Many power industry professionals are familiar with frequency converters. Compared to traditional electrical circuit control, frequency converters have a higher technological content and are devices that combine high-voltage and low-voltage electricity. Therefore, their faults are diverse, and we can only learn from experience by combining theory with practice. Below are 15 common problems about frequency converters (definition of frequency converter terms and some frequently occurring faults). Are you familiar with these?
1. What is variable frequency resolution? What is its significance?
For digitally controlled frequency converters, even if the frequency command is an analog signal, the output frequency is given in steps. The smallest unit of this step difference is called the frequency conversion resolution. Frequency conversion resolution is typically set to 0.015~0.5Hz. For example, with a resolution of 0.5Hz, 23Hz can be changed to 23.5Hz or 24.0Hz, thus the motor's operation also follows in steps. This causes problems for applications like continuous winding control. In such cases, a resolution of around 0.015Hz is sufficient for a 4-pole motor, where one step difference is less than 1 r/min. Additionally, some models have a different given resolution than their output resolution.
2. What is the significance of models where acceleration and deceleration times can be set separately, and models where acceleration and deceleration times can be set together?
For machine types where acceleration and deceleration can be set separately, this is suitable for short-time acceleration and slow deceleration applications, or for small machine tools where strict production cycle time needs to be set. However, for applications such as fan drives, where acceleration and deceleration times are relatively long, acceleration and deceleration times can be set together.
3. What is regenerative braking?
If the command frequency is reduced while the electric motor is running, the motor will operate as an asynchronous generator and act as a brake. This is called regenerative (electric) braking.
4. Is it possible 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%.
5. What are the protection functions of the frequency converter?
Protection functions can be divided into the following two categories: (1) Automatically performing corrective actions after detecting abnormal conditions, such as overcurrent stall prevention and regenerative overvoltage stall prevention. (2) Blocking the PWM control signal of the power semiconductor device after detecting an abnormality, so that the motor stops automatically. Such as overcurrent cut-off, regenerative overvoltage cut-off, semiconductor cooling fan overheating and instantaneous power outage protection, etc.
6. Why does the inverter's protection function activate when the clutch is used under continuous load?
When a clutch is used to connect a load, at the moment of connection, the motor changes drastically from an unloaded state to a region with high slip. The large current flowing through it causes the inverter to trip due to overcurrent and become unable to operate.
7. Why does the frequency converter stop when large motors in the same factory start running together?
When a motor starts, a starting current corresponding to its capacity will flow through it. The transformer on the stator side of the motor will generate a voltage drop. When the motor capacity is large, this voltage drop will have a greater impact. The frequency converter connected to the same transformer will make a judgment of undervoltage or momentary stop. Therefore, sometimes the protection function (IPE) will be activated, causing the motor to stop operating.
8. What does stall prevention function mean?
If the given acceleration time is too short, the change in the inverter's output frequency will far exceed the change in speed (electrical angular frequency), causing the inverter to trip due to overcurrent and stop operation; this is called stall. To prevent stall and allow the motor to continue running, the current magnitude must be detected for frequency control. When the acceleration current is too large, the acceleration rate should be appropriately slowed down. The same applies during deceleration. The combination of these two measures constitutes the stall function.
9. Are there any restrictions on the installation direction when installing a frequency converter?
The internal and rear structure of the frequency converter takes into account the cooling effect, and the vertical relationship is also important for ventilation. Therefore, for unit-type inverters that are inside the panel or mounted on the wall, the vertical position is taken, and they are installed as vertically as possible.
10. Inverter overvoltage
Overvoltage alarms usually occur when the machine stops, mainly due to a short deceleration time or problems with the braking resistor and braking unit.
11. Inverter temperature too high
In addition, inverters may also experience overheating faults. If an overheating alarm occurs and the temperature sensor is found to be normal, the problem may be caused by interference. In this case, the fault can be masked. The inverter's fan and ventilation should also be checked. For other types of faults, it is best to contact the manufacturer for a quick and feasible solution.
12. Overcurrent is the most frequent alarm phenomenon of frequency converter.
Inverter overcurrent phenomenon
(1) The circuit breaker trips immediately upon restarting and accelerating. This indicates a severe overcurrent. The main causes include: load short circuit, mechanical jamming, inverter module damage, and insufficient motor torque.
(2) The circuit breaker trips immediately upon power-on. This phenomenon is generally unresettable. The main reasons are: faulty module, faulty drive circuit, or faulty current detection circuit. The circuit breaker does not trip immediately upon restart but during acceleration. The main reasons are: the acceleration time is set too short, the current upper limit is set too low, or the torque compensation (V/F) is set too high.
13. Is it permissible to directly connect the motor to a frequency converter with a fixed frequency without using soft start?
It's possible at very low frequencies, but if the given frequency is high, the conditions are similar to starting directly from a mains frequency power supply. A large starting current (6-7 times the rated current) will flow through it, and the motor will not start because the frequency converter cuts off the overcurrent.
14. What issues should be considered when a motor operates at a frequency exceeding 60Hz?
The following precautions should be taken when operating above 60Hz.
(1) The machinery and equipment must be able to operate at this speed with full capacity (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.
15. What happens if a frequency converter is not used 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.
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