VFD operation of a wound rotor induction motor 3

[edison123]

My client recently operated non-VFD rated, 415 V, 150 HP, slipring motor with a vfd by short-circuiting the rotor winding. The motor came to me with the stator winding failed. The rotor winding has passed the megger and surge test at 1 KV in our shop.

We will rewind the stator with insulation suitable for vfd operation.

Should I recommend the rewinding of the rotor (which is permanently shorted across lines) also because of the vfd operation ? I do not want the rotor winding to fail due to vfd operation.

*Why a man thinks he outrun a chasing dog when it has twice as many legs?*

 

[edison123]

Another question, should I worry about the bearings due to vfd ? If yes, what should be done ?

(I do remember a thread about slipring motors on vfd's some time back, but I can't find it. If someone points it out, I would be thankful)

*Why a man thinks he outrun a chasing dog when it has twice as many legs?*

 

[LionelHutz]

The rotor will be fine. There will not be much voltage generated in the windings with it short circuited.

 

[edison123]

Thanks Lionel.

*Why a man thinks he outrun a chasing dog when it has twice as many legs?*

 

[aolalde]

Hi Edison123.

Since the coupling of the rotor winding to the stator winding is electromagnetic induction, the high frequency spikes due to the current chopping in the stator are "smoothed". For that reason I think the rotor winding does not experience the same severity of PD as the stator.So leave it as is.
In regard to the bearing circulating currents, high probability of capacitive coupled currents due to the high frequency of the carrier driver frequency exist. I suggest that the NDE bearing be insulated and a shaft grounding devise like those by "AEGIS" be added.

 

[edison123]

Thanks Alex.

I was kinda leaning towards to insulating both side bearings. Do you see any snags there ?

*Why a man thinks he outrun a chasing dog when it has twice as many legs?*

 

[aolalde]

Insulating both bearings will allow you to test the bearings insulation resistance and shaft-induced voltage. It will not hurt to have both bearings insulated (except for your pocket). Be aware that "Capacitive coupled currents" still could flow through insulation and shaft grounding will secure those currents not to flow into the bearings.

 

[edison123]

More info about the drive

Make : ABB AC drive Model : ACS 800
Output Frequency : 0 - 300 Hz
Output Current : 348 Amps
No filter on the output
Distance from Drive to Motor : 40 Meters
Motor Cable : XLPE armoured Aluminum cable
Normal motor Speed Range : 700 To 900 rpm
Motor poles : 6

Are the cable type and cable distance ok ?

Should I recommend a filter between the VFD and the motor ?



*Why a man thinks he outrun a chasing dog when it has twice as many legs?*

 

[ozmosis]

40M should be ok without the need of additional dv/dt filters. As long as the AC supply voltage is <500Vac then it should not create a problem at this distance, especially as you have rewound the motor suitable for VFD operation.

 

[edison123]

Thanks sed2.

What the VFD-to-Motor cable itself ? Does it have to be VFD rated ?

*Why a man thinks he outrun a chasing dog when it has twice as many legs?*

 

[ozmosis]

ABB, as well as most other drive companies recommend symmetrically shielded cable.

http://library.abb.com/global/scot/...5730900315da9/$File/en_800_01_hw_H_scrres.pdf

Page 48/49 of this manual will highlight the requirements.

 

[edison123]

Thanks sed2. That was an useful info.

*Why a man thinks he outrun a chasing dog when it has twice as many legs?*