Common Faults and Maintenance of Variable Frequency Drives
With the development of microelectronics, power electronics technology and asynchronous motor control theory, the innovation of variable frequency drive technology has been promoted. It has become an inevitable trend for AC variable frequency speed regulation devices to replace DC speed regulation devices. Thanks to the expanding sales volume, mass production and falling prices of variable frequency drives, the prices of AC variable frequency drives and DC speed regulation devices with the same capacity have become close. Moreover, the adoption of variable frequency speed regulation on some production machinery has become a symbol of equipment upgrading. Therefore, understanding its working principle, common faults and maintenance methods is essential.
Diagnosis and Handling of Common Variable Frequency Drive Faults
1. Parameter Setting Faults
For general variable frequency drives, parameter setting is critical to meet the requirements of the transmission system. Incorrect parameter settings will prevent the variable frequency drive from operating normally.
Once a parameter setting fault occurs, the variable frequency drive cannot run properly. Generally, parameters can be modified according to the operation manual. If this method fails, it is recommended to restore all parameters to factory defaults and reset them following the corresponding procedures. The parameter reset method varies for variable frequency drives from different manufacturers.
2. Overvoltage Faults
Overvoltage of variable frequency drives is mainly reflected in the DC bus voltage. Under normal operating conditions, the DC voltage of the drive is the average value after three-phase full-wave rectification. Calculated based on a 380 V line voltage, the average DC voltage Ud = 1.35 × Uline = 513 V. When overvoltage occurs, energy storage capacitors on the DC bus are charged. When the voltage rises to approximately 760 V, the overvoltage protection of the variable frequency drive is activated. Therefore, every variable frequency drive has a normal operating voltage range; exceeding this range may damage the unit. There are two main types of overvoltage faults:
2.1 Input AC Power Overvoltage
This refers to input voltage exceeding the normal range. It usually occurs during holidays under light load conditions with voltage fluctuation or line faults. In such cases, cut off the power supply for inspection and troubleshooting.
2.2 Regenerative Overvoltage
This fault occurs with relatively high probability. It happens when the actual rotating speed of the motor exceeds its synchronous speed, putting the motor into generating mode, while no braking unit is installed on the variable frequency drive. Two scenarios may trigger this fault:
(1) When the variable frequency drive drives a high-inertia load with a short deceleration time. During deceleration, the speed command output by the drive drops rapidly, while the load decelerates slowly relying on its own resistance. As a result, the load drives the motor to rotate faster than the speed corresponding to the output frequency of the drive, and the motor enters generating mode. Without an energy feedback unit, the DC bus voltage of the variable frequency drive rises beyond the protection threshold and triggers a fault. To resolve this fault, install a regenerative braking unit or modify drive parameters to extend the deceleration time.
(2) This fault may also occur when multiple motors drive the same load, mainly due to lack of load distribution control (master-slave coordination problem). Take two motors driving one load as an example: if the actual speed of one motor exceeds the synchronous speed of the other, the faster motor acts as the prime mover and the slower one operates in generating mode, leading to faults. This fault frequently occurs in the press section and wire section of paper machines. The solution is to add load distribution control and set softer characteristics for the variable frequency drives on the branches of the paper machine transmission speed chain.
3. Overcurrent Faults
Overcurrent faults are classified into overcurrent during acceleration, deceleration and constant-speed operation.
Overcurrent during acceleration or deceleration is caused by insufficient acceleration/deceleration time, sudden load variation, unbalanced load distribution, output short circuit and other factors. Corresponding solutions include extending acceleration/deceleration time, mitigating sudden load changes, installing additional dynamic braking components, designing load distribution schemes and checking circuits. If the overcurrent fault still exists after disconnecting the load, it indicates that the inverter circuit of the variable frequency drive is damaged and the drive needs to be replaced.
4. Overload Faults
Overload faults include variable frequency drive overload and motor overload.
Variable frequency drive overload is caused by too short acceleration/deceleration time (resulting in transient overload) or excessive DC braking magnitude. Maintenance solution: adjust internal parameters and extend braking time.
Motor overload may be caused by low grid voltage or excessive load. Maintenance solutions: inspect the power grid voltage. If the load is overweight, the selected motor and variable frequency drive cannot drive the load; poor mechanical lubrication (excessive resistance) may also lead to this fault.
5. Other Faults
(1) Undervoltage: Indicates problems with the power input section of the variable frequency drive. Inspection is required before restarting.
(2) Excessive temperature: If the motor is equipped with a temperature detection device, check the motor heat dissipation condition. If the variable frequency drive overheats, inspect its ventilation system.
6. Abnormal Motor Operation
(1) Motor fails to start
Main circuit inspection: detect power supply voltage, check whether the charging indicator light is on, whether the LCD displays an alarm screen, and verify correct wiring between the motor and variable frequency drive.
Function setting inspection: confirm correct configuration of all function codes.
Load inspection: check for excessive load or mechanical stalling.
Input signal inspection: confirm start signal, forward (FWD) / reverse (REV) signals are input, and verify the set frequency or upper limit frequency is not too low.
(2) Motor speed cannot be adjusted
Possible causes: incorrect setting of upper/lower frequency limits or excessively long timer setting during program operation. An excessively low maximum frequency setting will also result in failure to adjust frequency.
(3) Motor stalls during acceleration
This may be caused by insufficient acceleration time setting, excessive load or inadequate torque boost.
(4) Abnormal motor heating
Check whether the load is excessive, whether the motor runs continuously at low speed, and whether the torque boost setting is appropriate. If none of the above causes apply, the three-phase (U, V, W) output voltage of the variable frequency drive may be unbalanced.
Precautions for Variable Frequency Drive Maintenance
Several points should be noted regarding wiring when connecting a test load:
The test load shall be connected between capacitors and power modules, rather than between rectifiers and capacitors. Capacitor discharge current can burn out the power modules.
The test load should also be connected after the DC voltage detection point. In this way, when the indicator lamp lights up due to abnormal drive output, the variable frequency drive will not trip on undervoltage, making it possible to identify which output phase has a fault.
Conditions leading to rectifier module breakdown:
If only the rectifier module is damaged, it is usually caused by large power grid voltage fluctuation and instantaneous high voltage input to the drive. The withstand voltage of rectifier modules for 380 V input variable frequency drives is generally 1600 V; therefore, extremely high voltage is required to break down the rectifier module. In addition, short-circuit faults of loads behind the rectifier module (such as filter capacitors and output modules) generate excessive current and burn out the rectifier module. It is essential to install an air circuit breaker at the input side of the variable frequency drive.
Faults caused by defective capacitors:
For filter capacitors, reduced capacitance will lead to unstable DC voltage on the main circuit of the variable frequency drive, easily damaging power modules and triggering frequent undervoltage faults.
Conditions leading to braking unit damage:
Possible causes include excessively high braking current setting or control failure (excessive dust accumulation on the control board).
Summary
Variable frequency drives enjoy wide application. This chapter mainly introduces common maintenance and troubleshooting methods, helping technicians grasp basic daily maintenance practices.
Diagnosis and Handling of Common Variable Frequency Drive Faults
1. Parameter Setting Faults
For general variable frequency drives, parameter setting is critical to meet the requirements of the transmission system. Incorrect parameter settings will prevent the variable frequency drive from operating normally.
Once a parameter setting fault occurs, the variable frequency drive cannot run properly. Generally, parameters can be modified according to the operation manual. If this method fails, it is recommended to restore all parameters to factory defaults and reset them following the corresponding procedures. The parameter reset method varies for variable frequency drives from different manufacturers.
2. Overvoltage Faults
Overvoltage of variable frequency drives is mainly reflected in the DC bus voltage. Under normal operating conditions, the DC voltage of the drive is the average value after three-phase full-wave rectification. Calculated based on a 380 V line voltage, the average DC voltage Ud = 1.35 × Uline = 513 V. When overvoltage occurs, energy storage capacitors on the DC bus are charged. When the voltage rises to approximately 760 V, the overvoltage protection of the variable frequency drive is activated. Therefore, every variable frequency drive has a normal operating voltage range; exceeding this range may damage the unit. There are two main types of overvoltage faults:
2.1 Input AC Power Overvoltage
This refers to input voltage exceeding the normal range. It usually occurs during holidays under light load conditions with voltage fluctuation or line faults. In such cases, cut off the power supply for inspection and troubleshooting.
2.2 Regenerative Overvoltage
This fault occurs with relatively high probability. It happens when the actual rotating speed of the motor exceeds its synchronous speed, putting the motor into generating mode, while no braking unit is installed on the variable frequency drive. Two scenarios may trigger this fault:
(1) When the variable frequency drive drives a high-inertia load with a short deceleration time. During deceleration, the speed command output by the drive drops rapidly, while the load decelerates slowly relying on its own resistance. As a result, the load drives the motor to rotate faster than the speed corresponding to the output frequency of the drive, and the motor enters generating mode. Without an energy feedback unit, the DC bus voltage of the variable frequency drive rises beyond the protection threshold and triggers a fault. To resolve this fault, install a regenerative braking unit or modify drive parameters to extend the deceleration time.
(2) This fault may also occur when multiple motors drive the same load, mainly due to lack of load distribution control (master-slave coordination problem). Take two motors driving one load as an example: if the actual speed of one motor exceeds the synchronous speed of the other, the faster motor acts as the prime mover and the slower one operates in generating mode, leading to faults. This fault frequently occurs in the press section and wire section of paper machines. The solution is to add load distribution control and set softer characteristics for the variable frequency drives on the branches of the paper machine transmission speed chain.
3. Overcurrent Faults
Overcurrent faults are classified into overcurrent during acceleration, deceleration and constant-speed operation.
Overcurrent during acceleration or deceleration is caused by insufficient acceleration/deceleration time, sudden load variation, unbalanced load distribution, output short circuit and other factors. Corresponding solutions include extending acceleration/deceleration time, mitigating sudden load changes, installing additional dynamic braking components, designing load distribution schemes and checking circuits. If the overcurrent fault still exists after disconnecting the load, it indicates that the inverter circuit of the variable frequency drive is damaged and the drive needs to be replaced.
4. Overload Faults
Overload faults include variable frequency drive overload and motor overload.
Variable frequency drive overload is caused by too short acceleration/deceleration time (resulting in transient overload) or excessive DC braking magnitude. Maintenance solution: adjust internal parameters and extend braking time.
Motor overload may be caused by low grid voltage or excessive load. Maintenance solutions: inspect the power grid voltage. If the load is overweight, the selected motor and variable frequency drive cannot drive the load; poor mechanical lubrication (excessive resistance) may also lead to this fault.
5. Other Faults
(1) Undervoltage: Indicates problems with the power input section of the variable frequency drive. Inspection is required before restarting.
(2) Excessive temperature: If the motor is equipped with a temperature detection device, check the motor heat dissipation condition. If the variable frequency drive overheats, inspect its ventilation system.
6. Abnormal Motor Operation
(1) Motor fails to start
Main circuit inspection: detect power supply voltage, check whether the charging indicator light is on, whether the LCD displays an alarm screen, and verify correct wiring between the motor and variable frequency drive.
Function setting inspection: confirm correct configuration of all function codes.
Load inspection: check for excessive load or mechanical stalling.
Input signal inspection: confirm start signal, forward (FWD) / reverse (REV) signals are input, and verify the set frequency or upper limit frequency is not too low.
(2) Motor speed cannot be adjusted
Possible causes: incorrect setting of upper/lower frequency limits or excessively long timer setting during program operation. An excessively low maximum frequency setting will also result in failure to adjust frequency.
(3) Motor stalls during acceleration
This may be caused by insufficient acceleration time setting, excessive load or inadequate torque boost.
(4) Abnormal motor heating
Check whether the load is excessive, whether the motor runs continuously at low speed, and whether the torque boost setting is appropriate. If none of the above causes apply, the three-phase (U, V, W) output voltage of the variable frequency drive may be unbalanced.
Precautions for Variable Frequency Drive Maintenance
Several points should be noted regarding wiring when connecting a test load:
The test load shall be connected between capacitors and power modules, rather than between rectifiers and capacitors. Capacitor discharge current can burn out the power modules.
The test load should also be connected after the DC voltage detection point. In this way, when the indicator lamp lights up due to abnormal drive output, the variable frequency drive will not trip on undervoltage, making it possible to identify which output phase has a fault.
Conditions leading to rectifier module breakdown:
If only the rectifier module is damaged, it is usually caused by large power grid voltage fluctuation and instantaneous high voltage input to the drive. The withstand voltage of rectifier modules for 380 V input variable frequency drives is generally 1600 V; therefore, extremely high voltage is required to break down the rectifier module. In addition, short-circuit faults of loads behind the rectifier module (such as filter capacitors and output modules) generate excessive current and burn out the rectifier module. It is essential to install an air circuit breaker at the input side of the variable frequency drive.
Faults caused by defective capacitors:
For filter capacitors, reduced capacitance will lead to unstable DC voltage on the main circuit of the variable frequency drive, easily damaging power modules and triggering frequent undervoltage faults.
Conditions leading to braking unit damage:
Possible causes include excessively high braking current setting or control failure (excessive dust accumulation on the control board).
Summary
Variable frequency drives enjoy wide application. This chapter mainly introduces common maintenance and troubleshooting methods, helping technicians grasp basic daily maintenance practices.
