For motors connected to frequency converters, the maximum surface temperature shall be determined by test methods under the most unfavorable conditions
- Most unfavorable conditions
(1) Torque/speed characteristics
For motors used for variable torque loads, the maximum surface temperature shall be measured at the maximum power at the maximum rated speed; for motors used for linear loads and constant torque loads, the maximum surface temperature shall be measured at least at the minimum and maximum speeds; for motors used for complex loads, the maximum surface temperature shall be measured at least at the inflection point of the speed-torque curve.
(2) The maximum surface temperature shall be measured at the minimum and maximum speeds at constant power.
(3) Voltage drop
During the design and commissioning of the scheme, the voltage drop of all components needs to be considered. Therefore, information about the voltage drop of the frequency converter, filter, voltage drop along the cable, system configuration and frequency converter input voltage should be understood. The instructions prepared by the manufacturer in accordance with Chapter 30 of GB/T 3836.1-2021 "Explosive Atmospheres Part 1: General Requirements for Equipment" should provide all necessary relevant information to facilitate the calculation/setting of the operating range.
(4) Inverter output characteristics.
Low switching frequencies tend to increase motor temperature. Special operating conditions may be required to specify minimum switching frequencies; multilevel inverters (3 or higher) generally result in reduced motor heating.
(5) Coolant Maximum surface temperature measured with minimum rated flow/maximum rated coolant temperature; special operating conditions may be required to specify coolant requirements.
- Test methods
(1) Dedicated inverter motors should be tested with the intended inverter. If the inverter output voltage and the harmonic content of the output voltage waveform are effectively independent of ±10% input voltage variations while maintaining the rated motor input current (speed dependent) and voltage and frequency, the normal ±10% input voltage variation need not be applied.
(2) Similar inverters When there is sufficient information to determine similarity, the motor can be tested with similar inverters. Usually, additional safety factors are used to account for similarity as appropriate. If the inverter output voltage and the harmonic content of the output voltage waveform are effectively independent of ±10% input voltage variations while maintaining the rated motor input current (speed dependent) and the voltage and frequency ratio, the normal ±10% input voltage variation need not be applied.
(3) Sinusoidal voltage motors do not need to be tested with a similar inverter, but can be tested with sinusoidal voltage under all of the following conditions: the expected load torque is proportional to the square of the speed; the motor should be subjected to maximum load at rated speed; the motor speed range is between 40% and 100% of the maximum rated speed;
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