# Understanding generator wear and rotor electrical measurements

I have a generator that I am attempting to fix, and I am getting readings that are out of spec on the rotor brushes. Per some instructional videos I found, I should be measuring 5V AC on the rotor brushes. I am measuring 65-70V AC. The video I was watching said that a high voltage reading (Assuming the brushes were good) indicated a bad rotor.

How can a rotor go bad (besides bearings and I'm sure I would notice other physical issues with the operation - the motor runs fine) and what are the electrical properties that cause it the voltage to read so much higher than nominal?

Edit: Per the question in the comments, this is a Duromax xp10000e (10000W peak) generator. The video I mentioned can be viewed here. I intentionally did not include that information originally because I was attempting to better understand the theory and principles at work, not just information related to my specific generator.

• You have omitted the specification of the generator (V, kVA, make, model, etc.) and the link to the video. It's also not clear where and how you are measuring the "rotor brushes" voltages. A photo of the rating plate and your measurement setup would help. – Transistor Jun 23 '18 at 17:07
• 120/240V generator, 10000w peak (Duromax XP10000e). Video: youtube.com/watch?v=tC_LrrZcx38 – StephenH Jun 23 '18 at 17:10
• The video is long. slow and of poor sound quality. Also 'peak' power has little meaning to us. It would be better to state the continuous RMS power it can put out. – Sparky256 Jun 23 '18 at 18:09
• That's why I didn't originally link to it. I'm not looking for specific numbers, I'm looking for theory. A generator should produce voltage X based on windings and speed of a certain value, so when it's reading factors ABOVE what X should theoretically be, what physical failure conditions could be causing that? – StephenH Jun 23 '18 at 18:34
• This alternator is rated for 30A 10kW . Did you measure under load or no load and what instrument? Was it arcing badly? Can you measure DC milliohms across brushes when off? Or across rotor with brushes removed? Often arc noise gives a false reading that needs a small plastic cap across meter – Sunnyskyguy EE75 Jun 23 '18 at 18:34