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VOMs and AC reading

Technoid1

New Member
I am currently trouble shooting a 120/240 volt AC generator. It's a military surplus 10kw unit. There is a problem where AC current is getting into the DC controls. The AC invading voltage is erractic. So this makes it hard for my Fluke 86-5 digital meter to trace and read it. Digital meters are to slow when dealing with problems like this. I need a good analog meter that blocks the DC while reading the AC. With the added ability to read frequency. My Simpson 260 does not block the DC while on the AC position. So what brand and model analog meter would work?
 

Nigel Goodwin

Super Moderator
Most Helpful Member
Stick a capacitor in series with the meter to block the DC - that's all a different meter will contain, but most don't as there's usually no reason for it to need to block DC on AC ranges.

Or use an oscilloscope to see exactly what's happening - be aware of potential earth issues though.
 

Technoid1

New Member
Stick a capacitor in series with the meter to block the DC - that's all a different meter will contain, but most don't as there's usually no reason for it to need to block DC on AC ranges.

Or use an oscilloscope to see exactly what's happening - be aware of potential earth issues though.
That's ok if the meter has the right impedance to work with the cap. I don't think the Simpson 260 falls in that capacity.
 

Nigel Goodwin

Super Moderator
Most Helpful Member
That's ok if the meter has the right impedance to work with the cap. I don't think the Simpson 260 falls in that capacity.
You just need the correct capacitor for the meters impedance (or at least a large enough one), there's no need for a precise value.

Or, if the 500V range is OK for what you want, that gives the exact same impedance as a digital meter, or double on the 1000V range.
 

crutschow

Well-Known Member
Most Helpful Member
Just use a 0.1uF, 500V cap in series with the 260 input.
 

Technoid1

New Member
That's ok if the meter has the right impedance to work with the cap. I don't think the Simpson 260 falls in that capacity.
I found an easier method. I added A diode to the negative probe. It cuts the reading in half but that's ok. Thanks for your input and quick response.
 

unclejed613

Well-Known Member
Most Helpful Member
It's a military surplus 10kw unit.
check all of your ground connections in the gen set and control panel. most of them should be connected with braided wire. make sure all of the connections are free of corrosion. if it's military surplus, get the field manual for it and follow the preventive maintenance checklist in the manual. the checklist tells you what to look for, and also should have troubleshooting charts. if you get the -10 (basic maintenance) version of the manual, the maintenance info is geared towards the user. try to get the -14 or -15 version of the manual as it will cover everything including rebuilding the motor, alternator, etc...
 

gophert

Well-Known Member
Most Helpful Member
military Generator? Is it 60Hz or 400Hz? Single phase or three phase?
 

unclejed613

Well-Known Member
Most Helpful Member
military Generator? Is it 60Hz or 400Hz? Single phase or three phase?
we ground pounders use 60hz. it's most likely 3-phase, since they were phasing out the single-phase gensets in the early 1990s. when i was a teletype operator, they were phasing out our super noisy 5kW gas powered gensets for 10kW quiet diesel ones. our motor pool was trying to speed up the process, as they would put 5 weight oil in them when they repaired them (this is in the south, in the summer). obviously the genset wouldn't last very long before it need fixing again. i think they were trying to dry up their inventory of spare parts so they would be "forced" by necessity to order the new ones.
 

gophert

Well-Known Member
Most Helpful Member
we ground pounders use 60hz. it's most likely 3-phase, since they were phasing out the single-phase gensets in the early 1990s. when i was a teletype operator, they were phasing out our super noisy 5kW gas powered gensets for 10kW quiet diesel ones. our motor pool was trying to speed up the process, as they would put 5 weight oil in them when they repaired them (this is in the south, in the summer). obviously the genset wouldn't last very long before it need fixing again. i think they were trying to dry up their inventory of spare parts so they would be "forced" by necessity to order the new ones.
I only asked because someone was selling a 400Hz military generator on the local craigslist a few months ago - it looked quite old so... before too many questions were asked - ya never know.
 

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