As we go about our daily business, few may stop to admire the simple technology behind the inverter drives that run our machines. The concept is simple and the implementation has become more efficient over the years. Where older units were larger, heavier and relatively basic in their implementation, newer models are slim, sleek and offer precise control over performance parameters. No matter the changes or added bells and whistles, the concept remains the same: take a constant frequency AC power and supply a variable frequency to the output.
Line frequency will be either 50 or 60 Hz (with 60Hz being the standard in America along with a small handful of other countries). Assuming you have 240VAC on your line at 60Hz, if you were to directly connect a motor to the wall, you would simply run at a constant 60Hz. For a variety of reasons, this is not an efficient or recommended way to run your motors.
Enter the variable frequency inverter. A box that can take a constant input voltage and output a user-controlled variable frequency. Some can regenerate excess power back to the line; others use braking units to absorb power back. Some have thousands of parameters and all the bells and whistles; others use a switch and reference potentiometer to run. At the end of the day, the result is the same.
So what is the general principle behind the inverter? First, constant frequency AC voltage is input to a converter stage. This performs basic rectification of the AC signal to get a constant DC signal out. This DC signal comes out of a diode-bridge and is stored on the main DC bus. A series of transistors work to invert the DC signal back to a pseudo-AC, PWM signal. The benefit here is that the transistors are specifically controlled to produce an output of any frequency within a designed limit, usually up to a few hundred Hertz. This PWM signal is effectively an AC signal to the motor and can be manipulated by the opening and closing of the transistors.
Again, this is the basic concept behind most variable frequency drives. The implementation can vary to increase efficiencies and add functionality, but ultimately the idea stays the same. With so many inverters used in thousands of different applications, we find it helpful to understand the basics of operation. Hopefully, this proves to be a great starting place to dive further into different implantations of control and feedback for motor control.