If this is your first visit, be sure to check out the FAQ by clicking the link above. You may have to register before you can post: click the register link above to proceed. To start viewing messages, select the forum that you want to visit from the selection below. |
|
|
Thread Tools | Display Modes |
#11
|
|||
|
|||
Noise Problem. Both Comms Breaking Squelch
Lancair IV-P Flyer wrote:
On Mar 31, 5:59 pm, MikeMl wrote: Lancair IV-P Flyer wrote: On Mar 29, 4:45 am, " Steve, I do not have any specific Lancair experience, however, I know about Cessnas. The new glass-panel Cessnas have an alternator controller which includes an overvoltage detector which incorporates an electronic "crowbar" circuit. It purposely overloads and trips the Alternator Field Breaker if a momentary bus voltage transient is detected. Could your Lancair have a similar system? On the radios unsquelching, is it possible that the radios are just overly voltage sensitive? With the alternator on-line, the bus voltage should be nominally 28.5V. With the alternator off-line, the bus voltage will quickly sag to about 24V. Some radios do better than others at not having their squelch threshold change when the input power changes that much. MikeM Mike, The solid state voltage regulators used in many experimental category aircraft do in fact have "crowbar" type protection as well as over voltage protection. As I understand the operation of the crowbar trip from a simple over voltage trip is if the crowbar opens the circuit, it can only be reset by turning off the alternator switch. Once that occurs, you can reset the breaker, turn the alternator switch back on and the alternator will be back in business. No, what you are describing is how the 1976 to 1986 steam-gauge Cessnas worked. They have a latching relay in the OverVoltageProtection module which has to be reset by momentarily turning off the ALT side of the Master Switch. The glass panel Cessnas (post 2003?) actually have an electronic crowbar which artificially creates an overload current which blows the Alternator Field Breaker when an overvoltage is detected. The Breaker must be physically reset in order to bring the Alternator back on line. A momentary spike lasting only milliseconds will trip the Field breaker; a truly stupid system! This system has an AD against it; spurious tripping, what else. Many new Cessna owners have had issues with this, and have spent money replacing things like alternators when the root cause was an OV circuit which was too sensitive to short duration transients caused by things like an inductive gear pump motor turning off! My breaker trips are always resettable without resetting requirement of the alternator switch. So, I think something is shorting to ground that shouldn't be. I am thinking the main alternator may still be the culprit or possibly the battery, which is an AGM type, may be shorting as it heats up. I am going to try pulling the field breaker and sense breaker of the backup alternator to see if the problems occur during the isolation. If so, then running from the backup system and pulling the breakers of the main system. If the failures only occur on the main system I am going to replace the alternator with a completely different unit. If both tests show voltage excursions, I am going to replace the battery and see if that isn't the culprit. There is really not much left to try. Thanks for your help. Steve The only faults (other than the possibility we are discussing above) that would cause the Field Breaker to trip (in order of likelyhood) is: rubbed through insulation on wiring between the breaker and the alternator field, a short between a wire shield and its center conductor, a loose brush holder inside alternator, or a fault inside the Voltage Regulator. You have already looked at wiring and replaced the alternator. Since your trips occur many minutes into a flight, ask yourself what happens to the bus voltage as the alternator recharges the battery after the startup? The higher the bus voltage climbs, the closer it gets to the OVP's trip voltage, biasing it ever closer to its trip point. I'll take a bet that you have an oversensitive OVP. The battery has nothing to do with this; the VR should keep the voltage on the battery at no higher than 28.8V. Find out what voltage your OVP trips at. I would do the following ground test: Isolate the OVP/VR from the Field Breaker (just open it if it is the pullable kind). Connect a 3A, 28V regulated, current-limited dual-metered adjustable Power Supply to the wire downstream from the Field Breaker. Start with the supply set to about 24V with a short-circuit current set to 3A; you should see about 1.5A flowing which is going into the alternator field winding. Slowly increase the supply voltage; at about 28.5V, you should see the supply current suddenly drop to a few tens of mA. That is the cutout voltage at which the VR regulates the bus voltage (by turning off the field excitation). Slowly decrease the supply voltage until the field current jumps up again; that is the cutin voltage, and might be a few tens of mV lower than the cutout voltage (hysteresis). Now increase the supply voltage above the cutout voltage and watch what happens. Ideally, the OverVoltage trip should be at 32V or higher. Observe what happens when the OVP fires. Does it act like a short circuit to the supply? (i.e. did the supply go into current limiting and stay there until you disconnect a wire?). What you are looking for is at what voltage did the trip occur, and would the trip have blown the field breaker? MikeM (PhD EE, retired) Skylane, Pacer. |
#12
|
|||
|
|||
Noise Problem. Both Comms Breaking Squelch
MikeM (PhD EE, retired) Skylane, Pacer. I had a 172M that gave random overvoltage tripout problems too. First we spun the alternator with an electric motor on the gorund & looked at the output voltage with an oscilloscope. Sure enough it was tripping out at well less than 16 V. Maybe these units drift? We replaced it with new, but noted that there is an exposed tweekpot on the OV sensor that probably adjusts the trip point. |
#13
|
|||
|
|||
Noise Problem. Both Comms Breaking Squelch
On Tue, 01 Apr 2008 08:56:27 -0600, MikeMl wrote:
The glass panel Cessnas (post 2003?) actually have an electronic crowbar which artificially creates an overload current which blows the Alternator Field Breaker when an overvoltage is detected. The Breaker must be physically reset in order to bring the Alternator back on line. A momentary spike lasting only milliseconds will trip the Field breaker; a truly stupid system! This system has an AD against it; spurious tripping, what else. Many new Cessna owners have had issues with this, and have spent money replacing things like alternators when the root cause was an OV circuit which was too sensitive to short duration transients caused by things like an inductive gear pump motor turning off! Do you have an AD number for that? |
#14
|
|||
|
|||
Noise Problem. Both Comms Breaking Squelch
Peter Clark wrote:
On Tue, 01 Apr 2008 08:56:27 -0600, MikeMl wrote: Do you have an AD number for that? I don't own a 182T. Now that you mention it, it was a Cessna Service Bulletin that required replacement of the ACU. It was discussed extensively on the Cessna Pilot's Assoc. forums. |
#15
|
|||
|
|||
Noise Problem. Both Comms Breaking Squelch
On Tue, 01 Apr 2008 15:47:49 -0600, MikeMl wrote:
Peter Clark wrote: On Tue, 01 Apr 2008 08:56:27 -0600, MikeMl wrote: Do you have an AD number for that? I don't own a 182T. Now that you mention it, it was a Cessna Service Bulletin that required replacement of the ACU. It was discussed extensively on the Cessna Pilot's Assoc. forums. I'd like to know the SB number for that. The only SB I can find that references alternators is SB06-24-03 which is an alternator replacement for manufacturing defects in Kellly Areospace alternators made from 8/95 to 8/04. |
#16
|
|||
|
|||
Noise Problem. Both Comms Breaking Squelch
On Mar 27, 2:27 pm, Lancair IV-P Flyer wrote:
Would anyone have an idea about the cause of this? Problem: Intermittent noise (sounds just like a squelch volume test) on both communication radios. Lasts from 15 seconds to two minutes. Will occur at any RPM including idle. Changing frequencies eliminates the noise but the noise can and most likely will come back shortly on that same frequency. I have a back up alternator on this aircraft and when I have the main alternator turned off to check the operation of the backup alternator, the noise never seems to occur. Also, when the noise manifests, if I turn off the alternator the noise always goes away. I have done this several times on the ground and it has been 100%. The voltage regulator has just been replaced and the noise is still present. The alternator has been rebuilt three times in the last 30 hours of operation. The alternator shop is tired of hearing from me. This squelch break noise is new however in the last 15 hours of operation. The alternator has been having so much trouble because we have been trying to locate an alternator field circuit breaker popping issue. Typical scenario is the aircraft must fly for at least 40 minutes before the breaker will pop. Usually, it pops in conjunction with a small static crack in the headset and a voltage excursion of one to two volts. This most often happens in level flight. This same voltage excursion has been noted at least once when the main alternator is turned off and running on the backup alternator. All connections in the alternator/breaker/battery/ground circuit have been cleaned, tightened and cable ends replaced when suspect. All to no avail. This is a 24 volt single battery system. Running the 35 amp hydraulic pump to actuate flaps or gear will on occasion trigger a field alternator breaker pop but many times it won't either. Thanks in advance for any light you might be able to shine on this problem. Mike, The OV protection is part of the voltage regulator product. The one we are using is an LR3C 24 volt from B&C Specialties. It is widely used in the experimental market and has a bullet proof history of no problems. Since we had tried everything else the company sent us a replacement regulator to try just in case we had a problem. I flew the airplane last week with the new regulator and saw no change in the symptoms. So, I am pretty confident the OV protection is not causing the problem. Regarding the field wire integrity, I hooked up a multimeter in series to the field breaker and looked at the amperage to the breaker during a flight. I was hoping for a building amperage which would have indicated resistance building then I could have begun searching for what was building resistance. But the multimeter amperage reading was dead solid at 1.5 amps which is quite a cushion from the 5 amp rating. Something is causing a voltage spike. I just have to find it. I am grateful for your help please forward any additional ideas you may have on this. Steve |
#17
|
|||
|
|||
Noise Problem. Both Comms Breaking Squelch
Lancair IV-P Flyer wrote:
On Mar 27, 2:27 pm, Lancair IV-P Flyer wrote: Mike, The OV protection is part of the voltage regulator product. The one we are using is an LR3C 24 volt from B&C Specialties. It is widely used in the experimental market and has a bullet proof history of no problems. Since we had tried everything else the company sent us a replacement regulator to try just in case we had a problem. I flew the airplane last week with the new regulator and saw no change in the symptoms. So, I am pretty confident the OV protection is not causing the problem. And I am just as confident that it is!!! According to this description: http://www.bandcspecialty.com/QuickFacts_LR3C.pdf the LR3C has exactly the type of crowbar circuit that I have been describing! If it detects what it thinks is an "overvoltage" condition, then it responds by firing its "protection crowbar", which instantly overloads the aircraft's Field Breaker, causing it to overheat and trip, which removes power from the LR3C, and therefore removes excitation from the Alternator's field circuit, thereby taking the entire charging system offline until the Field Breaker is reset. I personally think this is a DUMB design that causes many more problems than it prevents. Regarding the field wire integrity, I hooked up a multimeter in series to the field breaker and looked at the amperage to the breaker during a flight. I was hoping for a building amperage which would have indicated resistance building then I could have begun searching for what was building resistance. But the multimeter amperage reading was dead solid at 1.5 amps which is quite a cushion from the 5 amp rating. This is normal behaviour. Think of the alternator as a current amplifier. Its output current is nominally about 25 times its field current. In other words, it takes about 1A of Field Current to produce 25A of output current. In steady flight, many minutes after engine start after the battery has recharged, the average electrical load in the aircraft is somewhere around 20 to 40A, so the alternator has to produce 20 to 40A, meaning its Field Current will be 0.8 to 1.5A. Almost all of the current that you measured at the Field Breaker is flowing through the Alternator Field to ground. The LR3C regulator effectively "regulates" the Field Current so that the Alternator output just matches the electrical load. The current that is tripping the Field Breaker is a momentary overload cause by the LR3's crowbar that lasts only a few 10s of msec. It would take a "peak-capture&hold" type of meter to display it! A minor nit. If there was a "building resistance" in the field circuit, that would reduce the field current thereby reducing the likelyhood that the Field Breaker would trip. It takes an unplanned shunt path (fault) to ground to increase the field current. The crowbar inside the LR3 is a "shunt" path to ground when it fires. Something is causing a voltage spike. I just have to find it. Yes, that is the root cause of your problem; its just not where you have been looking. You have one of three problems: 1. The OverVoltage detection level of the LR3 is set too low (too close to the actual bus voltage, assuming that is correct). Solution, raise the LR3's Overvoltage Threshold. My preference for a realistic Overvoltage Threshold is 31V. 2. The Bus voltage really is climbing to unsafe levels. For your AGM battery, the bus voltage should never get above 28.5V. Solution: adjust the regulated bus voltage to 28.5V or lower. 3. There is an inductive load somewhere in the aircraft (flap motor, gear pump motor, autopilot servo, trim servo) which during its normal cycling puts a short duration inductive electrical transient voltage spike onto the main bus. The spike is of sufficient energy that the OVP circuit sees it, and reacts to it by firing its crowbar. Solution: find the source of the spike and suppress it at its source, or make the LR3 less sensitive to short duration spikes, either by raising its Overvoltage detection threshold, or by "filtering" its sensing input to prevent it from "seeing" the short-duration spikes. I am grateful for your help please forward any additional ideas you may have on this. I outlined a method of testing the Regulation voltage and the Overvoltage Threshold voltage of the VR/OVP in situ (using a lab supply) in my other post. The only thing new is that the LR3 is a "linear" regulator, so you will see the Field Current decrease linearly between about 28V and 28.4V, rather than exhibiting a bistable on-off behaviour. MikeM |
#18
|
|||
|
|||
Noise Problem. Both Comms Breaking Squelch
On Apr 2, 9:26 am, MikeMl wrote:
Lancair IV-P Flyer wrote: On Mar 27, 2:27 pm, Lancair IV-P Flyer wrote: Mike, The OV protection is part of the voltage regulator product. The one we are using is an LR3C 24 volt from B&C Specialties. It is widely used in the experimental market and has a bullet proof history of no problems. Since we had tried everything else the company sent us a replacement regulator to try just in case we had a problem. I flew the airplane last week with the new regulator and saw no change in the symptoms. So, I am pretty confident the OV protection is not causing the problem. And I am just as confident that it is!!! According to this description: http://www.bandcspecialty.com/QuickFacts_LR3C.pdf the LR3C has exactly the type of crowbar circuit that I have been describing! If it detects what it thinks is an "overvoltage" condition, then it responds by firing its "protection crowbar", which instantly overloads the aircraft's Field Breaker, causing it to overheat and trip, which removes power from the LR3C, and therefore removes excitation from the Alternator's field circuit, thereby taking the entire charging system offline until the Field Breaker is reset. I personally think this is a DUMB design that causes many more problems than it prevents. Regarding the field wire integrity, I hooked up a multimeter in series to the field breaker and looked at the amperage to the breaker during a flight. I was hoping for a building amperage which would have indicated resistance building then I could have begun searching for what was building resistance. But the multimeter amperage reading was dead solid at 1.5 amps which is quite a cushion from the 5 amp rating. This is normal behaviour. Think of the alternator as a current amplifier. Its output current is nominally about 25 times its field current. In other words, it takes about 1A of Field Current to produce 25A of output current. In steady flight, many minutes after engine start after the battery has recharged, the average electrical load in the aircraft is somewhere around 20 to 40A, so the alternator has to produce 20 to 40A, meaning its Field Current will be 0.8 to 1.5A. Almost all of the current that you measured at the Field Breaker is flowing through the Alternator Field to ground. The LR3C regulator effectively "regulates" the Field Current so that the Alternator output just matches the electrical load. The current that is tripping the Field Breaker is a momentary overload cause by the LR3's crowbar that lasts only a few 10s of msec. It would take a "peak-capture&hold" type of meter to display it! A minor nit. If there was a "building resistance" in the field circuit, that would reduce the field current thereby reducing the likelyhood that the Field Breaker would trip. It takes an unplanned shunt path (fault) to ground to increase the field current. The crowbar inside the LR3 is a "shunt" path to ground when it fires. Something is causing a voltage spike. I just have to find it. Yes, that is the root cause of your problem; its just not where you have been looking. You have one of three problems: 1. The OverVoltage detection level of the LR3 is set too low (too close to the actual bus voltage, assuming that is correct). Solution, raise the LR3's Overvoltage Threshold. My preference for a realistic Overvoltage Threshold is 31V. 2. The Bus voltage really is climbing to unsafe levels. For your AGM battery, the bus voltage should never get above 28.5V. Solution: adjust the regulated bus voltage to 28.5V or lower. 3. There is an inductive load somewhere in the aircraft (flap motor, gear pump motor, autopilot servo, trim servo) which during its normal cycling puts a short duration inductive electrical transient voltage spike onto the main bus. The spike is of sufficient energy that the OVP circuit sees it, and reacts to it by firing its crowbar. Solution: find the source of the spike and suppress it at its source, or make the LR3 less sensitive to short duration spikes, either by raising its Overvoltage detection threshold, or by "filtering" its sensing input to prevent it from "seeing" the short-duration spikes. I am grateful for your help please forward any additional ideas you may have on this. I outlined a method of testing the Regulation voltage and the Overvoltage Threshold voltage of the VR/OVP in situ (using a lab supply) in my other post. The only thing new is that the LR3 is a "linear" regulator, so you will see the Field Current decrease linearly between about 28V and 28.4V, rather than exhibiting a bistable on-off behaviour. MikeM Mike, Your explanations are really helping me through this problem. Thank you very much for your continued involvement. Using your theories, I contacted the president of B&C Specialties today and asked him what the crowbar trip point is set at as the LR3C is a sealed box. The only user adjustable item is a pot screw for adjusting the voltage the regulator maintains. Bill, the president, said the crowbar trip point is set at 32 volts. You are absolutely correct about the voltage set point ideal is at 28.5 volts. I got that number from speaking with the Concorde battery tech. We have the regulator set so that at high RPM we are getting 28.5 and at idle rpm we get 28.2 volts. This has been the set point for several months. We have a JPI 930 installed that gives us voltage, load amperage and other useful information. Each of those have a user defined alarm limit. The voltage alarm I have set is at 29.5 volts. Any time I hear the telltale "crack" of static in the headset, I look to the JPI or the voltage readout on the Davtron timer and can see the voltage excursion. It is usually less than one volt. Whenever the voltage excursion goes above the alarm point I see an alarm post as the display color goes to red, and the word "alarm" appears. Most of the time when we have a trip event on the field breaker it occurs without the voltage spiking high enough to trigger the alarm set point of 29.5 volts. Regarding the idea that something in the aircraft is triggering a transient voltage spike onto the bus, this is the theory we have been working on for the most part since August. After we got the alternator fixed all of our attention was looking for the origin of the transient. The flap and gear are run by the hydraulic pump which is the highest draw in aircraft. The circuit breaker is a 35 amp size and when actuating the flap or gear one can see up to 35 amp increase in the load on the load meter shown on the JPI 930. There is a light on the panel that comes on anytime the hydraulic pump is actuating which it does on its own to maintain specified pressure. I have looked closely when we begin to hear static and see voltage excursions which are precursors to a trip event to see if they are accompanied by a cycling of the hydraulic pump. I have never seen that to be the case except on landing operations when the flap and gear are actuated. In those occasions, when the field breaker trips it is directly in response to hydraulic pump actuation. Mike, given this information, what do you suggest I do next to try and isolate the cause? Thanks again, you have a better grasp on this issue than anyone I have talked to since I began working on it 7 months ago. Steve |
#19
|
|||
|
|||
Noise Problem. Both Comms Breaking Squelch
Another long shot is might the master switch or master relay be
intermittent? Normally I'd think any inductive kickbacks from motors etc would be ballasted by the battery - as long as it stays connected to the main bus. But if there is anything intermittent in the master contactor system, any inductive load will kick into whatever electronics are down-circuit from the intermittent. A classic example of this is if a master contactor should stutter (like from a weak battery) when the starter is engaged, the collapsing field from the starter could generate enormous voltage spikes which could wipe out any electronics that are also turned on. This is probably why we are told to turn off radios etc before starting. |
#20
|
|||
|
|||
Noise Problem. Both Comms Breaking Squelch
Peter wrote:
MikeMl wrote If it detects what it thinks is an "overvoltage" condition, then it responds by firing its "protection crowbar", which instantly overloads the aircraft's Field Breaker, causing it to overheat and trip, which removes power from the LR3C, and therefore removes excitation from the Alternator's field circuit, thereby taking the entire charging system offline until the Field Breaker is reset. I personally think this is a DUMB design that causes many more problems than it prevents. I am an electronics engineer (35 years' design experience) and can't believe anybody would do something so stupid in an aeroplane. Overvoltage crowbars are used on switching power supplies which have instant acting short circuit protection features and whose output power is limited by the magnetic components anyway. But on an aeroplane you have a very powerful alternator and more to the point you have thermal circuit breakers which take a while to trip. They are not like the magnetic ones in one's house which trip really fast. The thermal ones have to heat up first. If one was going to do an overvoltage protector for an aeroplane, the way to do it is to put something in series with the alternator field winding (i.e. in series with the existing voltage regulator) which goes open circuit when the bus voltage reaches say 32V. That will kill the alternator output very fast. I think the problem with that approach is that the huge inductive surge of the field collapsing would add to the already present overvoltage. Crowbars are used in military and spacecraft design where a positive zero-volts shutdown is needed. |
Thread Tools | |
Display Modes | |
|
|
Similar Threads | ||||
Thread | Thread Starter | Forum | Replies | Last Post |
Tough noise problem in Arrow radios | Ray Andraka | Owning | 7 | February 18th 08 08:44 PM |
FAA paper Noise Attenuation Properties of Noise-Canceling Headsets | Jim Macklin | Piloting | 26 | January 13th 07 12:06 AM |
FAA paper Noise Attenuation Properties of Noise-Canceling Headsets | Jim Macklin | Instrument Flight Rules | 15 | January 13th 07 12:06 AM |
Static/Squelch Noise in Radio | Kensandyeggo | Home Built | 2 | April 13th 06 09:00 PM |
Radio "Squelch-type" Noise | Kensandyeggo | Owning | 7 | April 12th 06 07:20 PM |