Considerations for Alternator Upgrades: Running on Plane-Power

Written by Steve Ells
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June 2014-

     Replacing the original equipment DOFF-type alternator with a modern, aircraft-specific Plane-Power alternator is a good move. It's a very good move if you're still depending on a generator for electrical power.
I recently stepped in to the 21st century by replacing the 50 amp generator/regulator system in my other-than-Cessna airplane with a Plane-Power alternator/voltage regulator and I'm glad I did.

     Now I have a new, well-built power source that provides ample (and electrically quieter) power at idle engine speeds—something the generator couldn't do.
"But I got by with a generator," aided by rationalizations.

     I've always liked generators. Unlike alternators, they will provide electrical power even if the battery is dead since generators depend on residual magnetism to provide the lines of magnetic flux needed to produce output.
Generators are tough, too: unlike alternators, they don't need a rectifier bridge to convert alternating current (AC) output to useable direct current (DC) for our airplanes' electrical systems.

     And, they're cheap. I can buy a belt-driven replacement Delco generator (for almost any airplane that uses a belt-driven generator) by shopping the internet. The last one I bought—a 50 amp Delco 1101915—cost less than $50.
That unit wasn't ready for installation, though. (An identical but freshly overhauled generator would have set me back around $250.) I paid $50 because my great buy needed a little maintenance. I tuned it up by smoothing out the commutator, undercutting the segments, testing the armature on a growler and installing a set of brushes.

     I had already upgraded to new generator charging system technology by replacing the original vibrating point regulator/reverse current relay with a solid-state generator voltage regulator from Zeftronics. I just wasn't quite ready for an alternator.
What did this "good deal" get me? A weighty, current-limited DC generator that made a lot of electrical noise and didn't come online until around 1,400 engine rpm. (This magic number varies, but is always above 1,200 rpm.) That's okay for the day VFR flying I favor, but every pilot flying a generator-powered airplane knows these old-technology units have one or two drawbacks.

     Any time the engine rpm drops below the magic rpm number, the battery alone takes over the task of powering the avionics, the pitot tube heater, the landing and taxi lights, and the landing gear motor (if equipped). One wise soul once described a typical light aircraft battery as little more than a wide spot in the wires; for every minute the battery is carrying the full aircraft electrical load, the reservoir of "juice" in the battery bank is shrinking.
Let's say you're flying a GPS approach into the home 'drome on a dark and gusty night at the end of a long day and you realize you're high. Whaddya do? If the power reduction needed is slight, you're still good; but if you have to reduce power below your airplane's magic rpm threshold, you're on battery power. You'll know it because the landing lights will dim.

     Or, imagine this: you're in a departure hold waiting to launch into the clag on an IFR flight plan. Do you ride the brakes because you have to keep the rpm up to keep the generator online, or pull the power back, reduce the electrical load and taxi slowly (and dimly) until it's your turn to take off? (Don't forget to turn everything back on before launching!)

     And then there's the task of polarizing. After extended non-flying periods it seemed like my generator would never come online as I taxied out to launch. So I would have to I shut 'er down, get out, rummage in my toolbox for my jumper wire and do the polarization dance at the regulator. The nontechnical term for this is "flashing the field."

     It takes a jumper wire and less than 15 seconds to again set things right, and the process is relatively easy on my other-than-Cessna because the cowling opens quickly. But on fully-cowled airplanes, it's a real pain.
Recently my $50 generator started acting up, so I started shopping around for an alternator conversion kit. It had to be FAA approved for installation on my airplane, and it had to be made by a company that's been around and provides good customer service.
I found a belt-driven FAA approved generator-to-alternator conversion kit by Plane-Power out of Granbury, Tex. My research showed that the STC for this kit was issued in 2007, and the company website indicated that over 10,000 alternator kits had been shipped.
With so many kits out there, the industry had voted its thumbs up... so I bought a kit and took off the old generator in my airplane, too. It was way past time to go modern. (The author, a longtime A&P, is able to undertake a project like this alone. Aircraft owners who aren't A&Ps should check with their mechanic before purchasing a conversion kit. —Ed.)

Save weight?
     One of the advantages touted by the alternator replacement vendors is a marked weight savings when old-style components such as generators and Bendix-engaged starters are replaced with modern units.
I weighed everything I took off (mounting, tensioning arm, regulator, generator) and compared them to the new Plane-Power replacement parts. The weight savings must be on the starter conversions, because installation of the alternator only saved 4.4 pounds (17.4 pounds versus 13 pounds). Still, that's part of a gallon of gas.

Quality components
     I started my research on the Plane-Power website and, as a techie guy, I was impressed with the breadth of information available online. When my order arrived, I could see that every component included in the Plane-Power kit was well made and well finished.
The kit I purchased included a new alternator (12 VDC and 70 amp), two mounting brackets—one for each type of Lycoming engine mounting—a tensioning arm and bolt, an alternator controller (voltage regulator), two placards ("ALT FIELD" and "ALT INOP"), an alternator inoperative light and installation instructions. The steel mounting bracket and tensioning arm are tough and fit perfectly.
As I compared the old black generator to the shiny new alternator, I knew I'd done the right thing. After all, I'd spent the last five years slowly upgrading avionics, reconfiguring the instrument panel, overhauling landing gear components, overhauling the propeller and the engine—and an alternator was almost the final step in my goal of achieving low-maintenance airworthiness.
Plane-Power versus OEM alternators

     In the past, I've spent quite a bit of time helping association members troubleshoot single engine Cessna alternator problems. The original equipment alternators on single engine Cessnas were derivations of the Ford automotive alternators, and the biggest problem revolves around the rectifier bridge; specifically, one or more failed diodes in the rectifier bridge. When a single diode fails, two things happen: the voltage output of the alternator drops, and increased noise is generated.
When a diode in these older-technology alternators had failed, the battery would never fully charge and the pilot would report a whine in the headset. This is easy to troubleshoot. With the engine running and a comm radio on, turn off the alternator. If the whine goes away, you've found the problem.

     According to Jason Hutchison, general manager of Plane-Power, the two enemies of alternators are vibration and heat. Plane-Power takes care of the vibration by fine-tuning the balance of the rotating components. The buildup of heat is well controlled by the open configuration of the alternator body and by dual fans—one installed on the front of the rotor to cool the rotor, and one installed on the aft end of the rotor to cool the rectifier bridge.
These design features permit so much free airflow that no additional cooling is required. Hutchison said that there had only been two rectifier failures in the three years he's been with Plane-Power.
My Delco generator required a blast tube to direct a one-inch-thick column of ram air to the commutator/brush end to keep it cool but I blocked off the blast tube opening in the cowling when I removed the generator. According to Hutchison, some Plane-Power alternator installations do utilize blast tubes simply because they are approved for replacement of existing OEM units under PMA and as such, mustn't vary much from the original installation.

My installation
     Since the Plane-Power alternator is rated to produce 20 more amps (70 amps of output, versus 50 amps), I needed to do some calculations to determine if the existing wiring in my aircraft was big enough. A load analysis revealed a worst-case maximum electrical load—all lights on; pitot heat on; all avionics on; landing gear motor in action; and the assorted smaller loads, such as the LG solenoid—of just over 40 amps.
Then I measured the diameter of the existing generator output wiring and found it to be #4 AWG wiring that can handle a maximum continuous load of 60 amps. No wiring changes were needed. When I discussed this with Hutchison, he told me that there's almost never a need to change existing wiring unless additional electrical loads are installed or anticipated in the future.

     In addition to using the existing output wiring, I also used other existing wiring from the generator system to complete the installation. In fact, I only had to fabricate three short 18 gauge wires; a short jumper from the "enable" to the "aux" terminals on the regulator; and wires from the alternator inop lamp to an existing circuit breaker and to an existing wire that I connected to the lamp terminal of the regulator.
The regulator is set to vary the amperage to the alternator rotor slip ring as necessary to maintain 14.0 volts out. It senses the main electrical bus voltage through a field circuit breaker/switch (a five-amp circuit breaker needs to be installed) to the "enable" connection that turns the alternator on and off. A jumper from the "enable" connection to the "sense" connection completes the bus voltage detection wiring.

     I installed a Tyco W31 switch/circuit breaker in the existing row of electrical switches in my instrument panel and installed the ALT FIELD placard above the switch.
The installation instructions are very complete. Step four of Part 1 of the Installation Instructions reads: "Ensure that internal jumper #1 and internal jumper #2 of the R1224 regulator are set for 12V operation." This required me to remove the cover of the alternator to visually check the jumpers. Both were correct.

     The instructions also showed the location of the rheostat that controlled the regulator voltage set point (it is adjustable in the field). Plane-Power recommends upping the bus voltage slightly (14.2 VDC) if the majority of trips are short; 14.0 is the correct setting for longer flights.

     I asked Hutchison for Plane-Power's advice on operations. He replied that he personally starts his airplane with the alternator circuit breaker/switch in the off position for a couple of reasons.
This procedure permits a pre-start check of the ALT INOP light operation (it should be lit with the alternator switch off) and because there can be as much as three amps delivered to the field of the alternator if the switch is on during the start. Those three amps might make a difference in starting if the battery is weak.

Other products
     In addition to belt driven alternators, Plane-Power also produces STC approved gear driven alternator/regulator kits for Cessnas with small-block Continental engines (such as the O-200 in the 150), and STC approved upgrade/replacement kits for later-model Lycoming powered 28 volt singles.
Plane-Power hasn't forgotten Cessna twin owners, either. Gear-driven alternators for TCM 520- and 550-powered airplanes are also available in 14 and 28 volts with 100 and 150 amp output options.
The latest Plane-Power offering is the FAA approved ALT FLX. It's a belt-driven alternator that's a direct replacement for the original DOFF Ford-derived original equipment alternator—the difference is the ALT-FLX provides a whopping 100 amps of available power in both 14 and 28 volt versions. It is a solid option for Cessna flyers that need more power.

     In addition, Plane-Power also has a file of field approval paperwork detailing the installation of its systems on Cessna 120, 170 and 210B models.
2,000 Hours!

     Hutchison pointed out other advantages besides the increased cooling and precision rotor balancing when he talked about the slip rings. "We certify our alternators to 35,000 feet and during testing we found that the life span of the slip ring brush material we were using could be measured in minutes, so we made changes to the material of the brushes," he said.
He also pointed out another feature incorporated to slow brush wear. "The slip rings in other alternators are three-quarters to one inch in diameter; ours is half an inch. That may not sound like much, but it drastically reduces something called the foot travel per revolution," he said.

     According to Hutchison, the combination of the new brush material and the smaller slip ring has extended brush life so much that Hutchison tells buyers to install a Plane-Power alternator with a new engine installation. When it's time to send the engine in for overhaul at 2,000 hours, send the alternator back to Plane-Power for an overhaul. He's had one user who has reported no alternator problems after 3,000 hours of use.
Plane-Power has incorporated its brand of new technology in a wide range of retrofit and upgrade alternator installations for both direct drive and belt driven applications. Plane-Power alternators and regulators all come with a two-year warranty.
Overhaul and customer service.

     Plane-Power overhauls its alternators for a flat fee of $295 and in almost all cases can provide same-day turnaround. According to Hutchison, overhauled Plane-Power alternators are the same as new since many parts are not reused.
There are seven employees at Plane-Power. Hutchison addressed customer service by saying, "Everyone here has a green light to do whatever it takes for our customers. It's not unusual for one of us to swing by UPS after work to drop off a unit for next-day delivery."
Part of my initial reluctance to install an alternator related to the problems I'd seen when troubleshooting older Cessna alternator systems as a tech rep. From what I've learned about Plane-Power alternator systems, those concerns have been addressed by the newer technology in Plane-Power's alternator systems.

I'm glad I finally made the switch; I suspect you will be, too.

Steve Ells has been an A&P/IA for 39 years and is a commercial pilot with instrument and multi-engine ratings. Ells also loves utility and bush-style airplanes and operations. He's a former tech rep and editor for Cessna Pilots Association and served as associate editor for AOPA Pilot until 2008. Ells is the owner of Ells Aviation ( and lives in Paso Robles, Calif. with his wife Audrey. Send questions and comments to .


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