Method B, also known as the "Input-Output Loss Analysis Method," is an efficiency testing method required for motor energy-saving certification (primarily targeting product energy efficiency limits, as stipulated in the GB18613 standard). Ms. Can will discuss Method B testing with you today.
Explanation regarding Method B test
When using Method B to determine efficiency, several sets of input current, input power, output power (or torque), speed, core loss, and mechanical loss values need to be obtained under rated voltage and rated frequency through direct load tests and no-load tests. Then, relevant corrections are made to the test values, and the corrected input power and output power are calculated. Finally, the efficiency at each test point is calculated by comparing the output power with the input power, and the relationship curve between efficiency and output power is plotted. The efficiency value at rated output power is found on this curve, which is the "full-load efficiency".
Typically, the relevant data obtained during the calculation process at rated voltage and frequency are also used to plot the relationship curves of input current, input power, output torque, slip (or speed) to output power in the same coordinate system as the efficiency curve mentioned above. This is what is referred to as the " motor operating characteristic curve".
The source and processing of the required data
When using this method, the key is to obtain five loss values under several different load conditions at rated voltage and frequency through relevant tests, namely stator copper loss, rotor copper loss, core loss (referred to as "iron loss"), mechanical loss and stray loss.
●Stator copper loss Pcu1
The stator copper loss Pcu1 is calculated using formula (1) and corrected to the value at an ambient temperature of 25℃. The stator resistance RN is the value measured immediately after the end of the thermal test; θN is the hot temperature of the winding; θt is the ambient temperature at the hot state; K is a constant, which is taken as 235 for the copper winding.
●Rotor copper loss Pcu2
The rotor steel loss Pcu2 is calculated using equation (2). The slip is corrected to the specified temperature.
Pcu2=s(P1-Pcu1-PFe)………………(2)
●Core loss PFe
In Method B, the core loss PFe is obtained from the no-load test. It is considered to be a quantity that depends only on the stator voltage. During the load test, the stator voltage is constant at its rated value, so the core loss at each current point is assumed to remain unchanged.
Method B1 assumes that core loss is mainly related to stator voltage, but also to stator current and input power. First, a voltage is calculated using the stator current l1, stator winding resistance R1, and input power P1 according to the following formula. Then, the core loss corresponding to Ub is obtained through the relevant curves of the no-load test.
GB/T1032 specifies that Ub is calculated according to formula (3):
In formula (3)
U—Stator voltage (V) during load test;
I1—Stator current (A) during load test;
Rt——CDC resistance of stator winding when temperature rise is stable (Ω);
P1—Input power (W) during load testing.
Based on the above information and the relationship cos2ø+sin2ø=1, a simplified formula for equation (4) can be obtained.
●Mechanical loss Pm
Mechanical loss Pm is obtained from the no-load test and is a quantity related to rotational speed. During the load test, the rotational speed changes little at each current point, so it can be considered a metric.
● Stray loss Ps
The results were obtained through experimental measurements using the input-output method and determined by linear regression curves.
