I. Working Principle of Mitsubishi Frequency Inverters
The most widely used Mitsubishi inverters on the market are the A700 series and the E700 series. The A700 series is a general-purpose inverter, suitable for applications requiring high starting torque and high dynamic response. The E700 series, on the other hand, is suitable for applications with simpler functional requirements and lower dynamic performance requirements, and it is more cost-effective.
The working principle of Mitsubishi inverters is mainly as follows:
1. Main Circuit: The reactor's function is to prevent high-order harmonics generated by the Mitsubishi inverter from returning to the power grid through the power input circuit and affecting other electrical equipment. Whether a reactor is needed depends on the capacity of the Mitsubishi inverter. A filter is installed at the output of the Mitsubishi inverter to reduce high-order harmonics. A filter should be installed when the distance between the Mitsubishi inverter and the motor is long. Although Mitsubishi inverters have various protection functions, their phase loss protection is not perfect. Circuit breakers in the main circuit provide overload and phase loss protection; their selection should be based on the capacity of the Mitsubishi inverter. The overload protection of the Mitsubishi inverter itself can replace the thermal relay.
2. Control circuit: It has manual switching between power frequency and frequency conversion, so that the power frequency can be manually switched to operation when the frequency converter fails. Since the output terminal cannot be energized, the power frequency and frequency converter must be interlocked.
II. Load-bearing test operation of Mitsubishi inverters
① Manually operate the run/stop button on the Mitsubishi inverter panel, observe the motor running and stopping process and the inverter's display window, and check for any abnormal phenomena.
② If the Mitsubishi inverter trips its overcurrent protection during the start-stop process of the motor, the acceleration and deceleration settings should be reset. The acceleration during acceleration and deceleration depends on the acceleration torque, while the frequency change rate of the Mitsubishi inverter during start-up and braking is set by the user. If the motor's moment of inertia or load changes, insufficient acceleration 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 Mitsubishi inverter's output frequency are not coordinated, resulting in overcurrent or other problems. Therefore, the acceleration and deceleration times need to be set reasonably based on the motor's moment of inertia and load to ensure the frequency change rate of the Mitsubishi inverter matches the motor speed change rate. To check if this setting is reasonable, first select the acceleration and deceleration times based on experience. If overcurrent occurs during start-up, the acceleration time can be appropriately extended; if overcurrent occurs during braking, the deceleration time should be appropriately extended. However, the acceleration and deceleration times should not be set too long, as this will affect production efficiency, especially during frequent start-up and braking.
③ If the Mitsubishi inverter continues to operate under protection within the specified time, the start-stop curve should be changed from a straight line to an S-shaped, U-shaped, or S-shaped/reverse U-shaped curve. When the motor load inertia is large, a longer start-stop time should be used, and the operating curve type should be set according to its load characteristics.
④ If the Mitsubishi inverter still malfunctions, try increasing the maximum current protection value, but do not disable the protection; leave a protection margin of at least 10-20%. (Imported pumps and valves)
⑤ If the Mitsubishi inverter malfunctions, it should be replaced with an inverter of a higher power rating.
⑥ If the Mitsubishi inverter fails to reach the preset speed during startup, there may be two possibilities:
(1) The system is experiencing electromechanical resonance, which can be determined by the sound of the motor running.
By setting frequency jump values, resonance points can be avoided. Most frequency inverters can be set with three jump points. When a Mitsubishi frequency inverter controlled by VPf drives an asynchronous motor, the motor's current and speed may oscillate in certain frequency ranges. In severe cases, the system may fail to operate, or even trigger overcurrent protection during acceleration, preventing the motor from starting normally. This is more pronounced when the motor is lightly loaded or has low rotational inertia. Ordinary frequency inverters are equipped with a frequency skipping function. Users can set the skipping points and skipping widths on the VPf curve based on the frequency points where system oscillations occur. When the motor accelerates, it can automatically skip these frequency ranges, ensuring normal system operation.
(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 different frequency converter control methods, causing differences in motor load capacity; or differences in system output efficiency, leading to variations in load capacity. 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 temperature rise. If this still does not work, a new control method should be used. For example, Hitachi frequency converters use a constant VPf ratio method. If the starting requirement is not met, a sensorless space vector control method should be used, which has a greater torque output capability. For fan and pump loads, the torque reduction curve value should be reduced.
