Wheeler on HVACR:
What's So Good About True RMS?
If you haven't been asked that question before, you probably don't work the parts counter much.
Over the past couple of years we've seen a lot of manufacturers' top-of-the-line electrical meters advertised as reading "True RMS." And the fact is, they truly sell at a premium price. What's so special about these meters and why should service contractors and technicians shell out the extra dollars for these premium digital electric devices? Are they worth the extra cost?
Well, they truly are worth the extra cost. These meters will show up problems with electrical lines that most people never knew existed. For the first time they'll be able to read the reason for unexplained motor and compressor failures that couldn't be recognized when using the old-fashioned (and bottom-of-the-line) types of ammeters, voltmeters, etc.
I've been out on many of those jobs in the past. Compressors or fan motors will just overheat and kick off on overloads for no apparent reason. You read the amps and volts, and sometimes everything looks okay. At other times the voltage looks okay, but the amps on one three-phase leg look unusually high and the problem stays with the phase leg - in other words, it isn't a motor problem.
So, before telling a story of the use I found for true RMS meters, let me explain the problem with the readings that standard meters provide. The meters we have used for years just take averaged readings, which is fine when you're powering a light bulb or an electric heater because they can use averaged power. However, motors need a true, clean sinusoidal waveform to work properly, with the fewest losses. There are many electrical devices (specifically electronic devices), such as charging machines, variable-frequency drives, etc., which don't use electrical power smoothly. So when there are a lot of these devices on an electrical distribution system (in a house or from a power company), they distort the wave shape of the incoming power, and this creates a hardship for all electric motors that share the same power source.
Of course, the problems caused by distorted electrical wave shapes aren't anything new to power companies. This is why you will often see large capacitors on power lines, especially at distribution points. Capacitors, when they are properly matched to the amount of wave distortion on the line, tend to smooth out the wave shape and eliminate the problem. However, notice that the capacitors need to be matched to the amount of distortion. And when a huge source of that distortion is periodically turned on and off, the distortion may go unnoticed or the line may be even overcorrected when it is turned off.
So do problems with line wave shapes go unnoticed and create "mystery" problems with motors and compressors? Yes, they do. And until True RMS meters came along, technicians never had any tool to determine the source of this problem.
Recently I was out to look at a job with a large country club in central Florida where incoming power distortion was a problem. Several technicians had been out on the job to find out why 60 tons of three-phase compressors kept kicking out on electrical overload. They even changed one of the 20-ton compressors! Yet, despite fiddling with the overload settings many times, invariably on a Saturday morning (the club's busiest day), the unit would kick off and the managers would have to wait for a technician to come out and reset the overloads. This was quite a nuisance.
One day when I was out there on a courtesy visit, one of the technicians took me back into the mechanical room to tell me of a problem and to ask about some strange electrical readings he was getting. Joe - a top technician - took a lot of pride in his tools and had recently purchased a True RMS a meter. He had taken the electrical readings several times before with a regular meter, and that day was the first time he had used the new one. As he showed me, the readings were fine using the regular meter, but when he checked it with the True RMS meter, one leg showed more than 20% lower amps. And when he reversed the phase connections, the problem stayed with the incoming phase. So the problem obviously wasn't something wrong with the equipment - it was a problem with the incoming power.
To shorten a long story, I contacted the power company. Their man came out, and when he saw the results of our meter reading (his meter didn't read True RMS and he had no tools to check the power wave shape), he headed out, as he said, to check his "capacitor banks."
The power company later claimed that he found nothing wrong. However, strangely, the problem just went away. So, would I recommend True RMS meters to all contractors and service technicians? You bet!