A brief analysis of the application of variable frequency technology in air compressor systems

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I. Operating principle and analysis of air compressors: The company I work for is a specialist in distributing imported brand air compressors; as for the main equipment currently in use, they are mostly traditional screw-type air compressors. An air compressor is a load with a large moment of inertia. Its structure consists of a pair of parallel, meshing male and female rotors that rotate within the cylinder, causing periodic changes in volume between the rotor teeth and grooves. Air is then transported along the axis of the rotors from the intake side to the output side, thereby completing the entire process of suction, compression, and exhaust in a screw air compressor. There are two types of models that primarily drive the rotation of the compressor’s male and female rotors: one is driven by a diesel engine through a coupling and gear engagement, and the other is driven directly by an electric motor through gear engagement. The main reason why it is difficult to achieve energy savings is that the operating conditions of diesel engines are influenced by their speed and power output curves. Here, the electric air compressor is analyzed in detail, while the energy savings of diesel engines will be discussed later. II. Operational Analysis of General Electric Air Compressors 1. Power Source of the Air Compressor The power for the air compressor comes from an AC asynchronous motor, and the energy of this motor is derived from electrical power. To understand the relationship between the motor and electrical power, it is necessary to consider the principles of rotation; the formula for its speed is: n=60f(1-s)/p, where n represents the speed of the motor ； f is the power supply frequency ； P is the number of pole pairs in the motor ； s is the slip rate. Under power-frequency conditions, motors are generally started from a static state using a star-delta starting switch, and it takes about ten seconds for them to reach their rated operating speed. During the starting process here, the motor, the start switch, and the wires must withstand a current that is nearly seven times the motor’s rated current. Taking the P100 air compressor as an example: its motor power is 75 KW, it operates at 3-phase 380V, with a rated current of around 150 A; it has four poles (with 2 pole pairs), and a slip rate of about 2%. Using the formula above: Asynchronous speed = 60 × 50 Hz × (1 – 2%) / 2 = 1470 r/min. It takes approximately 15 seconds for the speed to increase from 0 r/min to the rated speed of 1470 r/min. The starting current is seven times the rated current (that is, 150 × 7 = 1050 A), and this current gradually decreases to 100 A (when the intake valve is closed, the load rate is around 66%). Although the current appears to be decreasing at this point, there is a phenomenon where excess power is being used – in other words, the reactive power increases. Electrical consumption = active power + reactive power. On the other hand, given that air compressors often operate at full load for extended periods of time, a larger margin is generally required when designing the motor drive power, and their energy consumption cannot be ignored. 2. Problems with the loading and unloading methods of air compressors: The control method for loading and unloading is based on the intake valve control mechanism – the intake valve is closed when the pressure reaches its upper limit, causing the compressor to operate at reduced load; the valve opens when the pressure drops to the lower limit, allowing the compressor to operate at full load. It can be seen that, under normal operating conditions, the speed of the motor in an air compressor does not change as pressure varies; in other words, the motor operates at its rated maximum speed, while the level of load it bears depends on whether the intake valve is open or closed. (1) Energy consumption analysis of the motor: The loading and unloading control mechanism causes the pressure of the compressed gas to fluctuate between an upper limit and a lower limit. The lower limit is the minimum pressure value, which is the lowest pressure required to ensure the user can work properly. Under normal circumstances, the relationship between Pmax and Pmin can be expressed by the following formula: Pmax = (1 + δ)Pmin, where δ is a percentage with a value roughly between 15% and 30%. In air compressors operating under load and unload air supply control modes, the energy wasted is primarily in two areas: a. Electrical power consumption during loading; once the pressure drops to its minimum value, due to the nature of the control mechanism, the pressure continues to rise until it reaches its maximum value. During the pressurization process, a large amount of heat must be released to the outside, resulting in energy loss. On the other hand, before the gas at the upper pressure limit enters the (user) pneumatic components, its pressure must be reduced using a pressure reducing valve, and this process is also energy-consuming. b. Power consumption during unloading: When the pressure reaches its maximum value, the air compressor unloads by reducing the pressure – the intake valve is closed to keep the motor under light load, while the excess compressed air in the separation tank is released through the vent valve. This adjustment method results in significant energy waste. It is estimated that, in operating conditions where the unloading time accounts for a small proportion, the energy consumption of the air compressor during unloading is approximately 10% to 25% of its energy consumption when operating at full load. In other words, the air compressor spends 20% of its time idling, doing no useful work. Based on this working principle, it is easy to see that there is significant potential for energy savings. (2) Equipment wear: The air intake volume is regulated by a cylinder-controlled butterfly valve, which prevents the supply pressure from remaining stable. As the gas consumption changes continuously, the supply pressure fluctuates accordingly. Therefore, components with high gas precision cannot meet the requirements of the production process. It also accelerates the wear of the cylinders, intake valves, and exhaust valves, increasing the amount of maintenance required and the associated costs, thereby compromising the safety of the equipment. In summary, during its operation, in addition to consuming energy, air compressors also suffer from the following issues: 1. Due to the large amount of electrical power required to start the motor, high requirements are placed on the power supply conditions. When the grid voltage is unstable or insufficient, the starting current increases significantly, preventing the unit from starting properly or forcing it to stop operating, which results in unnecessary economic losses ； 2. The reactive power loss generated by inductive loads increases the motor’s temperature rise, and harmonic interference has an adverse effect on the stable operation of the power grid ； 3. Under the influence of high currents, the wires overheat due to excessive current, which accelerates their aging ； 4. Under the influence of high currents, the arc generated by the switch reaches high temperatures, which causes severe ablation of the switch contacts, leading to their melting and loss of the switch’s functional capacity. In severe cases, this can result in the contacts sticking together and causing a short circuit, thereby leading to accidents ； 5. Under the influence of high starting currents, the force generated by the motor causes increased wear on the bearings. The instantaneous overcurrent in the stator windings leads to excessive heating, which reduces the motor’s power and performance; in severe cases, it can even cause the motor to burn out ； 6. The strong impact forces generated during startup can increase the wear on the coupling gears of the air compressor as well as other mechanical components, and the resulting damage cannot be underestimated. In summary, all of these six issues will directly affect the performance of the unit and shorten its service life. III. Energy-saving analysis of the air compressor system As mentioned earlier, in a system for supplying air, the fundamental parameters are the flow rate and pressure of the gas. In traditional systems, air compressors must adjust the opening and closing of the intake valve while operating in order to meet the user’s air supply requirements and achieve a desired gas flow rate. The intake valve is controlled by a pressure difference relay, which operates within a certain range defined by upper and lower pressure limits; there is a difference of several kilograms of pressure between these limits. Therefore, pressure changes are large and frequent. In gas supply with a continuously changing flow rate under pressure, the motor always operates at its maximum rated asynchronous speed. Since it is not possible to adjust the motor’s output power, the motor does not reduce its energy consumption as the load decreases; thus, there is little change in its energy consumption, and essentially no energy-saving effect is achieved. The frequency converter alters the working principle of traditional air compressors; it utilizes functions such as soft starting, elimination of harmonic interference, and reactive power compensation, along with the ability to adjust frequency smoothly, in order to regulate flow and pressure in a controlled manner. It replaces valve-based control methods by adjusting the speed of the motor, thereby ensuring that the supply of air matches the requirements in terms of flow, pressure, and power. At the same time, it reduces wear and the damage caused by shocks during startup to the equipment. Inverters not only save energy effectively but also overcome the six aforementioned problems caused by traditional air compressors. Of course, in addition to reducing the starting current, the energy-saving effect of frequency converters is conditional: the operating load rate of the unit must be below the rated gas flow rate, and the greater the variation in gas consumption, the more significant the energy savings. IV. Working principle of energy saving in air compressor systems
Using an inverter to control the speed of the air compressor is a relatively scientific method for achieving energy savings. Based on the characteristics of variable-frequency operation of air compressors, and through scientific analysis, it has been established that: Q1/Q2 = n1/n2, H1/H2 = (n1/n2)², P1/P2 = (n1/n2)³. Here, Q represents the air flow rate supplied by the air compressor ； H is the pipeline network pressure ； P is the power consumed by the motor ； n is the compressor speed. As can be seen from the above formula, when the motor speed drops to 80% of the rated speed, the flow rate supplied by the air compressor to the pipeline network decreases to 80%, the pressure in the pipeline network drops to (80%) squared, and the power consumed by the motor drops to 80% cubed; thus, the energy savings amount to 51.2%. After accounting for mechanical losses of the motor as well as copper and iron losses, the actual energy savings efficiency is around 40%. Therefore, speed regulation is where energy savings lie. V. Frequency conversion transformation and application of air compressors, selection schemes 1. Since air compressors are loads with high rotational inertia, their startup can easily cause the frequency converter to activate its overcurrent protection mechanism. Therefore, a speed-sensorless vector frequency converter should be used; it ensures the continuous operation of the unit while also guaranteeing stable and reliable performance of the equipment ； 2. Air compressors should not be operated at low frequencies for extended periods of time; due to the low speed, their operational stability is poor, and the lubrication of the compressor’s cylinders deteriorates, which increases mechanical wear. It is advisable to maintain a frequency of no less than 20 Hz ； 3. In factories and mines equipped with fixed machinery, frequency converters with AC reactors for output should be used to reduce interference caused by higher harmonics ； 4. For some customers with high gas requirements, a constant-pressure gas supply method is adopted ； 5. It is advisable for the control cable not to exceed 30M in length