# meg readings



## jwiehagen76 (Jan 5, 2009)

Working on 375ton d.c overhead crane. looking for some bad resistors in a large bank with no markings hot real dirty area. found 5 bad ones now the crane is running. the best reading ive got was 275m. One bank soon to be trouble i believe is reading 1.1m. What would you accept for a reading? 250vdc. I am still new at the game and do not want to question the old timers


----------



## John Valdes (May 17, 2007)

Why are you using a megger to check resistors?


----------



## jwiehagen76 (Jan 5, 2009)

Searching for a ground. too much current going to mtr killing the instantaneous OL. These are large resistors with low values .0036 ohms in a large banks.


----------



## John Valdes (May 17, 2007)

I do not understand what you mean. If theres a grounded condition your breaker should trip. High current is not the result of grounded conductors or as in your case resistors. Take each resistor bank and get the wattage and voltage. Use ohms law to determine the resistance. Then measure the resistance. You may need to ohm out each individual resistor if the resistor bank was assembled on site. If the resistor banks were purchased as whole units and connected to other banks there should be some markings as to the values required to measure the resistance.

What are these resistors used for? Braking?


----------



## jwiehagen76 (Jan 5, 2009)

Found the last bad one i believe. There is no breaker on this. connected to hot rails. this is for heavy industry reisistors or used to limit current to hoist control as you hoist removing resistence from the armature increase speed. contactor arrangement changes mtr from series to shunt adding resistence to the field to increase speed in the lower


----------



## John Valdes (May 17, 2007)

jwiehagen76 said:


> Found the last bad one i believe. *There is no **breaker on this* . connected to hot rails. this is for heavy industry reisistors or used to limit current to hoist control as you hoist removing resistence from the armature increase speed. contactor arrangement changes mtr from series to shunt adding resistence to the field to increase speed in the lower


What? Then you have fuses or a non-compliant and un-safe install.


----------



## micromind (Aug 11, 2007)

If the motors in question are AC with wound armatures, then the current of the resistors is limited by the motor design. In fact, the armature leads are shorted out for high speed. As long as the stator is protected, the resistors are also protected. 

If the motor is DC, and the resistors are placed in series with the armature for speed changes, then they are protected by the current limit of the DC supply. 

In my experience, anything over 100K at 500VDC is good. Because of all the dirt and assorted guck that is usually found in the resistor can (unless they're open, which is even worse), trying to get higher than a few megs is nearly impossible. 

These systems are pretty tough, and can handle a certain amount of ground-fault without danger to people, or damage to equipment.

Rob


----------

