor runs at variable speeds but does not use feedback. A vector motor integrates an encoder to ensure positioning accuracy. “That’s going to give you precision all the way down to zero speed. It’s got the encoder and that ties right into a motion control [application].”
So what, you might say. A servo motor can do the same, and more rapidly. A vector motor can provide one critical thing that a servo motor can’t, however, large amounts of power. “A brushless servo gives you low inertia and really fast speeds, but they’re limited right now in terms of how large of one you can get. Although they’re not rated in horsepower, around 30 hp, it’s harder to find a servo motor,” says Lovelace. “With a vector motor, you’re unlimited in horsepower.”
Of course, the vector motor isn’t a perfect solution. A bigger motor means more inertia, which limits acceleration and speed. “Its downfall is that it’s a big mass sitting out there that can’t do superfast moves,” Lovelace acknowledges. For certain applications, though, big is exactly what is required.
“Typically, a vector motion control application would be something a lot larger in size, because you may have an application [that needs] maybe a 100 hp motor,” says Baldor’s motion sales and support manager, Jesse Henson. As an example, he points to a recent application involving the use of four vector motors to control a rail-mounted crane that offload coal from barges. “We wouldn’t have a servo motor that could do that but the crane still needed positioning.”
Different horses for different courses, so the saying goes. “In probably 80 to 85% of the motion control applications we get involved with, it’s servo motors that get specced in,” says Henson. “Not very often do we use vector technology just because we don’t need the size.” But when only size will do, it’s the adjustable speed drive vector motor that fits the bill.