# Alternator simulator circuit

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#### MikeMl

##### Well-Known Member
I'm chasing alternator whine due to ground-loops in audio circuits in an aircraft. Since it is dangerous (and noisy) to work around a spinning prop, I'd like to be able to connect an "alternator simulator" in place of the real engine-driven alternator and have it produce about 14.2V D.C at about 30A (to run the normal aircraft loads), with an 1 to 2 kHz full-wave-rectified ripple component riding on the D.C., just like the real alternator would have.

An aircraft or automotive alternator is basically a three phase sinosoidal current source (high impedance output) feeding into a six diode full-wave rectifier. The rectification creates six overlapping current pulses, so the ripple is well defined and determined from the three phases.

After the battery is recharged, the average D.C. current from the alternator supplies the loads; but almost all the ripple current flows into and out of the battery. The minimum system bus voltage is determined solely by the battery.

Since the airframe is used as a ground return for everything in an airplane (or car), the ripple current can couple into any audio circuits that connect to the airframe (car body) in multiple places as a common-mode noise. The solution is to identify and break those ground loops. I would be using the alternator simulator box to create the ripple currents as would be flowing if the engine and alternator was turning.

I'm thinking I could do this with an electronically-regulated 30A D.C. power supply which starts about 1V higher than the battery voltage (nominally 14.25V with the engine running) in series with a giant NFET connected as a source follower, whose gate is driven with the synthetic ripple waveform.

The source follower can only source current (not sink it), but that is the way the real alternator works, i.e. if the instantaneous alternator voltage is less than the battery voltage, current flows out of the battery to the loads. During the part of the cycle when the alternator output is higher than the battery voltage, current flows from the alternator to the loads and to the battery to put back the charge that was "borrowed" during the previous low point. The battery acts as a giant filter capacitor, but has a finite series resitance.

The obvious problem would be the power dissipation in the NFET. It acts as a Class A amplifier, with an average current of 30A modulated by the ripple. The average drain to source voltage would be about 1V, so the dissipation would be about 30W. Should work as long as it is mounted on a large heatsink.

Can anybody suggest a better way to emulate what the alternator does?

##### Banned
Wouldn't it be a whole lot easier to just remove the prop? What's that take, a few tools in the hands of a mechanic that knows what they're doing? The worst you're going to get bumping into the cowl without a prop is a friction burn, unless you're wearing loose clothing =)

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#### MikeMl

##### Well-Known Member
Wouldn't it be a whole lot easier to just remove the prop?
No. The prop is variable pitch; fed with pressurized oil via a hollow crankshaft. Besides, the prop acts as the flywheel for an aircraft engine; it wouldn't even run without it.

#### Ubergeek63

##### Well-Known Member
hmm, i'll give it some thought ... last I knew aircraft ran on 28VDC, has something changed?

And what are you trying to get the audio out of? for relatively low power equipment it would be easier to add an isolation supply than try to track down something intrinsic to the airframe design... unless of course you the mfg trying to fix the design before it goes to production.

Dan

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#### crutschow

##### Well-Known Member
How about just driving an alternator/regulator with an electric motor. That way the simulation would be perfect.

#### MikeMl

##### Well-Known Member
hmm, i'll give it some thought ... last I knew aircraft ran on 28VDC, has something changed?

And what are you trying to get the audio out of? for relatively low power equipment it would be easier to add an isolation supply than try to track down something intrinsic to the airframe design... unless of course you the mfg trying to fix the design before it goes to production.

Dan
I'm retrofitting older aircraft with modern avionics, which usually includes stereo entertainment systems (IPOD, XMradio, CDs) piped through modern ANR (electronic noise reduction) stereo headsets. The ANR headsets reduce the acoustic noise to the point that alternator whine embedded in the entertainment audio becomes very noticeable. The alternator ripple currents circulating in the airframe gets into the headphone audio due to ground loops.

Fixing an improperly-wired aircraft requires hours of work rewiring the audio system harnesses, floating/isolating all mic and headphone jacks, isolating some audio sources, using audio coupling transformers to break ground loops, reconfiguring the audio system to a single-point ground, etc, etc. During this time, it would be really nice to be able to generate the circulating ripple currents on demand without starting the engine...

#### MikeMl

##### Well-Known Member
How about just driving an alternator/regulator with an electric motor. That way the simulation would be perfect.
I have thought about that, and this maybe the simplest way of doing it. To generate 30A at 14V (420W), I would have to start with a 3/4HP electric motor. Aircraft engines cruise at about 2500 rpm. The alternator is normally driven with a belt system that turns the alternator 2x or 3x the engine rpm, say 6000 rpm. Commonly available 2 pole 120V motors spin at about 3450 rpm, so the test alternator would have to be belt driven with a slight speed-up.

#### tcmtech

##### Banned
I used to do a lot of ground loop chasing on car audio systems years ago. One full car system work over could take all day and still could have internmitent alternator whine from time to to time.

I got lazy and just started putting a 10000uf to 40000 uf capacitor right behind the alternator then a high current toroid choke followed by another capacitor about 1/4 to 1/2 the size of the first one. 10 minute install, left the car sit to the next day charged the dummy my usual debug time fee plus parts. worked every time too!

Given the size of modern capacitors you could easily build a filter device like this that still wheighed under a pound. And could easily be built for $20 -$30. With a generous install fee of course!

#### Ubergeek63

##### Well-Known Member
I had not known. I designed aircraft instrumentation from '88 to '92. our 28 stuff was rated for 10-32V operation.
I'm retrofitting older aircraft with modern avionics, which usually includes stereo entertainment systems (IPOD, XMradio, CDs) piped through modern ANR (electronic noise reduction) stereo headsets. The ANR headsets reduce the acoustic noise to the point that alternator whine embedded in the entertainment audio becomes very noticeable. The alternator ripple currents circulating in the airframe gets into the headphone audio due to ground loops.

Fixing an improperly-wired aircraft requires hours of work rewiring the audio system harnesses, floating/isolating all mic and headphone jacks, isolating some audio sources, using audio coupling transformers to break ground loops, reconfiguring the audio system to a single-point ground, etc, etc. During this time, it would be really nice to be able to generate the circulating ripple currents on demand without starting the engine...
it would be much easier to break the ground loops with an isolated switcher.

Dan

#### crutschow

##### Well-Known Member
I used to do a lot of ground loop chasing on car audio systems years ago. One full car system work over could take all day and still could have internmitent alternator whine from time to to time.

I got lazy and just started putting a 10000uf to 40000 uf capacitor right behind the alternator then a high current toroid choke followed by another capacitor about 1/4 to 1/2 the size of the first one. 10 minute install, left the car sit to the next day charged the dummy my usual debug time fee plus parts. worked every time too!

Given the size of modern capacitors you could easily build a filter device like this that still wheighed under a pound. And could easily be built for $20 -$30. With a generous install fee of course!
So a customers that comes to you for help is a "dummy", eh. Remind me never to bring anything to you to fix...

Large electrolytic capacitors are not the most reliable. Don't know that I would want to install one on a aircraft alternator. If a cap shorted, it could have serious consequences.

#### Diver300

##### Well-Known Member
Large electrolytic capacitors are not the most reliable. Don't know that I would want to install one on a aircraft alternator. If a cap shorted, it could have serious consequences.
Fit a fuse in series with the capacitor. They might have them inside on large capacitors like that anyhow.

#### tcmtech

##### Banned
I should have pointed out those "dummys" I refer to were fellow high school kids that I had already told how to fix their problems. And gave them the places, cost of the parts and instructions how to install the filter system.

If you dont want to do it yourself or are just to lazy, I have very little sympathy.

Being knowlegable and creative makes me a bad service tech because I can profit from it? I dont want you as a customer either!

My service buisiness is geared towards specialty repair work. By knowing how to use creative or intelegent repair aproches I make $50 -$100 an hour on site.
And customers only care about if the problem was fixed or not. not how I fixed it!

#### fernando_g

##### New Member
I have thought about that, and this maybe the simplest way of doing it. To generate 30A at 14V (420W), I would have to start with a 3/4HP electric motor. Aircraft engines cruise at about 2500 rpm. The alternator is normally driven with a belt system that turns the alternator 2x or 3x the engine rpm, say 6000 rpm. Commonly available 2 pole 120V motors spin at about 3450 rpm, so the test alternator would have to be belt driven with a slight speed-up.
You know something............that is actually a pretty good idea.
Additionally, if you feed the electric motor from a variable frequency drive, you can also simulate the engine's varying RPMs....

##### Banned
I guess it would be down to which is harder. Removing the prop without disconnecting said pitch control hydraulics and loading the main output shaft to allow the engine to run, or disconnecting the drive belt to the alternator and putting in an electric motor to drive it. It's starting to sound like simply electrically disconnecting the alternator and putting in some kind of direct electric simulation would be smarter, if not easier. Honestly, I'd just come up with a practical way of working around the aircraft while running, even if you need a separate person for safety alone it sounds more practical than most methods so far, unless you plan on doing this repeatedly.

#### kchriste

##### New Member
Forum Supporter
If you had 400Hz 3Φ in your shop it would be easy.

#### tcmtech

##### Banned
I still say filter the alternator. Kill the noise at its source. why prune each leaf of a weed every time it grows a new one? Kill the root at its source. Still just a thought.

If the system is a 30 amp. the capacitor values could only need to be a few 1000 uf and possibly not need a filter choke.
Still just a thought.
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True lazyness is the real motivator to do it right the first time!
Doing it wrong, then doing it over is just misguided ambition!

#### MikeMl

##### Well-Known Member
I still say filter the alternator. Kill the noise at its source...

If the system is a 30 amp. the capacitor values could only need to be a few 1000 uf and possibly not need a filter choke.
Still just a thought.
I have tried "filtering" an alternator with just a large 47,000uF "computer grade" electrolytic capacitor, and it does NOT work. The effective series resistance of a large electrolytic capacitor is more than 10mOhms at 120Hz. I suspect that the ESR is much higher at kHz frequencies. The impedance being bypassed (30A loads, the battery) is much less than 10mOhms, so most of the ripple current still flows along the airframe to the loads and battery.

It would require putting a series choke of more than 100uH (that will not saturate at the max alternator current of 60A) between the alternator and the loads and battery.

Electrically that might help, but practically, it would never fly. It raises all kinds of FAA certification issues due to modifying an existing design, and introduces several failure modes. Those large electrolytics hate elevated temperatures (engine compartment is over 130degF), and they hate vibration. Fusing the path to the electrolytic wont work because the resistance of the fuse is a few hundred mOhms, which kills its effectiveness as a bypass for the ripple currents.

#### tcmtech

##### Banned
Ah. computer caps do have rather poor high frequency response.
Good point on the FAA regs though.
Chase the ground loops it is!
sure its just alternator? No feed back from the duel ignition circuits?