Discussion on the parameter setting of Siemens frequency converters

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Over the past decade or so, with the development of large-scale integrated circuits, computer control technology, and modern control theory – particularly the application of vector control technology – AC variable-frequency speed control technology has gradually acquired excellent technical properties such as a wide speed regulation range, high speed stability, fast dynamic response, and the ability to operate reversibly in all four quadrants. Its speed control performance can compare favorably with that of DC electric drives. In AC speed control technology, as the performance aspects such as speed control capabilities and reliability of frequency conversion speed control continue to improve and its cost keeps falling, especially given its significant energy-saving effects and the ease with which it enables speed control of AC motors, frequency converters are widely used in all applications that require speed control, thanks to their ease of use, compact size, and high control performance. Many of the problems that occur when using frequency converters are caused by improper setting of the converter’s parameters. The Siemens Micromaster 440 frequency converter has thousands of parameters that can be set; only by setting these parameters in a systematic, appropriate, and accurate manner can the full performance of the frequency converter be utilized. 1. Selection of control method The choice of the inverter’s control method is determined by the torque characteristics of the load. The mechanical load torque characteristics of the motor are given by the following formula: p = t × n / 9550, where p represents the motor power in kW, t represents the torque in N·m, and n represents the speed in revolutions per minute. The relationship between torque t and speed n can be roughly divided into 3 categories depending on the type of load. (1) Constant-torque loads in which the torque changes little even when the speed changes; such loads include conveyor belts, cranes, extruders, compressors, etc. (2) As the speed decreases, the torque decreases according to the square of the speed for such loads. Such loads include fans, various liquid pumps, etc. (3) A constant power load in which the torque decreases as the speed increases. Such loads include rolling mills, machine tool spindles, coilers, etc. The control methods provided by inverters include V/f control, vector control, and torque control. In v/f control, there are linear v/f control and parabolic characteristic v/f control. Setting the inverter parameter p1300 to 0 causes the inverter to operate in linear V/f control mode, which keeps the flux and excitation current essentially constant during speed regulation. Suitable for general constant-torque speed control applications whose operating speed is not in the low-frequency range. 　　 Set p1300 to 2, and the inverter will operate in the parabolic v/f control mode, which is suitable for loads such as fans and water pumps. The shaft power n of such loads is approximately proportional to the cube of the rotational speed n. Its torque m is approximately proportional to the square of the rotational speed n. For such a load, if the v/f characteristic of the inverter is linear, the allowable torque of the motor at low speeds is much greater than the load torque, resulting in a severe decrease in power factor and efficiency. To meet the requirements of such loads, the voltage decreases in a quadratic manner as the output frequency decreases, thereby reducing the magnetic flux and excitation current of the motor and keeping the power factor within an appropriate range. 　　 The v/f control curve can be further adjusted by setting parameters to suit the load characteristics. Setting an appropriate value for p1312 between 0 and 250 provides a start-up boost function. The output voltage at low frequencies is appropriately increased with respect to the linear v/f curve in order to compensate for the reduction in motor torque caused by the voltage drop resulting from stator resistance at low frequencies. Suitable for speed control objects with high starting torque. 　　 When a motor is driven using the V/f control mode of an inverter, at certain frequency ranges, the motor’s current and speed may oscillate; in severe cases, the system fails to operate. Overcurrent protection can also be triggered during acceleration, preventing the motor from starting properly. This problem is more pronounced when the motor is operating under light load or has a low torque inertia. Based on the frequency points at which oscillations occur in the system, jump points and jump bandwidths can be set on the v/f curve; when the motor accelerates, it can automatically bypass these frequency ranges to ensure the normal operation of the system. From p1091 to p1094, 4 different jump points can be set, and p1101 is used to determine the jump bandwidth. 　　 Some loads require the motor to deliver a specific torque at certain frequencies; by using programmable V/f control to adjust the parameters of the inverter, the desired control curve can be achieved. Set p1320, p1322, and p1324 to determine the programmable V/f characteristic frequency coordinates, while p1321, p1323, and p1325 correspond to the programmable V/f characteristic voltage coordinates. 　　 The parameter p1300 is set to 20, and the inverter operates in vector control mode. This type of control is relatively sophisticated: it offers a wide speed regulation range, high starting torque at low speeds, precision as high as 0.01%, and fast response. High-precision speed control is achieved through the SVMPWM vector control method. 　　 The parameter p1300 is set to 22, and the inverter operates in vector torque control mode. This control method is the most advanced one available internationally at present; other methods involve simulating the parameters of DC motors and performing conformal transformation for regulation control. Vector torque control, on the other hand, uses the parameters of AC motors directly for control, offering simplicity and high precision. 2. Quick debugging 　　 Before using an inverter to drive a motor, quick debugging must be carried out. Parameter p0010 was set to 1 and p3900 was also set to 1, allowing for a quick tuning of the frequency converter. After this quick tuning was completed, the necessary motor data were calculated, and all other parameters were restored to their default values. In vector or torque control modes, to achieve proper control, it is very important to input the motor data to the inverter correctly; moreover, the automatic detection parameter p1910 for the motor data must be performed when the motor is at normal temperature. When this function is enabled (p1910 = 1), an alarm signal a0541 is generated as a warning; upon issuing the on command thereafter, automatic detection of the motor parameters begins immediately. 3. Acceleration and deceleration time adjustment The acceleration time is the time required for the output frequency to rise from 0 to the maximum frequency, while the deceleration time is the time required for it to drop from the maximum frequency back to 0. The appropriateness of the selected acceleration and deceleration times has a significant impact on the motor’s start-up and stoppage, as well as on the response speed of the speed control system. The constraint on the acceleration time setting is to limit the current within the overcurrent range, so as not to trigger the overcurrent protection device. During the motor’s deceleration operation, the inverter will be in regenerative braking mode. The mechanical energy stored in the drive system is converted into electrical energy, which is then fed back to the DC side through an inverter. The feedback electrical energy will cause the voltage across the energy storage capacitor in the intermediate circuit to rise. Therefore, the constraint on the deceleration time setting is to prevent the DC circuit voltage from becoming too high. The formulas for calculating acceleration and deceleration times are as follows: Acceleration time: ta = (jm + jl)n / 9.56(tma – tl). Deceleration time: tb = (jm + jl)n / 9.56(tmb – tl). Where: jm is the inertia of the motor; jl is the inertia of the load; n is the rated speed; tma is the torque applied by the motor for driving; tmb is the braking torque of the motor; tl is the torque of the load. The acceleration and deceleration times can be calculated using these formulas, or they can also be determined through simple testing methods. First, operate the drive system at its rated speed (at line frequency), then cut off the power supply to put the drive system into a free-braking state, and use a stopwatch to measure the time it takes for its speed to drop from the rated value to zero. The acceleration and deceleration times can initially be set at 1/2 to 1/3 of the free braking time. By starting and stopping the motor, check for overcurrent or overvoltage alarms; adjust the setting values for acceleration and deceleration time, with the principle that no alarms should occur during operation. Repeat this process several times to determine the optimal acceleration and deceleration times. 4. Inertia setting 　　 The setting of the inertia of the motor and the load is often overlooked; it is believed that proper setting of the acceleration and deceleration times will ensure the proper operation of the system. In fact, an improper setting of the moment of inertia can cause the system to oscillate, and the speed control accuracy will also be affected. Moment of inertia formula: j = t/dω/dt. The moment of inertia of both the motor and the load is determined in the same way, by setting the operating frequency of the inverter to an appropriate value, between 5 and 10 Hz. Operate the motor without a load and with a load respectively, read the parameters r0333 for rated torque and r0345 for the motor’s starting time; then convert the frequency at which the inverter operates into the corresponding angular velocity, and use this value in the formula to calculate the rotational inertia of both the motor and the load. Set parameter p0341 (motor inertia) and parameter p0342 (ratio of total drive inertia to motor inertia), so that the frequency converter can regulate speed more effectively. Conclusion 　　 There are an increasing number of brands of frequency converters, and their functions continue to be improved and enhanced. Setting the parameters correctly is very important for the proper use of the frequency converter and for it to perform at its best.