vfd drives
Wednesday, August 18th, 2010
Can Variable frequency drives INCREASE rpm of an AC motor without using up too much current?
so the VFd changes the frequency of the current being used but does it pull more current when it changes the frequency?
for this i am not sure if it is used to step up rpm or only step down..thanks
Both posters are correct, frequency determines speed and current determines torque.
Many VFD’s can go up to 2x the normal speed, some go more. Most AC motors can do 1.5x the design rotational speed, many can do 2x, only a few can go faster than this.
Motors have thermal issues limiting them to the rated HP. Because power = speed * torque, if you overspeed them, you have to have less torque. So, if it’s going 2x faster, you can only get 1/2 the rated torque. The VFD has this limitation programmed in it, but on most you can also program lesser current limits. Thus, you can only overspeed the motor if the load is using less than the rated HP at the design speed.
By the way, since motors are designed to provide a certain torque, at lower speeds, they are limited to that torque (except for temporary overloads like starting), so they will produce less than their rated HP.
Modulation sound of IGBT variable frequency VFD motor drive on a train
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