Problem- testing spindle motors

[johnhob]

we recently had a problem with one of our fanuc spindle motors shorting down to earth, however the problem was not discovered untill a new amplifier unit was installed. prior to this the motor was fully tested and insulation teasted upto 1000v with no breakdown so it was asumed to be a faulty drive, with the motor disconcted the same no12 fault on the drive apeared. the drive was replaced and the spinlde ran upto full speed no problems occored however after 2 hours of running the new drive developed the same fault and the insultaion to earth on the motor down with 400v.
my question is
why did this not show up in testing earlia is there a special method for teasting the motors? just wanted to know for futur referance

btw alpha drives h405

[MysticMonkey]

Did you megga the cables ?

[underthetire]

Agree. You need a Mega-Ohmer. Tests both motor insulation and cable insulation to ground. There are some home brewed test procedures, but you can buy a mega ohmer for just over a hundred bucks now, not worth the effort. You need to check the cables while moving the head up and down. May be a short in the cable that doesn't show up till the heads in the right place.(if it's a mill).

[CNCZILLA]

You stated that you used 1000 volts for the test. Fanuc recommends using the rated voltage of the motor if possible. I.E. if your motor is 220 v then use that insted of 1000 v. You don't need to use super high voltage in a 220 v motor. If your meters selection is 500 then use that insted of the 1000. If there is a short at 1000 then there is a short at 500. Fanuc says you should have a reading of 100 meg ohms or higher, anything under that then replace or repair. Also as underthetire said, a lot of people look over the cables. Disconnect them from the motor and the drive and ohm them individualy like you would the windings. If not then faulty cables will give the illusion of a bad motor. Good Luck.

[johnhob]

yes as i exlained only the motor was down the cable were fine. also insulation testing at 400 v is fine i know but i did 1000v to see if there was any break down at all and there was non full >9999 ohmes on all phases star and delta windings. the report form the rewinde company said te ceramic bearing had been destroyed but still it did not fault out on first run up to top speed. my only guess is maby water ingerss form the throu coolent but still this should hae pulled up on the megger
i take it from the replies there is no actual special test other than the normal ones then