Energy saving of electric motors driven by frequency converters

1 Introduction

In recent years, environmental protection represented by reducing the global warming effect (reducing CO2 emissions) and various measures to cope with the depletion of energy such as oil have promoted the development of energy conservation on a global scale, and the energy-saving intention of motor inverter drive has increased. In China, with the formulation and strengthening of the Energy Conservation Law, the increasing number of factories and enterprises that have obtained ISO4001 certification, and the promotion of energy use rationalization plans, the demand for energy conservation has increased. On the other hand, about 70% of the electricity used in factories is consumed by motors, so the demand for high-efficiency motors is also increasing. In particular, inverter drive systems that combine inverters and motors into variable speed control, centered on fans and pumps, are becoming widely popular. This article explains the trend of motor high-efficiency technology, energy conservation caused by motor inverter drives, and related points of attention.

2. Improving the efficiency of electric motors

The electric motor is a device that converts input electrical energy into rotational mechanical energy. The purpose of improving the efficiency of the electric motor is to reduce the loss generated during this energy conversion process. The efficiency is defined as follows:

There are five types of motor losses: primary copper loss, secondary copper loss, iron loss, mechanical loss, and stray load loss, as listed in Table 1.

Table 1 Definition of loss

The motor driven by the inverter can be divided into permanent magnet motor (IPM) and induction motor. IPM means internal permanent magnet motor, also known as high-efficiency synchronous motor. Regarding the structure of the motor, the comparison of the rotor structure of the induction motor and IPM is shown in Figure 1.

IPM is a structure that installs permanent magnets inside the rotor. The built-in permanent magnets generate magnetic flux, so no excitation current is required, which reduces the primary copper loss. As a result, IPM can improve efficiency by up to about 10% compared to induction motors. IPM is a low-loss motor that can reduce heat capacity, so it is smaller and lighter than induction motors.

Induction motors are widely used in factories and enterprises because they have no permanent magnets, are easy to maintain, and have a strong structure. To improve the efficiency of induction motors, it is necessary to reduce the various losses listed in Table 1.

Primary copper loss accounts for a large proportion of the loss. By changing the winding method to shorten the length of the wire and using high-density filling winding technology (to increase the performance ratio), copper loss can be reduced. In addition, the re-examination and design of the rotor slot shape can reduce the secondary copper loss during rated operation. In addition, due to the popularity of low-loss, high-magnetic density core materials, the use of them can reduce iron losses. The optimized combination of stator and rotor slots, the optimized design of air gap length and rotor inclination can reduce stray load losses.

Compared with general-purpose motors, high-efficiency motors can reduce losses by (20~30)%. Reducing cooling air volume and using small-diameter fans can also reduce ventilation losses.

3. Frequency Converter Principle

The motor rotation speed is defined as follows:

In the formula: n is the motor speed (rpm); f is the frequency (HZ); p is the number of motor poles; s is the slip rate unique to induction motors, which indicates the ratio of the lag behind the synchronous speed. Under rated conditions, s ≈0.05.

From formula (2), we can know that changing the motor speed n can be achieved by changing the number of poles p of the motor or changing the frequency f. The frequency converter is a device that can adjust its output voltage frequency arbitrarily, allowing the three-phase AC motor to run at any speed and achieve stepless speed regulation.

Figure 2 shows the structure of the frequency converter. The frequency converter is mainly composed of a converter that rectifies the industrial frequency power supply into DC and an inverter that converts DC into AC of any frequency. In addition, the converter part is composed of a rectifier for three-phase full-wave rectification, a smoothing capacitor for stabilizing the pulsating component, and a control circuit that suppresses the inrush current when the smoothing capacitor is charged. The DC converted by the converter part is used to generate AC in the inverter part by means of pulse width modulation (PWM). It seems that in order to change the speed of the motor, it is better to change the frequency only, but the voltage remains constant. If the output frequency is below 50HZ, as the magnetic flux of the motor increases and reaches saturation, the motor will overheat due to the increase in current and eventually burn out.

To avoid this phenomenon, the magnetic flux must be kept constant. The magnitude of the magnetic flux is defined as follows:

From equation (3), we can see that the magnetic flux is proportional to the voltage and inversely proportional to the frequency. This relationship must always remain constant. The ratio of the inverter output voltage to the output frequency is called the mode. This relationship is an important factor in controlling the motor.

4 Energy saving examples

When controlling the speed of an installed chilled water pump, the method of using a frequency converter is simple, easy and economically advantageous.

As a specific example, the cooling water pump system for building air conditioning increases or decreases the circulation of cold water by changing the heat load. Correspondingly, only the output valve is used to adjust the pressure for the pressure change, resulting in large pressure loss and poor efficiency.

If the speed of the cooling water pump is controlled to maintain the optimal pressure, there will be no pressure loss due to decreased efficiency, and energy saving can be achieved.

The composition and structure of the system is shown in Figure 3. When the output water volume is below 2500L/min, it is operated by a 75kW motor; when the output water volume exceeds this, two 150kW motors are used, one of which is usually in operation, and the output valve is adjusted according to the change of heat load to increase or decrease the circulation of cold water.

Here (in the figure), the 75kW motor is stopped. Corresponding to the two 150k cooling water pumps for normal use and standby, one inverter is set to switch the operation of either of the two cooling water pumps. In addition, for the running cooling water pump, the water pressure of the top layer is detected, and the pressure is kept constant by PID adjustment metering instrument to control the speed.

In the case of water pumps, the smaller the ratio of the actual range to the full range, the greater the energy saving effect. That is, according to the relationship between flow rate and motor input shown in Figure 4, for example, at 50% flow rate, the speed of the chilled water pump is controlled by the inverter drive, and the input power of the motor may be reduced to less than half compared with the control of the output valve.

Table 2 lists the operation mode and energy saving effect of the cold water pump system for building air conditioning in one year. In the above-mentioned introduction example, the electricity consumption can be reduced by 49,200 kWh per year. The electricity cost is 0.8 yuan per kWh, and the CO2 reduction per 1 kW is 0.000422 tons. The electricity bill can be saved by nearly 40,000 yuan per year, and the CO2 reduction is 20.76 tons.

For buildings built more than 30 years ago, most of them use central air conditioning, so the introduction of inverters can achieve great power saving effects. However, when achieving the expected energy saving, the operating conditions of the equipment must be carefully investigated and studied in advance.

Table 2 Operation mode

5. Points to note when using a frequency converter to drive an installed motor

When the motor is driven by a frequency converter, compared with the case of driving with a sine wave (industrial frequency power supply), attention must be paid to the temperature rise of the motor and the surge voltage of the frequency converter due to the influence of higher harmonics contained in the frequency converter output waveform.

5.1 Temperature rise of the motor

The life of the insulation is shortened by about half when the temperature rises by 10°C, so the temperature rise of the motor is a very important issue. When the motor is driven by a frequency converter, the loss increases due to the influence of high-order harmonics. Compared with the general drive with industrial frequency power supply, the current increases by about 10% and the temperature rise increases by about 20%.

The following discusses the problem of reduced cooling effect during low-speed operation. When a cooling fan is installed at the end of the motor rotor shaft, the motor speed is low during low-frequency operation, and the cooling effect is greatly reduced. Generally, the relationship between the motor temperature rise and the cooling effect caused by the cooling air volume is that when the motor loss is the same, the temperature rise △t is inversely proportional to the speed n:

On the other hand, when the motor is operated above the power frequency, the output voltage of the inverter is controlled to be constant, so the motor has a constant output power characteristic. At this time, the motor current decreases with the increase of frequency, and the cooling effect is also improved, so the problem of temperature rise is not big. However, the maximum allowable speed value is limited by the allowable speed of the bearing, the strength of the rotating part, noise, vibration and other conditions.

5.2 Surge voltage of inverter

The inverter power supply generates surge voltage during commutation operation. Therefore, a surge voltage with a certain alternating period, which depends on the inverter frequency and control mode, is applied to the motor coil, which has a great impact on the insulation of the coil.

In general-purpose inverters, the voltage builds up rapidly. Due to differences in motor capacity, winding methods, etc., when a voltage is applied to the motor, the voltage distribution between the coils is such that the voltage on the first coil near the power supply is higher. Therefore, the insulation strength and coordination between the coils must be ensured.

Once the inverter rectifies the industrial frequency power supply into DC, the peak value of the output voltage is usually lower than the DC voltage E because it uses switch control (the DC voltage E is a certain multiple of the effective value of the industrial frequency power supply voltage, such as approximately DC620V when AC440V, and its multiple is 1.4).

The inductance (L) of the wiring between the inverter and the motor, the stray capacitance (C) between the wiring, and the surge voltage generated by LC resonance during switching will be superimposed on the output voltage of the inverter, and the result is shown in Figure 5. Compared with the peak output voltage of the inverter, the terminal voltage on the motor input side has increased. The terminal voltage peak of this motor theoretically reaches twice the maximum circuit voltage (peak inverter output voltage) (620 2=1240V), that is, due to the difference in switching speed and wiring length, the generated voltage is also different. According to its principle, especially
in PWM inverters, surge voltage is inevitable.

Figure 6 shows an example of actual measurement of the motor input terminal voltage corresponding to the wiring length between the 400 series inverter and the motor. As can be seen from Figure 6, the motor terminal voltage increases as the wiring length increases. It can be confirmed that saturation is reached when the inverter output voltage is about twice. In addition, the IGBT with a faster switching speed has a higher terminal voltage of the motor even if the wiring length is short. It can also be confirmed that the saturation voltage is roughly the same when the wiring length increases.

Below, we introduce the energy-saving effect of inverter drive in the case of installed chilled water pumps. Generally, the insulation life of motors is about 40,000 hours. It cannot be generalized according to the use environment and conditions. The use time of the motor is calculated as 8 hours a day, and the insulation life is roughly 15 years. In addition, for the installed motors, most of them have not yet taken measures to deal with the surge voltage of the inverter. In particular, when the 400V-class motor is converted to inverter drive, the surge voltage of the inverter will cause insulation degradation and burnout. Therefore, when introducing inverter drive, it is recommended to discuss the replacement of the motor at the same time.

References

1 Achiwa Norihiro. "Provincialization of the Province", "Provincialization of the Province" 2009.NO.11.P39-43.

2 Yasuhisa Seki, Toshiaki Idemitsu, Atsushi Koga, Masaki Nakai, Koji Iwshashi. Fan, Pump and Compressor Applications. "Yaskawa Electric" 2009.NO.02.P66■