Fluke 902 Service Guide

Why true-rms matters
for HVAC technicians

Non-linear loads need a true-rms
test tool for accurate readings

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.

Application Note

True-rms implications
for HVAC

Consider all the problems found
in complex HVAC and refrigera­tion systems. There’s a full range
of electrical and mechanical
issues, of course, but also control
system problems, air supply bal­ance, compressor performance
and the delicate balance of cool­ant temperature and pressure,

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

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 consid­ered a non linear load.

Normally when troubleshoot­ing an HVAC equipment failure

or nuisance trips due to an elec­trical problem, your first instinct
would be to check the panel for
tripped circuit breakers or over­loading.

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 over­loaded, or is the problem with
the load itself?

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 measure­ments affected by rms

Here’s a birds-eye view of the
HVAC electrical measurements
that require a true-rms test tool.

Measuring supply side current

•

and voltage as well as load
side current and voltage

Measuring current and volt-

•

age phase balance on 3 phase
systems

Troubleshooting compressor

•

electrical motor faults

Troubleshooting
compressor motor faults

Compressor failures are often
caused by electrical faults. The
best tool for this measurement
sequence is a true-rms clamp
meter like the Fluke 902 or a
true rms digital multimeter with
a current clamp accessory.

1. Allow the compressor to cool
down prior to the electrical
test. This allows the device
to reset to its normal position.
Then, remove the electrical
terminal cover.

2. Check line voltage at the load
center with the compressor off.
Low line voltage causes the
motor to draw more current
than normal and may result
in overheating and premature
failure. Line voltage that is
too high will cause excessive
inrush current at motor start,
again leading to premature
failure.

3. Check line voltage at the
motor terminals with the com­pressor running. The true-rms
voltage should be within 10 %
of the motor rating.

4. Check running current. The
readings should not exceed
manufacturers’ full load rated
amps during heavy load peri­ods. Low amps are normal
during low load conditions.
Excessive current may be due
to shorted or grounded wind­ings, a bad capacitor, a faulty
relay, or bearing fatigue.

Troubleshooting
compressor motor failures
caused by refrigeration
system problems

Compressor electrical problems
are often caused by mechani­cal system failure or installation
and service errors. Compressor
bearings can fail or lock up due
to improper lubrication or insuffi­cient oil return to the compressor
(largely due to poor piping). To
diagnose this problem, measure
the compressor current with a
true-rms test tool. The current
readings should not exceed man­ufacturers full load ratings. Worn
bearings will cause higher than
normal current readings.

Checking for voltage
imbalance in a three-phase
compressor motor

Voltage imbalance in three­phase motors causes high cur­rents in the motor windings.

These higher currents generate
additional heat that degrades
and destroys winding insula­tion. A 10 °C rise in motor tem­perature can reduce motor life
by half.

Voltage unbalance is usually
caused by adding single phase
loads on the same circuit used
by the compressor, although
sometimes component failure is
the culprit. Voltage unbalance
for three phase motors shouldn’t
exceed one percent, otherwise
the motor load capacity should
be derated. To calculate voltage
unbalance, use this formula:

% Voltage Unbalance = 100 x
(maximum deviation from average) /
Average voltage

For example, given true-rms
voltages of 449, 470, and 462,
the average voltage is 460. The
maximum deviation from the
average is 11 volts. The percent
unbalance is 100 x 11 / 460 =

2.39 %. That result indicates a
voltage unbalance problem. The
closer the motor is matched to
the load, the less reserve power
it has and the more important it
becomes to periodically check
motor supply voltages.

Checking for current
unbalance in a three-phase
compressor motor

For accurate results, measure
current with a true-rms clamp
meter or true rms DMM with
a clamp attachment. The goal
behind measuring current on a
three-phase compressor motor is
to ensure that the full load rat­ing on the motor nameplate isn’t
exceeded and to verify that all
three phases are balanced.

Unbalanced current can be
caused by voltage imbalance
between the phases, a shorted
motor winding, or a high resis­tance connection. To calculate
current unbalance, use the same
formula as above but substitute
current in amps. Maximum cur­rent unbalance for three-phase
motors is typically 8 % to10 %.
A 1 % voltage unbalance will
cause an 8 % current unbalance.

2 Fluke Corporation Why true-rms matters for HVAC technicians