Abstract: This paper introduces the current status and composition of energy consumption of electrical equipment, and elaborates on the energy-saving principles of frequency converters and soft starters through calculations. It also illustrates the energy-saving advantages of frequency converters in fan and pump control, and the process by which soft starters effectively combat voltage spikes in the power grid.
Keywords: Inverter soft start energy saving
I. Introduction
With the accelerating pace of industrialization, the challenges of energy conservation and emission reduction are becoming increasingly severe, leading to the emergence of electronic products that can reduce industrial losses. The application areas of frequency converters and soft starters are expanding, and their use is becoming more frequent. In the control of motors and fans, variable frequency speed regulation and soft starting offer significant energy-saving effects.
II. Power Consumption Analysis
To ensure production reliability, all production machinery is designed with a certain margin of safety in its power drive system. Motors cannot operate at full load; beyond meeting the power drive requirements, excess torque increases active power consumption, resulting in energy waste. During production, due to the wide range of motor load variations, the flow rates of fans, pumps, etc., must be adjusted in real time. Currently, flow regulation is mostly achieved through regulating valves. This method merely changes the flow resistance of the channel, without significantly altering the motor's output power. Often, the motor runs at full speed, and the regulating valve controls production needs by throttling, essentially artificially increasing resistance to achieve the regulation purpose. This throttling method wastes a significant amount of energy. When airflow decreases and fan speed drops, the motor's input power decreases rapidly. For example, when airflow drops to 80% and speed n also drops to 80%, the shaft power drops to 51% of the rated power; if airflow drops to 50%, the shaft power will drop to 13% of the rated power, demonstrating significant energy-saving potential.
When a motor starts at full voltage, it draws seven times its rated current from the grid to meet the required starting torque. This large starting current wastes electricity, significantly impacts grid voltage fluctuations, and increases line and transformer losses. Excessive starting torque generates mechanical shock, causing significant stress on driven equipment, shortening its lifespan, and affecting accuracy. This can lead to damage such as couplings and belt tears. It also causes abnormal wear and impact on mechanical transmission components, accelerating aging, shortening their lifespan, and increasing maintenance workload. Hard starting of a motor severely impacts the grid and places excessive demands on grid capacity. The large current and vibration generated during startup cause significant damage to baffles and valves, severely impacting the lifespan of equipment and pipelines. Using a soft starter, however, utilizes the inverter's soft start function to start the starting current from zero, with a maximum value not exceeding the rated current. This reduces the impact on the grid and the demand on power supply capacity, extending the lifespan of equipment and valves and saving on equipment maintenance costs. When the motor starts, the starting current can gradually increase from 0 to the rated current of the motor, which reduces the impact of the starting current on the power grid, saves electricity costs, and also reduces the impact of starting inertia on the large inertia speed of the equipment, thus extending the service life of the equipment.
III. Variable Frequency Energy Saving Analysis
3.1 Working principle of frequency converter
The expression for the synchronous speed of an AC motor is as follows:
n=60 f（1-s）/p Equation (1)
In the formula, n — the speed of the asynchronous motor;
f — the frequency of the asynchronous motor;
s — Motor slip;
p — Number of pole pairs of the electric motor.
As shown in equation (1), the rotational speed n is directly proportional to the frequency f. Changing the frequency f will change the motor's rotational speed. When the frequency f varies within the range of 0–50 Hz, the motor's speed adjustment range is very wide. A frequency converter achieves speed regulation by changing the motor's power supply frequency, making it an ideal, highly efficient, and high-performance speed control method.
A frequency converter changes the speed of a motor by altering the frequency of the power supply; this is commonly known as variable frequency speed control. Frequency converters are broadly classified into two categories: AC-DC-AC and AC-AC converters. Currently, AC-DC-AC converters are more widely used. They consist of three parts: an inverter, an intermediate filter, and a converter. The inverter converts constant-voltage, constant-frequency AC power into adjustable DC power, providing DC power to the inverter through a voltage-source or current-source filter. The inverter converts the DC power into AC power with an adjustable frequency. Both the inverter and the inverter use thyristor three-phase bridge circuits. The filter, composed of capacitors or reactors, provides a stable voltage or current source to the inverter.
3.2 Energy-saving methods of frequency converters
3.2.1 Variable frequency energy saving:
To ensure production reliability, all production machinery is designed with a certain margin of safety in its power drive system. Motors cannot operate at full load; beyond meeting the power drive requirements, excess torque increases active power consumption, resulting in energy waste. When pressure is high, the motor's operating speed can be reduced to save energy while maintaining constant pressure. The change in motor shaft power P as the motor speed changes from N1 to N2 is as follows:
P=CN 3 ; Equation (2)
In the formula: P—host transmitter
N - Engine speed
C — constant
When the motor speed decreases from N1 to N2, its power also decreases from P1 to P2. The power change formula is (3).
Equation (3)
This shows that reducing the motor speed can achieve cubic-level energy savings.
If the pump efficiency is constant, when the required flow rate decreases, the motor speed N can decrease proportionally, and the shaft output power P decreases cubically. That is, the power consumption of the pump motor is approximately cubically proportional to the motor speed. Therefore, when the required flow rate Q decreases, the inverter output frequency can be adjusted to proportionally reduce the motor speed n. At this time, the motor power P will decrease significantly according to a cubic relationship, saving 40-50% more energy than adjusting baffles or valves, thus achieving the purpose of energy saving.
For example, a centrifugal pump motor with a power of 55 kilowatts consumes 28.16 kilowatts of electricity when its speed drops to 4/5 of the original speed, saving 48.8% of electricity. When its speed drops to 1/2 of the original speed, its power consumption is 6.875 kilowatts, saving 87.5% of electricity.
3.2.2 Dynamically adjust energy saving:
With the development of electronic technology and the maturity of frequency converter technology, frequency converters can quickly adapt to load changes and supply voltage at maximum efficiency. The variable frequency drive (VFD) has a software-defined monitoring and control output function of 5000 times/second to ensure the motor always operates at high efficiency.
3.2.3 Energy saving through the inverter's own V/F function:
While ensuring the motor's output torque, the V/F curve can be automatically adjusted. This reduces the motor's output torque and lowers the input current, achieving energy savings.
3.3.4 Improving power factor for energy saving:
An electric motor generates torque through electromagnetic interaction between its stator and rotor windings. Due to their inductive reactance, the windings exhibit an inductive impedance characteristic relative to the power grid. Consequently, the motor absorbs a significant amount of reactive power during operation, resulting in a very low power factor. However, by employing a variable frequency drive (VFD), the load characteristics change after rectification and filtering, as the VFD's performance transforms into AC-DC-AC. The VFD then exhibits a resistive impedance characteristic relative to the power grid, resulting in a high power factor and reduced reactive power losses.
IV. Soft-start energy-saving characteristics:
When a motor starts at full voltage, it draws seven times its rated current from the grid to meet the starting torque requirements. This large starting current wastes electricity, significantly impacts grid voltage fluctuations, and increases line and transformer losses. With soft starting, the starting current can be reduced from 0 to the motor's rated current, minimizing the impact on the grid, saving on electricity costs, and reducing the impact of starting inertia on the high-speed rotation of equipment, thus extending its lifespan. The soft starter controller can automatically determine the motor's load rate based on its power factor. When the motor is unloaded or under very low load, phase control can change the conduction angle of the thyristors, thereby altering the power input to the motor and achieving energy savings. The soft starter uses three anti-parallel thyristors as voltage regulators, connected between the power supply and the motor stator. This circuit resembles a three-phase fully controlled bridge rectifier circuit. When starting a motor using a soft starter, the output voltage of the thyristor gradually increases, and the motor gradually accelerates until the thyristor is fully turned on. The motor then operates at its rated voltage mechanical characteristics, achieving a smooth start, reducing starting current, and preventing overcurrent tripping. Once the motor reaches its rated speed, the starting process ends, and the soft starter automatically replaces the thyristor with a bypass contactor to provide the rated voltage for normal motor operation. This reduces heat loss from the thyristor, extends the soft starter's lifespan, improves its efficiency, and prevents harmonic pollution from the power grid.
4.1 Soft start control mode:
4.1.1 Current-limiting soft-start control mode: When the motor starts, its output voltage increases rapidly from zero until the output current reaches the set current limit value Im. Then, while ensuring that the output current does not exceed this value, the voltage gradually increases, and the motor gradually accelerates. When the motor reaches the rated speed, the bypass contactor engages, and the output current rapidly drops below the rated current Ie, completing the starting process. As shown in Figure 1.
4.1.2 Voltage Ramp Start Control Mode: When the motor starts, within the range where the motor current does not exceed 400% of the rated value, the output voltage of the soft starter rapidly rises to the set value U1, and then gradually increases at a set rate. The motor accelerates smoothly and continuously as the voltage rises until it reaches the rated voltage and rated speed. The bypass contactor then engages, completing the starting process. Generally, the voltage ramp start mode is suitable for applications where starting current requirements are not stringent but starting smoothness is highly demanding. The output characteristic curve is shown in Figure 2.
4.1.3 Jump Start + Current Limiting or Jump Start + Voltage Starting Mode: Figure 3 shows the output waveform changes in the jump start mode. This starting mode can be used in some heavy-load applications when the motor cannot start due to static friction. During startup, a relatively high, fixed voltage is applied to the motor and maintained for a limited time to overcome the static friction of the motor load and allow it to rotate. Then, the motor starts by limiting the current or using a voltage ramp. Before selecting this mode, a non-jump start mode should be used first. If the motor cannot rotate due to excessive static friction, then this mode should be selected; otherwise, this mode should be avoided to reduce unnecessary high-current surges.
4.1.4 Current Ramp Start Mode. Figure 4 shows the output current waveform in the current ramp start mode, where I1 is the current limiting value and T1 is the set time value. The current ramp start mode has strong acceleration capability, is suitable for two-pole motors, and can also shorten the start-up time within a certain range.
V. Common Misconceptions about Using Variable Frequency Drives for Energy Saving
It's true that frequency converters operate at the mains frequency and have energy-saving functions. However, this is contingent on three conditions: first, high-power loads, such as fans/pumps; second, the device itself must have energy-saving capabilities (software support); and third, long-term continuous operation. These are the three conditions for demonstrating energy-saving effects. Beyond these, the concept of energy saving is meaningless. Generally, the mechanical characteristic curve of an AC motor is fixed. Both theory and practice have proven that when the load power is less than the motor's rated power, its efficiency decreases as the load torque decreases. In other words, a lightly loaded motor consumes relatively more electricity. A frequency converter automatically adjusts the V/f value (where V is the voltage of the motor's stator windings and f is the frequency of voltage change in the stator windings) according to the load size, changing the motor's mechanical characteristic curve to adapt to the load, thereby improving efficiency and achieving energy savings.
VI. Conclusion
Energy conservation in motor systems is one of the ten key energy-saving projects launched by the National Development and Reform Commission. National development plans require the promotion of variable frequency speed control (VFD) technology, meaning that general mechanical systems such as fans, pumps, and compressors should adopt VFD speed control measures, while industrial machinery should use AC motor VFD speed control technology. Motor system energy conservation is currently the most commercially promising area in China's energy-saving market. With the development of microelectronics, power electronics, computers, and automatic control theory, VFD technology has entered a new era, and its fully mature technology has led to a new surge in its application. It achieves the goal of reducing input power and saving energy by changing the shaft output power through variable frequency speed control.
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