16 common problems with frequency converters.
1. 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.
2. Why does the frequency converter stop when large motors in the same factory start running together?
When a motor starts, the starting current corresponding to the flow rate and capacity will cause a voltage drop in the transformer on the stator side of the motor. 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, so sometimes the protection function (IPE) will be activated, causing the operation to stop.
3. What is variable frequency resolution? What is its significance?
For digitally controlled frequency inverters, 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 inverter resolution. The inverter resolution is usually taken as 0.015~0.5Hz. For example, with a resolution of 0.5Hz, 23Hz can be changed to 23.5Hz or 24.0Hz.
4. 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.
5. What issues should be considered when a motor operates at a frequency exceeding 60Hz?
The following precautions should be taken when operating at frequencies above 60Hz:
(1) The machinery and equipment must be able to operate at this speed with sufficient 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.
For medium-capacity and larger motors, especially 2-pole motors, it is necessary to discuss the operation frequency with the manufacturer carefully when operating at 60Hz or higher.
6. Can a frequency converter drive a geared motor?
Depending on the structure and lubrication method of the speed reducer, several issues need to be considered. For gears, a maximum speed limit of 70-80Hz is generally considered. When using oil lubrication, continuous operation at low speeds can lead to gear damage.
7. Can a frequency converter be used to drive a single-phase motor? Can it use a single-phase power supply?
Single-phase motors are generally unusable. For single-phase motors with speed controller switch start, the auxiliary winding will burn out when operating below the operating point; for capacitor-start or capacitor-run motors, it will induce capacitor explosion. Inverter power supplies are typically three-phase, but some small-capacity models operate on single-phase power.
8. How much power does the frequency converter itself consume?
It depends on the type of inverter, its operating status, and the frequency of use, but it's difficult to answer definitively. However, inverters below 60Hz have an efficiency of approximately 94% to 96%, from which losses can be estimated. But for regenerative braking (FR-K) inverters, if braking losses are also taken into account, the power consumption will increase, which must be considered in the design of the control panel, etc.
9. Why can't it be used continuously across the entire 6~60Hz frequency range?
Generally, motors use external fans mounted on the shaft or blades on the rotor end rings for cooling. If the speed decreases, the cooling effect decreases, and therefore the motor cannot withstand the same heat generation as when running at high speed. It is necessary to reduce the load torque at low speeds, or use a combination of a high-capacity frequency converter and the motor, or use a special motor.
10. What precautions should be taken when using a motor with a brake?
The power supply for the brake's excitation circuit should be taken from the input side of the frequency converter. If the brake operates while the frequency converter is outputting power, it will cause an overcurrent trip. Therefore, the brake should only be activated after the frequency converter has stopped outputting power.
11. I want to use a frequency converter to drive a motor with a capacitor for improving the power factor, but the motor does not move. Please explain why.
The inverter's current flows into the capacitor used to improve the power factor. Because the charging current of the capacitor causes an overcurrent (OCT) in the inverter, it cannot start. As a countermeasure, please remove the capacitor and run the inverter. In addition, connecting an AC reactor to the input side of the inverter is effective in improving the power factor.
12. How long is the lifespan of a frequency converter?
Although frequency converters are static devices, they also have consumable components such as filter capacitors and cooling fans. If they are maintained regularly, they can be expected to have a lifespan of more than 10 years.
13. The inverter has a built-in cooling fan. What is the direction of the airflow? What happens if the fan breaks down?
For small-capacity models without cooling fans, and for models with fans, the airflow is from bottom to top. Therefore, avoid placing any mechanical equipment above or below the inverter installation location that might obstruct airflow. Also, do not place heat-sensitive components above the inverter. In case of fan failure, protection will be provided by fan stop detection or overheat detection on the cooling fan.
14. Since filter capacitors are consumables, how do you determine their lifespan?
As a capacitor used for filtering, its capacitance decreases slowly over time. The capacitance is measured periodically, and its lifespan is judged based on the point at which it reaches 85% of the product's rated capacity.
15. Are there any restrictions on the installation direction when installing a frequency converter?
Ideally, the contents should be primarily stored within a tray. However, the problem is that trays with a fully enclosed structure are large in size, take up a lot of space, and are relatively expensive. Solutions include:
(1) The design of the tray should be based on the heat dissipation required by the actual device;
(2) Increase the cooling area by using aluminum heat sinks, finned coolants, etc.;
(3) Use heat pipes.
In addition, a type of inverter with an exposed back has been developed.
16. If you want to increase the speed of an existing conveyor belt to operate at 80Hz, how should you select the capacity of the frequency converter?
Assuming a base speed of 50Hz, speeds above 50Hz exhibit constant power output characteristics. For loads with constant torque characteristics, such as conveyor belts, increasing speed requires a capacity increase of approximately 1.6 times (80/50 ≈ 1.6 times). The motor capacity also increases in the same way as that of the frequency converter.
