Four basic steps to pay attention to when commissioning a frequency converter :
I. No-load power-on test of the frequency converter
1. Ground the inverter's grounding terminal.
2. Connect the power input terminal of the frequency converter to the power supply via a leakage protection switch.
3. Check if the factory display of the inverter's display window is normal. If it is incorrect, try resetting it. If it is still incorrect, you should request a refund or replacement.
4. Familiarize yourself with the inverter's operating keys. (See figure)
Most frequency inverters have six keys: RUN, STOP, PROG, DATA/ENTER, UP (▲), and DOWN (▼). The definitions of these keys are generally the same across different inverters. In addition, some inverters may have function keys such as MONTTOR/DISPLAY, RESET, JOG, and SHIFT.
II. Variable frequency drive with motor running under no-load
1. When setting the motor 's power and number of poles, the inverter's operating current must be taken into account.
2. Set the inverter's maximum output frequency, base frequency, and torque characteristics. The V/f type selection includes the maximum frequency, base frequency, and torque type. The maximum frequency is the highest frequency at which the inverter-motor system can operate. Since the inverter's own maximum frequency may be higher, if the motor's maximum allowable frequency is lower than the inverter's maximum frequency, it should be set according to the requirements of the motor and its load. The base frequency is the dividing line between constant power control and constant torque control of the motor by the inverter, and should be set according to the motor's rated voltage. The torque type refers to whether the load is a constant torque load or a variable torque load. Users should select one type based on the V/f type diagram in the inverter's instruction manual and the load characteristics. General-purpose inverters provide multiple V/f curves for users to choose from; users should select the appropriate V/f curve according to the nature of the load. For fan and pump loads, the inverter's torque operation code should be set to variable torque and reduced torque operation characteristics. To improve the inverter's low-speed performance during startup and ensure that the motor's output torque meets the startup requirements of the production load, the starting torque should be adjusted. In asynchronous motor variable frequency speed control systems, torque control is relatively complex. At low frequencies, the effects of resistance and leakage reactance are not negligible. If V/f is kept constant, the magnetic flux will decrease, thus reducing the motor's output torque. Therefore, appropriate voltage compensation is necessary at low frequencies to increase torque. Generally, this compensation is manually set by the user.
3. Set the frequency converter to the built-in keyboard operation mode, press the run button and stop button, and observe whether the motor can start and stop normally.
4. Familiarize yourself with the protection codes when a fault occurs during inverter operation. Observe the factory default values of the thermal protection relay and the overload protection settings, and modify them as needed. Inverter operators can set the electronic thermal relay function according to the inverter's instruction manual. The threshold value of the electronic thermal relay is defined as the ratio of the rated current of the motor and the inverter, usually expressed as a percentage. When the inverter's output current exceeds its allowable current, the inverter's overcurrent protection will cut off the inverter's output. Therefore, the maximum threshold value of the inverter's electronic thermal relay must not exceed the inverter's maximum allowable output current.
III. Load-bearing trial operation
1. Manually operate the run/stop button on the inverter panel, observe the motor running and stopping process and the inverter's display window, and check for any abnormal phenomena.
2. If the inverter trips its overcurrent protection during motor startup or shutdown, the acceleration and deceleration times should be reset. The motor's acceleration during acceleration and deceleration depends on the accelerating torque, while the inverter's frequency change rate during startup and braking is set by the user. If the motor's moment of inertia or load changes, insufficient accelerating torque may occur when accelerating or decelerating according to the preset frequency change rate, leading to motor stall. This means the motor speed and the inverter's output frequency are not synchronized, resulting in overcurrent or overvoltage. Therefore, the acceleration and deceleration times need to be set reasonably based on the motor's moment of inertia and load to ensure the inverter's frequency change rate matches the motor's speed change rate. A method to check the rationality of this setting is to first select and set the acceleration and deceleration times based on experience. If overcurrent occurs during startup, the acceleration time can be appropriately extended; if overcurrent occurs during braking, the deceleration time should be appropriately extended. On the other hand, the acceleration and deceleration times should not be set too long, as this will affect production efficiency, especially with frequent startup and braking.
3. If the frequency converter continues to operate under protection within the specified time, the start/stop operating curve should be changed from a straight line to an S-shaped, U-shaped, or inverse S-shaped or inverse U-shaped curve. When the motor load inertia is large, a longer start and stop time should be used, and the operating curve type should be set according to its load characteristics.
4. If the inverter still has operational faults, try increasing the maximum current protection value, but do not cancel the protection. At least 10%-20% protection margin should be left.
5. If the inverter malfunctions, it should be replaced with an inverter with a higher power rating.
6. If the frequency converter fails to reach the preset speed during startup, there may be two situations:
(1) Electromechanical resonance in the system can be detected by the sound of the motor running. The resonance point can be avoided by setting a frequency jump value. Generally, frequency converters can be set with three jump points. When a V/f controlled frequency converter drives an asynchronous motor, the motor current and speed will oscillate in certain frequency ranges. In severe cases, the system cannot operate, and overcurrent protection may even occur during acceleration, preventing the motor from starting normally. This is more serious when the motor is lightly loaded or has low rotational inertia. Ordinary frequency converters are equipped with a frequency jump function. Users can set the jump point and jump width on the V/f curve according to the frequency points where the system oscillates. When the motor accelerates, it can automatically skip these frequency ranges, ensuring the system can operate normally.
(2) Insufficient torque output capability of the motor. Different brands of frequency converters have different factory parameter settings, resulting in different load capacities under the same conditions. This may also be due to differences in the frequency converter control methods or the system's output efficiency. In this case, the torque boost value can be increased. If this is insufficient, the manual torque boost function can be used, but it should not be set too high, as this will increase the motor's temperature rise. If this still doesn't work, a new control method should be used. For example, Hitachi frequency converters use a constant V/f ratio method. If the starting requirement is not met, a sensorless space vector control method can be used, which has a greater torque output capability. For fan and pump loads, the torque reduction curve value should be reduced.
IV. Connecting the frequency converter to the host computer for system debugging
After completing the basic manual settings, if the system has a host computer, connect the inverter's control lines directly to the host computer's control lines and change the inverter's operating mode to terminal control. Adjust the inverter's frequency signal receiving terminal range (0-5V or 0-10V) and the inverter's response speed to analog frequency signal sampling according to the host computer system's requirements. If additional monitoring meters are needed, select the analog output monitoring quantity and adjust the inverter's output monitoring quantity terminal range.
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