moonshineJ
Member
- Location
- USA
The brushless exciter of a 3-phase motor-generator has (per manual) 2.61-3.19 Ω resistance of its stationary exciter field winding.
Exciter field voltage (no-load) is 23 VDC @ 5.7 A
Exciter field voltage (full load) is 25 VDC @ 6.25 A
The rated output voltage is 3-phase, 120 VAC, 400 Hz
After someone did repairs on the generator (one of 6 diodes of the rotating rectifier was blown), the generator output was 150 VAC (instead of its rated 120 VAC), and could not be regulated. After I verified that nothing else is damaged (the wiring, the components inside the voltage regulator, etc.), I started the MG and the output of generator was 150 VAC.Motor runs at its rated 1200 rpm, nothing else is wrong. I verified voltage shown by the cabinet voltmeter with my Fluke; still 150 VAC.
Here is the question: when I checked voltage going to exciter’s filed winding (F1, F2 terminals…sounds familiar?) I saw 13.3 VDC. I put clamp-on meter and it showed 7.7 going to the field. Yet, voltage output of the MG was 150 VAC (no control), not 120 VAC. Also, I checked the resistance of stationary exciter field winding…3 Ω, which is perfectly good per tech manual.
Unfortunately, I was diverted to work on another MG set which also had a recent casualty. I replaced a blown motor starter with the one from the MG I was working initially on. Changed wiring, did all checks, then started the MGset. There was 150 VAC (no control) instead of 120 VAC. After some playing withthe voltage regulator, I made it to work. Due to no parts for another MG set, I could not continue work on it.
A few weeks later other folks installed a new motor starter (instead of the one which I used on another MG), and replaced the whole voltage regulator, having fixed the problem with unregulated 150 VAC.
Since it was repaired, and I was not there, I could not replicate the problem and figure out how 13.3 VDC @ 7.7 A going to the field would buildup 150 VAC instead of 120 VAC. When I called the company, and asked how this is possible, they tried to say that by the Ohms law I should have the low resistance of the exciter’s field winding. But it was not 1.8 Ω (per G. Ohm), but 3 Ω (and later on this MG was fixed merely by replacing the whole voltage regulator).
The only one thing I could think of, was a situation which I saw a couple of times with AC power distribution. When 115 VAC (hot and neutral) has a hot wire touching a ground somewhere (I believe there was water intrusion into HOT wire of 115 VAC underwater cable), I saw hot wire having half of 120 VAC. Replacing the cable fixed the problem. Here, with the MG, I saw 13 VDC, which is roughly half of 23-25 VDC required to build the rated output.
I’m still really puzzled; any ideas?
Exciter field voltage (no-load) is 23 VDC @ 5.7 A
Exciter field voltage (full load) is 25 VDC @ 6.25 A
The rated output voltage is 3-phase, 120 VAC, 400 Hz
After someone did repairs on the generator (one of 6 diodes of the rotating rectifier was blown), the generator output was 150 VAC (instead of its rated 120 VAC), and could not be regulated. After I verified that nothing else is damaged (the wiring, the components inside the voltage regulator, etc.), I started the MG and the output of generator was 150 VAC.Motor runs at its rated 1200 rpm, nothing else is wrong. I verified voltage shown by the cabinet voltmeter with my Fluke; still 150 VAC.
Here is the question: when I checked voltage going to exciter’s filed winding (F1, F2 terminals…sounds familiar?) I saw 13.3 VDC. I put clamp-on meter and it showed 7.7 going to the field. Yet, voltage output of the MG was 150 VAC (no control), not 120 VAC. Also, I checked the resistance of stationary exciter field winding…3 Ω, which is perfectly good per tech manual.
Unfortunately, I was diverted to work on another MG set which also had a recent casualty. I replaced a blown motor starter with the one from the MG I was working initially on. Changed wiring, did all checks, then started the MGset. There was 150 VAC (no control) instead of 120 VAC. After some playing withthe voltage regulator, I made it to work. Due to no parts for another MG set, I could not continue work on it.
A few weeks later other folks installed a new motor starter (instead of the one which I used on another MG), and replaced the whole voltage regulator, having fixed the problem with unregulated 150 VAC.
Since it was repaired, and I was not there, I could not replicate the problem and figure out how 13.3 VDC @ 7.7 A going to the field would buildup 150 VAC instead of 120 VAC. When I called the company, and asked how this is possible, they tried to say that by the Ohms law I should have the low resistance of the exciter’s field winding. But it was not 1.8 Ω (per G. Ohm), but 3 Ω (and later on this MG was fixed merely by replacing the whole voltage regulator).
The only one thing I could think of, was a situation which I saw a couple of times with AC power distribution. When 115 VAC (hot and neutral) has a hot wire touching a ground somewhere (I believe there was water intrusion into HOT wire of 115 VAC underwater cable), I saw hot wire having half of 120 VAC. Replacing the cable fixed the problem. Here, with the MG, I saw 13 VDC, which is roughly half of 23-25 VDC required to build the rated output.
I’m still really puzzled; any ideas?
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