The energy consumption of motors is mainly reflected in the following aspects.
First, the motor load rate is low. Due to improper motor selection, excessive margin, or changes in production processes, the actual working load of the motor is far less than the rated load. Approximately 30% to 40% of the installed capacity of motors operate at 30% to 50% of their rated load, resulting in excessively low operating efficiency. Second, the power supply voltage is unbalanced or too low. Due to the imbalance of single-phase loads in a three-phase four-wire low-voltage power supply system, the three-phase voltage of the motor is unbalanced, causing the motor to generate negative sequence torque, increasing losses during operation. In addition, the long-term low grid voltage causes the motor to have a higher current during normal operation, thus increasing losses. The greater the three-phase voltage imbalance and the lower the voltage, the greater the losses. Third, old and obsolete motors are still in use. These motors use Class E insulation, are large in size, have poor starting performance, and low efficiency. Although they have undergone years of upgrades, they are still used in many places. Fourth, maintenance and management are inadequate. Some units do not maintain and repair motors and equipment according to requirements, allowing them to operate for a long time, resulting in continuous increases in losses. Therefore, it is worthwhile to study which energy-saving solutions to choose based on these energy consumption patterns.
1. Select an energy-saving motor
Compared to ordinary motors, high-efficiency motors have optimized overall design, utilize high-quality copper windings and silicon steel sheets, and reduce various losses by 20% to 30%, while increasing efficiency by 2% to 7%. The investment payback period is generally 1 to 2 years, sometimes even a few months. Comparatively, high-efficiency motors are 0.413% more efficient than the J02 series motors. Therefore, replacing older motors with high-efficiency motors is imperative.
2. Select appropriate motor capacity to achieve energy saving.
The state has stipulated the following three operating zones for three-phase asynchronous motors: an economic operating zone with a load rate between 70% and 100%; a normal operating zone with a load rate between 40% and 70%; and an uneconomical operating zone with a load rate below 40%. Improper motor capacity selection will undoubtedly lead to a waste of electrical energy. Therefore, using a suitable motor to improve the power factor and load rate can reduce power loss and save electrical energy.
3. Replace the original slot wedge with a magnetic slot wedge.
Magnetic slot wedges primarily reduce no-load iron losses in asynchronous motors. No-load additional iron losses are generated in the stator and rotor cores by harmonic flux induced by the cogging effect within the motor. The high-frequency additional iron losses induced in the stator and rotor cores are called pulsation losses. Additionally, the stator and rotor teeth sometimes align and sometimes misalign, causing fluctuations in the magnetic flux of the tooth clusters, which can induce eddy currents in the tooth surface layer, generating surface losses. Pulsation losses and surface losses are collectively called high-frequency additional losses, accounting for 70%–90% of the motor's stray losses. The remaining 10%–30% is called load-related additional losses, generated by leakage flux. Although using magnetic slot wedges reduces starting torque by 10%–20%, motors using magnetic slot wedges can reduce iron losses by 60kJ compared to motors using ordinary slot wedges, and are well-suited for retrofitting motors that require no-load or light-load starting.
4. Adopt a Y/△ automatic conversion device
To address the energy waste caused by lightly loaded equipment, a Y/Δ automatic transfer switch can be used to save power without replacing the motor. This is because different load connections in a three-phase AC power grid result in different voltages, thus affecting the energy drawn from the grid.
5. Power factor and reactive power compensation of the motor
Improving the power factor and reducing power loss are the main objectives of reactive power compensation. The power factor is equal to the ratio of active power to apparent power. Generally, a low power factor results in excessive current. For a given load, with a constant supply voltage, the lower the power factor, the greater the current. Therefore, a higher power factor is preferable to save energy.
6. Variable frequency speed control
Most fan and pump loads are selected based on full-load operating requirements, but in actual applications, they are not operated at full load most of the time. Because AC motor speed regulation is difficult, baffles, return valves, or start-stop times are commonly used to adjust airflow or flow rate. Furthermore, frequent starting and stopping of large motors at power frequency is challenging, resulting in significant power surges and inevitably causing energy losses and current spikes during start-up and shutdown. Using frequency converters to directly control fan and pump loads is the most scientific control method. When the motor operates at 80% of its rated speed, energy efficiency approaches 40%, and closed-loop constant voltage control can be achieved, further improving energy efficiency. Since frequency converters can achieve soft stop and soft start for large motors, they avoid voltage surges during startup, reduce motor failure rates, extend service life, and also reduce grid capacity requirements and reactive power losses.
7. Wound-rotor motor with liquid speed regulation
Liquid resistance speed control technology is developed based on traditional liquid resistance starters. It still achieves stepless speed regulation by changing the resistance value of the electrode spacing. This gives it good starting performance. However, its continuous operation leads to heat generation issues. Due to its unique structure and reasonable heat exchange system, its operating temperature is limited to a reasonable range. Liquid resistance speed control technology for wound-rotor motors has been rapidly adopted due to its advantages such as reliable operation, convenient installation, significant energy savings, easy maintenance, and low investment. For wound-rotor motors with low speed control accuracy requirements, narrow speed range requirements, and infrequent speed adjustments, such as large and medium-sized wound-rotor asynchronous motors in equipment like fans and water pumps, liquid speed control is particularly effective.
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