Abstract: For variable torque loads, improper selection may result in excessively high output voltage at low speeds, wasting electrical energy, or even high starting current when the motor is under load, resulting in insufficient speed.
I. Acceleration and deceleration time
Acceleration time is the time required for the output frequency to rise from 0 to the maximum frequency, while deceleration time is the time required for the frequency to drop from the maximum frequency to 0. Acceleration and deceleration times are typically determined by the rise and fall of the frequency setting signal. When the motor accelerates, the rise rate of the frequency setting must be limited to prevent overcurrent, and when decelerating, the fall rate must be limited to prevent overvoltage.
Acceleration time setting requirements: The acceleration current should be limited below the inverter's overcurrent capacity to prevent overcurrent stall and inverter tripping. The key point for deceleration time setting is to prevent excessive voltage in the smoothing circuit to avoid regenerative overvoltage stall and inverter tripping. Acceleration and deceleration times can be calculated based on the load, but during commissioning, it is common practice to initially set a longer acceleration/deceleration time based on the load and experience, observing for overcurrent and overvoltage alarms by starting and stopping the motor. Then, the acceleration/deceleration time is gradually shortened, ensuring no alarms occur during operation. Repeating this process several times will determine the optimal acceleration/deceleration time.
II. Torque Boost
Also known as torque compensation, it's a method to increase the f/V ratio in the low-frequency range to compensate for the torque reduction at low speeds caused by the resistance of the motor stator windings. When set to automatic, it automatically increases the voltage during acceleration to compensate for the starting torque, allowing the motor to accelerate smoothly. When using manual compensation, the optimal curve can be selected through testing based on the load characteristics, especially the starting characteristics. For variable torque loads, improper selection can lead to excessively high output voltage at low speeds, wasting electrical energy, and may even result in high starting current and insufficient speed during motor start-up under load.
III. Electronic thermal overload protection
This function is designed to protect the motor from overheating. The inverter's CPU calculates the motor's temperature rise based on the operating current and frequency, thus providing overheat protection. This function is only applicable to "one-to-one" applications. In "one-to-many" applications, thermal relays should be installed on each motor.
Electronic thermal protection setting value (%) = [motor rated current (A) / inverter rated output current (A)] × 100%.
IV. Frequency Limitation
This refers to the upper and lower limits of the inverter's output frequency. Frequency limiting is a protective function to prevent damage to the equipment caused by misoperation or a malfunction of the external frequency setting signal source, which could result in an excessively high or low output frequency. It should be set according to the actual situation in application. This function can also be used for speed limiting. For example, some belt conveyors, due to the relatively small amount of material being conveyed, can be driven by an inverter to reduce wear on the machinery and belt. The upper limit frequency of the inverter can be set to a certain value, allowing the belt conveyor to operate at a fixed, lower speed.
V. Bias Frequency
Some also call it frequency deviation or frequency deviation setting. Its purpose is to adjust the output frequency when the frequency is set by an external analog signal (voltage or current), as shown in Figure 1, when the frequency setting signal is at its lowest. Some inverters allow the deviation value to operate within the range of 0 to fmax when the frequency setting signal is 0%. Some inverters (such as Meidensha and Sanko) also allow setting the bias polarity. For example, during commissioning, if the inverter output frequency is not 0Hz but xHz when the frequency setting signal is 0%, then setting the bias frequency to a negative xHz will make the inverter output frequency 0Hz.
VI. Frequency Setting Signal Gain
This function is only effective when setting the frequency using an external analog signal. It is used to compensate for the inconsistency between the external setting signal voltage and the inverter's internal voltage (+10V); it also facilitates the selection of the analog setting signal voltage. When setting, when the analog input signal is at its maximum (e.g., 10V, 5V, or 20mA), calculate the percentage of the frequency that can output the f/V graph and use this as the parameter for setting; if the external setting signal is 0-5V, and the inverter output frequency is 0-50Hz, then the gain signal can be set to 200%.
VII. Torque Limitation
It can be divided into two types: drive torque limiting and braking torque limiting. Based on the inverter's output voltage and current values, the CPU performs torque calculations, which significantly improves the recovery characteristics of impact loads during acceleration, deceleration, and constant speed operation. The torque limiting function enables automatic acceleration and deceleration control. Even if the acceleration/deceleration time is less than the load inertia time, it can still ensure that the motor automatically accelerates and decelerates according to the torque set value.
The drive torque function provides strong starting torque. During steady-state operation, the torque function controls motor slip and limits the motor torque within the maximum set value. Even when the load torque suddenly increases, or when the acceleration time is set too short, the inverter will not trip. Even with a short acceleration time, the motor torque will not exceed the maximum set value. A high drive torque is beneficial for starting; setting it to 80-100% is recommended.
A smaller braking torque setting results in greater braking force, suitable for applications requiring rapid acceleration and deceleration. However, setting the braking torque too high can trigger an overvoltage alarm. Setting the braking torque to 0% reduces the total regenerative capacity applied to the main capacitor to near zero, allowing the motor to decelerate to a stop without a braking resistor and without tripping. However, under certain loads, a 0% braking torque setting can cause brief periods of idling during deceleration, leading to repeated inverter starts, significant current fluctuations, and in severe cases, inverter tripping. This should be noted.
8. Acceleration/Deceleration Mode Selection
Also known as acceleration/deceleration curve selection. Generally, frequency converters have three types of curves: linear, non-linear, and S-curve. The linear curve is usually chosen. Non-linear curves are suitable for variable torque loads, such as fans. S-curves are suitable for constant torque loads, where acceleration and deceleration are relatively slow. The appropriate curve can be selected based on the load torque characteristics, but there are exceptions. When debugging a frequency converter for a boiler induced draft fan, I initially selected a non-linear acceleration/deceleration curve. Upon startup, the frequency converter tripped. Adjusting many parameters had no effect. After switching to an S-curve, it functioned normally. The reason is that before startup, the induced draft fan rotates due to the flow of flue gas and reverses, becoming a negative load. Selecting an S-curve slows the frequency rise during startup, thus preventing the frequency converter from tripping. Of course, this method is used for frequency converters without a DC braking function.
IX. Torque Vector Control
Vector control is based on the theoretical assumption that asynchronous motors and DC motors have the same torque generation mechanism. Vector control decomposes the stator current into a defined magnetic field current and torque current, controls them separately, and simultaneously outputs the combined stator current to the motor. Therefore, in principle, it achieves the same control performance as a DC motor. Using torque vector control, the motor can output maximum torque under various operating conditions, especially in the low-speed operating range.
Modern frequency converters almost universally employ feedback-free vector control. Because the converter can compensate for slip based on the load current magnitude and phase, it imparts a very stiff mechanical characteristic to the motor, which is sufficient for most applications, eliminating the need for an external speed feedback circuit. This function can be enabled or disabled depending on the specific situation.
The related function is slip compensation control, which compensates for speed deviations caused by load fluctuations by adding a slip frequency corresponding to the load current. This function is mainly used for positioning control.
10. Energy-saving control
Fans and water pumps are both torque-reducing loads, meaning that as the speed decreases, the load torque decreases proportionally to the square of the speed. Inverters with energy-saving control functions are designed with a dedicated V/f mode. This mode can improve the efficiency of the motor and inverter. It can automatically reduce the inverter's output voltage according to the load current, thereby achieving energy saving. It can be set to be effective or ineffective depending on the specific situation.
It should be noted that parameters nine and ten are very advanced, but some users cannot enable these two parameters during equipment modification. That is, after enabling them, the frequency converter trips frequently, but everything is normal after disabling them. The reasons are as follows: (1) The original motor parameters are too different from the motor parameters required by the frequency converter. (2) Insufficient understanding of the setting parameter functions, such as the energy-saving control function can only be used in V/f control mode and cannot be used in vector control mode. (3) Vector control mode is enabled, but the motor parameters are not manually set and automatically read, or the reading method is improper.
