01 Selection of Frequency Converters
Generally, the selection of a frequency converter should be based on the actual operating current of the motor. Of course, if the most suitable one cannot be found in a short time, a larger model can be chosen, because a large frequency converter can power a small motor, but a small frequency converter certainly cannot power a large motor!
02 The significance of inverter resolution
For digitally controlled frequency converters, even if the frequency command is an analog signal, the output frequency is given in stages. The smallest unit of this stage difference is called the frequency resolution.
The frequency conversion resolution is usually set to 0.015~0.5Hz; for example, if the resolution is 0.5Hz, then 23Hz can be changed to 23.5 or 24.0 Hz, so the motor's operation is also stepped and follows the frequency.
03 The principle of regenerative braking of electric motor
When the rotor speed of the motor exceeds the rotational speed of the motor's synchronous magnetic field, the direction of rotation of the electromagnetic torque generated by the rotor winding is opposite to the direction of rotor rotation. At this time, the motor is in a braking state, which is called regenerative braking.
04 How to obtain greater braking force
The energy regenerated from the motor is stored in the inverter's filter capacitor. Due to the capacitance and voltage rating of the capacitor, the regenerative braking force of a general-purpose inverter is approximately 10% to 20% of the rated torque. If an optional braking unit is used, it can reach 50% to 100%.
05 What situations will trigger the protection function?
When the clutch engages the load, the motor changes drastically from an unloaded state to a region with high slip at the instant of engagement. The large current flowing through the motor causes the inverter to trip due to overcurrent and stop operating, thus triggering the inverter's protection function!
What is the stall prevention function of the 06 motor?
Stall: This refers to a situation where, when the acceleration time is too short, the change in the inverter's output frequency far exceeds the change in speed (electrical angular frequency). The inverter will trip due to overcurrent and stop operating. To prevent stall and allow the motor to continue running, the magnitude of the current must be detected for frequency control.
07 Causes of Inverter Overvoltage Alarm
Overvoltage alarms usually occur when the machine stops, mainly due to a short deceleration time or problems with the braking resistor and braking unit.
What precautions should be taken when operating a 08 motor above 60Hz?
(1) The machinery and equipment must be able to operate at this speed with full capability (mechanical strength, noise, vibration, etc.).
(2) When the motor enters the constant power output range, its output torque must be able to maintain operation (the shaft output power of fans, pumps, etc. increases proportionally to the cube of the speed, so attention should be paid when the speed increases slightly).
(3) The issue of bearing life should be fully considered.
(4) Special attention should be paid to motors with medium or large capacity, especially 2-pole motors, when operating at 60Hz or above.
What happens if a 09 frequency inverter is left unused for a long time?
(1) The lubricant in the inverter fan bearing has dried up, affecting its use.
(2) High voltage filter capacitors are prone to bulging if they are not used for a long time, and low voltage electrolytic capacitors are prone to leakage.
(3) Low-voltage electrolytic capacitors may also experience electrolyte leakage, which may even corrode the printed circuit board.
10. When should a DC/AC reactor be installed?
(1) When the frequency converter is connected to a power transformer of 600KVA or above or when switching the phase capacitor.
(2) When the same power supply system is connected to thyristor converters such as DC drivers, a DC reactor or an AC reactor must be installed.
11. Reasons for frequent alarms from frequency converters
(1) If the ambient temperature is too high, the internal components of the frequency converter will become too hot. In order to protect the internal circuit of the frequency converter, the frequency converter will report a high temperature fault and shut down.
(2) Poor ventilation of the inverter, such as when the air duct of the inverter itself is blocked or the air duct of the control cabinet is blocked, will affect the heat dissipation inside the inverter and cause the inverter to overheat alarm.
(3) The fan is stuck or damaged, and a lot of heat accumulates inside the inverter and cannot be dissipated.
(4) When the load on the frequency converter is too heavy, it will generate excessive current and a lot of heat. Sometimes the frequency converter will also overheat and alarm.
12. Points to note during the installation of frequency converters
(1) Heat dissipation problem: The heat generated by the frequency converter is due to internal losses. Among the various losses in the frequency converter, the main circuit accounts for approximately 98%, while the control circuit accounts for 2%. To ensure the normal and reliable operation of the frequency converter, heat dissipation is necessary. We usually use fan cooling.
(2) Electromagnetic interference issues: When there are high-frequency impact loads in the system, such as welding machines or electroplating power supplies, the frequency converter itself may trigger protection due to interference. In this case, the power quality of the entire system should be considered. In addition, if the power of the frequency converter is very large, accounting for more than 25% of the entire system, anti-interference measures for the control power supply need to be considered.
(3) Protection issues:
1. Dust Control: All air inlets must be equipped with dust screens to prevent fibrous debris from entering. The dust screens should be designed to be removable for easy cleaning and maintenance. The mesh size of the dust screens should be determined based on the specific site conditions, and the connection between the dust screens and the control cabinet must be tightly sealed.
2. Resistance to corrosive gases: This is common in the chemical industry, in which case the frequency converter cabinet can be placed in the control room.
3. Waterproof and anti-condensation: If the frequency converter is placed on site, attention should be paid to whether there are pipes or other leaks above the frequency converter cabinet, and there should be no splashing water near the frequency converter. In short, the protection level of the on-site cabinet should be above IP43.
