I. Overview
Statistics show that approximately 60% of the world's electricity consumption is driven by electric motors. Due to factors such as starting, overload, and safety systems, highly efficient electric motors often operate inefficiently. Using a frequency converter (VDC) to regulate the speed of AC asynchronous motors can bring them back to a more efficient operating state, thus saving a significant amount of energy. The types of loads on electric motors in production machinery vary greatly. For ease of analysis, loads are categorized into several mechanical characteristics, such as square torque, constant torque, and constant power. This paper only estimates the energy savings of square torque and constant torque loads. This estimation refers to the calculation and prediction of the energy-saving effect after using the VDC before it is put into operation. Once the VDC is operational, measuring the system's energy savings with electrical instruments is more accurate. It is assumed that the power factor of the motor system is essentially the same before and after using the VDC for speed regulation, and that the VDC efficiency is 95%.
Excessive consideration of differences in long-term process requirements before and after construction during the design process leads to overly large margins. For example, the thermal power design code SDJ-79 stipulates that the air volume margin for blowers and induced draft fans of coal-fired boilers should be 5% and 5-10%, respectively, and the air pressure margin should be 10% and 10-15%, respectively. However, it is difficult to calculate the resistance of the pipeline network and consider various problems that may occur during long-term operation. Typically, the maximum air volume and air pressure margin of the system are used as the basis for selection. However, the range of fan series is limited, and often, when a suitable fan model cannot be selected, an oversized model is chosen, often exceeding the margin by 20% to 30%. In actual production operation, valves and baffles are commonly used for throttling and regulating centrifugal fans and pump loads, which increases the damping of the pipeline system and wastes electrical energy. Electromagnetic speed controllers and hydraulic couplings are commonly used for regulating constant torque loads. These two speed regulation methods are inefficient, and the lower the speed, the lower the efficiency. Because the current of a motor varies with the load, and therefore the power consumed by the motor also varies with the load, accurately calculating the energy savings of the system is difficult, which to some extent affects the implementation of variable frequency speed control for energy saving. This article introduces the following formula for estimating energy savings.
II. Energy Saving Estimation
1. Energy-saving variable frequency speed control for fans and pumps with square torque loads: The electricity consumption of general-purpose fans and pumps accounts for about 50% of the electricity consumption of electric motors, which means it accounts for 30% of the national electricity consumption. Using variable frequency speed control to regulate flow can save 20% to 50% of electricity compared to using baffles or valves. If we calculate an average of 30%, the saved electricity is 9% of the total national electricity consumption, which will generate huge social and economic benefits. In production, valves and baffles are commonly used for throttling regulation of fans and pumps, increasing pipeline damping while the motor still runs at its rated speed, resulting in significant energy consumption. If a frequency converter is used to control the speed of fans and pumps, there is no need for throttling regulation using valves and baffles. Opening the valves and baffles to their maximum opening minimizes pipeline damping, greatly reducing energy consumption. The energy savings can be calculated using the formula in the implementation supervision guide of the mandatory national standard GB12497 "Economic Operation of Three-Phase Asynchronous Motors", namely:
We should first calculate the electrical energy consumed during the original system's throttling adjustment, and then subtract it from the electrical energy consumed after the system's frequency conversion speed regulation. This is exactly the expression of the numerator in equation (2). Therefore, to accurately calculate energy saving, we also need to use equation (1) to calculate the electrical energy consumed during the system's throttling adjustment.
2. Speed regulation and energy saving for constant torque loads
Constant torque load variable frequency speed control is generally used to meet the speed regulation requirements of processes. If variable frequency speed control is not used, other speed regulation methods must be adopted, such as voltage regulation speed control, electromagnetic speed control, and rotor resistance speed control of wound-rotor motors. Since these speed regulation methods are energy-consuming and inefficient, using high-efficiency variable frequency speed control can save the slip power consumed by speed regulation, and the energy saving rate is also considerable.
3. Electromagnetic speed control system
The electromagnetic speed control system consists of a squirrel-cage induction motor, a slip clutch, a tachometer motor, and a control device. Speed regulation is achieved by changing the excitation current of the slip clutch. The losses of the slip clutch itself are caused by the wind resistance and friction losses of the driving part and the mechanical friction losses of the driven part. If these losses are considered to be balanced with the excitation power of the slip clutch and are negligible, the input and output power of the slip clutch can be calculated by the following formula:
The electromagnetic speed-regulating motor is a squirrel-cage motor. Since the input power and torque remain constant, the power of the squirrel-cage motor remains constant. The losses are expressed in the form of active power, which is generated by the eddy currents of the slip clutch and dissipated by the fan blades on the armature.
As can be clearly seen from the power loss formula (10), the lower the speed of the electromagnetic speed-regulating motor, the greater the energy wastage. However, the speed of production machinery is usually not at the maximum speed. Variable frequency speed regulation is a method to change the synchronous speed of the rotating magnetic field. It is an efficient speed regulation method that does not consume energy. Therefore, using variable frequency speed regulation will have a very good energy-saving effect. The energy saved can be directly calculated using formula (10).
4. Hydraulic coupling speed control system
A hydraulic coupling transmits energy from a motor by controlling the change in the angular momentum of the working fluid within its working chamber. The motor drives its driving impeller via the input shaft of the hydraulic coupling, accelerating the working fluid. This accelerated fluid then drives the driven turbine of the hydraulic coupling, transferring energy to the output shaft and the load. Hydraulic couplings are classified into speed-regulating and torque-limiting types. The former is used for speed regulation in electrical drives, while the latter is used for motor starting. In this system, the hydraulic coupling acts as a buffer during motor starting. Since the structure of a hydraulic coupling is similar to that of an electromagnetic slip clutch, the efficiency can be calculated using the same method as for electromagnetic speed governors:
5. Wound-rotor motor series resistance speed control system
The most common method for speed regulation of wound-rotor motors is to change the series resistance of the rotor circuit. As the series resistance of the rotor increases, it is not only easy to change the forward speed of the motor, but also to make the motor rotate in reverse and change the reverse speed under potential energy load. Therefore, this speed regulation method is widely used in the hoisting and metallurgical industries.
For wound-rotor motors, using rotor series resistance will result in energy loss during starting, braking, and speed regulation. This loss increases as the speed decreases and the slip S increases. In addition, as the series resistance increases, the mechanical characteristics soften, making it difficult to achieve the static speed regulation target.
In equation (14), if S=0.5, half of the electromagnetic power is consumed in the rotor resistance, and the efficiency of the speed regulation system is less than 50%. Using equation (14), as long as the speed of the motor is known, the electrical energy consumed by the series resistance speed regulation of the wound-rotor motor can be easily calculated, and the calculation of energy saving is very simple.
When carrying out frequency converter energy-saving retrofits, the input and output must be carefully considered, and the output involves calculating the energy savings. Calculating energy savings before the frequency converter is put into operation is quite difficult, and a simple and practical calculation method is often desired for predicting energy savings. With the above calculation formula for energy savings, the input and output become clear.
III. Relationship between energy saving of variable frequency speed regulation and system power factor
It was previously assumed that the power factor of the motor system remained essentially the same before and after using the frequency converter for speed regulation, thus the impact of the system power factor could be disregarded when calculating energy savings. However, in reality, the power factor may differ before and after the frequency converter is put into operation. Therefore, should the calculated energy savings take into account the change in power factor before and after the frequency converter speed regulation?
In a sinusoidal circuit, the power factor is determined by the phase angle difference between voltage U and current I. In this case, the power factor is often expressed as . The active power P in the circuit is its average power, i.e.:
When measuring actual energy savings using an electricity meter, the meter measures the active power consumed by the motor system. If the original motor system had a low power factor, using a frequency converter to operate at a constant speed of 50Hz improves the power factor. However, the motor's operating state remains unchanged, and its active and reactive power consumption remains the same. The filter capacitor in the frequency converter exchanges reactive power with the motor, thus reducing the actual input current to the frequency converter. This reduces line losses between the grid and the frequency converter, as well as copper losses in the power transformer, and also reduces reactive current flowing into the grid. Therefore, when calculating energy savings, the energy savings resulting from improving the power factor should be considered.
After improving the power factor, the rate of decrease in current in the power distribution system is:
The current drop rate and loss drop rate of the power distribution system refer to the current and loss of a single motor before and after compensation, not to the actual changes in the current and loss of the power distribution system.
The current drop rate and loss drop rate of the power distribution system refer to the current and loss of a single motor before and after compensation, not to the actual changes in the current and loss of the power distribution system.
Here is a typical example.
Example 2: A material handling machine has a 200kW motor, installed more than 100 meters away from the power distribution room. The metering instruments—voltmeter, ammeter, and active power meter—are all located in the power distribution room. At power frequency, the motor's no-load operating current is 192A; under load, the motor's operating voltage is 356V and the current is 231A. Due to the light load, the motor's load factor and efficiency are both low. The motor's power factor can be calculated using the following formula:
In this example, if the power factor is simply improved, there is no need to use a frequency converter; local compensation can be achieved using a power capacitor. However, if the speed regulation needs of the process also need to be met, using a frequency converter for speed regulation is the best energy-saving method. In this case, the energy saving should be the sum of the energy consumption on the line and the energy saving of frequency conversion speed regulation.
If the power factor of the original motor system is high and the power factor does not change much after the frequency converter is put into operation, the influence of line loss after the power factor change can be ignored, and the energy saving can be calculated using (1) to (14) in this article.
IV. The efficiency of the frequency converter must be considered when calculating energy saving for variable frequency speed control.
GB12668 defines a frequency converter as an electrical energy conversion device that converts electrical energy and can change its frequency. Energy conversion inevitably involves losses. Inside the frequency converter, the switching losses of the inverter's power devices are the largest, followed by heat losses from electronic components and fan losses. The efficiency of a frequency converter is generally 95%-96%, therefore, when calculating energy savings in variable frequency speed control, the 4%-5% losses of the frequency converter must be taken into account. If the frequency converter losses are considered, the energy saving rate calculated in Example 1 of this article would not be 36%, but rather 31%-32%, a result closer to the actual energy saving rate.
V. Conclusion
Generally, frequency converters are used for 50Hz speed control. Whether the load has a square torque characteristic or a constant torque characteristic, speed regulation is necessary for energy saving; operating at the mains frequency without speed regulation will not result in any energy savings. Sometimes, even with a very low system power factor, using a frequency converter can still lead to energy savings. This is not due to frequency conversion speed regulation itself, but rather to power factor compensation. The energy-saving calculation method for frequency conversion speed regulation described in this article has excellent practicality.
References:
(1) Peng Youyuan GB12497 "Economic Operation of Three-Phase Asynchronous Motors" Mandatory National Standard Implementation Supervision Guide
(2) Du Junming, Peng Haiyu. Research on energy saving of variable frequency speed regulation for certain constant torque loads. Automation Expo.
