harmonics

Status
Not open for further replies.
R

RowE

Member
Location
Dumas Tx. 79029
We have lost a few motors bearings powered by VFD's. The company that manufactures the skids is saying the bearing loss is due to harmonics and poor grounding. What affect does grounding have on harmonics? I thought harmonics was a result of non-linear loads such as a VFD. As far as I know there are no line reactors or filters installed to reduce harmonics unless they are built into the drive. Equipment Grounding conductors are all #2 400A OCP. thanks for all your feedback
 
RowE said:
We have lost a few motors bearings powered by VFD's. The company that manufactures the skids is saying the bearing loss is due to harmonics and poor grounding. What affect does grounding have on harmonics? I thought harmonics was a result of non-linear loads such as a VFD. As far as I know there are no line reactors or filters installed to reduce harmonics unless they are built into the drive. Equipment Grounding conductors are all #2 400A OCP. thanks for all your feedback
Click to expand...
FWIW, we got a call to fix a problem with a VFD controlled ventilation motor that was eating up bearings. Turned out the problem was fan belt was way too tight.

I am in no way saying that VFD's or harmonics can't damage bearings, I'm just saying sometimes the problem can be quite mundane and a sexy answer is easier than sending someone to get their hands dirty on site.
 
What is an acceptable level of current total harmonics distortion. I have measured the THD at a few sites and have gotten several different readings. I think the standard is around 3-5%. most of the sites have been anywhere from 2-7% THD, but one site was at 170% THD
 
RowE said:
What is an acceptable level of current total harmonics distortion.
Click to expand...

As long as the current distortion does not cause a voltage distortion, the level of its harmonics is basically a non-issue. This is why present methodology typically uses TDD (Total Demand Distortion) which involves the amount of harmonic current and the size of the system.
 
RowE said:
We have lost a few motors bearings powered by VFD's. The company that manufactures the skids is saying the bearing loss is due to harmonics and poor grounding. What affect does grounding have on harmonics? I thought harmonics was a result of non-linear loads such as a VFD. As far as I know there are no line reactors or filters installed to reduce harmonics unless they are built into the drive. Equipment Grounding conductors are all #2 400A OCP. thanks for all your feedback
Click to expand...
It's a dv/dt (rate of change of voltage) issue rather than a harmonics issue that affects the bearings. The bearings operate with a very thin layer of lubricant, often less than 20um and induced shaft voltages can easily break that down. This can result in the "fluting" effect sometimes see.

Harmonics is a line side issue and line reactors won't help. It is a load side issue. Load side reactors will reduce output dv/dt, may help, and are usually recommended/required for motor conductors greater than 200 metres (about 600 ft).

Motors not specifically designed for VFD operation may need that fact taken into consideration.
 
To add a little more to the above.
The PWM (pulse width modulated) waveform fed to the motor has very fast switching edges. The voltage can get from zero to DC link level, usually about 1.35 times the AC input voltage. in a fraction of a microsecond. That's what causes the currents.
i=Cdv/dt
 
Status
Not open for further replies.
Top