Understand these VFD definitions to become a VFD expert!

Variable frequency drives (VFDs) are likely familiar to many electrical professionals. Compared to traditional electrical circuit controls, VFDs have a higher level of technological sophistication, combining strong and weak electrical components. Therefore, they can experience a variety of faults, requiring continuous experiential learning combined with theoretical knowledge. Below are listed 15 common issues related to VFDs (definitions of VFD terminology and some frequently occurring faults). Do you understand all of these?

1. What is frequency resolution? What is its significance?

For digital control variable frequency drives (VFDs), 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 frequency resolution. The frequency resolution typically ranges from 0.015 to 0.5 Hz. For example, with a resolution of 0.5 Hz, frequencies above 23 Hz can be set to 23.5, 24.0 Hz, and so on, causing the motor's action to follow in steps. This can pose problems for applications like continuous winding control. In such cases, a resolution of around 0.015 Hz can be sufficient, where for a 4-pole motor, one step corresponds to less than 1 rpm. Additionally, in some models, the given resolution may differ from the output resolution.

2. Why are there two types of models: those where the acceleration and deceleration times are separately specified, and those where the acceleration and deceleration times are specified together?  What is the significance of this?

Models with separately adjustable acceleration and deceleration times are suitable for situations requiring short acceleration times and slow deceleration, or where strict production cycle times need to be defined for small machine tools. However, for applications such as fan drives where both acceleration and deceleration times are longer, it is appropriate to specify acceleration and deceleration times together.

3. What is regenerative braking?

Regenerative braking refers to the operation of an electric motor when the command frequency is reduced during operation. In this state, the electric motor operates as an asynchronous generator, acting as a brake.

4. Can greater braking force be achieved?

The energy regenerated from the motor is stored in the filtering capacitors of the variable frequency drive.  Due to the relationship between the capacity and voltage endurance of the capacitors, the regenerative braking force of standard variable frequency drives is typically about 10% to 20% of the rated torque.  However, by using selected braking units, it is possible to achieve 50% to 100% of the braking force.

5. What are the protection functions of a variable frequency drive?

Protection functions can be divided into two categories:

(1) Automatic corrective actions taken upon detecting abnormal conditions, such as Overcurrent and under-speed prevention, and regenerative over-voltage prevention. 

(2) Locking of power semiconductor device PWM control signals upon detecting abnormalities, causing the motor to stop automatically. Examples include Overcurrent cutoff, regenerative over-voltage cutoff, semiconductor cooling fan overheating, and momentary power outage protection.

6. Why do the protection functions of a variable frequency drive activate when continuously loaded with a clutch?

When a clutch is used to connect the load, a sudden change occurs in the motor's operating state from unloaded to a region with a large difference in slip rate, causing a large current to flow, leading to Overcurrent tripping of the variable frequency drive, preventing it from operating.

7. Why does the variable frequency drive stop while large motors start together within the same factory?

When a motor starts, a starting current corresponding to its capacity flows, causing a voltage drop across the motor's stator, which is significant for large-capacity motors. Variable frequency drives connected to the same transformer may interpret this as under-voltage or momentary stoppage, leading to the activation of protection functions (IPE), resulting in the drive stopping.

8. What does under-speed prevention function mean?

If the set acceleration time is too short, the output frequency of the variable frequency drive changes far more rapidly than the speed (electrical angular frequency), causing the drive to trip due to Overcurrent, stopping its operation. To prevent under-speed and keep the motor running, the current magnitude is detected for frequency control. When the acceleration current is too high, the acceleration rate is appropriately slowed down. The same applies during deceleration. The combination of these two is the under-speed prevention function.

9. Is there any restriction on the installation direction when installing a variable frequency drive?

The internal and back structures of the variable frequency drive are designed with cooling in mind, and the orientation is important for ventilation. Therefore, for standalone units installed in cabinets or wall-mounted units, they should be installed vertically as much as possible.

10. Over-voltage in variable frequency drives

Over-voltage alarms typically occur during shutdown, mainly due to too short deceleration times or problems with braking resistors and braking units.

11. High temperature in variable frequency drives

In addition to Over-voltage, variable frequency drives also have high-temperature faults. If a high-temperature alarm occurs and the temperature sensor is found to be normal upon inspection, it may be caused by interference. In such cases, the fault can be masked, and the fan and ventilation of the variable frequency drive 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 in variable frequency drives

Occurrences of Overcurrent in variable frequency drives: 

(1) Tripping immediately upon restarting. This is a severe Overcurrent phenomenon. The main reasons include short-circuiting of the load, mechanical jamming, damage to the inverter module, and insufficient torque of the motor.

(2) Tripping upon power-on, which generally cannot be reset. The main reasons include module failure, driver circuit failure, and current detection circuit failure. Tripping during acceleration instead of immediately upon restarting is mainly due to too short acceleration time, too small current limit setting, or a high torque compensation (V/F) setting.

13. Is it possible to directly start a motor at a fixed frequency without using soft start when connected to a variable frequency drive?

It is possible at very low frequencies, but if the given frequency is high, it is similar to starting with mains frequency power. Starting the motor with a large starting current (6~7 times the rated current) will cause the variable frequency drive to cut off the Overcurrent, preventing the motor from starting.

14. What should be noted when the motor operates above 60Hz?

When operating above 60Hz, the following points should be noted:

(1) Mechanical and equipment operation at this speed should be as thorough as possible (mechanical strength, noise, vibration, etc.). 

(2) The motor should enter the constant power output range, and its output torque should be able to sustain work (for example, in fans and pumps, the shaft output power increases proportionally to the cube of the speed, so even with a slight increase in speed, attention should be paid). 

(3) Consideration should be given to the lifespan of the bearings.

15. What happens if the variable frequency drive is not used for a long time?

The lubricant in the variable frequency drive fan bearings may dry up, affecting its operation.

Long-term disuse of the high-voltage filtering capacitors can cause them to swell, while low-voltage electrolytic capacitors may develop leakage.