Megger Test Voltages
Megger Test Voltages
(OP)
We have been testing/inspecting 250 to 750 HP motors (mainly DC with separately excited fields) for several years at a customer of ours. Between my colleague and myself (both of us engineers) have developed a process to visually inspect the communtator, brush lengths, and motor insulation. We typically use a megger to test the insulation and have used the "twice the voltage" idea to safely perform the test. For example, if the output to the armature of the drive is 500VDC we output 1000V from the megger and record the results after a couple of minutes.
Likewise with the field, if the field voltage is 300VDC we output 500V from the megger.
Are these megger output values correct for testing DC motors?
I have yet to find a specific documented table of test voltages vs. motor voltages.
Likewise with the field, if the field voltage is 300VDC we output 500V from the megger.
Are these megger output values correct for testing DC motors?
I have yet to find a specific documented table of test voltages vs. motor voltages.





RE: Megger Test Voltages
RE: Megger Test Voltages
RE: Megger Test Voltages
RE: Megger Test Voltages
RE: Megger Test Voltages
Still, most well thought out test regimens specify a megger test before applying the much higher voltage of a hipot test for cables.
As far as motors are concerned, 1000 volts for a 480 volt motor is the standard approach, but again, if you suspect your motor to be wet, dirty, or damaged, a quick test at a lower voltage won't hurt, either.
They still pay us to think about what we're doing, after all...
old field guy
RE: Megger Test Voltages
RE: Megger Test Voltages
Gunnar Englund
www.gke.org
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: Megger Test Voltages
That's what I told them: "Better it should fail now than at two AM on a Saturday morning in the midst of a critical production effort..." Somehow that statement did little to soothe the guy who had to set up for the replacement job.
NETA Maintenance Test Specification 2005 gove the DC voltage for insulation tests for 600-volt class equipment as 1000 VDC.
Again, though, a thoughtful approach to investigating a piece of equipment that was functional but that also might be badly deteriorated is to use a lower voltage for evaluation purposes.
More than once I've told a client "It's getting pretty bad. You REALLY want to start lining up resources to replace this thing because it hasn't failed YET, but it's looking like it could go at any time..."
old field guy