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| SEW Frequency Converters |
Frequency converters have numerous setup parameters, each with a specific range of selectable values. It is common to encounter situations where improper settings of individual parameters can lead to the converter failing to operate normally. Therefore, it is crucial to correctly configure the relevant parameters.
The control method refers to speed control, torque control, PID control, or other modes. Once the control method is selected, static or dynamic identification is typically performed based on the required control accuracy.
This refers to the minimum rotational speed at which the motor operates. When the motor runs at low speeds, its heat dissipation performance is poor, and prolonged operation at low speeds can lead to motor burnout. Additionally, at low speeds, the current in the motor's cables increases, which can also cause the cables to overheat.
Standard frequency converters typically have a maximum frequency of up to 60Hz, with some capable of reaching even 400Hz. Operating at high frequencies causes the motor to spin rapidly. For conventional motors, their bearings are not designed to sustain extended periods of operation above their rated speed. Additionally, it must be considered whether the motor's rotor can withstand the centrifugal force generated at such high speeds.
The higher the carrier frequency setting, the greater the high-order harmonic components it generates. This is closely related to factors such as cable length, motor heating, cable heating, and inverter heating.
The frequency converter is configured with the motor's power, current, voltage, speed, and maximum frequency within its parameters. These parameters can be directly obtained from the motor's nameplate.
Resonance phenomena may occur at certain frequency points, particularly when the entire installation is relatively tall. When controlling compressors, it is necessary to avoid the compressor's resonant frequency points.
Acceleration time refers to the duration required for the output frequency to rise from 0 to the maximum frequency, while deceleration time is the duration required for the frequency to drop from the maximum to 0. Typically, the acceleration and deceleration times are determined by the rising and falling edges of the frequency setpoint signal. During motor acceleration, it is necessary to limit the rate of increase of the frequency setpoint to prevent overcurrent, while during deceleration, the rate of decrease is limited to prevent overvoltage.
Key considerations for setting acceleration time:
- Limit the acceleration current to below the overcurrent capacity of the frequency converter to prevent overcurrent-induced trips.
Key considerations for setting deceleration time:
- Prevent excessive voltage in the smoothing circuit to avoid regenerative overvoltage-induced trips.
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. Gradually reduce the acceleration and deceleration times, repeating the process several times while ensuring no alarms occur during operation. This approach will help determine the optimal acceleration and deceleration times.
Also known as torque compensation, torque boost is a method of increasing the low-frequency range (f/V) to compensate for the torque reduction at low speeds caused by the resistance of the motor's stator windings. When set to automatic, the voltage during acceleration is automatically increased to compensate for the starting torque, ensuring smooth motor acceleration. When using manual compensation, 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 result in excessive output voltage at low speeds, leading to wasted energy. In extreme cases, it may even cause high starting current with little or no increase in rotational speed when the motor is starting with a load.
This function is designed to protect the motor from overheating. The CPU within the frequency converter calculates the motor's temperature rise based on the operating current and frequency values, thereby providing overheat protection. This function is only applicable in "one drive for one motor" scenarios. In "one drive for multiple motors" scenarios, thermal relays should be installed on each individual motor.
The setting value for electronic thermal protection (%) is calculated as: [Motor Rated Current (A) / Inverter Rated Output Current (A)] × 100%.
The frequency limit refers to the upper and lower boundary values of the output frequency of the frequency converter. This feature serves as a protective function to prevent equipment damage caused by excessively high or low output frequencies due to misoperation or malfunctions in external frequency setting signal sources. It can be set according to actual application scenarios. Additionally, this function can also be used for speed limiting. For example, in some belt conveyors where the amount of conveyed material is relatively low, to reduce wear and tear on the machinery and belts, a frequency converter can be used to drive the conveyor, and the upper limit frequency of the frequency converter can be set to a specific value. This allows the belt conveyor to operate at a fixed, lower working speed.
Offset Frequency
Also known as deviation frequency or frequency deviation setting, this feature is used to adjust the output frequency when the frequency is set by an external analog signal (voltage or current) at its lowest setting. Some frequency converters allow the deviation value to be adjusted within the range of 0 to fmax when the frequency setting signal is at 0%. Additionally, some converters (such as Meidensha and Sanken) permit the setting of offset polarity. For instance, during commissioning, if the frequency output from the converter is not 0Hz but xHz when the frequency setting signal is at 0%, setting the offset frequency to -xHz will adjust the output frequency to 0Hz.
This function is only effective when using an external analog signal to set the frequency. It is utilized to compensate for any inconsistencies between the external setting signal voltage and the internal voltage (+10V) of the frequency converter. Additionally, it 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), the frequency percentage that can be output from the f/V graph is calculated and used as a parameter for the setting. For instance, if the external setting signal ranges from 0 to 5V and the frequency converter's output frequency ranges from 0 to 50Hz, the gain signal should be set to 200%.
Torque limitation can be classified into two types: drive torque limitation and brake torque limitation. It involves torque calculations performed by the CPU based on the output voltage and current values of the frequency converter. This feature significantly improves the shock load recovery characteristics during acceleration, deceleration, and constant speed operation. The torque limitation function enables automatic acceleration and deceleration control. Even when the acceleration or deceleration time is shorter than the load inertia time, it ensures that the motor accelerates or decelerates automatically according to the torque setpoint.
The drive torque function provides robust starting torque. During steady-state operation, the torque function controls the motor slip to limit the motor torque within the maximum setpoint. 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 does not exceed the maximum setpoint. A higher drive torque setting is beneficial for starting, with a range of 80 to 100% being recommended.
A lower brake torque setting results in greater braking force, suitable for rapid acceleration and deceleration scenarios. However, excessively high brake torque settings can lead to overvoltage alarms. If the brake torque is set to 0%, the total amount of regeneration fed back to the main capacitor approaches zero, allowing the motor to decelerate to a stop without using a braking resistor or causing the converter to trip. However, with certain loads, when the brake torque is set to 0%, transient coasting may occur during deceleration, causing the converter to repeatedly start, resulting in significant current fluctuations. In severe cases, this can lead to converter tripping, which should be noted.