Updated: Apr 2, 2020
In the manufacturing and industrial world, Variable Frequency Drives (VFDs) are like bread in a sandwich shop. They are an ordinary and necessary component in industrial and manufacturing environments. In the MEP world, however, VFDs have a much lower profile. They are usually only seen in booster pumps or sophisticated HVAC applications and are not particularly well understood. The purpose of this article is to introduce the reader to AC motor speed control with a VFD.
The VFD is king in the motor speed control department; they are used to control 3 phase AC induction motors. If you ever took a power class, you have probably seen the equation for AC motor speed. It is
where n = speed, f = frequency, and P = the number of poles.
An AC motor always has a nameplate attached to it with all of its pertinent information on it. Typically, the nameplate RPM is lower than the ideal given in the equation due to a factor called slip, which is beyond the scope of this article. In North America, f is always 60 Hz, so the possibilities are:
Note that if you connect the same motor to a 50 Hz European power grid, the speeds will lower.
The European chart looks like:
The image below is a typical AC motor nameplate. Note that the RPM value is only 1730 at 60 Hz and that this doesn’t fit precisely into the previous chart. That’s due to the slip factor mentioned before; it is, however, a 4 pole motor. Note also the voltage is given as 230/460, which means that it can be wired as a 230 V or as 460 V motor at the peckerhead per the wiring diagram in the lower right corner of the nameplate. Sometimes the wiring diagrams can also be found on the inside of the peckerhead cover. The horsepower information is also useful in specifying a VFD.
Figure 1 – Typical AC induction motor nameplate.
So for this example, if we were wiring it into a 480 V circuit, we would spec the VFD as a 460 V or 480 V input, 460 or 480 V output, rated to handle 5 hp(3.75 kW). Now that we have specified a VFD for this motor, let’s take a look at how the VFD works. Most commonly, the VFD takes the incoming power and rectifies it. That means it runs through a network of diodes and filters that convert to DC. The DC bus connects to an inverter, which converts the power back to AC. The inverter is usually made of Isolated Gate Bi-Polar Transistors (IGBTs), which can switch power at a variable rate, which allows the VFD output to be at any frequency you want. If we go back to the N=(1*f*60)/P, we can see that as f approaches 0, n also 0 and as f approaches 60, n approaches nameplate, and there you have it. You can now adjust the motor between 0 RPM to nameplate RPM.
Figure 2-VFD Model.
There are several ways to control VFDs. They typically have input and output terminals on the front cover to allow sensor inputs to be connected directly to the VFD. The VFD has dozens to hundreds of different parameters internal to it that are accessible and programable through an input module attached to the front of the VFD. The VFD can be programmed to run based on sensor data or based on user preference. Additionally, the VFD can be a part of a much more extensive control network that is managed by Programmable Logic Controllers (PLCs). So, in a nutshell, the VFD is a rectifier and inverter that can invert a DC signal to and any frequency of AC signal.
There are other functions that a VFD can perform as well. In Figure 2, you see that the motor gets its power from an inverter acting on the DC bus. The DC bus does not care if the power feeding it is 1 phase or three-phase, which gives the VFD the ability to act as a phase converter as well. The VFD’s ability to operate at variable frequency means that it can be used as a frequency converter as well. For example, a VFD could be used on European 50 Hz electrical equipment so that it could run on the US 60 Hz power grid. These uses of VFD for these purposes have caveats and limitations, and that will be the topic for the next article.