The output voltage of a frequency converter can be selected according to the rated voltage of the motor. According to national standards, it can be divided into two series: 220V and 400V. For a 3kV high-voltage motor, using a 400V-class frequency converter involves installing an input transformer on the input side and an output transformer on the output side. This first reduces the 3kV to 400V before supplying it to the frequency converter, and then increases the frequency converter's output voltage back to 3kV to supply the motor.
Abnormal grid voltage will harm the frequency converter. Excessive voltage, such as a line voltage rising from 380V to 450V, will cause damage. Therefore, if the grid voltage exceeds the range specified in the user manual, a transformer must be used to adjust it to ensure the safety of the frequency converter.
The maximum output frequency of a frequency converter varies greatly depending on the model, ranging from 50Hz/60Hz, 120Hz, 240Hz, or higher. 50Hz/60Hz is typically used for speed regulation below the rated speed; most large-capacity general-purpose frequency converters fall into this category. Frequency converters with a maximum output frequency exceeding the power frequency are mostly small-capacity. Above 50Hz/60Hz, the output voltage remains constant, exhibiting constant power characteristics. It's important to note the decrease in torque at high speeds. However, machine tools such as lathes can adjust the speed according to the workpiece diameter and material, operating within the constant power range. Using high speeds under light loads can improve productivity. However, it's crucial not to exceed the maximum permissible speed of the motor and load.
When starting a motor with a general-purpose frequency converter, the starting current is usually lower than when starting with a mains frequency power supply. Depending on the starting torque characteristics of the load, it may sometimes fail to start. Furthermore, the torque in the low-speed operating range is typically lower than the rated torque. If the selected frequency converter and motor cannot meet the starting torque and low-speed torque requirements of the load, the capacity of the frequency converter and motor needs to be increased. For example, if at a certain speed, 700/0 of the initially selected rated torque of the frequency converter and motor is required, but the output torque characteristic curve indicates that only 50% of the torque can be obtained, then the capacity of the frequency converter and motor needs to be reselected to be at least 1.4 (70/50) times the initially selected capacity.
The inverter's internal structure generates a large amount of heat. To improve the economy of heat dissipation, except for small-capacity inverters which are all open structures, the rest use forced cooling fans. When the inverter is installed outdoors or in a harsh environment, it is best to install it on an independent panel with a fully enclosed structure equipped with a cooling heat exchange device.
When switching from the power grid to the frequency converter, the motor running on the mains power grid must be completely stopped before switching back to the frequency converter to restart. Otherwise, excessive inrush current and torque will inevitably be generated, causing the power supply system to trip and damage equipment. However, some equipment cannot be completely stopped when switching from the power grid to the frequency converter. For these devices, a frequency converter model equipped with appropriate control devices (optional) must be selected so that the motor can be switched to the frequency converter without stopping. That is, after disconnecting from the power grid, the frequency converter synchronizes with the freely running motor before outputting power.
Considerations for Selecting Inverter Capacity
(1) The most suitable time is when the power value of the frequency converter is comparable to that of the motor, which is conducive to the high-efficiency operation of the frequency converter.
(2) When the power rating of the frequency converter is different from that of the motor, the power of the frequency converter should be as close as possible to the power of the motor, but should be slightly greater than the power of the motor.
(3) When the motor is frequently started and braked or is under heavy load and operates frequently, a larger inverter can be selected to facilitate the long-term and safe operation of the inverter.
(4) After testing, the actual power of the motor is indeed sufficient. It is advisable to select a frequency converter with a power less than that of the motor. However, attention should be paid to whether the instantaneous peak current will cause the overcurrent protection to activate.
(5) When the power of the frequency converter and the motor are different, the energy-saving program settings must be adjusted accordingly to achieve a higher energy-saving effect.
(6) The rated capacity and parameters of the frequency converter are specified for a certain altitude and ambient temperature, generally referring to an altitude below 1000m and a temperature below 40℃ or 25℃. If the ambient temperature exceeds these specifications, the resulting derating factor should be considered before determining the model based on the frequency converter parameters. Prolonged high ambient temperatures, especially when installed in poorly ventilated and cooled cabinets, will shorten the lifespan of the frequency converter. Electronic components, especially electrolytic capacitors, will have their lifespan halved for every 10℃ increase above their rated temperature. Therefore, the ambient temperature should be kept at a low level. In addition to setting up a complete ventilation and cooling system to ensure the normal operation of the frequency converter, a larger capacity level should be selected to reduce the temperature rise during rated operation. In high-altitude areas, due to the reduced air density, the heat sink cannot achieve the rated heat sink effect. Generally, above 1000m, the capacity decreases by 10% for every 100m increase. If necessary, the capacity level can be increased to prevent the frequency converter from overheating.
(7) When the motor requires instantaneous stop and restart, it is necessary to confirm that the selected frequency converter has this function. Because the frequency converter stops running when the power is off, if the motor frequency is not appropriate when it is suddenly restarted after a power outage, it will cause overvoltage and overcurrent, resulting in a fault shutdown.
(8) When a sensor is used in conjunction with the speed control of the frequency converter, attention should be paid to whether the type and magnitude of the signal output by the sensor are consistent with the speed control signal used by the frequency converter.
