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In speed control systems composed of general-purpose frequency converters, startup failures often occur due to incorrect settings of the torque boost function. Installation and debugging personnel sometimes resort to incorrect solutions because they are unable to identify the root cause of the problem, which can result in wasted funds, delays in project completion, or protective deficiencies in the system. This article discusses the setting of the torque boost function by referring to several typical cases. Inverter torque enhancement yunrun.com.cn/tech/709.html 1. Failures caused by incorrect settings of the torque enhancement function. Most general-purpose inverters have a torque enhancement function, though the meaning of this function varies among different brands; for example, Fuji’s products define torque enhancement 1 and torque enhancement 2 ； The products of American company A-B define DC boost, start-up boost, operation boost, and operation/acceleration boost. In the torque enhancement function, there are various enhancement modes available for users to choose from. Products of different series from the same manufacturer have different default settings. If the setting of torque enhancement parameters is neglected during system calibration, high starting torque of the load can cause an overcurrent trip, resulting in a failed start. ①Starting failures of pump loads due to changes in process conditions: During the commissioning of the slurry pump system for the pipeline-based melting process at Shanxi Aluminum’s Second Aluminum Plant, two slurry pumps were selected; these were solid-liquid two-phase flow slag pumps equipped with 90kW motors, having a rated current of 164A. Fuji FRN90P9S-4CE frequency converters were used, with a rated current of 176A. The supplier was responsible for the commissioning process. During startup, the motor stalled at around 12Hz, after which an overcurrent trip occurred, resulting in several failed startups. The same issue occurred with both pumps. Later, the commissioning team increased the motor’s overload limit from 164A to 185A, and the motor was able to start successfully. Upon learning of this, I considered such an approach to be incorrect, as it led to a deficiency in the motor’s overload protection mechanism (the current exceeded the frequency converter’s rated value), which could result in significant losses. By checking the set parameters, it was found that the frequency converter’s function code (displayed on the LCD) for torque enhancement was set at 0.1, maintaining the factory default value. The torque enhancement function was set to provide strong torque reduction. Under normal circumstances, this setting should be sufficient to meet the starting requirements of pump loads. However, investigations revealed that due to the effects of the process flow, there was initial pressure at the pump’s outlet, which increased the starting torque required by the slurry pumps and caused starting failures. Subsequently, the torque enhancement value was changed to 0.0, and the automatic torque enhancement mode was selected, allowing the motors to start properly. ②For constant torque loads, the torque increase curve must be selected appropriately. The bauxite alumina plant installed flat-plate filters from the German company Bell; the drive motor has a power of 15 kW and is controlled by a Fuji FRN22G9S inverter. The local equipment suppliers were responsible for the commissioning process. During testing, overload protection was triggered due to an excessively high torque increase setting (the actual load torque was relatively low). The technician resolved this issue by increasing the overload threshold by 1.5 times, thereby enabling successful startup. After switching to the automatic torque boost setting, the inverter is able to automatically determine the appropriate boost value based on the load conditions during startup, thereby ensuring a smooth startup process. This also guarantees that the motor’s overload rating does not exceed its rated current. ③The importance of torque enhancement settings for starting special loads: At an academic conference on frequency conversion technology, an engineering expert from a company in Shenzhen that distributes imported frequency converters explained that during the technical renovation of the No. 4 cement kiln at Shanxi Aluminum Cement Plant, the 63.6×65m cement kiln was converted from DC drive to AC drive. The original DC motor of type Z2-111 with a capacity of 155 kW was replaced by a Y315L2-8 motor with a capacity of 110 kW. A frequency converter model 1336S-B250HP from American company A-B was used for control. During testing, the cement kiln started normally, but when it stopped after discharging material and was started again, overcurrent trips occurred around 10 Hz due to motor stalling; the maximum current reached 530 A. The technicians determined that the reason was the large angle of material accumulation during startup, which caused load eccentricity and increased the starting torque of the kiln. They adjusted the V/F curve so that the rated voltage was delivered at 37 Hz, and startup was successful. However, after startup, the frequency converter entered constant-power operation, and the high magnetic flux caused the motor core to saturate and overheat; the current reached 380 A at 20 Hz, with reactive current accounting for about 80% of the total current. To ensure proper operation of the motor, the technicians adjusted the V/F curve back to its normal setting after startup. But due to the requirements of the cement kiln’s operation process, frequent temporary shutdowns were necessary, making it inconvenient to keep adjusting the V/F curve. As a result, the company’s technicians collaborated with a university to develop specialized software using PLCs to address this technical issue and expand the use of frequency conversion speed control technology for cement kilns. However, company A-B rejected this idea due to high investment costs. In response, those technicians worked together with a university to come up with a control scheme that involved using PLCs to manage the process, with 9 different startup frequency combinations available, and buttons to adjust the operating curve after startup, thereby meeting the control requirements for frequency conversion speed regulation of cement kilns. Based on years of experience in the construction of rotary kilns, and after discussing the matter with the speaker following the meeting, the author demonstrated the errors in his analysis of the startup process of the rotary kiln as well as the inefficiency of the proposed final control scheme. As early as 1986, during the construction project of Kiln No. 6 at the Shanxi Aluminum Alumina Plant, the author conducted tests and analyses on the starting characteristics and operating conditions of the rotary kiln. This rotary kiln (φ4.5×90m) was driven by two ZD2-132-1B 200kW/440V DC motors; after starting up, its normal operating load was only 30%. The excess capacity of these motors was primarily used to overcome the high starting torque associated with the rotary kiln’s large moment of inertia, resulting in significant waste of funds and energy. Therefore, if it is possible to effectively increase the starting torque of the motors, the capacity of those motors used to drive the rotary kiln can be greatly reduced. For variable-frequency speed control systems, the output voltage in the low-frequency range (around 1/3 of the fundamental frequency) is very low, and the output torque is insufficient; this is the main reason for failed startup of such systems. To address this issue, inverter manufacturers have implemented an effective torque enhancement function in this frequency range (achieved by increasing the output voltage). When the torque enhancement setting is not sufficient to meet the torque required for motor startup, overcurrent occurs. The inverter detects this condition and blocks the output once the set threshold is reached, resulting in a failed startup. It can be shown that, during the operation of a rotary kiln, under the action of the material lifting plate, the material remains distributed on one side. The load torque remains relatively constant during startup and is not greatly affected by the angle at which the material is piled up. When the 4# cement kiln failed to start, the frequency converter always activated its protection mechanism at the same frequency, but the set values for the startup time varied significantly, and the height reached by the material each time was also different. Therefore, the main reason for the failure to start the cement kiln is not, as the speaker suggested, the eccentric loading caused by the piled-up material, which leads to the kiln rotating to a certain angle and resulting in an excessively high starting torque that triggers an overcurrent trip. Instead, it is due to the insufficient starting torque provided by the frequency converter at low speeds; this torque is not enough to overcome the additional resistance forces generated by the eccentricity of the material and the thermal bending of the kiln body, thus leading to a failure in starting the kiln under load. For a scheme that uses a PLC for control, it is highly uneconomical. In the author’s opinion, by providing sufficient torque enhancement in the low-speed range below 1/3 of the fundamental frequency, and maintaining a constant slope for the V/F curve at other frequencies, it is possible to achieve satisfactory speed control of the rotary kiln. It was later found that the technicians at the cement plant, after 2 months of operation of Kiln No. 4, successfully resolved this issue by conducting repeated tests to adjust the torque enhancement parameters. Currently, the system has been operating normally for half a year; it features simple control and significant energy savings. 2. Characteristics of the starting process and torque increase settings when variable frequency speed control is used for different loads: Due to the variable frequency and voltage capabilities of inverters, the starting process varies depending on the load. When setting the torque increase value, it is necessary to first analyze the characteristics of the starting process; the starting current can be monitored, and adjustments can be made accordingly. Generally, the smaller the increase value, the better, as this helps to reduce the impact on the system or prevent overcurrent protection from being triggered. ①Fan loads are also known as square-torque loads (torque is inversely proportional to the square of speed). Under favorable starting conditions, manufacturers generally provide a variety of load-reduction characteristic curves ranging from mild to severe for users to choose from; these can be set according to the principles outlined in point 2. However, for fan loads with an initial positive pressure at the system outlet, the effect of additional load torque must be taken into account in order to reduce the magnitude of the torque reduction. ②A constant torque load is an approximately ideal constant torque load; since its load torque remains constant, an appropriate torque boost can be determined from among a set of V/F curves with constant slopes, based on the actual value of that load torque. ③The special load associated with rotary kilns is, under normal operation, a constant-torque load with a large rotational inertia. However, during startup while the kiln is carrying material and is in a hot state, due to the eccentricity of the material and slight thermal bending of the kiln body, the torque applied at this time becomes a complex torque that varies with time. This results in worse conditions for starting the system. It is precisely this particularity of starting rotary kilns while they are hot that leaves many industry professionals at a loss and deterries them from taking action. This is also the key reason why variable-frequency speed control has not been widely adopted in rotary kiln drives despite its use for over a decade. As can be seen from the above analysis, to address the difficulties in starting a rotary kiln while it is hot, as well as the conflict between its wide speed control range and the need for energy efficiency during normal operation, it is necessary to fully understand the torque characteristics of the load under various starting conditions. It is also important to use inverters with high torque enhancement capabilities; generally, when setting torque enhancement, the voltage increase is usually 10% or less – for example, the enhancement provided by the Mingdian THY-FREC-VT210S model is only 5%. For constant-torque loads, the V/f curve consists of a series of lines with different slopes, which makes it difficult to meet the requirements for starting the rotary kiln and operating it in an energy-efficient manner. Therefore, it is essential to properly set the slope of the V/f curve at different frequencies (rotational speeds) in order to satisfy the needs related to starting the rotary kiln, achieving wide-range speed control, and ensuring energy efficiency (some brands of inverters allow a voltage increase of up to 20% in the low-frequency range). Currently, some frequency converters have an automatic torque increase function, which is expected to solve this problem effectively. 3. Points to consider regarding the torque boost setting of inverters: ① For transmission systems with poor starting conditions, when selecting an inverter, it is necessary to check whether the torque boost function of the chosen brand and model can meet the system’s requirements, as well as whether the slope of the constant torque U/F curve is adjustable. This is important to avoid losses resulting from incorrect selection, especially when the motor may operate at low speeds (below 1/3 of the fundamental frequency) for extended periods of time. ②When setting the torque boost function of the inverter, it is necessary to first analyze the torque characteristics and starting conditions of the load, select the appropriate boosting (reducing) method, and make adjustments while monitoring the starting current displayed on the monitor, so as to bring the system into an optimal starting condition. ③If the system trips during startup, it is necessary to determine whether the startup acceleration time is appropriate or if the mechanical load is too high (due to mechanical braking), etc., in order to set the torque increase appropriately. ④When setting the torque boost, care should be taken to minimize the boost amount as much as possible while still meeting the starting requirements, in order to avoid shocks to the system and overcurrent conditions. Authors: Jiang Pu, Zhang Jing