and Motor Testing
Understanding IEEE 112 Method B (or CSA C390)
goal of all of EPACT (Energy Policy Act) legislation is, of
course, energy savings. This is to be accomplished by improving
energy efficiency or using less energy to perform the same
task. To demonstrate or prove a product's energy efficiency,
in this case an electric motor, obviously requires a method
of testing. And to compare one product to another requires
that this method of testing be standardized, preferably to
some recognized national or international standard. Discussions
about how EPACT applies to electric motors, presently three
phase industrial motors, therefore have always included mention
of test standard IEEE 112B, or the Canadian Standards Association's
C390, which can be considered equivalent. But what are these
standards and what do they mean to the average user?
understand this we must first realize that there are a number
of ways to determine an electric motor's efficiency. The most
obvious way, or so it would seem, is to connect the motor
to a known load and measure the electrical power into the
motor. Assuming we now know both the power (or work) output
and power going in, the ratio of these is efficiency. A mathematical
way of stating this is shown below:
output + losses
output + losses
of the total input power
that produces work or output
value of efficiency is then normally converted from a
decimal fraction to a % for convenience.)
of these losses produce the heat given off by the motor during
operation. This energy is therefore not available to perform
motor losses are normally broken into these categories :
or steel losses: energy lost in the magnetizing of the steel
laminations, and to keep them magnetized.
copper losses: heat generated due to the resistance of the
wire as the current flows through it.
copper losses: heat generated due to the resistance in the
rotor conductors or bars and end rings as current also flows
and windage losses: energy lost in bearing friction, energy
needed to turn the cooling fans and windage of other rotating
losses: other energy lost that does not directly fall into
one of the above categories. These are related to the construction
of the motor, parts that don't produce output power in or
near the magnetic fields in the motor, and interaction of
magnetic fields in the motor.
why not simply test the motor, as mentioned, to determine
the efficiency and total losses? First consider that the efficiency
of an electric motor changes as the grease "breaks in" (warms
and flows), as the motor materials heat up, and so on. Therefore
a procedure must be established to define which efficiency
measurement will be considered the "real" or "steady state"
efficiency. Secondly, there is the accuracy of the measurements
to consider. The output and input power are relatively large
numbers that differ by as much as 15% but as little as 4 to
5%. For lower efficiency motors, a slight error in measurement
would have a relatively smaller effect on the efficiency.
However, with higher efficiency motors (those for EPACT, higher
horsepower motors, etc.) where cost decisions are based on
only a few tenths of a percent difference in efficiency, great
accuracy is critical. Accuracy is another issue that must
be addressed in a standard way to get consistent results.
Third, a rotating motor and load constitute a "dynamic" system.
of speed, torque, volts, amperes, watts, and temperature are
not steady or constant values. There are fluctuations, though
small, that must be dealt with during testing in order to
be consistent. Basing motor efficiency on essentially one
reading could be misleading.
address all these concerns and more, IEEE (the Institute of
Electrical and Electronics Engineers) set out to write standards
to define how best to test electric motors. In IEEE standard
112, several methods are described. Efficiency determination
is only part of this standard, although it is an important
one. Some of the key (there are a total of 10) test methods
for efficiency are:
A: simple input-output
B: input-output with loss segregation (or separation)
F: equivalent circuit (model) calculation
other methods, C, E, E1, F1, C/F, E/F and E1/F1 are variations
of these. Very early discussions about efficiency testing
concluded that simple input-output was not accurate enough.
Method F, using an equivalent circuit approach was considered
an indirect determination rather than a direct measurement.
There was also a need, by motor design engineers, to know
how the total losses were distributed among the various types
or categories. This information would allow them to determine
how best to improve efficiency and where to focus their efforts.
all the methods outlined, only one, Method B, measures input
and output and attempts to determine and separate each type
of loss. Because of this IEEE 112 Method B became a popular
tool and is commonly used in the U.S. motor industry (and
in Canada by way of standard C390). Further work has improved
the accuracy and repeatability of this method over the years.
Experience with the method and this additional work have led
to its adoption as the standard and most accurate method for
determining motor efficiency.
Does Method B Relate to the "Real World"?
for efficiency to IEEE 112 Method B is broken down into sections
or types of testing. These are no load testing, temperature
testing, and testing under load.
no load testing the motor is connected to rated voltage
and frequency and allowed to run without a connected load
until the input watt readings stabilize. This can take from
over one to perhaps four hours to occur. Once the motor has
"loosened up," or stabilized, testing proceeds by adjusting
the motor voltage and taking a series of readings from approximately
125% of rated voltage down to a minimum voltage where motor
current no longer continues to drop with voltage. Using these
readings -- combined with winding and ambient temperatures
and winding resistance -- two of the motor losses, iron loss
and friction loss, can be determined. The iron loss will vary
with voltage while the friction loss will stay relatively
constant because motor speed is constant (within a few RPM).
The process involves first subtracting out the stator copper
loss that can be calculated from the current and resistance
(we are not interested in these at this time) and plotting
(or mathematically curve-fitting) the remainder. By extrapolating
the low voltage data to zero, the constant loss (friction)
can be determined. Subtracting this out of the higher voltage
readings will allow the iron loss to be calculated for exactly
temperature testing (or full load heat run) simply
involves testing the motor under rated load conditions at
rated voltage and frequency and monitoring temperature. The
specifications define how precise the load and input voltage
must be held. This test must continue until the motor temperature
is stable (thermal equilibrium). This is defined as when the
motor temperature rise does not change more than 1°C over
a period of 30 minutes
under load is done by maintaining rated voltage and frequency
while applying six different loads to the motor in one-fourth
rated load increments from approximately a quarter load to
1.5 times rated load. Readings such as current, torque, RPM,
and temperature will provide information about how the motor
performs under load. At this point motor testing is complete
and analysis begins.
or directly calculated losses are determined by tabulating
the readings, making any adjustments to these readings so
they are at the correct operating temperature (from the temperature
test), and determining each individual loss for each of the
six load readings mentioned previously. The iron loss and
friction losses were determined in the no-load test. These
are held constant for all load points. The winding resistance,
corrected for temperature, and measured current will yield
the stator copper loss.
the motor speed (and therefore slip), power input, and other
losses at each load will allow the rotor copper loss to be
calculated. All of these losses are therefore directly determined
or calculated from measured values. However, there is one
remaining loss category that has not been addressed, that
being stray loss.
Stray loss cannot be calculated directly from
an equation based on input or output readings. It is actually
the sum of several smaller losses that are dependent primarily
on motor (or part) geometry and variation. For the purposes
of this test method, stray loss is determined indirectly by
subtracting the directly calculated losses and output power
from the input power. Because all power (or energy) must be
accounted for, this difference is considered to be the stray
loss. (There are special tests designed to "measure" or isolate
stray loss, but the added time and complexity of testing would
not improve the accuracy of this method.) The value of stray
loss is typically in the 1% range of motor output power. Even
with very accurate measurements, the calculated value of stray
loss can vary significantly from reading to reading. This
is compounded by the fact that this loss being "what's left"
will include small errors or inaccuracies in the measurements
or calculation of other losses. This presents a problem.
entire test method is based on the premise that all losses
follow some smooth function of motor load, with other parameters
held constant. This potential variation or error often will
not allow the stray values to fall on a smooth curve. To address
this, the standard outlines a "smoothing" calculation where
these remainder values (stray losses) are fit to an agreed
upon equation form based on theory. The equation must go through
zero, have a positive slope, and the value of stray loss is
to vary by the value of torque squared. This smoothing is
also intended to "improve" accuracy under the assumption that
the true value of stray loss should be closer to values calculated
from this smooth equation than the actual test difference
values. Therefore, the values calculated must "closely" fit
this curve. Specifically, to determine if the readings are
correct and/or the test is valid, the values of stray loss
must fit this type of curve with a correlation coefficient
of 0.9 (90%) or better. If not, there is an allowance provided
that a maximum of one point can be ignored if necessary to
bring the correlation coefficient up to this level (thus allowing
for the possibility of one bad test point). But if this still
does not improve the correlation coefficient enough, the test
must be taken over.
calculation of efficiency is done by applying the equation
we started with. The efficiency value is determined by first
subtracting the losses, now corrected to the operating temperature
and smoothed in the case of stray loss, from the known input
power to get the corrected output power. Then this corrected
output power divided by input power is efficiency as defined
by IEEE 112B.
Time, Accuracy, Comparison with Other Methods
first thing one may notice from this discussion is that these
methods require much time, very accurate measurements, as
well as patience and experience. Including the preparation
of the motor (adding thermocouples to measure temperature,
mounting it to a dynamometer of some sort for loading,and
hooking up leads and meters), taking readings, waiting for
temperatures to stabilize, and performing calculations these
tests easily take eight hours or more. The standard defines
the accuracy of all measurement equipment to the point of
also including a dynamometer correction calculation (not discussed
here) to check and account for small errors in output measurement
resulting from friction in the dynamometer or other factors.
Even so, one can see that there are a relatively large number
of measurement calculations which do result in some "calculation
error." This includes round off, slight errors in temperature
corrections, stray smoothing, and simply error compounding
as measurement values are multiplied, squared, and so on.
And, one cannot ignore the "human" or procedural errors and
limitations in measuring a dynamic system.
IEEE, and CSA have worked to address these issues over time
with updates to the standard. The latest improvements dealt
with tightening up the equipment accuracy and more clearly
defining the procedure of testing to minimize "human error."
But even with these improvements, variation does exist. A
report, published by NEMA, on a "round robin" test program
involving several different motors, shows that the variation
in total losses on the same motor tested at different facilities,
all with proper equipment and using the best practices is
still about 10%. Even without a study you can show that even
with ± 0.2% instruments and ± 1 RPM speed measurements (both
as required by the latest standard) the resulting change in
losses can be 4-25% and the range in efficiency calculated
for any single load point is almost 1% (0.8-0.9%). See chart
above for an example.
should be noted that one of the original considerations in
the development of IEEE 112 Method B was to reduce errors
due to measurement limitations that existed in older test
equipment. Today, with more accurate equipment available,
some question if this method is still the best for efficiency
determination. But, in spite of the apparent shortcomings,
most feel that it is still the best method of testing and
determining efficiency for 3 phase AC motors. It is certainly
standardized, widely used in the North America for motor engineering
purposes, and perhaps studied the most. It also segregates
individual losses. And, if followed properly, the method will
be a fair comparison of one motor to another. So it is not
likely we will see a change any time soon.
does NOT however allow for easy verification by the average
user. And, one must understand that the tested efficiency
is NOT an exact single static number. Although the efficiency
calculated from a single rated load input-output reading should
come close to the IEEE 112B value, it will seldom be the same.
True verification will require a full test. And as we just
found out, that too has some variability.
6/19/97 - Technical Note 97-1