The application of frequency converters in industrial production is extremely important. Apart from speed regulation and soft start functions, their most significant advantage lies in energy conservation. Frequency converters possess numerous functional parameters, often numbering in the dozens or even hundreds, for users to select from. In practical applications, it is not necessary to set and adjust every parameter, as most can simply adopt the factory default values. However, some parameters are closely related to actual usage scenarios and may even be interconnected, necessitating customization and adjustment based on reality.
Given the functional differences among various types of frequency converters, even the names of identical functional parameters may vary. Nevertheless, basic parameters are ubiquitous across all types of frequency converters, allowing for easy generalization. The following parameters are commonly utilized:
I. Acceleration and Deceleration Time
Acceleration Time: This refers to the time required for the motor to reach its operating frequency from its starting frequency.
Deceleration Time: It can be set to determine how long the motor takes to stop from its operating frequency.
Acceleration time signifies the duration for the output frequency to rise from 0 to the maximum frequency, while deceleration time denotes the time taken for the frequency to drop from the maximum to 0. Typically, the acceleration and deceleration times are determined by the rising and falling rates of the frequency setting signal. During motor acceleration, it is crucial to control the rate of frequency increase to prevent overcurrent, and during deceleration, to manage the rate of decrease to avoid overvoltage.
Acceleration Time Setting Requirements: The acceleration current should be set **below the overcurrent capacity of the frequency converter to prevent the converter from tripping due to overcurrent and resultant loss of speed. The key points for deceleration time setting are to prevent excessive voltage in the smoothing circuit, thus avoiding regenerative overvoltage and resultant loss of speed that could cause the frequency converter to trip. The acceleration and deceleration times can be calculated based on the load, but during commissioning, it is common to initially set longer acceleration and deceleration times based on the load and experience. By starting and stopping the motor, observe for any overcurrent or overvoltage alarms. Then, gradually shorten the acceleration and deceleration settings, adhering to the principle of no alarms occurring during operation. Repeat this process several times to determine the optimal acceleration and deceleration times.
II. Motor Parameter Settings
The parameters in the frequency converter can be set according to the rated voltage and rated current specified on the motor's nameplate to ensure correspondence.
Rotation Direction: Primarily used to set whether reverse rotation is prohibited.
Stop Mode: Determines whether the motor will stop with braking or in a free-running manner.
Voltage Upper and Lower Limits: Sets the voltage limits based on the motor's specifications to prevent overheating and potential damage to the motor.
III. Torque Boost
Also known as torque compensation, this feature is a method of increasing the f/V ratio within the low-frequency range to compensate for the decrease in torque at low speeds caused by the resistance of the motor's stator windings. When set to automatic, it automatically raises the voltage during acceleration to compensate for the starting torque, ensuring smooth acceleration of the motor. When manual compensation is employed, optimal curves can be selected through testing based on the load characteristics, particularly the starting characteristics of the load. For variable torque loads, improper selection can lead to excessive output voltage at low speeds, resulting in wasted energy, or even situations where the motor struggles to start with a load due to high current but insufficient rotational speed.
IV. Frequency Setting Signal Gain
This function is only effective when using an external analog signal to set the frequency. It is used to address inconsistencies between the voltage of the external setting signal and the internal voltage (+10V) of the frequency converter. It also facilitates the selection of the analog setting signal voltage. During setup, when the analog input signal is at its maximum (e.g., 10V, 5V, or 20mA), calculate the frequency percentage of the output f/V graph and use this as a parameter for setting. For example, if the external setting signal ranges from 0V to 5V and the desired output frequency of the converter ranges from 0Hz to 50Hz, the gain signal should be set to 200%.
V. Torque
Torque can be classified into two types: drive torque and braking torque. Based on the output voltage and current values of the frequency converter, the CPU performs torque calculations, which significantly improve the shock load recovery characteristics during acceleration, deceleration, and constant speed operation. The torque function enables automatic acceleration and deceleration control. Even when the acceleration and deceleration times are shorter than the load inertia time, the motor can still accelerate and decelerate automatically according to the set torque values.
The drive torque function provides robust starting torque. During steady-state operation, the torque function controls the motor slip to keep the motor torque within the maximum set value. When the load torque suddenly increases or even if the acceleration time is set too short, it prevents the frequency converter from tripping. When the acceleration time is set too short, the motor torque will not exceed the maximum set value. A higher drive torque is beneficial for starting, and a setting of 80-100% is generally recommended.
The smaller the braking torque setting, the greater the braking force, making it suitable for rapid acceleration and deceleration scenarios. However, setting the braking torque too high can result in an overvoltage alarm. If the braking torque is set to 0%, the total amount of regeneration fed into the main capacitor approaches zero, allowing the motor to decelerate to a stop without using a braking resistor and without tripping the frequency converter. However, in some loads, setting the braking torque to 0% may cause brief idling during deceleration, leading to repeated startups of the frequency converter, significant current fluctuations, and potentially tripping the converter. This should be taken into consideration.
VI. Acceleration and Deceleration Mode Selection
Also known as acceleration and deceleration curve selection, frequency converters typically offer three types of curves: linear, non-linear, and S-shaped. Linear curves are commonly chosen; however, non-linear curves are suitable for variable torque loads such as fans. The S-shaped curve is suitable for constant torque loads, offering a more gradual change in acceleration and deceleration. When setting, the appropriate curve can be selected based on the load torque characteristics, though there may be exceptions. During the commissioning of a boiler induced draft fan's frequency converter, the author initially selected a non-linear curve, resulting in immediate tripping upon startup despite numerous parameter adjustments. Switching to an S-shaped curve resolved the issue.
The underlying reason was that before starting, the induced draft fan rotated in reverse due to the flow of flue gas in the duct, effectively acting as a negative load. By selecting the S-shaped curve, the initial frequency increase was slower, preventing the frequency converter from tripping. This approach is particularly useful for converters without a DC braking function during startup.
VII. Electronic Thermal Overload Protection
This feature is designed to protect motors from overheating. The CPU within the frequency converter calculates the motor's temperature rise based on the operating current and frequency, thereby implementing overheat protection. This function is only applicable in "one-to-one" scenarios. In "one-to-many" configurations, thermal relays should be installed on each motor. The electronic thermal protection setpoint (%) is calculated as follows: [Motor Rated Current (A) / Frequency Converter Rated Output Current (A)] × 100%.
VIII. Frequency
This refers to the upper and lower limit values of the output frequency of the frequency converter. Frequency limitation serves as a protective function to prevent excessively high or low output frequencies caused by misoperation or malfunction of the external frequency setting signal source, thereby safeguarding the equipment from damage. In application, these limits can be set according to actual needs. Additionally, this function can also be utilized for speed limitation. For instance, in the case of belt conveyors transporting relatively light loads, to reduce wear and tear on the machinery and belts, a frequency converter can be employed to drive the conveyor, and the upper limit frequency of the converter can be set to a specific value, allowing the conveyor to operate at a fixed, lower working speed.
Panel Speed Regulation: Adjusting the frequency via buttons on the panel.
Sensor Control: Controlling the frequency by using changes in sensor voltage or current as signal inputs.
Communication Input: Regulating the frequency through communication with superior controllers such as PLCs.