Frequency converters are crucial in industrial production. Besides speed control and soft starting, their most important function is energy saving. Frequency converters offer numerous functional parameters, typically dozens or even hundreds, for users to select. In practical applications, it's unnecessary to set and adjust every single parameter; most can be left at the factory default values. However, some parameters are highly dependent on actual usage conditions, and some are even interrelated, thus requiring specific setting and adjustment based on the specific circumstances.
The Importance of Inverter Parameter Adjustment
The installation and parameter adjustment of production equipment must be based on its specific conditions before it can be put into use. For example, many problems encountered during the maintenance of frequency converters are soft faults, which are closely related to parameters or may be due to problems with other components of the equipment. Therefore, it is essential to learn how to adjust parameters in order to determine the location of the fault in various application scenarios.
Adjustment of basic parameters of frequency converter
Inverter functions have many parameters, but in practical applications, it's unnecessary to set and adjust every single one; most can simply use the factory default values. However, some parameters are highly dependent on actual usage conditions, and some are even interrelated, so they need to be set and adjusted according to the specific situation. Because different types of inverters have different functions, and the names of parameters with the same function may vary, the basic parameters are almost universally present in all types of inverters, allowing for a general understanding across different types. The following parameters are commonly used:
I. Acceleration and deceleration time
1. Acceleration time: Acceleration time is the time from its startup frequency to its operating frequency.
2. Deceleration time: The time required for the motor to stop from its operating frequency can be set.
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. During motor acceleration, the rise rate of the frequency setting must be controlled to prevent overcurrent, while during deceleration, the fall rate must be controlled to prevent overvoltage.
Acceleration time setting requirements: The acceleration current should be kept 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, gradually shorten the acceleration/deceleration time, ensuring no alarms occur during operation. Repeating this process several times will determine the optimal acceleration/deceleration time.
II. Motor Parameter Settings
The parameters can be set in the frequency converter according to the rated voltage and rated current on the motor nameplate, and correspond accordingly.
1. Rotation direction: Mainly used to set whether to prohibit reverse rotation.
2. Stopping mode: Used to set whether to brake or stop freely.
3. Voltage upper and lower limits: Set limits according to the motor voltage of the equipment to avoid burning out the motor.
III. 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.
IV. 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%.
V. Torque
It can be divided into two types: driving torque and braking torque. It is calculated by the CPU based on the inverter's output voltage and current values, and it can significantly improve the recovery characteristics of impact loads during acceleration, deceleration, and constant speed operation. The torque function can realize 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 the motor slip, keeping the motor torque within the maximum set value. Even when the load torque suddenly increases, or if 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.
VI. 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.
VII. 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 single-motor setups. For multi-motor setups, thermal relays should be installed on each motor. Electronic thermal protection setting (%) = [Motor rated current (A) / Inverter rated output current (A)] × 100%.
VIII. Frequency
This refers to the upper and lower limits of the inverter's output frequency. The frequency setting is a protective function to prevent damage to the equipment from excessively high or low output frequency caused by misoperation or a malfunction of the external frequency setting signal source. It should be set according to the actual 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.
1. Panel speed adjustment: The frequency can be adjusted using the buttons on the panel.
2. Sensor control: The frequency can be controlled by using changes in the voltage or current of the sensor as a signal input.
3. Communication input: The frequency is controlled by a host computer such as a PLC .
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