1. Analog Signal-Based Forward and Reverse Control: Many frequency converters have the function of directly controlling forward and reverse rotation using analog signal inputs. There are two main methods:
(1) The forward and reverse directions are controlled by the positive and negative values of the given signal. For example, the given signal can be preset to (-10~+10)V, where (-10~0)V is the reverse signal and (0~+10)V is the forward signal.
(2) Use the midpoint of the given signal as the dividing point between forward and reverse rotation. For example, when the given signal is (0~+10)V, it can be preset that (0~5)V is the reverse rotation signal and (5~10)V is the forward rotation signal.
For this forward and reverse control method, some frequency converters are equipped with the following functions:
(1) Dead-zone function: When using analog input signals for forward and reverse control, "0" speed control is difficult to stabilize, often resulting in a "creeping" phenomenon in either forward or reverse rotation. To prevent this "creeping" phenomenon, a dead zone needs to be set near "0" speed. Specifically, a minimum frequency dead zone is preset in both the forward and reverse directions.
The dead zone range is defined by the ratio "Xmin f" and "-Xmin f".
(1) The dead zone is in the shape of a loop, as shown in Figure 1(a); while in the TVF2000 series inverter (China) and ABB-ACS600 inverter (Sweden), the above-mentioned (2) method is used, and the dead zone is shown in Figure 1(b).
Figure 1. Simulation of given forward and reverse control (a) Positive and negative signal control (b) Positive signal control
(2) Effective "0" Signal Function In the control method described above (2), there is a special problem. That is, if the given signal is "lost" due to poor circuit contact or other reasons, the signal obtained at the given input terminal of the frequency converter will be "0". According to the curve shown in Figure 2(b), its output frequency should be -50Hz, and the motor will be in a high-speed reverse state. In actual work, this situation is often very harmful and may even damage the production machinery.
To address this, the frequency converter is equipped with a valid "0" function. For example, the valid "0" can be preset to 0.5V or higher. Then, when the given signal is less than the preset valid "0", the frequency converter's output frequency will drop to 0Hz.
To address this, the frequency converter is equipped with a valid "0" function. For example, the valid "0" can be preset to 0.5V or higher. Then, when the given signal is less than the preset valid "0", the frequency converter's output frequency will drop to 0Hz.
2. Proportional interlocking setpoint
Figure 2. Proportional interlocking setpoint (a) Wiring diagram (b) Logic relationship
In actual production, the basic speed of the frequency converter is given by the main setpoint signal. However, during production, it is often necessary to correct (fine-tune) the motor speed based on other signals. For example, the basic speed of the cooling fan of a certain machine is adjusted by the operator according to the specific situation. At the same time, when the ambient temperature changes, the speed needs to be automatically fine-tuned. General frequency converters can achieve this control, but the Meiden VT230S series frequency converter (Japan) has specially set up an auxiliary signal input terminal and provided a very clear calculation method, as shown in Figure 2. Figure 2(a) shows the arrangement of the external setpoint terminals, where FSV is the main signal setpoint terminal and AUX is the auxiliary signal setpoint terminal. The logical relationship is shown in Figure 2(b), and the relationship between the quantities in the figure is:
Y = AX + B + C
In the formula, Y is the overall given signal; X is the main given signal; A is the frequency gain; B is the bias frequency; and C is the auxiliary given signal.
