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Selection and Practice of Low-Voltage High-Power Frequency Converters Abstract: The proper selection of frequency converters is crucial for the proper operation of the electrical control systems in mechanical equipment. When selecting an inverter, it is first necessary to consider the actual conditions of the enterprise and choose an appropriate voltage level for the inverter. Within a certain power range (below 1000 Kw), low-voltage inverters should be preferred. Meanwhile, depending on the type of mechanical equipment, the characteristics of the load torque, the speed control range, the static speed accuracy, the starting torque, and the requirements of the operating environment, it is determined which control method and protection level of inverter to use. Keywords: Inverter, precision, selection Inverters are primarily used for regulating the speed of AC motors, making them an ideal solution for speed control. With the rapid development of China’s economy, the demands on drive products in the market have been increasing. Thanks to their advantages such as a wide speed control range, high precision, and good dynamic response, inverters play an increasingly important role in various speed control applications. In addition to their excellent speed control capabilities, they also offer significant energy-saving benefits. This is especially true when inverters are used in fans and pumps – types of loads that are widely used across various sectors of the national economy, consume large amounts of electricity, and have limited speed control ranges (usually 50%-100%). Therefore, this is a field with great potential for further development. Taking power plants as an example, the drive systems of numerous boiler auxiliaries generally have a large capacity, resulting in serious electricity waste when operating at a constant speed. By using high-power frequency converters, it is possible to equip or upgrade the boiler fans and pump systems in power plants – such as boiler blowers, boiler exhaust fans, boiler feed pumps, condensate pumps, slag pumping pumps, and slurry pumps – resulting in a very significant energy-saving effect. It also enables a high level of automated control, making the operation of the boiler safer and more reliable; for example, variable frequency speed control of the exhaust fans helps to maintain a more stable negative pressure in the furnace ； The variable-frequency speed control of the primary fan ensures more stable air supply, resulting in more complete combustion of coal ； Variable frequency speed control of the grout pump can protect the motor of the grout pump, preventing issues such as pump clogging, overload, and motor burnout. I. Current status of high-power motors in domestic power plants In domestic power plant boilers, the power of most fans and pumps used as auxiliary equipment ranges from 200 kW to 2000 kW. According to domestic practices, motors with a power rating of over 200 kW use 6 kV systems; as a result, all the main auxiliary equipment in power plants employs 6 kV motors. From the perspective of reducing line losses, the electricity supply sector aims to increase the voltage level and promotes the use of 10 kV systems. On the other hand, users, seeking to simplify system design, prefer that motors and frequency converters with a power rating of over 200 kW use 6 kV or 10 kV systems. The advantage of this configuration is that it allows for savings in both the cost of purchasing motors and the amount of work required for installation ； A frequency converter bypass switch can be installed directly; in the event of a failure of the frequency converter, it is possible to switch to the 6kV or 10kV plant bus through this bypass switch, without having to interrupt operation. However, using 6kV or 10kV “direct” frequency conversion is unreasonable from both technical and economic perspectives. Currently, all “direct” high-voltage frequency converters are not truly direct frequency converters; they all have transformers on their input side or rely on the series connection of electronic components. As a result, it is not necessary for the motor and the frequency converter to have the same voltage as the grid voltage. From an economic perspective, the price of a 560kW high-voltage inverter is more than twice that of a low-voltage 660V or 690V inverter, exceeding 1.2 million yuan. It shows that within a certain power range, even when taking into account the costs of replacing the motor and adding a rectifier transformer, the low-voltage approach is much more economical than the high-voltage approach; moreover, the subsequent equipment maintenance costs and the technical requirements for repairs are lower. II. Comparison of the Structures of High- and Low-Voltage Frequency Converters In recent years, various high-voltage frequency converters have emerged. However, to date, high-voltage frequency converters do not yet have a nearly uniform topology similar to that of low-voltage frequency converters, which consists of a diode rectifier circuit, an intermediate DC circuit, and an inverter bridge. The inverter bridge is usually made up of 1.2 kV (for 380 V frequency converters) or 1.7 kV (for 690 V frequency converters) IGBTs, with generally only one power device used per bridge arm. High-voltage frequency converters have a relatively complex structure. Through years of continuous research, a type of high-voltage frequency converter has been developed, represented by Beijing LDHF, which utilizes multiplexing technology; this is a method for achieving high-power conversion using low-power devices. The so-called multiplexing technique involves each phase being composed of several low-voltage PWM power units connected in series or parallel at their outputs using some method such as a transformer. Each power unit is powered by an isolated multi-winding transformer, and by connecting the low-voltage units in series, high voltage output is achieved; or by connecting them in parallel, a large capacity output is obtained. Another type is the high-voltage frequency converter with a two-level conversion circuit featuring multiple tubes connected in series, represented by Chengdu Jialing. This type of converter utilizes power devices in both series and parallel configurations to meet the requirements of high-voltage frequency conversion. The various devices connected in series and parallel are treated as a single unit, and the control strategies and methods used in more mature low-voltage frequency converters are applied to develop high-voltage frequency converter products. The difficulty in doing this lies in the fact that the series-connected switching transistors require both dynamic and static voltage equalization, which imposes high demands on the overall drive control circuit. Moreover, many technical issues such as du/dt effects, common-mode voltage rejection, and sine wave filtering need to be addressed. Another type is the high-voltage frequency converter with a diode-clamped three-level conversion circuit, represented by the Department of Electrical Engineering at Tsinghua University. Its purpose is to overcome the voltage equalization problem that arises when power devices are connected in series directly; by introducing the concept of clamping diodes alongside the direct series connection of multiple devices, this approach avoids the dynamic voltage equalization issues caused by such direct series connections. In addition to the three main types of topologies for high-voltage frequency converters mentioned above, various other types have emerged in recent years, such as flyback capacitor-clamped three-level conversion circuits and capacitor-buck multi-level conversion circuits. Each of these types comes with its own corresponding control methods and theories. Any of these control methods is more complex than those used in conventional low-voltage two-level frequency converters, and they involve a series of challenges that need to be addressed, including the selection of power devices, handling of stray parameters, insulation issues, electromagnetic interference, heat dissipation, field analysis of components, energy control, effects of du/dt and di/dt, and control of transient processes. These are issues that either do not arise at all or only rarely occur in low-voltage frequency converters, or they are easier to handle in such converters. Therefore, using low-voltage frequency converters makes the system more reliable and simpler. III. Selection of Frequency Converters – Proper Selection of Frequency Converters The proper selection of frequency converters is crucial for the proper operation of the electrical control systems in mechanical equipment. When selecting an inverter, it is first necessary to consider the actual conditions of the enterprise and choose an appropriate voltage level for the inverter. Within a certain power range (below 1000 Kw), low-voltage inverters should be preferred. Meanwhile, depending on the type of mechanical equipment, the characteristics of the load torque, the speed control range, the static speed accuracy, the starting torque, and the requirements of the operating environment, it is determined which control method and protection level of inverter to use. www.Ｅxamda.CoM 3.1 Selecting the appropriate voltage level for inverters: At present, the appropriate voltage level for an inverter is determined by the voltage level that the power semiconductors can withstand. The possible power range of an inverter at a certain voltage level, on the other hand, is determined by the current-carrying capacity of those power semiconductors ； Due to the voltage constraints of power electronic devices and the allowable du/dt values for motors, 10 kV frequency converters must be multi-level with multiple devices connected in series. This results in complex circuits, high costs, poor reliability, and high maintenance expenses. For 10kV inverters, if 1700V IGBT devices are used, 10 strings are required; three phases supply a total of 120 devices ； If 3300V devices are used, 5 strings are also required to supply 60 devices, resulting in a large quantity whose reliability will inevitably be affected. On the other hand, the components used in high-voltage frequency converters generally handle low currents, resulting in underutilization of their current-carrying capacity. Taking a 710kW system as an example, the current in a 10kV motor is only around 50A. Currently, IGBTs with a voltage rating of 1700V can handle currents of up to 2400A, while those with a 3300V rating can handle 1600A. Since components capable of handling high currents are not used, a large number of low-current components have to be connected in series, which makes the design highly unreasonable. Even when the motor power reaches 2000 kW, the current is only around 150 A, which is still quite low. In practical applications, for the purposes of level isolation, improving the shape of the input current waveform, and reducing harmonics, an input transformer is usually installed on the input side of most high-voltage frequency converters. Since a transformer is present, it is not necessary for the voltage of the frequency converter and the motor to be identical to the grid voltage; 6 kV or 10 kV is not mandatory. Thus, there is the issue of appropriate voltage levels for inverters and motors. In the past, for low- and medium-voltage motors of 200kW class, the main consideration was direct motor starting; the starting current was 5–7 times the rated current. If a 10kV/380V power transformer with a capacity of 2000kVA and a short-circuit impedance of around 6% was used, the instantaneous tripping value of the circuit breaker would be very high. To improve the stability of the power system and the quality of power supply, it is desirable to further reduce the impedance of the transformers ； However, the lower the impedance, the greater the short-circuit current in the system, which makes it more difficult to select appropriate electrical equipment. Moreover, the breaking capacity of the switches in the circuits must be increased, which inevitably leads to higher selection costs and increased expenses. If the voltage drop on the 380V bus during motor startup is limited to around 50%, it will be necessary to increase the capacity of the transformer; otherwise, the short-circuit current will be too high and the low-voltage switch will not be able to handle it. With speed control using frequency converters, the starting current is kept within the rated current level. The capacity of low-voltage frequency converters can be quite large nowadays; companies such as ABB have mastered the technology for parallel output of inverter units, allowing low-voltage frequency converters to reach a capacity of 2900 kW. This makes it possible for us to use a large number of high-power low-voltage frequency converters. In practice, we can replace the incoming transformers with a voltage rating of 10Kv/6kV or 6Kv/6KV, which previously served only an isolation function, with incoming transformers of 6Kv/690V or 6Kv/380V. This allows us to obtain power supply voltages of 690V and 380V, thereby making it possible to use a large number of low-voltage, high-power frequency converters. Currently, the capacity of low-voltage motors at 660V or 690V in China has reached 1000–1200Kw, making selection easier. Source: www.examda.com 3.2 Load torque characteristics of mechanical equipment In practice, production machinery is often classified into three categories based on their different load torque characteristics: constant torque loads, constant power loads, and loads such as fans and water pumps. 3.2.1 Constant torque load: In this type of load, the load torque TL is independent of the rotational speed n; it remains constant or essentially constant at any speed, while the load power increases linearly as the speed increases. Frictional loads such as conveyor belts, mixers, extruders, and the feeding mechanisms of mechanical equipment, as well as gravitational loads such as cranes, hoists, and elevators, all belong to constant torque loads. When a frequency converter drives loads with a constant torque characteristic, it must provide sufficient output torque at low speeds, as well as adequate overload capacity. If long-term steady operation at low speeds is required, the heat dissipation capacity of standard cage asynchronous motors should be taken into account to prevent excessive temperature rise in the motors. 3.2.2 Constant power load: The characteristic of this type of load is that it requires? ? It seems to remain unchanged. The spindles of metal cutting machines, as well as winders and unwinders in rolling mills, paper mills, and film production lines, all belong to constant power loads. The constant power characteristic of the load should be considered within a certain range of speed variations. When the speed is very low, due to limitations in mechanical strength, TL cannot increase indefinitely, and it behaves as a constant torque at low speeds. The constant power region and constant torque region of the load have a significant impact on the selection of the transmission scheme. When the motor is controlled at a constant magnetic flux, the maximum allowable output torque remains unchanged; this constitutes constant-torque speed control ； Beyond the weak magnetization point, the system enters the weak magnetization speed control region; in this area, the output voltage of the inverter does not increase as the speed rises, while the torque decreases. The maximum allowable output torque is inversely proportional to the speed, which constitutes constant power speed control. 3.2.3 Loads such as fans and water pumps: The torque of such loads is proportional to the square of the speed, while their power is proportional to the cube of the speed. Various fans, water pumps, and oil pumps all belong to typical fluid-type loads. Fluid-based loads can regulate air volume and flow rate by using frequency converters to control speed, thereby saving a significant amount of electrical energy. Since the power demand of fluid-type loads increases too rapidly at high speeds, being proportional to the cube of the load’s rotational speed, such loads should not be operated above their rated frequency. Such load devices account for a large proportion in power plants. Knowing the load type of the equipment is not enough when selecting an inverter; it is also necessary to choose the correct voltage level. Knowing the type of load driven by the motor and selecting the appropriate supply voltage are important; when choosing an inverter, the motor’s rated current value should be used as the main criterion, while the motor’s rated power can only serve as a reference. Furthermore, it should be fully considered that the output of the inverter contains abundant high-order harmonics, which can degrade the motor’s power factor and efficiency. Therefore, when powering the motor with an inverter compared to using a power supply at line frequency, the motor’s current increases by 10%, while the temperature rise increases by about 20%. Therefore, when selecting an inverter, this situation should be taken into account, and an appropriate margin should be reserved to prevent excessive temperature rise from affecting the inverter’s service life. Furthermore, when there is a large distance between the inverter and the motor, measures should be taken to mitigate the effect of the coupling capacitance of the long cable to ground, in order to prevent insufficient output from the inverter. Therefore, when selecting an inverter, it is necessary to choose a higher capacity model or install an output reactor at the inverter’s output side. Only by selecting the right frequency converter can the smooth progress of the renovation and production be ensured. The following uses a specific example to illustrate the successful application of low-voltage, high-power technology in new power plant projects. IV. Practical Application of Low-Voltage, High-Power Frequency Converters With economic development giving priority to thermal power generation, against this backdrop, Qinhuangdao Tonghe Thermal Power Co., Ltd. carried out a second-phase expansion project in 2004. The project entailed the installation of 1×24MW double-condensing steam turbine generators paired with a 170t/h circulating fluidized bed boiler, as well as the construction of the main plant building and its foundations. The boiler was equipped with four high-power electric motors; the rated power of the induced draft fan motor was 1120KW ； Rated power of the supply fan motor: 710KW ； Rated power of the secondary blower motor: 450KW ； Rated power of the circulation pump motor: 680 KW. If high-voltage 6kV frequency converters were used, it would inevitably increase the initial investment costs for the expansion project. Thanks to our strong recommendation for low-voltage, high-power frequency converters, and after on-site inspections by the manufacturers, ABB low-voltage frequency converters were ultimately chosen. The specific inverter models and applications are as follows: ACS800-07-0580-7 ； The specific current value is 488A. It is used to drive the variable-frequency motor of the 450KW/690V secondary fan ； Motor current: 441A. ACS800-07-0870-7 ； The specific current value is 729A. It is used to drive the variable-frequency motor of the 680KW/690V circulation pump ； Motor current: 705A ACS800-07-1060-7 ； The specific current value is 885A; it is used to drive the variable-frequency motor of the 710KW/690V primary fan ； Motor current: 736A ACS800-07-1500-7 ； The specific current value is 1208A. IV. Detailed descriptions of the accessories for the four sets of equipment are as follows: 1) All four sets of equipment are equipped with input reactors that function as AC-DC-AC inverters. These input reactors not only help to reduce harmonics in the incoming power supply but also minimize current surges to the equipment, thereby ensuring its reliability. ABB incorporates the input reactor as a standard feature within the rectifier module ； Its main function is to suppress or eliminate the high-order harmonics in the input current of the frequency converter from being transmitted through the power grid, thereby preventing interference with electronic circuits and devices. 2) All four units are equipped with fuse combination switches; their main function is to protect the electronic components of the rectifier input unit. To provide operators and maintenance personnel with a clear cut-off point, an isolating switch must be installed on the input side of the frequency conversion unit as a distinct operational cut-off point ； Ensure the safety of maintenance personnel, and that there is a clear break in the main circuit during shutdown and startup ； It is strictly prohibited to disconnect this switch under load. 3) All four sets of equipment are equipped with contactors + emergency stop switches. Their main function is to ensure the reliable protection of the equipment: in the event of a malfunction, pressing the emergency stop switch causes the incoming line contactor to act as an automatic tripping device, thereby cutting off the main circuit and safeguarding both the equipment and human safety ； It makes the system safer and more convenient, facilitating remote operation. 4) All four units are equipped with Du/dt filters, whose main function is to prevent the inverter from generating significant winding insulation stress voltages and asymmetric bearing currents, which can have a negative impact on the motor’s performance and lifespan ； Unless it is a specially designed motor, frequency conversion units used with 690VAC motors must be equipped with a du/dt filter to ensure the reliable operation of the motor ； It also offers benefits in terms of reducing noise generated by the motor, lowering electromagnetic radiation, significantly attenuating high-frequency components, ensuring a clean electromagnetic environment, and extending the motor’s lifespan. 5) All four units are equipped with 12-pulse rectifiers; each unit also has two sets of 6-pulse rectifiers, together forming 12-pulse rectifiers – these can effectively eliminate harmonics below the 11th order in the current, reduce harmonic interference with the power grid, and improve the quality of the power grid environment. 4.1 Working Principle and System Structure The auxiliary equipment of the 170-ton boiler at Qinhuangdao Tonghe Power Plant uses high-power ACS800 frequency converters produced by ABB. These converters employ direct torque control (DTC); they abandon the decoupling concept used in vector control, instead continuously detecting the amplitude of flux and torque values to compare them with the desired flux and torque values. The flux and torque regulators then directly output the required voltage vector values. The direct torque control system consists of a speed control loop and a torque control loop. ABB combines DTC technology with fuzzy control theory to produce high-performance, low-cost frequency converter speed control products, whose performance is **superior to that of vector control frequency converters. In DTC, stator flux and torque are used as the main control variables. The combination of a high-speed digital signal processor and advanced motor software models allows the motor’s status to be updated 40,000 times per second. Since the motor status and the comparison value between the actual value and the set value are continuously updated, each switching state of the inverter is determined independently. This means that the drive can generate the optimal switching combination and respond quickly to dynamic changes such as load disturbances and instantaneous power outages. It responds accordingly based on the set conditions; thanks to the intermediate DC link, voltage drops can be effectively suppressed, thereby ensuring that the inverter can determine whether to continue operating or engage fault protection. A PWM modulator that does not require separate control of voltage and frequency is needed in DTC. Therefore, there is no fixed chopper frequency; in actual operation, it does not generate the high-frequency noise produced by other inverters driving motors, and it also reduces the power consumption of the inverter itself. The rich and flexible input/output port definition capabilities allow for meeting user requirements in most application scenarios without the need for any additional circuits. 4.1.1 Function of the three fans: Circulating fluidized bed boilers use a balanced ventilation system, with one primary fan and one secondary fan assigned to each boiler. The air required by the combustion chamber is drawn in by the primary and secondary fans and then sent to the air preheater for heating. The heated primary air is fed into the fluidized bed and the return reactor, while the secondary air enters the furnace. The fuel burns in the combustion chamber after mixing with air; the primary air is evenly distributed into the furnace from the bottom through the grate, thereby facilitating combustion. The secondary air is preheated in the furnace’s interlayer before entering the secondary combustion chamber, where it contributes to the second stage of combustion. By adjusting the positions of the butterfly valves, as well as using the frequency converters of the primary and secondary fans, it is possible to change the speed of the fan motors and thus adjust the amount of air used. The induced draft fan is used to maintain negative pressure in the furnace for combustion. 4.1.2 Function of the exhaust fan: The exhaust fan is used in conjunction with the boiler control system to save fuel, improve the combustion efficiency of the boiler, and reduce environmental pollution. By utilizing modern automatic control technologies and a hierarchical DCS-based distributed control approach, and by taking into account the combustion conditions of the fuel, it is possible to monitor various parameters such as furnace pressure, flue gas oxygen content, and exhaust gas temperature. This enables reliable control of the frequency converter, allowing the operating speed of the exhaust fan to be adjusted at any time, thereby automatically monitoring furnace pressure and maintaining negative pressure during combustion. It is required to maintain a constant main steam pressure under different load conditions, and by adjusting the amount of fuel and air supplied to the furnace, ensure that the steam output of the boiler is in balance with the steam consumption and heat supply requirements of the turbine. If the negative pressure in the furnace chamber is too low or even positive, flame spraying can occur in certain areas, which is not conducive to safe production nor to environmental hygiene ； If the negative pressure is too high, a large amount of cold air will leak into the furnace, increasing the load on the exhaust fan and the heat loss due to smoke extraction, which is not conducive to efficient combustion. Must the furnace have negative pressure? The frequency is controlled by the exhaust fan inverter, and the air supply volume is used as a feedforward signal to participate in the overall control. The automatic control process is carried out by the host computer. 4.1.3 Function of the primary air blower: The primary air blower is used in conjunction with the boiler control system to save fuel, improve the combustion efficiency of the boiler, and reduce environmental pollution. Through the upper-level DCS system, its operating speed can be adjusted at any time, thereby automatically monitoring the oxygen level in the furnace and ensuring normal combustion. Maintaining an appropriate ratio of air to fuel is the optimal operating condition for the combustion process, and it is a key measure to improve the efficiency and economy of boilers. To maintain economic operation, the key is to keep an appropriate air-coal ratio. When the volumetric flow rate of coal changes, the air supply volume should be adjusted according to a certain ratio. Changing the air volume is achieved through the frequency converter of the primary fan. First, adjust the opening degrees of the dampers in each air chamber beneath the grate, and then adjust the frequency of the fan’s variable frequency drive as a means to control the air volume. The automatic control process is carried out by the host computer. Using ABB’s frequency converters enables soft start control of the blower, which not only reduces the impact on the power grid during motor startup but also lowers the wear associated with starting the blower as well as the energy consumption resulting from shocks, thereby extending the electrical and mechanical lifespan of the equipment. 4.1.4 Function of the secondary fan: The secondary fan is used in conjunction with the boiler control system to save fuel, improve the combustion efficiency of the boiler, and reduce environmental pollution. Through the upper-level DCS system, its operating speed can be adjusted at any time to maintain an appropriate ratio between air and fuel, thereby creating the optimal conditions for the combustion process. It is a key measure for enhancing the efficiency and economic viability of the boiler. To maintain economic operation, the key is to keep an appropriate air-coal ratio. When the volumetric flow rate of coal changes, the air supply volume should be adjusted according to a certain ratio. It is difficult to achieve this merely by changing the fan volume once ； A secondary fan is required to ensure thorough combustion of the coal in the secondary combustion chamber. Since the secondary fan is affected by the primary fan, it is necessary to pay close attention to this aspect during operation, and the minimum speed of the motor should be restricted in order to make full use of the secondary air flow and achieve optimal combustion, thus enabling the boiler to operate at full capacity. The use of a frequency converter with the aforementioned fans not only meets the requirements of the production process, but also enables soft start control of the fans. This reduces the impact on the power grid during motor startup, as well as the wear associated with fan startup and the energy consumption resulting from such impacts, thereby extending the electrical and mechanical lifespan of the equipment. Additionally, the output of the inverter contains certain higher harmonics, which may accelerate the insulation aging of the motor; this aspect should be taken into consideration during use. V. Power Supply Input The power supply for each unit is 690V. All four large frequency converters are equipped with input switchgear, contactors, and emergency stop switches. Optional accessories for the incoming line contactor include an emergency stop button on the cabinet door; an external emergency stop button can be connected to the terminal block inside the cabinet. When the emergency stop switch is pressed, the emergency stop command disables the inverter’s semiconductors and cuts off the main contactor, causing the motor to stop automatically. The device is equipped with a mechanism to prevent accidental startup. A 12-pulse rectifier is used in this case, consisting of two 6-pulse rectifier bridges connected in parallel. By using a 12-pulse rectifier, the total harmonic distortion can be reduced, eliminating the fifth and seventh harmonics. Because EMC filters cannot be used with 12-pulse rectifiers (the transformer’s secondary side is floating!) It is recommended to install a power supply transformer with a shielding layer to reduce transmission radiation. The rectification unit includes: two sets of diode rectification units (DSU), model: ACS800-507-0680-7. VI. Data on the rectification transformer A multi-winding transformer is required in order to achieve phase shift in a 12-pulse mode. Another purpose of the transformer is to provide sufficient impedance to keep the grid-side harmonics within the limits specified by IEEE519. We use dry-type transformers manufactured in Shunde, Guangdong. The isolation transformer has two secondary windings: one connected in a Y configuration and the other in a Δ configuration. This results in a 30° phase difference between the two secondary windings, providing 12 pulses to meet the requirements of a 12-pulse input rectifier bridge. Overload factor: 1.5 times (1 minute for every 10 minutes). No-load voltage ratio: 6 kV ± 2×2.5% taps / 0.72 kV / 0.72 kV. Rated frequency (Fn): 50 Hz ± 2%. Connection type: Dyn 11 d0. Short-circuit impedance voltage drop: The short-circuit impedance voltage drop for each of the two secondary windings is 8%. Deviation in short-circuit impedance voltage drop: The deviation between the short-circuit impedance voltage drops of the two secondary windings is ≤ 3%. Voltage deviation: The voltage deviation of the two secondary windings under rated load is ≤ 0.3% of UN. Ambient temperature: Up to 40 degrees Celsius. Other details: A shielding layer is provided between the high-voltage and low-voltage windings, with connection terminals for this shielding layer. A terminal is led out from the center point of the secondary Y winding. The voltage withstanding capacity, insulation, and safety of each winding, shielding layer, and terminal meet the corresponding **standards. Air-cooled temperature control device equipped with an aluminum alloy protective cover and a PT100 temperature sensing element (low-voltage winding + core). VII. Inverter Tuning 7.1 Setting of Inverter Functions The parameters of the inverter are adjusted to ensure the proper operation of the auxiliary equipment in the power plant’s boilers. These parameters can be set using the CDP312 control panel that comes with the inverter, or a pre-programmed application macro can be selected; this allows for a quick and easy startup of the ACS800. Standard application macros include factory macros, manual/automatic macros, PID application macros, torque control macros, sequence control application macros, etc. 7.2 Control Modes The frequency converter offers the following three control modes: Local control: This is carried out through the accompanying CDP312 control panel. Remote control: Receives digital or analog control signals from the field or control room via the inverter terminals. Bus control: The bus adapter connected via the inverter’s expansion port receives communication signals from the PLC or a higher-level computer. The ACS800 features various serial communication interfaces, which allow it to communicate with CDP312 control panels and PCs; it can also communicate with other higher-level control systems by using appropriate area bus adapters, thereby enabling operations, debugging, diagnosis, and control. This project uses remote control of the host computer for operation. Parameter setting is carried out via the CDP312 control panel. 7.3 Setting of acceleration and deceleration times: Since the rotational inertia of the fan is relatively high, the acceleration and deceleration times are generally set to be long, in order to prevent the inverter from reporting overcurrent or overload errors during startup. A too short deceleration time can lead to errors related to excessive DC voltage; therefore, these two time periods are usually more than 90 seconds. 7.4 After the parameter settings for the variable frequency drive speed control system have been completed, motor parameter identification is carried out first; once this is done, the system can be put into trial operation. First, carry out low-frequency operation on the control panel to check whether the motor rotates in the correct direction, whether its speed is stable, whether the temperature rise is normal, and whether the acceleration and deceleration are smooth. Then proceed with trial operations at frequency levels of 20, 30, 40, 50 Hz, etc. If the trial run goes well, the frequency converter can be put into trial production. VIII. Conclusion Through practical examples, this article demonstrates that using high-power frequency converters for auxiliary equipment in power plants is not only feasible but also economical; the control and wiring are very simple. Coupled with the comprehensive fault diagnosis and display functions of these frequency converters, the reliability and maintainability of the entire speed control system are greatly improved, and the automation level of power plants is further enhanced. This approach has received recognition from users, offering very promising market prospects. This post was last edited by LZ Gas Station on 2009-4-5 21:10]