Today’s VFD are available in compact form factors and can reliably operate motors of all sizes, across a span that may range from nearly 0Hz all the way up to 400Hz. This is quite a range compared to the nominal 60Hz fixed speed. Not only that, but precise speed control of 0.01Hz resolution with an accuracy of 1% of maximum output frequency is possible. Similarly, torque control can apply up to 200% rated torque at as low a speed as 0.5Hz.

These horsepower and torque capabilities allowed frequency drive and AC motors to be used in many more applications. However, the original frequency drive were intended simply to drive the motor, and not to replace more advanced control functions.

As technology continued to advance, motor drives become more capable. Basic analog devices with elementary ramp algorithms and simple PID loop control functions have transitioned into fully digital controllers capable of advanced motor control functions. Fundamental control devices have evolved to include built-in programmable logic control (PLC) functionality. Many frequency drive gained extensive data communication abilities.

Advanced motor drive features, particularly those providing integrated PLC functionality, are still subject to some limitations, such as the number of connected input/outputs (I/O) and the program memory size. However, for many applications like tank farms, lift stations, there is a need for this type of simple local control.

Advanced control modes

The most common and simple frequency drive control algorithm is Volts per Hertz (V/Hz or V/f). Most modern VSD will still default to this control mode because it requires the fewest steps to get the system up and running, and it’s still appropriate for more than 80% of the motor control applications. But those other 20%? This is where these more advanced control methods come into play.

In many cases these algorithms require the frequency drive to perform an auto-tuning procedure while connected to the motor. This allows it to set some internal parameters to provide more precise control. Technologies with names like sensorless vector, field-oriented control, and full closed loop flux vector are specific modes using implied or actual encoder feedback, enabling the VFD to provide more precise control. Certainly, these capabilities promote VFD into the grey area of almost-servo-like functionality. For the right applications, applying these advanced motor drive control modes can save the user hundreds or thousands of dollars when compared to a comparable servo system.

Connected frequency drive

In addition to control and safety, the networking and communications aspects of VFD is another area that has advanced rapidly. Original VFD relied on simple discrete and analog hardwired I/O signals. This progressed to industrial digital field-bus connectivity, enabling the networking of many frequency drive on a single network

Recently, Ethernet communication using various industrial protocols like Modbus TCP and EtherNet/IP has become the standard for networking multiple frequency drives, and in many cases is now more common than hardwired I/O. More and more, VSD are receiving their command signals from and providing status information to associated PLC or other controllers via Ethernet. Of course, Ethernet-connected VFD can also provide extensive operating data and statistical information, and accept configuration information from higher-level components, such as a PLC.