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
The VFD will provide as much power as the motor needs providing those needs are within specs. As the load on the motor increases or decreases, the current supplied by the VFD changes accordingly, again, providing the current demand is within the VFD's specs.
First answer here is correct, but I don't think it is exactly clear. The speed of an AC motor is determined only by the frequency, not by the current. So increasing speed does not increase current needed. The torque produced by an AC moter is determined by the current. As the load against the motor increases, therefore the torque needing to be produced by the motor increases, the current required increases. So a VFD will deliver whatever current is required to turn the motor as long as it is within the specifications (ability) of the VFD.
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.
The problem with starting any motor is the high starting current. For small motors up to say 4Kw this is not considered a problem and they are often wired to a DOL direct on line starter. That is the full Voltage and Frequency is applied to the motor all the way through. Basically it is on/off. With a VFD it is possible to limit the starting current by electronically varying the Supply parameters particuarly the frequency. This can be simply explained by comparing the motor to a light bulb. A normal light switch turns the light on/off with nothing in between eg DOL starter A dimmer switch allows you to vary the amount of power to the light bulb. Therefore the VFD is similar to the dimmer switch slowly increasing the speed of the motor. As mentioned by others to use a VFD just to start a motor is an expensive method with Soft start electronic conmtrollers more economical. However if the motor requires variable speed operation as part of a process control system then the VFD is justified
