Factors affecting motor loss

Update:26-10-2019
Summary:

Input voltage changes. Compared with the rated voltage […]

Input voltage changes. Compared with the rated voltage of the motor, the input voltage will have different degrees of deviation during actual operation. When the operating voltage is slightly lower than the rated voltage (but not less than 10%, between 2-3%), the motor is guaranteed to operate normally. At the same time, the motor loss is small; on the contrary, when the motor running voltage is higher than the rated voltage, the motor loss will increase. If it exceeds too much, the motor current will increase too much or even cause burnout.
Uneven voltage loss. If the input voltage is unbalanced, the asynchronous motor current will be uneven. The torque of the motor is reduced due to the presence of a negative sequence current.
The power frequency changes. For the fan pump type load, the shaft torque is proportional to the square of the speed. After the frequency is reduced, the speed decreases, the torque also decreases, and the stator and rotor currents decrease. Therefore, the motor efficiency is improved, and the shaft power is large. As the amplitude decreases, the input power of the motor also drops drastically, so the fan pump type load adopts frequency conversion speed regulation, and good energy saving effect can be obtained at low speed.
Non-sinusoidal waveform power supply. Most inverter output voltage waveforms are non-sinusoidal, generating harmonic currents and harmonic magnetomotive forces in asynchronous motors, which inevitably lead to an increase in motor stray losses.
Motor starting and stopping losses. Some loads require intermittent operation, or even frequent start-ups. This will cause the stator windings to be subjected to frequent impacts. The squirrel-cage rotor will also generate thermal stress due to uneven heating. Repeated starting will cause the rotor bars to break. When starting, the motor generates more heat and the heat dissipation condition is worse than steady state operation. Multiple starts will also cause the motor to overheat. Therefore, the number of starts and the time interval are specified within the specified time. The use of high rotor resistance motors can reduce the starting current of the stator and rotor, so it can reduce the impact of energy consumption and current impact.

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