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Thread: PT6 Charging Issues

  1. #1
    CenterHillAg's Avatar
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    PT6 Charging Issues

    Curious if anyone is familiar with the older charging systems on the PT6. I have a turbine Ag Cat with the Mills PT6 conversion, which is essentially the firewall forward off an early King Air. The charging system consists of the starter generator, and the ancient reverse current relay and voltage regulator that have been around since WWII.

    The plane was completely rewired in 2014 and everything is very well done and good grounds. I put a new engine on it this year and put a overhauled starter generator, new battery, plus an EI voltmeter/ammeter. The voltage would either show 24 volts and discharging, or turn the voltage regulator one click and it would go to 31 volts and overvolt. Changed the voltage regulator and it seemed to settle in at 27-28 volts on the test flight, but once I started working the volts would drop to 24.5 and the discharge light on the EI gauge would come on, but the generator warning light on the dash was still off. You could cycle the generator switch and it would start charging again, but after a few minutes it would start discharging. Hereís the kicker, once youíve been flying a couple hours and everything is warm and the air conditioner is running, itíll charge at 27.8 all day with no issues.

    My mechanic thinks the EI gauge is just too sensitive and as long as the voltage is showing over 24 itís ok, but admittedly he isnít real familiar with this style of charging system, and I donít like that answer. The voltage at startup the next day is 24.8 when the system is giving me fits, compared to 25.3 if the system has been charging normally. Thatís a difference of 30 degrees on the ITT at start, still well below redline but the cooler the better. Is the solution just to run with all the lights, gps, ac, etc. on to keep a load on the charging system at all times and cycle the generator switch as needed to keep it charging, or is there something else Iím missing? I have a reverse current relay on the shelf I can install, I just donít know if it could be the source of the intermittent issues.

    Thanks for any help you might have.
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    wireweinie's Avatar
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    First off, go back and re check connections at the starter/gen and the regulator. VERY easy to flip a couple of them. Then check condition of the brushes. If all checks correct swap out a regulator for a flight or two. Better if you can get ahold of a newer style replacement reg.

    What you're describing sounds like the reg is not sensing the voltage correctly. Which makes me think bad connection or bad reg.

    Keep that EI volt/amp meter. As long as it was installed correctly it will last as long as the airplane.

    Let us know what you find.

    Web
    Life's tough . . . wear a cup.
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    CenterHillAg's Avatar
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    Thanks, I’m gonna start looking through it all again tomorrow morning. A buddy as 3 more regulators on the shelf so I’ll borrow one to try if it’s still giving me problems.

    We were talking about trying the Jasco voltage regulator, I have one that was used with a 70 amp alternator on a 985. Neither of us is brave enough to try it on our planes, and there’s the whole legality issue. Seems like it would work better than these ancient carbon pile regulators though.

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    The adjustment rheostats on the regulators get dirty/worn in the sweet spot. Your best bet would be to find an overhauled regulator. When setting the voltage you need to let them get up to operating temperature first.
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    And please don't try the Jasco reg, you'll let the smoke out of everything.
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    PT6 Charging Issues

    If your voltage regulator is a carbon pile regulator, you need to wire the generator so it can use it. A style or B style, forget which it is, but with a carbon pile regulator, the regulator is a resistor on the field ground lead. With a vibrator or electronic regulator, the field goes direct to ground, and the voltage going into the field is adjusted with the regulator.


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    wireweinie's Avatar
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    All regs are 'A' or 'B' style. "A' style has one brush connected to power and the other to the reg, which controls current flow to ground. 'B' style has one brush tied directly to ground and the other connected to the reg, which controls current flow from the positive source. Doesn't matter whether they are old school or solid state, output of the alternator or generator is a function of the amount of current flowing in the field coils. Low amps through the field = low voltage. Higher amps through the coils = higher voltage. A good example of this is when you bypass an external regulator, with a jumper lead, from the battery. This puts a full 12 volts (momentarily!) to the field, which can result in about 30 volts if the alternator is not inop.

    Web
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    Quote Originally Posted by wireweinie View Post
    All regs are 'A' or 'B' style. "A' style has one brush connected to power and the other to the reg, which controls current flow to ground. 'B' style has one brush tied directly to ground and the other connected to the reg, which controls current flow from the positive source. Doesn't matter whether they are old school or solid state, output of the alternator or generator is a function of the amount of current flowing in the field coils. Low amps through the field = low voltage. Higher amps through the coils = higher voltage. A good example of this is when you bypass an external regulator, with a jumper lead, from the battery. This puts a full 12 volts (momentarily!) to the field, which can result in about 30 volts if the alternator is not inop.

    Web
    Since the OP is talking PT-6, I pretty sure he has a 28 volt system.

    My point was if his generator is a B style connected to a carbon pile A style regulator, it isnít going to work. He needs to verify what system he has and be sure the generator is wired correctly for the system.


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    CenterHillAg's Avatar
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    Web set me straight over PM’s on the Jasco regulator, I won’t be getting anywhere near my plane with it. I have a B style system, I was able to trace the cable from the generator to the ground. Looking over it all today, everything appeared to be wired correctly, 2 of the nuts holding the cables on the back of the generator took a quarter turn to feel as snug as the others. The regulator is a Bemdix 1589-1D and has been on the plane 2 weeks, it’s running good enough once warmed up that I don’t think I need to swap it out right now. Everything else appeared to be right. I’ll be flying again in a couple days so I’ll report back if there’s any issues.

    Thanks for all the help.

  10. #10
    SuperCub MD's Avatar
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    I'm guessing the jasco would smoke something, not worth trying.. I have had the start/gen connections mixed up, that usually is a complete failure. Because it works better when warm I would clean all the connections firewall forward, make sure everything is tight and correct. Put in a fast start if you have multiple battery's, batterys are a lot cheaper than hot sections.

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    CenterHillAg's Avatar
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    Don’t think there’s space for a fast start on a Mills, it’s pretty tight under the cowling. A buddy has it on a Frakes Cat and it’s a nice setup. I’m buying a 36 volt golf cart to use, make the aux cable pulling from 5 batteries and it works real well.

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    wireweinie's Avatar
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    Quote Originally Posted by CenterHillAg View Post
    Don’t think there’s space for a fast start on a Mills, it’s pretty tight under the cowling. A buddy has it on a Frakes Cat and it’s a nice setup. I’m buying a 36 volt golf cart to use, make the aux cable pulling from 5 batteries and it works real well.
    That's an idea I've been trying to get the new turbine operators, up here, to use. Every start off a cart of cheap batteries is one less start off the ships batteries. Every battery made has a finite number of starts in it, so use the cart as much as possible.

    For one or two aircraft, get a small four wheel wagon that can be pulled around by one person. Fill it with diesel pickup batteries, strapped to get the voltage you want (usually 24 volt but can be higher if desired, just shut off any electronics). Then install a small charger directly on the batteries. Plug the charger into the wall until the cart is needed. Automotive batteries are cheap compared to aircraft batteries.

    Web
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    Quote Originally Posted by wireweinie View Post
    That's an idea I've been trying to get the new turbine operators, up here, to use. Every start off a cart of cheap batteries is one less start off the ships batteries. Every battery made has a finite number of starts in it, so use the cart as much as possible. For one or two aircraft, get a small four wheel wagon that can be pulled around by one person. Fill it with diesel pickup batteries, strapped to get the voltage you want (usually 24 volt but can be higher if desired, just shut off any electronics). Then install a small charger directly on the batteries. Plug the charger into the wall until the cart is needed. Automotive batteries are cheap compared to aircraft batteries. Web
    ......................... to add my .02 cent, it is very common to see home made "battery cart / wagon " to use on jump planes " ,"at many parachute airfields " ..... good day .../rick

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    A lot of guys will tap 30v off a golfcart, make sure and rotate the batterys on it regularly

  15. #15
    CenterHillAg's Avatar
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    Had a successful morning, flew for a couple hrs with no issues at all, charged without ever showing any discharge or having to cycle the generator. Got my golf cart set up at 30v for starting and it started the engine at 600 degrees. It had been starting at 660-690 on the planes system, very happy with that.
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