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Understanding Terms - Low Pass Filter

When eagerly going through the specs on multimeters (like most people like to do in their spare time😁), have you ever come across the function LPF, or Low Pass Filter,  and wondered what it was for? 

It’s one of those functions we often hear about, without fully understanding it…  so let's shed some light on it. 

Firstly, we'll look at a typical situation where LPF might be used - with Variable Speed Drives.

In today’s industrial environment, variable speed drives are popping up everywhere.  Variable speed drives (VFDs) are used to precisely control the speed of electrical motors. For process manufacturing, and mechanical system control, variable speed drives are a powerful way to operate electrical plant and machinery using exact motor speeds to gain precise timing and outcomes during a particular process. That’s probably a fancy way to say absolute speed control. 

This is all very well, but often technicians have to measure what voltages are coming out of the controller, to determine if the signals being sent to the motor are correct. It is here that we run into some measuring difficulty. To understand why, here’s a little bit on how the variable speed drive works. 

First, the drive takes in an AC supply from the mains. This can be single or three phase, and rectifies it to a DC supply.  The variable speed driver then switches the DC on and off for variable lengths of time to create an average voltage output.

Variable Speed Driver circuit diagram  

In order to create a simulated waveform output at a particular frequency, the signal changes the time it is on compared to the time it is off as well as changes the total time for the on and off cycle to simulate the average increasing and decreasing voltage of the wave.

I’ll attempt to describe this in words, then show a graph for visual assistance. 

At zero on the wave, the signal is off, then the signal goes on for a short time, then off again, next it goes on for a bit longer then off again, then on a bit longer again, then off.  What it is doing here is simulating the rising side of a wave form by turning the DC supply on for gradually increasing time intervals to gain an overall average of what would be the increasing positive side of the wave. When it gets to the wave peak, the DC supply is on. To simulate the decline of the positive side of the wave, the signal starts to turn on for consecutively decreasing time periods until it gets to being off just off where it started.  

At this point, the voltage is reversed, and the same process is repeated, but now in the opposite direction. The total result of both processes is a simulated waveform.

Variable Speed Drive Pulse Width Output  

The total output frequency is adjusted accordingly based on the input request.  This can be via electronically controlled signals, or manually adjustable inputs or both. 

For three phase VFD output, three of the above simulated wave forms are emitted simultaneously 120 degrees offset to each other, as would be on a traditional three phase sinusoidal output. 

Here is a graph to show a typical 3 phase VFD output along with a basic schematic of the set up.

Variable Speed Drive 3 Phase Pulse Width Output 

As you can imagine, there is a lot of switching going on with all these on and offs. Think about a normal AC wave form.   This occurs at 50Hz in Australia.  That’s 50 times per a second.  Now chop that into heaps of little parts to simulate this wave form.  All these little parts are actually a measurable on and off.

This is where the problem with measuring comes in. These ons and offs create what is called noise on the wave form.  This noise adds to the simulated wave form to give an incorrect measurement of the true average output.  

This is where the Low Pass Filter comes in.  The filter in this measuring circuit blocks out high frequency values, and only measures the simulated wave form.  This filter is usually set at 100Hz.  That is, it will filter out the high frequency switching used to create the simulated wave form, and only measure the simulated form itself. 

As can be seen, this feature is very handy when you need to know the actual output voltage and frequency as simulated, so that the technician can adjust and troubleshoot accordingly. 

Variable speed drives are an example here to illustrate the use of a low pass filter, but this function can be used with any output that has a noise placed on it due to high frequency switching. Switch mode transformers are another good example of noise producing electronics that can be hard to measure accurately.