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Vsd Induced Shaft Currents


andrewhl62

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Hi,

 

I've no field experience with VSDs but learned a great deal from the forum discussions and articles posted. This is my first posting - I hope that you are able to assist with my queries.

 

(i) Providing a low impedance path from motor frame to VSD chassis mitigates the build-up of voltage between the VFD and motor frame. However, capacitive coupling between stator and rotor also causes stray currents to flow from the rotor through the motor bearings, eventually discharging to earth through the motor frame. It is well documented that these stray discharge currents do cause premature bearing failure through a process called "Electrical Discharge Machining". The fact that a low impedance path is provided from the motor frame to the VSD chassis actually accentuates this phenomenon, I feel.

 

So how can these bearing-damaging currents be mitigated? The use of a bearing protection ring is often suggested. Are there any other ways?

 

(ii) Would anyone who has had experience with VSD-associated premature bearing failure share on the magnitude of this problem - is the phenomenon so serious that without mitigation techniques, bearing life would be seriously shortened, or is someone out to make a quick buck of bearing protection rings? How much shorther bearing life - 50%, 25% ??

 

(iii) Would steel armoured cables, say PVC/SWA/PVC cabling qualify as a screened cable for the low impedance path mentioned in (i) above?

 

(iv) A single earthing point for both motor and VSD is recommended. However, the motor frame is inherently earthed, by virtue of it being anchored to ground through its plinth. Any major consequence of this?

 

(v) With stray currents caused by capacitive coupling discharging to earth, VSD driven motors are inherently "leaky". Any probems associated with nuisance tripping of residual current devices, and if so how is this mitigated?

 

(vi) Could you please point me in the right direction as far as EMC standards are concerned - what are the currently available standards to refer to?

 

Regards & thank you.

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Have a look at http://www.lmpforum.com/forum/topic/4534-causes-of-edm/

Also keep an eye on http://www.vfd-edm.co.nz which is a sebsite dedicated to edm. - still currently under construction.

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