three phase motor insulation resistance test

[Buzz1980]

Hi all wondering if you could help clear something up for me. I was recently working on a three phase motor rated for 480volts a.c and wired in delta. So before powering up I done my dead testing and done my insulation resistance test at 500volts d.c which I thought was correct but read up on it that it has to be twice the operating voltage so I done a megger test at 1000volts d.c which came up that I picked up some bad insulation readings of 65mega ohms but now looking deeper in and asking a few more people about it I should have just kept to 500volts d.c as phase to ground is only 240volts where as it is 480volts across phase to phase. So by doing my megger test at 1000volts could I have damaged any of the equipment or cable. Everything worked when I powered the equipment up. Would be much appreciated for any advice?

Thanks
Trig

 

[Darkwood]

Firstly why is 65million ohms a bad reading?

Who exactly are you asking these questions to and are they Electricians or Engineers.
What were you testing between when you did the tests Give order of test and results + voltage tested at.
Why are you testing the motors in the first place if I may ask ?

 

[GMES]

As above ^^

Also was this a new motor it would be nice to see Darkwood's questions answered first.

 

[Hawk81]

65Mohms bad???

 

[Buzz1980]

Firstly why is 65million ohms a bad reading?

Who exactly are you asking these questions to and are they Electricians or Engineers.
What were you testing between when you did the tests Give order of test and results + voltage tested at.
Why are you testing the motors in the first place if I may ask ?

Electricians i am asking I am testing each phase to the motor down to earth to make surd there are not shorts etc. I have done a insulation test (megger) on the cable going to the motor and the cable from the main panel azwell . I have set the range at 1000volts as the three phase motor is 480volt a.c operating. I have went between each phase and ground but was wondering if this range was to high and could damage the motor or cable? Help please

 

[Dizzy_Maskell]

Do you know why you test IR? and what reading would you expect ?

 

[Richard Burns]

Putting 1000V dc on a standard mains cable for fixed wiring would not damage the cable unless it was already damaged.
Your insulation resistance test would be performed without the motor connected so the motor would be unaffected.
(You should also be checking between live conductors as well as checking live conductors to earth)
65MΩ is an OK reading and would not need any action to be taken other than recording the value.

 

[Meggerman]

Putting 1000V dc on a standard mains cable for fixed wiring would not damage the cable unless it was already damaged.
Your insulation resistance test would be performed without the motor connected so the motor would be unaffected.
(You should also be checking between live conductors as well as checking live conductors to earth)
65MΩ is an OK reading and would not need any action to be taken other than recording the value.

If he I/R tested between live conductors he would be testing across the windings. Also OP what values were you expecting to see?

 

[Darkwood]

The OP suggests he is testing the motor itself, his profile says he is a handyman, he does not understand the results he gets nor the voltage the motor will be subject to in this country ..the 480v is the max running voltage rating of the motor and unless run through a TX it more than likely going to be 415v.

If the Motor is new a 1000v test will not be an issue between phases (Motor fully disconnected for testing), if the motor isn't new a test of 500v should be done first of all to check for spurious reading in which case 1000v test may damage the winding insulation further more so refrain from taking the test at 1000v and draft up a maintenance schedule to monitor the motor or have it sent for the winding dipping.

VSD's on older motors too can cause breakdown in winding insulations and can be further damaged by IR testing at 1000v.

The above IR test is the quick check method to indicate obvious issues but a full insulation check requires a meter that can maintain the test voltage for long periods and you subject the motor for a Insulation soak test and record the results at intervals with a routine spec'd to each motor and its use.

 

[micknew]

Are you an electrician?

 

[Darkwood]

Are you an electrician?

Check his profile.. hence his question and lack of knowledge of his results.

 

[micknew]

Oh I see he's not an electrician, he's a handyman.
No handyman section so OP should be in the DIY section surely.

 

[benji]

tested a motor when i was about 17 (1962) with a big wooden megger connected to a big wooden wheatstone bridge and was told "keep turning and don,t stop till we tell you"

 

[DPG]

tested a motor when i was about 17 (1962) with a big wooden megger connected to a big wooden wheatstone bridge and was told "keep turning and don,t stop till we tell you"

Are you still turning it? Daz

 

[Engineer54]

tested a motor when i was about 17 (1962) with a big wooden megger connected to a big wooden wheatstone bridge and was told "keep turning and don,t stop till we tell you"

Ah those were the days.... lol!!