The performance of an electric motor is closely related to the characteristics of its frequency converter. The motor should be powered by a suitable frequency converter and tested at the same carrier frequency. In principle, the test power should be provided by a frequency converter that is compatible with the actual operation of the motor under test; otherwise, the difference between the test results and the performance data during field operation should be considered.
The test items for AC variable frequency speed control motors are specified in CB/T22670 and JB/T7118. Most of the test methods and requirements are basically the same as for ordinary asynchronous motors, but there are some specific requirements. Ms. Can has summarized the main points and is sharing them with everyone.
The test items for this type of electric motor are the same as those for ordinary electric motors, but the requirements for some aspects are increased. These items include: no-load test, locked rotor test, load test and thermal test.
(a) No-load test
The variable frequency drive (VFD) powered speed control motor should be tested under both sinusoidal power supply and VFD power supply at the same fundamental frequency.
●When testing under a sinusoidal power supply, the method and procedures for conducting no-load tests at the fundamental frequency, as well as the relevant regulations, are exactly the same as for ordinary motors.
● During the test under the inverter power supply, the motor under test is subjected to the rated voltage at the reference frequency, and the three-phase no-load line voltage Ub0, line current lb0, input power Pb0, and stator winding line resistance Rb0 are measured.
● Determination of core loss Pfe and mechanical loss Pm. (1) The calculation, plotting of curves and separation of mechanical loss, etc., of core loss Pfe and mechanical loss Pm under sinusoidal power supply are the same as those for ordinary motors. (2) For motor harmonic loss Pbh under voltage-type frequency converter power supply, it is calculated according to formula (1).
(ii) Stall test
● Rated frequency stall test. Powered by a sinusoidal power supply, all contents are the same as those of a regular electric motor.
●Low-frequency stall test. This test is only performed when calculating motor performance using the circle diagram method or equivalent circuit method. The relevant regulations are the same as for ordinary three-phase asynchronous motors.
● Starting torque test under inverter power supply. The test frequency and maximum starting current shall be as specified in the product standard or the agreement between the manufacturer and the customer. During the test, the inverter parameters shall be set as specified, and the inverter shall apply voltage to the motor to ensure that the rotor is stationary. The motor torque and the DC resistance of the stator winding shall be measured.
(iii) Thermal test
The "Standard Rating Thermal Test" clause in GB/T22670 is entirely the same as the thermal test for ordinary electric motors. However, the thermal test content specified in JB/T7118 differs from that for ordinary electric motors.
● Thermal test at 50Hz. First, start the cooling fan and adjust the inverter's output frequency to 50Hz. Run the motor under load at rated torque. After the temperature rise stabilizes to the specified time or cycle, stop the machine (but the cooling fan should continue to run) and measure the hot DC resistance of the windings and the temperature of bearings and other components. Calculate the relevant temperature rise and temperature values using methods for ordinary motors.
● Motor heating test at 5Hz. After completing the heating test at 50Hz, immediately start the motor, adjust the inverter output frequency to 5Hz, and run the motor under load at rated torque. After the temperature rise stabilizes or reaches the specified time or cycle, stop the machine (but the cooling fan should continue to run) and measure the hot DC resistance of the windings and the temperature of bearings and other components. Calculate the relevant temperature rise and temperature values using the method for ordinary motors.
(iv) Load characteristic test
After the above heating test was completed, the test motor was restarted and operated under load. For a 50Hz motor, the inverter output frequency was adjusted to 3 (5)Hz, 15Hz, 30Hz, and 50Hz respectively. At each frequency point, the voltage, current, input power, output torque, and speed of the test motor at 100%, 110%, and 80% of its rated torque were measured. Subsequently, at frequencies of 60Hz, 80Hz, and 100Hz, the torque values of the test motor at 100%, 110%, and 80% of its nominal power were measured. Finally, the load characteristic curve of the test motor was plotted.
During the test, the motor should run smoothly without any obvious torque pulsation.
(v) Loss test
GB/T22670 "Determination of fundamental loss" is exactly the same as that for motors powered by ordinary power supply. It should be noted that the recommended value of stray loss is different depending on the power of the motor; the total loss value is calculated according to formula (2).
In formula (2):
PT—fundamental frequency loss;
Pbh — Harmonic loss.
(vi) Noise and vibration tests
The cooling fan is in operation. With the variable frequency power supply, measure the noise level or maximum effective vibration velocity at the rated frequency or the minimum and maximum frequencies specified in the product standard. The test methods and regulations are the same as for ordinary motors. JB/T7118 specifies that the maximum noise level of the motor should be measured under no-load conditions at three frequency points: 20Hz, 50Hz, and 100Hz (other frequency points may be added if required).
(vii) Overspeed test
GB/T22670 stipulates that during the test, the motor speed should be increased to 1.2 times the maximum operating speed or the speed specified in the standard for various types of motors, or the maximum speed. The motor should then run under no-load for 2 minutes.
JB/T7118 stipulates that after a 2-minute overspeed test with no-load operation, the motor with rated voltage of 4kW and below and frequency of 150Hz, or above 4kW and frequency of 120Hz, should not experience harmful deformation.
(viii) Insulation resistance, withstand voltage and impulse voltage withstand tests
GB/T22670 specifies the same test method as ordinary motors. The difference lies in the assessment standards or test voltage values specified in JB/T7118. The insulation resistance, AC voltage withstand test voltage value, and inter-turn impulse voltage withstand test voltage value (peak value) are all different from those of ordinary motors. For specific values, please refer to the standard.
