industry standards for minimum Megger test acceptability

[rick56]

We recently had an 84" diameter electromagnetic chuck (from a Blanchard rotary surface grinder) repaired with new coils wound and installed. As I understand, these are considered "random wound DC coils". The chuck tested 11.1 megohms when we received it after rebuild. Information I have received from various sources indicates that this is an extremely low insulation value for a "complete rebuild" . So, I am wondering if there are industry standards for a minimum acceptable Megger test value for a "total rebuild" of the chuck.

 

[Tony S]

How long did you keep your finger on the test button? You will find the value rises over time due to capacitance.

 

[zog]

Besides the duration of the test what was the temperature of the windings at the time of test? Is your value as read or temp corrected?

 

[zog]

From NETA/ANSI standard

The dielectric absorption ratio or polarization shall be compared to previously obtained results and should not be less than 1.0. The recommended minimum insulation resistance (IR 1 min) test results in megohms should be corrected to 40° C and read as follows:
1. IR 1 min = kV + 1 for most windings made before 1970, all field windings, and others not described in 2.2 and 2.3. (kV is the rated machine terminal-to-terminal voltage in rms kV)
2. IR 1 min = 100 megohms for most dc armature and ac windings built after 1970 (form-wound coils).
3. IR 1 min = 5 megohms for most machines and random-wound stator coils and form-wound coils rated below 1 kV.

 

[Ingenieur]

what voltage?

 

[rick56]

How long did you keep your finger on the test button? You will find the value rises over time due to capacitance.

Tested for approximately 2 minutes.

 

[rick56]

Besides the duration of the test what was the temperature of the windings at the time of test? Is your value as read or temp corrected?

Temperature of windings at time of test was approximately 65 degrees Farenheit. Value was "as read", not temperature corrected.

 

[rick56]

what voltage?

tested at 1000 volts

 

[rick56]

tested at 1000 volts

 

[zog]

Temperature of windings at time of test was approximately 65 degrees Farenheit. Value was "as read", not temperature corrected.

So your multiplier is about 0.37 11.1x.037= 4.1Meg

 

[Ingenieur]

1000 v to gnd ~ 0.24 mA at 4.1 Mohm
I assume machine voltage is 480 so 277 to ground or 0.07'm
ok imo

a rebuild may not include a rewind
only a cleaning and baking

 

[rick56]

Quote for rebuild stated: removal of old coils and potting compound, winding of new coils, new potting compound, new bolts, contact ring included. Chuck will be tested before shipping.
So, am I to assume this rebuild does not meet the minimum industry standard?

 

[Ingenieur]

what is the operating voltage of the machine?

 

[Ingenieur]

act R = k x 11.1

0.22 from chart or 1/2 for every 10 C drop
65 F = 18.33
40 - 18.33 = 21.67
k = 10/21.67 x 1/2 = 0.22 or 0.5^(21.67/10) = 0.22

act R = 0.22 x 11.1 = 2.47 Mohm
std is 5 Mohm

ref std IEEE Recommended Practice for Testing Insulation Resistance of Rotating Machinery, IEEE Standard 43-2000.

 

[rick56]

can you email me a copy or direct me to the "chart" where you got the .22 value you used in the formula to correct for temperature? Thank you.

 

[Ingenieur]

can you email me a copy or direct me to the "chart" where you got the .22 value you used in the formula to correct for temperature? Thank you.

here's one used in a whitepaper http://www.netaworld.org/sites/default/files/public/neta-journals/NWsp08 - Bishop.pdf
the chart uses act temp in deg C

the equations yield the same result
act temp in deg C
corrected temp = 40 - act temp

k = (10/corrected temp) x 1/2
k = 0.5^(corrected temp/10)