Throughout the many applications and instances of automated industrial systems, motor control is a hot topic. One particular component that has a prominent role in this space is the variable frequency drive (VFD). There are three basic ways to control motors – an electromechanical starter, a reduced voltage soft-starter, and the VFD. The electromechanical starter or across-the-line starter, is strictly a full voltage, on/off device. As for the reduced voltage soft starter and the VFD, the main difference is that a VFD can vary the frequency continuously while delivering optimal torque over a wide range of rotational speeds.
As you consider applications and how the VFD will integrate into control systems, a common valuable consideration is understanding the difference between constant torque and variable torque. Ok, so what exactly is the difference between the two? We’ll use the standard fan and conveyer example to explain. When a fan just begins to turn from being stationary it is overcoming friction and gravity to initially begin to rotate. Once the fan is up and running at operational speed, the system begins to experience decreased frictional and gravitational resistance while simultaneously experiencing increased air resistance. These changes in loading demand the ability to vary the torque. On the other hand, a conveyer that is loaded with ten 100lb widgets will need an exacting, precise amount of torque each startup to move the ten widgets. Now, with this knowledge in mind, how do we go about actually selecting a VFD?
Most VFD’s will have specifications for both normal and heavy-duty applications. While motors can withstand the heat generated from experiencing 2-3 times nominal torque, VFD’s, on the other hand, have Insulated Gate Bipolar Transistors (IGBT’s). IGBT’s are current microswitches that get very hot, very fast – this can easily damage a VFD. Therefore, it is critical to start the specification process with a clear understanding of the process and application to accurately assess duty type. On a typical fan or pump application, the motor will likely not draw over its rated full load amperage (FLA) often. In this instance, we would likely go with a normal duty specification which allows for 110% overcurrent for 60 seconds. Conversely, in the loaded conveyor example, we would likely go with a heavy-duty specification which allows for 150% overcurrent for 60 seconds. The additional protection here allows for increased torque in the motor on those constant torque applications.
Looking into sizing the drive, this is where you need that key piece of information, we all know so well, the motor nameplate. This is where all the good stuff is – motor voltage, motor rated amperage and horsepower, rpm’s, service factor, etc. This information is not only critical to sizing the VFD, it is also key in programming the VFD. For special case applications such as severe duty or 6+ pole motors, the motor will draw more amperage for the like horsepower ratings than a standard inverter duty motor. This certainly plays a big part in properly sizing our drive.
As we venture on into this topic, we really need to talk about overload and short-circuit protection. Are there special considerations that need to be taken when electrically protecting a VFD versus a reduced voltage soft-starter or an electromechanical starter? – Not particularly. With that being said, we should always review the VFD manufacturers specified methods of electrical protection for a fully warrantable installation. It doesn’t hurt to have a look at NEC Article 430 along with UL 508A for further, in-depth information on standards and applications.
Rounding things out we get down to options that can vary greatly across different manufacturers. Some common options would include communication and networking options, line and load reactor options, and signal filtering options. These can be integrated into and alongside various control and host/client architectures to achieve the desired solution. The bottom line – it’s always the best practice to get full nameplate data and as much knowledge of the process and application as possible. By following these general considerations, you’ll be well on your way to selecting the correct VFD for your application.