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Hello everyone. My name's Jay and I'm a tech at Rapid Repair Inc. We are currently under contract with Cablevision (Time Warner, Comcast) to repair their Cable trucks. The truck in question is a 2000 Ford F550, with an Altec boom, and a PTO driven Genec AC Compound Generator. The truck came in because the boom is in-operational.

The PTO driven generator, supplies AC voltage to quite a few smaller, electric PTO pumps that operate the boom and bucket (Seems silly to me...Why not just run the boom off the trucks PTO. Why the added weight and complexity of all these extra electric motors and PTOs.

The generator is a Getech HMt-13m, circa 2000, I believe. Originally the generator had 2 open windings. Cablevision opted to send this generator out for rebuild verses buying a new generator for $10,000. The generator came back with all 3 windings repaired, but still no output. If I give 12v to the brushes in the generator, I will get 40-45v AC between all 4 of the output wires, A-B-C and Common. Any 2 will give me 40ish volts. Now is my train of thought correct that, 3 phases at 40v each will give me 120v at my output? Or would A-B-C and Common all have 120v no matter which 2 are connected?

Looking at the scematic, which I can't provide here right now, sorry, There are 2 diodes, and a thermistor right at the brushes. My logic is telling me that a diode, and/or the thermistor is shot, which in turn will not supply voltage to the Shunt winding, which in turn will not power up the series windings.

All these theories I've posted here I came up with today, and I don't claim to know ANY of it as truth.

Here's a copy of the spec sheet for the new 2011 version of this generator. Not much info here, but maybe someone with more generator experience can make some sense of the info.

3600 RPM Hydraulically Driven AC Generators Three Phase HMP-13T

Any and all help is appreciated, My boss, co-workers, and I would really like to put this truck back on the road. Thanks :)

Jay
Rapid Repair Inc.
 

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If there are 2 diodes, and a thermistor right at the brushes, most likely the field voltage to that point is A/C. The 2 diodes, are there to induce half wave D/C into the rotating field, which is most likely much greater than the 12 D/C you applied. If you can post the wiring diagram, I may be able to walk you thru it. Many small units use half wave, some use A/C into the fields, controlled via caps. You can test the diodes, with most any meter, but if your looking for the forward opening voltage you will need a meter that has a diode function test. The thermo can be bypassed for testing. Thermistors are widely used as inrush current limiters, temperature sensors, self-resetting overcurrent protectors, and self-regulating heating elements. Are you sure it's a Thremo, or is it a MOV? Hope this helps.
 
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