Can Variable frequency drives INCREASE rpm of an AC motor without using up too much current?

Question

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

Answer 1

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.

Answer 2

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.

Answer 3

As you can see from the other answers, this question is more complicated than you might expect. You must consider both the capability of the motor and the characteristics of the load.

When a motor is operated over a range of frequencies, the voltage is usually increased or decreased to maintain a constant ratio of voltage to frequency, constant volts per hertz (V/Hz). With constant V/Hz, the motor can deliver a constant torque over the frequency range with a constant current. Since speed is proportional to speed and power is torque X speed, available power increases in proportion to frequency.

The output voltage of a VFD is limited to the input voltage (except for a few models equipped with a voltage doubler circuit). That means that it is usually not possible to increase to voltage to maintain constant V/Hz above the rated frequency of the motor. If the frequency is increased without increasing the voltage, the operation is called "field weakened" operation. As the frequency is increased, the V/Hz is reduced and the available torque is reduced. Up to about 150% to 200% of rated frequency, the available torque is sufficient to provide constant power.

Many loads require approximately the same torque at all speeds. Since power = torque X speed, those loads require steadily increasing power as the speed increases. A motor that has decreasing torque available at higher speeds will not work on that type of load unless it is greatly oversized.

Some loads require high torque only to get the load moving from standstill. A motor with low torque at high speed may be OK in that situation.

When accelerating inertia, such as an electric vehicle, high torque is only needed while the speed is increasing. Once a steady operating speed is reached, a lower torque is sufficient to overcome friction and wind resistance. In that situation, the decrease in torque means that the vehicle doesn’t accelerate as quickly at higher speeds, but that is probably OK.

Fans and pumps require increasing torque as speed increases. Motors with decreasing torque at high speed are rarely used for fans and pumps.

It is possible to obtain constant torque above the normal operating frequency by using a motor that has a rated voltage that is lower than the available supply voltage. That is somewhat more complicated, so I won’t try to explain it here.

In all cases, there may be speed limitations imposed by the bearings, rotor balance, centrifugal stress etc. The applicable motor standards may set a maximum speed that is within the speed range desired. Specially designed motors are can be obtained for requirements exceeding the limits of standard motors.

Answer 4

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.