Application Note

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Application Note

Why true-rms matters

for HVAC technicians

Non-linear loads need a true-rms

test tool for accurate readings

F r o m t h e F l u k e D i g i t a l L i b r a r y @ w w w . f l u k e . c o m / l i b r a r y

For today’s HVAC technician, troubleshooting

electrical problems is becoming more difficult

without the use of true rms test tools. This is

due in part to the proliferation of new solid state

adjustable speed motor drives and heating

controls containing power semiconductors or

rectifiers. These loads are referred to as “non

linear.” Non linear loads draw current in short

pulses rather than the smooth sine wave drawn

by a linear load such as an induction motor. The

current wave shape can have a drastic effect

on a test tool reading.

There are two types of electrical test tools

commonly available: “Average responding-rms

indicating” and “true-rms.” The average

responding units give correct readings for

linear loads such as induction motors, resistive

heaters, and incandescent lights.

But when loads are non-linear, average

responding meters may read anywhere from

5 % to as much as 40 % low when measuring

line side currents.

True-rms implications

for HVAC

Consider all the problems found

in complex HVAC and refrigera-

tion systems. There’s a full range

of electrical and mechanical

issues, of course, but also control

system problems, air supply bal-

ance, compressor performance

and the delicate balance of cool-

ant temperature and pressure,

Checking current on a compressor controller with a true-rms Fluke 902 HVAC

Clamp Meter.

superheat, subcooling and air

flow that is fundamental to air

conditioning and refrigeration

performance.

Now consider how many of

those involve non-linear loads.

Essentially, any control or system

containing semiconductors in the

power supply would be consid-

ered a non linear load.

Normally when troubleshoot-

ing an HVAC equipment failure

or nuisance trips due to an elec-

trical problem, your first instinct

would be to check the panel for

tripped circuit breakers or over-

However, if a non-linear load

is on that circuit, you’ll need a

true-rms test tool to accurately

measure the true load current to

determine where the problem

is—is the circuit faulty, is it over-

loaded, or is the problem with

the load itself?

Summary of content (3 pages)

PAGE 1
Why true-rms matters for HVAC technicians Non-linear loads need a true-rms test tool for accurate readings Application Note For today’s HVAC technician, troubleshooting electrical problems is becoming more difficult without the use of true rms test tools. This is due in part to the proliferation of new solid state adjustable speed motor drives and heating controls containing power semiconductors or rectifiers. These loads are referred to as “non linear.
PAGE 2
One current—two readings. Which do you trust? The branch circuit above feeds a non-linear load with distorted current. The true-rms clamp on the right reads correctly but the average responding clamp reads low by 32 percent. HVAC electrical measure- 3. Check line voltage at the motor terminals with the comments affected by rms Here’s a birds-eye view of the HVAC electrical measurements that require a true-rms test tool.
PAGE 3
What is true-rms? “RMS” stands for root-meansquare. It comes from a mathematical formula that calculates the “effective” value (or heating value) of any ac wave shape. In electrical terms, the ac rms value is equivalent to the dc heating value of a particular waveform— voltage or current. For example, if a resistive heating element in an electric furnace is rated at 15 kW of heat at 240 V ac rms, then we would get the same amount of heat if we applied 240 V of dc instead of ac.