【Daily Question 20090218】What is the purpose of adjusting the U/F curve in an inverter?

V/F control refers to adjusting the voltage-to-frequency ratio (U/f ratio) on the output side of the inverter. To change the control method for the mechanical characteristics of the motor during speed regulation. 　　An asynchronous motor when the load torque remains constant ； Reducing the frequency lowers the motor speed. It will result in a decrease in output power ； There is no direct relationship between the motor’s input power and frequency. That is, the input power of the motor does not automatically decrease as the frequency drops. Therefore, a decrease in frequency leads to a severe imbalance between the input power and the output power, resulting in a relatively large increase in the electromagnetic power and flux used for energy transfer. The magnetic circuit of the motor becomes severely saturated, the waveform of the excitation current is greatly distorted, and large peak currents are generated. Therefore, the inverter must reduce the output voltage accordingly while lowering the frequency, in order to maintain the balance between input power and output power. 　　To reduce the output voltage while operating at low frequencies, it is still necessary to ensure that the motor can generate sufficient torque to drive the load; this requires us to adjust the U/f ratio appropriately according to the characteristics of the load, in order to achieve the desired mechanical properties of the motor. 　　I. The U/f wires of the inverter 2. Criteria for selecting the U/f wires The inverter can provide multiple U/f wires for the user to choose from, or the desired U/f wires can be obtained by setting the functional parameters. When in use, the corresponding U/f line of the frequency converter should be selected or set based on the low-speed characteristics of the load. 　　For constant-torque loads, the torque that resists rotation remains unchanged regardless of the speed; for example, in belt conveyors, it is required that the motor generate a high torque even at low speeds, as shown in Figure 1. In this case, the U/f ratio should be set to a higher value; as shown in line 1 of the U/f graph in Figure 4, the voltage is increased to UX1 when the frequency is at fx, thereby enabling torque compensation and voltage elevation during low-frequency operation. 　　Some loads exhibit significant changes in braking torque at high and low speeds, such as centrifugal casting machines. Its torque characteristics are shown in Figure 2. The motor does not require much torque when operating at low frequencies; therefore, the U/f ratio can be set to a lower value, as indicated by the U/f line in Figure 4. II. Selecting the U/f line through functional parameters Any selection of the U/f line for a particular characteristic must be achieved through the setting of functional parameters. The following uses the Fuji G1*1S series frequency converters as an example to explain the method of parameter setting. 　　Figure 5 shows the constant torque characteristic, Figure 6 shows the torque characteristic with quadratic decay, and Figure 7 shows the proportional torque characteristic between the torque characteristic with quadratic decay and the constant torque characteristic. As can be seen from the schematic diagrams of various U/f torque curves. The slope of the curve in the lower part (low-frequency region) of Figure 7 is between those of the corresponding regions in Figures 5 and 6; that is, it is steeper than the low-frequency region in Figure 6, but gentler than that in Figure 5. The U/f line marked as “#2 . O” in the figure is the curve obtained when the parameter F09 is set to 2.0; the numbers with the prefix “#” have the same meaning. 　　The F09 parameter name for this inverter is “Torque Boost 1”; setting it to 0.0 enables the automatic torque boost feature, which automatically adjusts the torque boost value for constant-torque loads, allowing it to vary automatically between the U/f lines of #2.0 and 5#20.0. 　　If the load is a centrifugal fan or a water pump, which are devices with torque that decreases following a quadratic law, then the F09 setting should be between 0.1 and 0.9. Since the set value can be increased in increments of 0.1, there are a total of 9 U/fl lines available for selection. During specific setup, the smaller value should be chosen first; if the starting torque is insufficient, it can then be increased gradually to avoid low-frequency over-excitation and even tripping during the starting process. 　　If the load has a constant torque characteristic. If the automatic torque boost function is not to be used, parameter 1709 can be set to a value within the range of 2.0 to 20.0, as shown in Figure 5. There are a total of 180 U/f wires to choose from here. The setting principle remains the same: the values should increase gradually, as long as the starting torque is satisfied. 　　For other loads that fall between torque decay following a quadratic law and constant torque behavior, as shown in Figure 7, the parameter F09 should be selected within the range of 1.0 to 1.9. This set of U/f lines consists of 10 in total. 　　Some frequency converters allow the U/f curve to be set in a broken-line format. After setting the intermediate voltage and intermediate frequency parameters, the corresponding points of these two parameters on the U/f curve represent the turning points of the curve. Due to space constraints, further details are not provided; for specific applications, refer to the relevant inverter manuals. This post was last edited by tuubt on 2009-2-17 22:00]

To maintain a balance between input power and output power, the inverter reduces the output voltage accordingly while lowering the frequency. Moreover, at this time, in order to ensure that the motor can generate sufficient torque to drive the load, it is necessary to set a proper V/F curve that meets the requirements of the load. This is also the function of the V/F curve.

To maintain a balance between input power and output power, the inverter reduces the output voltage accordingly while lowering the frequency. Moreover, at this time, in order to ensure that the motor can generate sufficient torque to drive the load, it is necessary to set a proper V/F curve that meets the requirements of the load. This is also the function of the V/F curve.

In practical applications, the criterion for setting the preset V/F is usually the ability to carry the load even under heavy loading conditions. However, when the load is light, it can lead to excessive compensation, causing saturation of the magnetic circuit and resulting in tripping under no-load conditions. Therefore, during debugging, V/F should be adjusted gradually from low to high, and it is necessary to test whether the system can handle heavy loads and whether it trips under no-load conditions, until both requirements are met.

My answer is: The function of the inverter in adjusting the U/F curve is to use the ability to drive the load even under heavy load conditions as a basis for setting the preset V/F value in actual operation. However, when the load is light, it can lead to excessive compensation, causing saturation of the magnetic circuit and resulting in tripping under no-load conditions. Therefore, during debugging, V/F should be adjusted gradually from low to high, and it is necessary to test whether the system can handle heavy loads and whether it trips under no-load conditions, until both requirements are met.

To maintain a balance between input power and output power, the inverter reduces the output voltage accordingly while lowering the frequency. At this point, in order to ensure that the motor can generate sufficient torque to drive the load, it is necessary to set a proper V/F curve that meets the requirements of the load.

What is the purpose of adjusting the U/F curve with an inverter? When U/F = K (a constant), it represents a constant-torque speed control method. Depending on the load, the inverter automatically and effectively adjusts the optimal U/f ratio, ensuring that the voltage output by the inverter matches the required value. This allows for stable control of the output power, and this is key to the high efficiency of the inverter. High efficiency in power utilization means that the inverter can help save energy.

The control methods of frequency converters include: 1) the sine wave pulse width modulation mode with v/f = c, 2) the vector control (VC) mode, and 3) the direct torque control (DTC) mode. The basic principle of V/f control in frequency converters is u/f = c; therefore, the expression for ux at a frequency of fx is ux/fx = c, where c is a constant, known as the “voltage-to-frequency ratio coefficient”. To maintain a balance between input power and output power, the inverter reduces the output voltage accordingly while lowering the frequency. Moreover, at this time, in order to ensure that the motor can generate sufficient torque to drive the load, it is necessary to set a proper V/F curve that meets the requirements of the load. This is also the function of the V/F curve.