What is a variable frequency drive
A VFD is used to control the speed of an AC induction motor it can do this by varying the motors supplied voltage and changing the frequency of the power. Three major designs are used in VFD they are pulse width modulation, current source inverter, and voltage source inverter
It has become the most common drive controller it is because a VFD works very well with motor ranging, it is highly reliable, affordable, and it only reflects the least amount of harmonics back to the power source.
Fault conditions in VFD (motor fault analysis)
- Rolling element bearing faults
- Sleeve bearing faults
- Imbalance
- Misalignment
- Soft foot conditions
- Mechanical resonances
- Rotor winding failure
- Stator winding failure
- VFD carrier frequencies
- Air gap issues
Troubleshooting VFDs
If you have an unidentified wide frequency band vibration fault it is possible that it is a VFD problem. It can be corrected by additional tuning of torque profiles, carrier frequencies, or smoothing parameters may correct the issue
How does a motor is affected In case of poorly designed or faulty drive system
- Fast voltage rise times can break down the motor winding insulation which is not designed for this condition
- Motor winding insulation could be break by a reflected wave over-voltage condition
- Electrical damages in the bearing could happen
How to keep the VFDs and electric motors running
VFDs are very sensitive device so it must be checked regularly, but it has a lot of advantages a VFD provides better energy saving, in a mechanical application they provide fine adjustment which is not possible by other methods
Start with baseline readings
Before starting a motor take insulation resistance readings from phase to phase and phase to ground. Measure the insulation resistance of the winding, measure the starting and running amperage, measure the temperature at first startup and after a period of use
Check the protection
Check the protection systems like the overload contractors and fusing, check if the overload is set for full load amperes or is it set too high or low. Check if the fusing is correct for the applications and check if they are sized according to the load if it does not then replace it with a higher rated fuse….
Don’t change parts instead of troubleshooting
The parts must not be changed until the trouble goes away it is an expensive way to troubleshoot, the same motor may work in other application and by changing the device we will burden ourself with finding the real fault because it was only temporary it may be because of the loading or other factors
Cabling could be an issue
The line at the motor should be checked, power lines may cause the fault conditions power lines in the high-temperature area could fail even by the protection of the conduit. The voltage at the panel and the motor must be checked without checking all these. Parts must not be changed
Check if it is the right motor
In certain case, motors are used for certain applications for which they are not designed. The duty cycle of the motor could determine for which case the motor is suited
Checking power problems
Most of the drive failures are caused by power spikes phase loss or under voltages. After a power issue, it’s important to measure the power to check if it is corrected or not otherwise the drives may be damaged.