There are several methods for determining the efficiency of a three-phase asynchronous motor, which can be broadly classified into three categories: the first is the direct measurement method; the second is the indirect measurement method (also known as the loss analysis method); and the third is the method of using theoretical calculations after simple experiments. Today, we will briefly analyze the different experimental methods, focusing on the A-method.
Characteristics and applicability of various test methods
The first type of method is intuitive, simple, and relatively accurate, but it is not suitable for specific analysis of motor performance and targeted improvements. The second type of method involves more tests, is time-consuming and labor-intensive, and has a large amount of calculation, so its overall accuracy is not as good as the first type. However, it can show the specific situation of each major component that determines motor efficiency, thus facilitating targeted analysis of problems in motor design, process, and manufacturing, and improving the motor performance to meet or further enhance the requirements. The third type is the method used when testing equipment is insufficient, and its accuracy is the worst. Currently, the efficiency limits specified in the technical conditions of most motors require the use of the second type of method (loss analysis method) to determine the efficiency.
Classification of test methods
GB/T1032 references relevant IEC and NEMA standards and further classifies the methods for determining the efficiency of three-phase asynchronous motors, using codes to represent them.
Method A – Input-Output Method. This method is typically limited to motors with a rated power ≤ 1kW or an efficiency rating ≤ 80%.
Method B – Input and output loss analysis method for measuring stray losses. This method requires high accuracy of the measuring instruments (generally not lower than 0.2 class).
Method C – Indirect method for loss analysis (feedback) – Measured stray loss
E-method – Loss analysis method and measured stray loss
E1 method – Loss analysis method, recommended for stray loss.
F-method—Equivalent circuit method and measured stray loss
F1 Method – Equivalent Circuit Method and Recommended Stray Loss
Method G – Reducing Voltage Load and Measured Stray Loss
G1 Method – A Method to Reduce Voltage Load and Recommended Stray Losses
H Method – Calculation Method using a Circle Chart
Method A for determining efficiency—a direct method for measuring efficiency.
Method A is known as the "direct efficiency measurement method" or "input-output method" because it directly yields two data points for determining efficiency during the experiment: input electrical power P1 and output mechanical power P2.
●Test Equipment Requirements
The key to this method lies in having a dynamometer that can directly measure the output mechanical power (or torque) of the motor. The power of the dynamometer used (or the nominal torque of the torque sensor) should not exceed twice the rated power (or torque) of the motor under test at the same speed as the motor under test.
When using a calibrated DC motor , the calibration should be performed under generator conditions. During the test, the direction of rotation and excitation current of the DC motor should be the same as during calibration. The excitation current should remain constant throughout the test.
●Explanation of test methods
With a specified load applied to the test motor , run it until the temperature rise stabilizes or after a specified time (when using the latter method, the difference between the temperature reached by the test motor windings and the actual temperature reached after the temperature rise stabilizes should not exceed 5K). Adjust the load within the range of 1.5-0.25 times the rated power, and measure two operating characteristic curves when the load decreases and increases. Take no fewer than 6 readings on each curve, each reading including: three-phase line voltage (which should be maintained at the rated value), three-phase line current I1 (A), input power P1 (W), speed n (r/min), and output torque T (N·m). If possible, also record the output power P2 (W). Finally, disconnect the power and stop the machine. Within a specified time (as specified in the standard), measure the DC resistance R1 (Ω) of the stator windings; otherwise, extrapolate and correct according to the relevant regulations for obtaining the thermal resistance after the thermal test. When possible, prioritize using live measurement (superposition method) or pre-embedded thermocouples or copper (platinum) resistance thermometers in the windings to obtain the temperature or resistance of the stator windings at each point. During the experiment, the ambient temperature θ (°C) should also be recorded.
● Methods for processing test results
(1) Output torque correction
The output torque values at each point measured by the dynamometer should be supplemented by the torque value consumed by the dynamometer's wind friction to obtain the output torque of the tested motor, as shown in equation (1).
TC=T1+Tfw………………(1)
In formula (1):
TC — Corrected motor output torque (N·m):
T1—Torque value (N·m) displayed by the dynamometer during the test;
Tfw—the wind-induced friction loss torque (N·m) of a dynamometer, which is obtained according to formula (2):
In formula (2):
P1——Input power (W) of the motor when driving the dynamometer at rated voltage. At this time, the armature and excitation circuits of the dynamometer should be open.
Td—The torque value of the dynamometer during the wind friction loss torque test (Nm);
P0—No-load loss of the motor (W);
nt—Motor speed (r/min) during windage loss torque test.
(2) Output speed correction
The temperature of the cooling medium during the test should be converted to 25℃. At this time, the speed of the motor should be converted according to formula (3).
In formula (3):
nref — Rotational speed (r/min) converted to a cooling medium temperature of 25°C;
nt—The rotational speed (r/min) measured during the test;
Δθ2 — Rotor temperature rise (K) during the test. If it cannot be measured, the stator temperature rise can be used instead.
θt — Temperature of the cooling medium during the test (°C):
ns—Synchronous speed of the tested motor (r/min);
K – a calculation constant; 235 for copper conductors and 225 for aluminum conductors.
(3) Stator winding loss correction
The correction amount ΔPcu1(W) for the stator winding loss I2R is calculated according to formula (4):
In equation (4):
It—Stator current during the test (A);
Δθ1 — Stator temperature rise during the experiment (K);
Rt——Stator line resistance (Ω) during the test.
(4) The corrected output power of the tested motor
The corrected output power P2 (W) of the tested motor is calculated according to formula (5).
(5) Calculate the efficiency of each measuring point
The efficiency η (%) at each test point is calculated using the following formula, as shown in formula (6).
(6) Plotting the efficiency curve and determining the full-load efficiency
Plot the efficiency-output power relationship curve η=f(P2) using the efficiency values calculated above. For tests that produce two curves for load increase and decrease, an "average curve" should also be plotted. Find the efficiency value corresponding to the rated output power P2=PN of the tested motor from the average curve; this is the actual full-load efficiency value of the motor.
●The method for processing test results and some viewpoints as specified in GB/T755.2.
GB/T755.2 stipulates that "when a motor is running at its rated speed, rated voltage and rated current, the ratio of output to input is the efficiency. The test should be carried out at the temperature reached at the end of the rated time as close as possible to the temperature reached at the end of the specified time, and the winding resistance does not need to be corrected for temperature."
The standard does not address testing at other current points, efficiency calculations, or plotting the efficiency-output power curve. Furthermore, the two data points used for efficiency calculations are obtained "under rated speed, rated voltage, and rated current." Literally, this can be interpreted as the efficiency at rated power.
However, in actual tests, the motor's output power under the above operating conditions may not be the rated value (and of course, the input power will not be the rated value either), so the efficiency obtained from this will not necessarily be the value at the rated power. This difference can sometimes be quite large in some prototype products.
The biggest advantage of the "input-output method" is that it directly provides the rated output power or torque. However, if efficiency is not directly calculated, this advantage diminishes. Therefore, "when the motor operates at rated speed, rated voltage, and rated current" should be changed to "when the motor operates at rated voltage, rated frequency, and rated output power (or rated torque)." Additionally, the standard stipulates that resistance does not require temperature correction, simplifying the calculation. However, this may cause some differences in test values under different ambient temperatures.
