alternators and the battery
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I just checked the DA-40 POH (for the 1.7 Thielert-engined version). Indeed it's only got one (90A) alternator, but it's got no less than three batteries in the VFR edition:
- Main battery 12V, 35h Ah
- Alternator excitation battery 12V, 1.3 Ah
- ECU backup battery 12V, 12 Ah (powers ECU-B only)
The ECU backup battery should supply power to the ECU, if all else fails, for at least 30 minutes.
The IFR models also have a non-rechargeable dry battery to power the cabin floodlight and (electric) AH for 1 hour, completely independent of the electric system.
Now I'm not an electrical engineer but, although complex for such a small aircraft, the electric system of the DA40 does seem to be well thought out. My main worry would be if a failure elsewhere plus a failure of a critical relay to open/close a circuit somewhere would occur. For instance a short in the main bus, and a failure for the relays to isolate the ECU bus. Or something along those lines.
- Main battery 12V, 35h Ah
- Alternator excitation battery 12V, 1.3 Ah
- ECU backup battery 12V, 12 Ah (powers ECU-B only)
The ECU backup battery should supply power to the ECU, if all else fails, for at least 30 minutes.
The IFR models also have a non-rechargeable dry battery to power the cabin floodlight and (electric) AH for 1 hour, completely independent of the electric system.
Now I'm not an electrical engineer but, although complex for such a small aircraft, the electric system of the DA40 does seem to be well thought out. My main worry would be if a failure elsewhere plus a failure of a critical relay to open/close a circuit somewhere would occur. For instance a short in the main bus, and a failure for the relays to isolate the ECU bus. Or something along those lines.
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These look interesting.
In fact, they look awfully similar to the GAMI Supplenator which is not yet certified for anything.
I reckon GAMI are using the same generator assembly as B&C. They claim theirs works without a battery, and while GAMI did not answer a direct question on this in one forum a while ago, it seems obvious that they are modulating the field current sufficiently to generate a smooth output voltage without a battery. With a 3 phase AC generator and full wave rectification, and a decent closed loop regulator, this is completely possible.
Certifying this type of product cannot be hard, so IMHO GAMI are obviously holding back till the market is ready for their FADEC system.
What I don't understand is why aircraft makers don't simply fit 2 alternators, 2 batteries, two buses, and then have diode-fed batches of avionics. That's what the Cessna 400 does, IIRC, and it is very simple. Chance of a total electrical failure is practically zero. The extra weight is maybe 20kg.
In fact, they look awfully similar to the GAMI Supplenator which is not yet certified for anything.
I reckon GAMI are using the same generator assembly as B&C. They claim theirs works without a battery, and while GAMI did not answer a direct question on this in one forum a while ago, it seems obvious that they are modulating the field current sufficiently to generate a smooth output voltage without a battery. With a 3 phase AC generator and full wave rectification, and a decent closed loop regulator, this is completely possible.
Certifying this type of product cannot be hard, so IMHO GAMI are obviously holding back till the market is ready for their FADEC system.
What I don't understand is why aircraft makers don't simply fit 2 alternators, 2 batteries, two buses, and then have diode-fed batches of avionics. That's what the Cessna 400 does, IIRC, and it is very simple. Chance of a total electrical failure is practically zero. The extra weight is maybe 20kg.
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Put it another way, the alternator is the main source of power to the aircraft systems after engine start, not the battery.
Just to re-cap!
Put it another way, the alternator is the main source of power to the aircraft systems after engine start, not the battery.
Just to re-cap!
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There are a multitude of references to this on the net.
It seems Mr Cessna doesnt know as much about alternators as Mr BMW, but there again Cessna dont build engines!
BMW Alternator
Your BMW needs a source of energy for it to run and that energy is being supplied by the fuel stored on the vehicle’s fuel tank. But aside from fuel, the vehicle also needs an electrical energy source for the essential electric components like the distributor, electronic fuel injector and the engine management computer to work. The battery is often the primary source of this energy. The battery, however, would only function well if it is properly charged. To keep the battery charged, your BMW is equipped with another electrical energy source—the BMW alternator.
An alternator is an electrical machine or generator that converts mechanical energy to electrical energy in the form of alternating current. Alternators have a wide area of applications, although they are more popularly used for vehicle electrical systems. The main function of the alternator in a vehicle is to charge the battery while the engine is running. As a secondary function, the alternator also serves as the primary power source for the various electrical and electronic components installed in the vehicle while it is running.
It seems Mr Cessna doesnt know as much about alternators as Mr BMW, but there again Cessna dont build engines!
BMW Alternator
Your BMW needs a source of energy for it to run and that energy is being supplied by the fuel stored on the vehicle’s fuel tank. But aside from fuel, the vehicle also needs an electrical energy source for the essential electric components like the distributor, electronic fuel injector and the engine management computer to work. The battery is often the primary source of this energy. The battery, however, would only function well if it is properly charged. To keep the battery charged, your BMW is equipped with another electrical energy source—the BMW alternator.
An alternator is an electrical machine or generator that converts mechanical energy to electrical energy in the form of alternating current. Alternators have a wide area of applications, although they are more popularly used for vehicle electrical systems. The main function of the alternator in a vehicle is to charge the battery while the engine is running. As a secondary function, the alternator also serves as the primary power source for the various electrical and electronic components installed in the vehicle while it is running.
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What I don't understand is why aircraft makers don't simply fit 2 alternators, 2 batteries, two buses, and then have diode-fed batches of avionics. That's what the Cessna 400 does, IIRC, and it is very simple. Chance of a total electrical failure is practically zero. The extra weight is maybe 20kg.
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Sure, but carrying the extra engine works out awfully expensive, which is why almost nobody has been buying piston twins in the last couple of decades...
The DA42TDI came with a chance to change this, by having a reasonable fuel flow rate...
The DA42TDI came with a chance to change this, by having a reasonable fuel flow rate...
Last edited by IO540; 12th Mar 2010 at 11:13.
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I just checked the DA-40 POH (for the 1.7 Thielert-engined version). Indeed it's only got one (90A) alternator, but it's got no less than three batteries in the VFR edition:
- Main battery 12V, 35h Ah
- Alternator excitation battery 12V, 1.3 Ah
- ECU backup battery 12V, 12 Ah (powers ECU-B only)
The ECU backup battery should supply power to the ECU, if all else fails, for at least 30 minutes.
- Main battery 12V, 35h Ah
- Alternator excitation battery 12V, 1.3 Ah
- ECU backup battery 12V, 12 Ah (powers ECU-B only)
The ECU backup battery should supply power to the ECU, if all else fails, for at least 30 minutes.
However it is well worth having the backup battery under the back seat checked / replaced regularly. It should power the ECU for 30 minutes but, perhaps surprisingly, this assumes the battery is operating close to its maximium performance. In reality the batteries are often found to be way below their rated performance and 30 min is optomistic.
The sytem is OK but needs to be understood. I am not entirely sure you ever want to be worrying about the engine stopping on you while in IMC on partial panel and worrying whether you will get to your diversion quickly enough. Its fine for pottering around the local cabbage patch, maybe not really up to serious IFR touring.
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An alternator, with its voltage regulator, is a constant voltage power supply.
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The alternator with its voltage regulator forms a constant voltage power supply.
The alternator itself, with a constant field current, and a constant resistance load, will generate a voltage roughly proportional to RPM. This is no good, which is why the voltage regulator is added, and this varies the field current so as to keep the output voltage relatively constant.
The regulator is usually rather primitive which is why the resulting voltage is not perfectly constant. However, in my TB20 I am getting 28.something and it varies over a fraction of a volt only.
The regulator doesn't drop the alternator output voltage. The output voltage still comes directly from the alternator output. The regulator controls that voltage by varying the field current, which is much lower than the normal load current.
The alternator itself, with a constant field current, and a constant resistance load, will generate a voltage roughly proportional to RPM. This is no good, which is why the voltage regulator is added, and this varies the field current so as to keep the output voltage relatively constant.
The regulator is usually rather primitive which is why the resulting voltage is not perfectly constant. However, in my TB20 I am getting 28.something and it varies over a fraction of a volt only.
The regulator doesn't drop the alternator output voltage. The output voltage still comes directly from the alternator output. The regulator controls that voltage by varying the field current, which is much lower than the normal load current.
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REGULATOR
The regulator has two inputs and one output. The inputs are the field current supply and the control voltage input, and the output is the field current to the rotor. The regulator uses the control voltage input to control the amount of field current input that is allow to pass through to the rotor winding. If the battery voltage drops, the regulator senses this, by means of the connection to the battery, and allows more of the field current input to reach the rotor, which increases the magnetic field strength, which ultimately increases the voltage output of the alternator. Conversely, if the battery voltage goes up, less field current goes through the rotor windings, and the output voltage is reduced.
The regulator has two inputs and one output. The inputs are the field current supply and the control voltage input, and the output is the field current to the rotor. The regulator uses the control voltage input to control the amount of field current input that is allow to pass through to the rotor winding. If the battery voltage drops, the regulator senses this, by means of the connection to the battery, and allows more of the field current input to reach the rotor, which increases the magnetic field strength, which ultimately increases the voltage output of the alternator. Conversely, if the battery voltage goes up, less field current goes through the rotor windings, and the output voltage is reduced.
Though an observed meter may show a 'steady 28 volts from your alternator/regulator, an oscilloscope may well show many high transient spikes.
It's these the battery [and sometimes a capacitor] flatten.
Without them in the circuit the electronics may suffer.
It's these the battery [and sometimes a capacitor] flatten.
Without them in the circuit the electronics may suffer.
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the alternator output voltage isnt always constant.
The battery then takes care of its own charging, by drawing a current which is according to the difference between the charging voltage (14 or 28V) and its internal cell voltage (typically 12/24V).
I am an electronics hardware/software designer and used to design solid state regulators for motorbikes in the 1970s, back in the days when the regulator was essentially a relay with a vibrating contact and thus lasted somewhere south of no time at all.
It's a very simple system.
Doing it so that the system runs without a battery present (i.e. the alternator delivers a relatively smooth 14/28V) is more of a challenge and I don't think anybody bothers. It can be done by controlling the field current with a fast feedback loop, but may possibly require a customised alternator with a low inductance field winding. This is what GAMI claim to have done. It would be a neat thing to have because it would not matter if the battery became disconnected in flight, etc.
Though an observed meter may show a 'steady 28 volts from your alternator/regulator, an oscilloscope may well show many high transient spikes.
It's these the battery [and sometimes a capacitor] flatten.
Without them in the circuit the electronics may suffer.
It's these the battery [and sometimes a capacitor] flatten.
Without them in the circuit the electronics may suffer.
Moderator
I was thinking just the same thing!
I was wondering what would have become of this thread, if the question had been "How does the alternator work, while I momentarily push the aircraft over into near zero G, while electricity is being used?" Or, "What would happen if as a pilot, I used the battery to start the aircraft at LFAT?". Heaven help us, we'd be into our 6 th page, with some posts having been deleted by a vigilent moderator!
But, Aslan cunningly avoided using the words "cheap" or "spam can Cessna 150" in their post, and aside from capitalizaton errors, got the words spelled right as far as I noticed - so my duty is complete!
And, thanks everyone, I did learn a few things worth knowing - so there you go! I'm off now to lurk for the next transgression, then pounce, with editorial errors in my own critical rant about language perfection!
Pilot DAR
I was wondering what would have become of this thread, if the question had been "How does the alternator work, while I momentarily push the aircraft over into near zero G, while electricity is being used?" Or, "What would happen if as a pilot, I used the battery to start the aircraft at LFAT?". Heaven help us, we'd be into our 6 th page, with some posts having been deleted by a vigilent moderator!
But, Aslan cunningly avoided using the words "cheap" or "spam can Cessna 150" in their post, and aside from capitalizaton errors, got the words spelled right as far as I noticed - so my duty is complete!
And, thanks everyone, I did learn a few things worth knowing - so there you go! I'm off now to lurk for the next transgression, then pounce, with editorial errors in my own critical rant about language perfection!
Pilot DAR
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I think the famous "grocer's apostrophe" is the worst by far.
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Just to be pedantic. :- In automotive applications, Alternators come in "battery-sensed" and "machine-sensed" versions.....the net result is the same, the BATTERY should be maintained at a nominal 14.2 volts,for a "12V" lead-acid battery.
The posher alty's also have temperature-compensation built in,
The machine is set to the nominal voltage as a compromise,- to maintain a charge and replace a "surface-charge" (discharged by starting), yet not to overcharge, which would "boil" off the distilled-water part of the sulphuric-acid solution which forms the L/A battery's electrolyte.
Often, people are baffled,when , after starting with a flat /depleted battery, a short run restores it to full vigour,However, after a few hours' non-use, the surface-charge has dissipated and the battery is again depleted.
That is why an external charger is advised ,to deep-charge a battery and maintain it with a "float-charge"....cheap chargers will poke in a full charge,their output being reduced by the battery's rising voltage opposing the charger,-but an unregulated charger will gaily carry on "stoking" a battery and electrolyse the water (hence the warnings about not smoking and switching-off before disconnecting ) the liberated hydrogen and oxygen can give a hell of a bang and casing and acid get everywhere
Work it out, folks....a 36 AH battery will, at peak,deliver that amount...BUT USUALLY AT THE 12-HOUR RATE.....that means that THEORETICALLY a mint system would give a continuous 3 amps for 12 hours....we already established the load is typically~40 amps....so you'd be very lucky to get 30 minutes from alty -failure to loss of power due to voltage-drop.
As others stated, the battery acts as a small "buffer-tank" to provide starting energy and even-out the load on the alty.
Petrol V diesel !....the magneto is completely self-contained. engine will run without any external power.
Diesels commonly use a fuel-shutoff solenoid to switch off.....when the volts are inadequate,the valve closes and the donk starves. Modern Diesels also use electronics for injection, as opposed to the old mechanical injector-pumps with the pull-wire to stop.---same problems with lack of volts....the brain dies and kills the donk.
The posher alty's also have temperature-compensation built in,
The machine is set to the nominal voltage as a compromise,- to maintain a charge and replace a "surface-charge" (discharged by starting), yet not to overcharge, which would "boil" off the distilled-water part of the sulphuric-acid solution which forms the L/A battery's electrolyte.
Often, people are baffled,when , after starting with a flat /depleted battery, a short run restores it to full vigour,However, after a few hours' non-use, the surface-charge has dissipated and the battery is again depleted.
That is why an external charger is advised ,to deep-charge a battery and maintain it with a "float-charge"....cheap chargers will poke in a full charge,their output being reduced by the battery's rising voltage opposing the charger,-but an unregulated charger will gaily carry on "stoking" a battery and electrolyse the water (hence the warnings about not smoking and switching-off before disconnecting ) the liberated hydrogen and oxygen can give a hell of a bang and casing and acid get everywhere
Work it out, folks....a 36 AH battery will, at peak,deliver that amount...BUT USUALLY AT THE 12-HOUR RATE.....that means that THEORETICALLY a mint system would give a continuous 3 amps for 12 hours....we already established the load is typically~40 amps....so you'd be very lucky to get 30 minutes from alty -failure to loss of power due to voltage-drop.
As others stated, the battery acts as a small "buffer-tank" to provide starting energy and even-out the load on the alty.
Petrol V diesel !....the magneto is completely self-contained. engine will run without any external power.
Diesels commonly use a fuel-shutoff solenoid to switch off.....when the volts are inadequate,the valve closes and the donk starves. Modern Diesels also use electronics for injection, as opposed to the old mechanical injector-pumps with the pull-wire to stop.---same problems with lack of volts....the brain dies and kills the donk.
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All of you guys are technically correct here so let's not get into a confusing tizzy about details...
The battery is used primarily to start the engine and also in most cases to provide field excitation for the alternator / (generator if it's older). Some voltage regs are that good (prestlite etc) that the output is good enough alone without the battery in circuit - provided the system will let you do it!!
The alternator in the a/c is really no different from that at Sizewell! It doesn't need a battery!!
The battery is used primarily to start the engine and also in most cases to provide field excitation for the alternator / (generator if it's older). Some voltage regs are that good (prestlite etc) that the output is good enough alone without the battery in circuit - provided the system will let you do it!!
The alternator in the a/c is really no different from that at Sizewell! It doesn't need a battery!!