in situ motor testing

[jimmy2times]

we have 6.6kv induction motor for bow thruster of ship (1.4MW), difficult access for cleaning and removal.

we conduct PI insulation resistance measurements and have low PI reading ratio. 1min IR reading is just over 100Mohm though (102Mohm) so not awful by any means.

We ran anti condensation heaters for few days but no real improvement in result.

just wondering what the view is in terms of doing additional in situ testing in such case to determine condition of winding. I recall reading thread on here on Baker testing a while back and some advocating it with a Go/No-Go outcome, whilst others not. Just wondering if views are still the same or if any new developments out there in this space.

 

[waross]

By "in situ" I understand that you will be testing with the supply cables connected to the motor and forming part of the circuit under test.
See this thread, and in particular the post by Edison123 and the link that he posted.
This has to do with surface leakage and how surface leakage interferes with an insulation test.
The link explains the use of the guard temrinal on the Megger to nullify the effect of surface leakage.
HV motor running with low insulation
thread237-460539


Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[jimmy2times]

I’ll have to check with technicians whether they meggered from the switchboard end or whether they dropped cables off at motor terminal box. I think the maintenance task requires cables to be dropped off at motor terminal box but I’ll check with them tomorrow.

By in-situ I was referring to fact that access to motor is limited and needs to be removed (big job) just to clean winding. So was wondering if other diagnostic test with motor in-situ might be warranted

 

[edison123]

PI (10 min/1 min IR) is an indicator of dryness and cleanliness (no oil, grease, carbon dust etc.) of motor winding (without power cables, as Bill says). A PI value > 2 is considered good for modern F class machines.

As for motor innards cleaning frequency, it would depend on type of enclosure (completely closed - once in 5 to 10 years when bearings are changed or open type - as often as contaminant accumulation exceeds certain level defined by you) and the environment (clean - less or dirty - often).

The anti condensation heaters are just that, they prevent moisture condensation settling on winding surfaces by keeping the winding 2 to 5 deg C above the ambient. Their watt rating is too small to dry out a moist winding.

IR & PI tests are non-destructive tests along with DC resistance measurement. Other tests like Baker surge test, AC/DC hipot, tan delta test etc. are potentially destructive tests and the user has to weigh the pros and cons of applying these tests, especially in critical motors.

Finally, while a good IR & PI are preferred, they are not an absolute indication of the dielectric strength of the winding. A machine with good IR & PI can still fail.

Muthu

http://www.edison.co.in

 

[panter]

Jimmy ,
measured value at 15 and 60 seconds is a good parameter for insulation state for that motor and it should be more than 1.3 .
100 Mohms is very good final value of insulation resistance for a 6.6kV motor and I don't see why the coil is drying .
Good luck !