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Alternator failure modes (long)
List message posted by: Scott Bilinski
Is there any way to tell if your alternator is on its last leg? What are the signs? Stator: Failure in Stator rectifiers. I have had diodes in the stator rectifier fail open-circuit, meaning that the alternator delivers only one-of-three or two-of-three phases. This causes a reduction of max potential output current, so that the alternator may not be able to carry the normal load. The charging system still sort-of-appears-to-work, making diagnosis difficult. If you monitor bus voltage with an accurate dc voltmeter measured with engine running 1500rpm, most of the loads switched off (light load), then the bus voltage would be normal (14.2-14.5V). The Voltage Regulator (VR) is still doing its job by chopping the field current down to a duty cycle of 5-50%. A +- ampmeter would show no net charge or discharge in the steady state (proper indication). If you turn on all of the loads (especially landing/taxi/nav lights), then the bus voltage will sag due to the inablility of the alternator to supply these loads. Under these conditions, the bus voltage may sag to 12.6-13.5V, and the ammeter would show a net discharge from the battery in the steady state. The VR is likely doing its job by applying full (100%, unchopped) field to the rotor, but with one third to two thirds of the stator gone, the alternator cant keep up with the load... Eventually, the battery will be chronically undercharged, leading to cranking problems, especially in cold weather. There is also a marked increase in bus ripple. If you normally hear a (hopefully faint if you did your audio grounds right) alternator whine in your headphone audio, then after the stator/diode failure, the whine gets a lot louder, and lower pitched. Most stator failures are caused when the soldered electrical connection between the stator wire and the diode stack comes apart due to heat/vibration. Sometimes this is the only problem, and can quite easily be repaired, but this requires total disassembly of the alternator. You have to use a "hard solder" to repair these connections (not Pb/Sn electronic solder). Rotor: The brushes can wear down to where they no longer "ride" on the rotor slip rings. As they approach the wear limits, the contact pressure between the brush and the slip ring is insufficient to keep the slip ring clean, meaning that the brush contact to the slip ring gets intermittent and electrically "noisy". This reduces the available field current. This manifests as "reduced max output current", with similar behavior of the dc voltmeter and ammeter when all of the loads are switched on. The noise in the headsets may sound different, however. As before, this is not the VRs fault. It will probably still try to do its job of keeping the bus voltage at 14.25V by applying 100% field duty cycle, but the high resistance of the brushes against the slip rings prevents the alternator from developing much field current, thereby reducing output. This can also manifest as the "pulsating ammeter syndrome", along with unnaturlly high resistance in the external field wiring... The alternator whine in headset audio will sound less musical, more like hash (bacon sizzling). If you have an ADF or AM radio, tune to the low end of the band (200Khz or 540Khz, respectively) and you might hear the alternator hash. If you do remember to this before you have alternator problems, the increase in direct audio and/or ADF/AM radio RF noise as the brushes wear down can give you a clue as to what is going on. I have seen Lorans quit as the RF hash from the alternator increases over time and finally overwhelms the Loran signals at 100KHz. The fix is disassembly of the alternator, installing new brushes, cleaning or possibly turning the rotor slip rings on a lathe. This is part of a normal alternator overhaul done by overhaul shops. Mechanical: As bearings wear, the sideways pull of the belt can cause the rotor to hit the stator pole piece causing catastrophic failure and total destruction. Bearings are normally replaced during overhaul. At annual, remove the belt, and see how much play is in the bearings. MikeM Skylane '1MM Pacer '00Z |
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mikem writes:
List message posted by: Scott Bilinski Is there any way to tell if your alternator is on its last leg? What are the signs? This is a good post to file away. I agree with all of it, with minor qualifiers.. Stator: Failure in Stator rectifiers. I have had diodes in the ...... You can go a long time minus one diode and never know it.. There is also a marked increase in bus ripple. WhatHeSaid. The best test is a portable oscilloscope on the alternator output. You can SEE the hole in the waveform. Most stator failures are caused when the soldered electrical connection between the stator wire and the diode stack comes apart due to heat/vibration. Here I'll mildly defer. The {auto} ones I've seen are always dead diodes, not bad solder joints. But it never hurts to try. Note some diodes do fail only when hot, and "repair" themselves when cool. Rotor: The brushes can wear down to where they no longer "ride" on the rotor slip rings. GM alternators used to have a "full field test tab" to bypass the regulator. Ground it & the output voltage SHOULD go up to 15++ volts and the engine/belt alternator all GROAN. If you are doing a full-field test in-situ, be SURE all the avionics are off-line. Some brushes can be replaced without major work. Even if the rings are dirty, with more brush pressure they often clean up. At annual, remove the belt, and see how much play is in the bearings. And SPIN the pulley! It should be smooth and sweet. BTW, there is an alternator parts supplier in most major cities. These are the folks who sell parts to other rebuilders and work on marines, 300A 28VDC truck units etc. (NOT an auto-parts retailer in a strip mall..) If you don't require FCC blessed parts [1], you might seek such out. They stock and/or can order brushes, diode arrays, etc. One last rant! Grounds, grounds, grounds!!! Inspect/clean the ground bonds, be they engine to alternator, alternator to firewall, whatever. Grounds are Murphy's way of making troubleshooting electrical issues an adventure. 1] Your '540 is used in an airboat... -- A host is a host from coast to & no one will talk to a host that's close........[v].(301) 56-LINUX Unless the host (that isn't close).........................pob 1433 is busy, hung or dead....................................20915-1433 |
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