motor condition in centrifugal chiller

[ESS CHILL WATER]

Hello guys new question related to a motor on centrifugal chillers
YORK is open drive motor so no question about MEGG the motor
TRANE low pressure R-123 is a semi-hermetic motor whats the best condition to perform motor MEGGER ??? according to your experience
CARRIER & MC Quays 134a is semi-hermetic motor whats the best condition to perform motor MEGGER???? according to your experience.
if you guys have any chart or manual that explain how is the best method to MEGG the motor and under what condition ,i will appreciate your Help.

Regards

[Tech Rob]

Why wouldn't you test an open drive Y motor with a megger? It has windings with insulation that can deteriorate the same as any other motor.

As far as T, C, and M chillers... I don't know of any special procedures for any of them, other than checking from phase to phase in addition to phase to ground. Don't meg any of them in a deep vacuum. Don't use a megger with a solid state starter or vfd connected. Megger readings don't do a whole lot of good without a history. I think T has a chart in the blue service guide?

As far as a go/no-go rule of thumb for standard motors, I've always used 1 megohm per 1kv + 1 megohm.

[ESS CHILL WATER]

Rob you right that we need to check all the motors of the chillers , my question is around to the fact of york is open chiller compresor and motor are separate so the motors are under ambient condition , now trane R-123 OR 11 are low pressure and normally with refrigerant the units stoped are in vaccum my question is according to your experience we have to increase the pressure to 0 or 1 psi and then apply the megger or is no problem to megg the motor if the units are in vaccum but with the load of refrigerant???
Now on the R-134a C,M, or any other centrifugal with positive pressure you can megg the motor of the unit under pressure condition??
Do you have the blue service book for share??

regards

oscar

[Tech Rob]

The reason they say not to meg a motor in a vacuum is because in a deep vacuum, there is no atmospheric insulation between the phases and from phase to ground. Arcing can occur inside the motor. Even in a low pressure machine that's in a vacuum when it's sitting idle, the refrigerant is the atmosphere that helps to insulate the phases from one another and ground.

The chart is in CTV-SG-1 and CTV-SB-61. My copy of it is not much good and is mostly illegible.

[ESS CHILL WATER]

K ROB i will be on tampa next february , if you have time we can meet and buy you lunch do you work for a company or you do yourself??? may be you can give me some advices and recommendations in a chiller side , we have tranes ,mcquay,yorks and carrier under our responsability most of all are screw or centrifugals

thanks again for your support

[Tech Rob]

Going to a class or something around here? Tampa's pretty cool, you'll enjoy it. I am always for doing lunch and talking shop with other folks in the business. Glad I could be of assistance, Oscar.

[ESS CHILL WATER]

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[w~p]

The reason they say not to meg a motor in a vacuum is because in a deep vacuum, there is no atmospheric insulation between the phases and from phase to ground. Arcing can occur inside the motor. Even in a low pressure machine that's in a vacuum when it's sitting idle, the refrigerant is the atmosphere that helps to insulate the phases from one another and ground.

The chart is in CTV-SG-1 and CTV-SB-61. My copy of it is not much good and is mostly illegible.

Motor is to be megged with the stator at ambient temperature after 24 hours of idle standby.

Meg Phase to Phase with remaining Phase connected to Ground.

Meg Phase to Ground with remaining Phases connected to Ground.

[jayguy]

...Meg Phase to Phase with remaining Phase connected to Ground.

Meg Phase to Ground with remaining Phases connected to Ground.

nope.

[spinning wheel]

In the IOM manuals the manufactures tell how to meg their motors and the limits that are allowed. Sometimes we need to read. See any York OM manual around page 58.

[supertek65]

x2

nope.

[klove]

Motor is to be megged with the stator at ambient temperature after 24 hours of idle standby.

Meg Phase to Phase with remaining Phase connected to Ground.

Meg Phase to Ground with remaining Phases connected to Ground.

Where did this instruction come from?

[RichardL]

Something of far greater importance than a simple "Megger" reading is called a "Polarization Index". What with the windings of most centrifigal motors being bathed in both refrigerant and oil mixture a "PI" test can and will give a far better deternination of good/bad than any simple "Meg" reading. The megger voltage is applied, the first reading is taken, then for 10 minutes with the megger still connected a second reading is taken.

In summary, if the insulation system is clean and dry, the insulation resistance will increase with time( a smaller current). An insulation system that is contaminated with water, oil or is weak will show little if any increase with time.

James G. Biddle (now AVO Corp. of Dallas, TX) publishes the following table for the Dielectric Absorption Ratio (Polarization Index)

Condition 10:1 Min Ratio

Dangerous < 1
Poor <1.5
Questionable 1.5-2.0
Fair 2-3
Good 3-4
Excellent > 4

I have given the ratios for the 10/1 min. test. A 60sec/30 sec. test is also made and the corresponding values arefrom top to bottom) <1.1, 1.1-1.25, 1.25-1.4, 1.4-1.6 and > 1.6. Many technicians prefer the shorter test times especially if they have a hand crank test set.

Some References

"A Stitch In Time..." by James G. Biddle pub. 21-P8
Insulation Testing by D-C Methods by James G. Biddle pub. 22t1a-1975
Principles of Dielectric Engineering, by Andrew Blanck, Rutherford Research Products Comapany
The Characteristics of Insulation Resistance by Sydney Evershed, presented to Institute of Electrical Engineers, London, 27 Nov., 1913

[ESS CHILL WATER]

thanks Richard very good and understandable explanation also thanks for the literature reference
regards
oscar

[DITAW]

some intersting reading.
Attachment 238911

[jayguy]

thanks for posting Richard...i was getting tired of having this same conversation...that and the myth of not megging a motor in a vacuum...there is a huge difference between megging a motor in a vacuum versus applying line voltage to a motor in a vacuum. i use the PI method (1 minute / 10 minute). a 6 lead compressor can take about an hour so i have to plan for it!

[heavymetaldad]

x2

nope.

gotta love the open section

[w~p]

Where did this instruction come from?

It is from Fluke.