VFD repairs should beginwith checking the electric motor connections and VFD terminals. Motor and VFD connections can be missed, installed loosely or wired to the incorrect terminals, so checking all connections is recommended before pulling the VFD for a repair evaluation. Heat cycles and mechanical vibration can lead to sub-standard connections. Reusing torque screws is not a good idea, and further tightening an already tight connection can ruin the connection. Bad connections eventually lead to arcing. Arcing at the drive input terminal can result in over voltage faults, clearing of input fuses, or damage to protective components. Arcing at the VFD input or output terminals may result in over-current faults or damage to the power components. Loose connections can cause erratic operation. For example, a loose START/STOP signal wire can cause uncontrollable starting and stopping of the motor. A loose speed reference wire can cause the drive speed to fluctuate, resulting in scrap, machine damage, and personnel injury.
Diode and IGBT Testing
Prior to applying power during the VFD repairs, the input and output power sections need to be tested. If there is a short on the input side or output side of the VFD, further damage can be caused to the unit if power is applied to it. For this reason, special meters should be used to properly test the input and output power sections of the drive prior to applying power to the VFD unit. If a short is found, the VFD can be disassembled and the cause of the short can be diagnosed and quoted for repair. If the VFD repairs are too costly, VFD replacement should be considered.
If the input and output power sections test healthy during the VFD repairs process, power can be applied to the VFD unit to perform amp reading and output frequency testing. Slowly increasing the power voltage during this step of the VFD repairs process is suggested to gather accurate meter readings and to protect the motor. The power voltage is slowly increased on the VFD until the rated input voltage of the drive is reached. Depending on whether or not the VFD provides a display will determine what further action will be taken. If display is unavailable, disassemble and diagnosis of the internal power supply of the control section of the drive is necessary to further evaluate cause of failure and establish cost and lead time of the VFD repairs.
If the previous three tests have passed during the VFD repairs process, a basic JOG/ RUN function of the drive can be performed with a simple template program. When a VFD comes into our facility, repair techs will backup all existing programs stored in the drive prior to installing a test template program for testing. Backing up the VFD program ensures that we have a backup copy of the program to install after the repair is complete. The best method for backing up VFD programs will depend on the series and model of drive that’s being repaired. After the VFD programs have been backed up, the VFD is reset to factory default through the keypad and then recommissioned to a basic START / STOP function of the VFD connected to its motor. Closed loop testing will be necessary when an encoder or tachometer is involved. If the motor will not run, the output voltages and motor current ratings are tested to verify that the drive is functioning properly to rotate the motor.
VFD repairs should only be performed by trained technical staff during regular routine maintenanceor at a certified repairs facility.Routine maintenance prevents excessive VFD failure and production loss due to machinery downtime. VFD repairs are costly and lead times can also be extensive; and with regular routine maintenance, cost and lead times diminish. VFD repairs can be expensive depending on board and component damages. Most manufacturers keep spare VFD units in stock to prevent production downtime while the VFD repairs are being processed.
Manufacturers are encouraged to ensure that their VFD repairs are being performed at the actual site of whoever is offering the service. VFD repairs should be taken with extremecaution and only be performed by technicians who have required training and experience to work with electrical equipment. Precision Electric strongly recommends to consult an expert in the field when repairing all industrial electrical equipment. Many drive controllers have an internal DC bus that retains a charge after power has been cut to the drive, and as a result, electrical current is still present afterpower is shut down. Technicians performing VFD repairs must always take extra precautions to ensure proper safety measures are taken, or injury or even death may occur.
Precision Electric receives VFD repairs everyday from companies who offer VFD repairs; those third party repair companies will have Precision Electricperform the repair, and then they will mark up the repair cost to their customers. Going straight to Precision Electric for VFD repairs will prevent these added costs and prevent extensive lead times.For VFD repairs and VFD replacements, contact Precision Electric.