Variable-frequency drives, sometimes called AC (alternating current) drives, are commonly used in industrial automation to provide precise control over a variety of AC devices.
Why We Need Them
Most basic AC devices operate at a speed defined by the frequency of their input voltage. Given that power is supplied at a fixed frequency from sources like junction boxes, these devices would only be able to operate at a fixed speed when connected directly to a power source. This is where VFDs come in.
These wonderful devices can take a fixed input voltage, modify its frequency, and send it to whatever AC device it is connected to. This modified frequency can be adjusted as needed to give fine control over the speed of devices like motors, compressors, pumps, and fans.
How They Work
VFDs manipulate an input signal in three stages.
They first take the incoming voltage (such as a three-phase, 480V, AC waveform) and rectify it through several diodes placed in parallel with each other. This removes the negative portions of the input voltage, with the resulting waveform resembling a series of hills.
The waveform is then filtered via a capacitor that removes the bumps. At this point, the waveform has been effectively converted into a positive DC (direct current) signal.
The final stage, known as the inverter, consists of an array of transistor switches that open and close at intervals to convert the DC signal back into an AC signal. These switches are connected to a controller, which dictates the speed at which the switches operate. The controller can adjust the timings of the switches to manipulate the frequency of the AC output.
Note: Transistor switches shown here are not connected to any controller.
Applications
There are many advantages to using a VFD in a manufacturing environment. For example, say one were to connect a VFD to an AC motor to drive a conveyor belt. The speed of the belt could be adjusted as needed through the VFD to match production in other parts of the facility.
Configuring the VFD to perform slower changes to the output frequency would allow for the belt to start and stop more gradually, which reduces wear on the motor and leads to long-term cost savings on maintenance.
Furthermore, many VFDs (such as those available through MISUMI) incorporate power regeneration; this feature converts some torque from the motor into electrical power that feeds back into the VFD and power grid, reducing overall energy consumption and helping save on electrical costs.
In summary, VFDs are a must-have for any situation requiring precise control over an AC device. Be sure to check out Mitsubishi Electric VFD products, available through MISUMI.
