regulating generators

[Tim]

What's the reason that the voltage in generators are regulated to less than their output? I.e. the gens on a BE20 produce 30V but are regulated to 28.25V.

[CID]

Do you mean 30V as the generator's rating?

It's just a standard generator rating that encompasses the 28 VDC generator systems used in aircraft.

Generator output voltages are set according to the parameters of the generating system they are installed in. It's not like de-rating an engine. It's just setting one of the output parameters, in this case the output voltage.

[Tim]

So in other words, the gens are capable of a 30V output, but are just set to 28.25V, as opposed to going through some sort of mechanism that provides a lower voltage?

[CID]

In a way.

Voltage regulators don't work as some sort of restriction at the output. They are a little more fundamental in that they set the voltage that the generator produces. More like a governor than a brake.

[Colonel Sanders]

You want to make sure you don't cook your batteries.

Not sure what technology of battery you are using.

[CID]

Be careful on the advice you get here. If you set the generator for the voltage prescribed by the proper AMM or AMM supplement, you don't have to be worried about "cooking" anything.

Some people will tell you to adjust the voltage for different technologies of battery but if your AMM says 28.25 volts at a specific test point and your airplane is in conformity, don't change it.

The "meter" voltage you measure at the designated test point isn't necessarily the voltage your battery is seeing.

[GyvAir]

A number of aircraft have the option of installing either lead acid or ni cad batteries. It's been a while, but I don't recall having to change generator output voltage when switching battery types.

[CID]

The point is that if you should use the voltage the AMM states. Not the battery AMM, the aircraft AMM or if modified the supplemental AMM that goes with the STC that changed the battery.

You need to be a little careful in some cases. For example, the Twin Otter prescribes a slightly higher generator voltage when operating in the cold. This is to allow the ni-cad batteries to charge across a slightly larger temperature range. Unfortunately, it does nothing to improve the performance of lead acid batteries and in fact will likely "cook" them so if your Twin Otter has them installed, that original AMM instruction is null and void. Refer to the new AMM for voltage setting instructions.

[Tim]

Why not simply put a 28.25V gen on instead of regulating a 30V gen down. Going back to the BE20 example, the gens were no different from the lead acid to NiCad batts, both 30V-->28.25V. Maybe an explanation on how voltage regulators works would help me understand it?

Thanks for the responses so far.

[Colonel Sanders]

http://en.wikipedia.org/wiki/Voltage_regulator

[DonutHole]

why the concern over the 30v generator?

As long as it is set to provide the proper voltage why does it matter what the max rated output of the generator matter?

[CID]

Colonel, a pot shot at a Wikipedia on voltage regulators in general really didn't help. Voltage regulators in generator circuits are unique. Mixing things up with electronic regulators for electric circuits is just wrong.

Tim, generators convert mechanical work into electrical power by having a coil of wire rotate in a magnetic field. When that happens, current is "induced" into the coil. The voltage that the current is delivered is primarily dependent on the speed of turning speed of the coil and the strength of the magnetic field. (Electrical current can be compared to the the amount of water flowing in a hose whereas voltage is comparable to the water pressure pushing the water down the hose. The more pressure, the more water. The more voltage, the more electrical current)

For the sake of argument, you can produce electricity by keeping the coil stationary while rotating the magnet. Alternators tend to use that method and generators tend to use the rotating coil configuration.

Since the generator can be turned at various speeds throughout the engine RPM range, voltage is regulated by adjusting the magnetic field. The magnetic field is provided by a "field coil". The field coil is "excited" with field current to make a magnetic field.

The voltage regulator senses the output voltage of the generator and basically switches the field current between maximum and an intermediate value at a rate that will keep the output voltage at the rated level. Basically like someone flipping a switch on and off turning the field current rapidly between fully on and half way on. Generators use commutators to produce DC current or slip rings to generate AC current. Google those (if you're interested) to find out what they mean but generally it's not important in this high level explanation. Just be aware that there are AC and DC generators and that the "alternators" used in cars and smaller aircraft are essentially AC generators that use a diode bridge to convert the output to DC.

Now you may be thinking how the heck does the generator start working if there's no field voltage available to start making a magnetic field. We rely on a residual magnetic field in the iron core of the field winding to start things up.

Some times the generator loses this residual magnetic field so we need to "flash it". Flashing a field is done by momentarily passing current through the field coil in the proper polarity to re-establish the residual magnetism.

I've described the simplest generator. More complex aircraft utilize more complex systems. Some aircraft don't need to rely on residual magnetism to start things up. Some aircraft have much more complicated "regulators". For example, your Kingair will have a "Generator Control Unit" rather than a simple regulator. The GCU performs many functions including protection and regulation and switching as well as starter/gen configuration for starting cycles.

Here's a link I think would be valuable:

http://www.navymars.org/national/traini ... 77_ch1.pdf

[Tim]

im actually fairly well versed in electronics, im not asking how a gen works. i wanted to know why they regulate them and was hoping that explaining how a voltage regulator works might help.

why the concern over the 30v generator?

As long as it is set to provide the proper voltage why does it matter what the max rated output of the generator matter?

because i want to know, thats what the 'whats the reason' part of the question was for...thanks for the input i take it you dont know the answer.

[DonutHole]

Hmm nice reply

Start with difference of potential

Then see resistance

Finally read constant voltage charging

Then make conclusion

But a guy pretty well versed in electronics should already know all of this stuff rendering further thought on the matter futile.

If you knew how a generator works you would know why they need to be regulated


Grab the spoons!!! Time for a feeding

[mcrit]

The batteries produce a nominal voltage of 24V. I say nominal because if you measure the voltage at the terminals it could be anywhere from 20~26V. If the generators produce a voltage less than that of the battery, then the battery will discharge. If the generators produce a voltage equal to that of the battery, the battery will neither charge nor discharge. If the generator produces a charge greater than that of the battery then the battery will charge. Generally you need about 2V of over-potential to overcome the electrode resistance and charge the battery. Which is to say, for a 24V battery you need about 26V to make it charge. The greater the over-potential, the faster the battery charges; to a point. Too much over potential and you start to boil the electrolyte (bad). A further 2 volts or so does a nice job of this; hence most 24V systems like to be at 28V.
So, why are the generators set to produce 30V if you only need 28V? There are a couple of reasons. Generators do not produce a smooth, stable voltage. If you were to look at a generator's output on an oscilloscope you would see that it is quite ragged, with surges and lulls (29-31V for example). Most electronics do not like this. The solution is a voltage regulator. The regulator acts like a wood planer; It takes a high, variable voltage and reduces it, (slightly), to a smooth voltage. That extra 1.5V is where all the noise is, by shaving it off you are left with a steady 28.5V. To carry on the wood working analogy; let's say you needed a finished board 1" thick. If you start with a 1" thick rough board, and then run it through a planer set to 1", you are going to wind up with a rough board. If you need 28.5V and set the noisey generator to produce exactly 28.5V you are going to get a spikey signal.
The other reason alternators are set to produce higher than system voltage is the need for rectification. This is the process by which AC is converted to DC. It is commonly done via a full bridge rectifier, (not going to explain that here). A full bridge rectifier will always drop the voltage by about 1.6V. So, if you want 28.5V to run your electronics, you need to start with 28.5+1.6 ~=30V.

[single_swine_herder]

Excellent reply mcrit.

Well written.

Single_Swine_Herder

[Tim]

Thanks mcrit!

[CID]

mcrit, sorry but there are so many technical inaccuracies in that post I need to address them.

1. Batteries require a voltage higher than their quiescent voltage for two main reasons. Neither of those is "electrode resistance". The first is simple physics. Current flows from a higher potential to a lower potential. The second reason is "internal battery impedance". That is the characteristic impedance of the battery. This internal impedance is dependent on the chemistry (ni-cad, lead acid, etc) and temperature. Ni-Cad batteries have a lower internal impedance than lead acid batteries at moderate temperatures so they can deliver serious starting current with little voltage drop at the terminals. Think of the internal impedance as a resistor in series with an ideal battery.

The more current you draw from the battery, the more voltage is dropped across the internal resistor and the less voltage is available at the terminals.

The internal impedance of a ni-cad battery has a negative temperature coefficient as compared to a lead acid battery which has a positive coefficient. That's the main reason they act so differently. A ni-cad battery has a lower impedance with higher temperature which makes it prone to thermal runaway. It has a high impedance in the cold making it impossible to charge in extreme cold.

2. You can't say that batteries are a "nominal 24 VDC" without some context. The "quiescent" voltage, that is the characteristic voltage of a battery at rest is dependent on three main things. The chemistry, the number of cells and the charge state. A lead acid cell, just by it's chemistry is naturally about 2.105 volts in a charged state. A ni-cad cell is 1.2 volts. You can do the math for lead acid batteries with 12 cells or for ni-cad batteries with 19 or 20 cells. You will find that the "range" for properly maintained and charged cells is not as you stated.

3. I'm not crazy about the word "boil" in this context. Unless you are talking about thermal runaway, nothing is "boiling". If you overcharge a lead acid battery, it reaches a state where the chemical conversion that happens inside can no longer build the plates and the water in the electrolyte breaks down into hydrogen and oxygen which is released in non sealed batteries. This explosive mixture can be quite dangerous if it's allowed to accumulate. As the water is converted, the electrolyte level goes down and eventually portions of the plates are exposes causing them to become distorted as the battery charge cycles.

This can cause premature failure of the battery and a very stinky rotten egg smell.
In ni-cad batteries, the electrolyte doesn't change chemically during the charge discharge cycle but the effective level changes. Overcharge can cause the electrolyte to vent making a huge mess of potassium hydroxide inside the battery case. Again, no "boiling".

4. DC generators produce very noisy electrical power. True dat’. But (surprise) in part it’s because of the regulator. There are 2 basic sources of distortion (noise) in a traditional DC generator. One, source is brush noise. As the brushes slide over the commutator segments they spark like crazy. It’s not difficult to imagine why. This noise tends to be in the higher frequency spectrum and can reach well into the radio frequencies. The capacitors you often see in the generator housing or in the connection junction box are there to suppress that radio noise.

The second main source is from the very noisy field current. Generators are sort of regulated like your furnace. Your furnace is turned on and off to maintain a temperature. In cold weather the on-off cycle is “on” more often.

Generator regulators basically turn field current on and off (although off isn’t all the way off) at a rate relative to the demands of the generator. If the generator turns faster, the “on” time is shorter.

This rapid on-off-on-off switching is translated to the output in noise that is in the audio spectrum. In faulty systems it can be heard through headphones as a whine.

The battery helps a great deal in this regard as a “shock absorber” much in the same way a hydraulic accumulator or those water system reservoirs do. Without the battery in the system, it would be MUCH noisier electrically.

5. Modern electronics are pretty good at working with “dirty power”. In the old days it was unheard of to start your airplane with the avionics on but more and more aircraft are being developed without a radio master. This was especially a problem back when hybrid radios were developed that used a combination of transistors and tubes. The early transistors, especially of the germanium variety, really couldn't tolerate electrical transients. If you started your airplane with the avionics powered, there was a good chance you’d be making a visit to the radio shop.

Modern electronics make use of advanced transient protection and power supplies so the problem is nowhere near as prevalent.

6. No, voltage regulators in generator systems are absolutely not like a “wood plane”. That entire analogy is technically incorrect. It would be somewhat correct if the subject was series regulators used in electronic circuits but we’re talking generators here. The furnace analogy works much better. Mcrit, you have to ask yourself, if a generator voltage regulator worked like a wood plane and shave off the excess, what happens to all the excess power? In a planar series regulator, that excess power is diverted from the load and turned in to heat.

Using the same regulating technique in a generator would sure produce a lot of heat, and waste. Generators already have issues with heat. Think about it. A 400 Amp generator working at capacity with just 2 volts being “planed” off would generate an additional 800 watts of heat!

7. Full bridge rectifier in an alternator? Sort of, but the term is usually full “wave” rectifier. The assembly with the diodes is usually called a diode bridge. Dropping about 1.6 volts? I hope not. You’re thinking of small signal diodes. Larger power rectifiers have much lower forward bias voltages closer to 0.5 volts. And yes, in an alternator the voltage generated will be reduced across the diode bridge.

Alternators produce 3 phase AC power. The beauty of this design is truly in how those three waves which are 120 degrees apart in phase, add up mathematically to “null”. Try it. Take any angle and add the SIN of that angle, plus the SIN of that angle+120, plus the SIN of the original angle+240. You just don’t get that simplicity from a single phase full wave rectifier.

[B-rad]

CID,

Mcrit laid things out in laymen terms so people can understand and give reasoning to the operation of a genny. I think he did a wonderful job. I appreciate your technical understanding and desire to further explain the situation but the way I see it Mcrit completed the objective of teaching people who didn't know about a voltage output to a level that quenches the curiosity.

[GA MX Trainer Dude]

Actually none of the above is the reason the generator is rated at 30 Volts.

Back in the bad old days when multi-engine aircraft used carbon pile regulators and differential voltage relays for connecting the generators to the aircraft system provision had to be made to allow the generators to connect to the system.

When the first engine was started and was ready for the generator to be put on line – the switch was moved to the “ON” position – in many cases the generator would connect automatically to the system as it was already being regulated at 28.5 Volts and the battery was at about 24 Volts.

The differential voltage relay control needed about .5 to 1 Volt difference between the generator voltage and the system voltage to operate and connect the generator. Once connected the generator would raise the system voltage from battery @ about 24 Volts to regulated 28.5 volts.

Now start the second engine – generator is ready to come on line but won't – its' Voltage is 28.5 and the aircraft system voltage is at 28.5 so there is no voltage differential to operate the differential voltage relay.

What we have right now is both generators producing 28.5 Volts – but only the first one is connected to the aircraft system.

The method used to connect the second generator to the aircraft system is by the use of the “RESET” position of the generator switch. When “RESET” is selected it boosts the voltage of the generator above 28.5 – towards 30 Volts and allows the generator differential voltage relay to operate and close the "Line Contactor" to place the generator on line. Once we have boosted the generator voltage that .5 to 1 volt above the system voltage the differential voltage is sufficient to activate the relay - but not sufficiently high enough to operate the Reverse Current Relay and disconnect the first generator.

A generator that will not produce the 30 Volts would not be able to do this.

The switch is spring loaded so once released from the “RESET” position it removes the extra voltage and the 2 generators are now on line. It only takes about a half second to “RESET” the generator. Both generators should be connected to the system - the warning lights should be out and the aircraft system regulated to 28.5 Volts.

Once connected the generator can be disconnected by placing the switch to "OFF" or if there is a reverse current condition - like a generator fail the reverse current relay will open and remove the generator from the system.

On most systems the “GEN FAIL” light is connected to a set of auxiliary contacts in the Line Contactor Relay and is actually telling you that the generator is “ON or OFF LINE”. Sometimes the Line Contactor Relay is also the Reverse Current Relay - just depends on the system design

The newer solid state regulators work differently but they came way after the early systems.

For what it is worth – many aircraft generators are capable of higher voltages than just the 30 Volt rating. I have a 400 Amp generator from a large aircraft that is my primary stick welder – I drive it with a nice little 2 cylinder diesel. No voltage regulation – just current regulation and open circuit voltage before striking an arc is about 40 Volts. Once the arc is stabilized it sits at a nice 29 to 30 volts. Makes really nice welds.

Hope this helps.

PS – Yes I know the “RESET” position is also used to flash the field as well.

[CID]

B-rad,

I respectfully disagree. It's not even valuable as a "layman's" reference. It's perfectly OK to dumb things down to appeal to a broader audience but it's not helpful to present the subject matter incorrectly.

What do you say to a student who asks for further clarification? Do you give him/her an answer that simply expands on the original inaccurate concept? At what point do you need to end the lie?

To be effective and helpful, don't make up things to explain complex concepts. Just remove detail as appropriate.

[Colonel Sanders]

If you want to understand voltage regulators (OP)
I might suggest starting with this:

http://en.wikipedia.org/wiki/Servomechanism

A Servo-mechanism is a pretty neat idea. Your
toilet uses one, to maintain the level of water in
the toilet tank. When you flush the toilet, the
servo in the toilet tank refills the water to the
correct level, and thus keeps it there.

A voltage regulator does pretty much the same
thing - it's a servo, just in your toilet tank.

For most people, using the analogy of water
for DC electricity works very well. You know,
voltage is the height of a waterfall, and amps
is the width of the waterfall. Multiply the two
and you get watts, or the amount of work
being done.

Back to the voltage regulator. Consider the
generator (or alternator) to be a "black box"
with a pulley or gear, a big wire and little wire.
We really don't care what goes on inside, for
the purpose of this discussion. That's why we
call it a "black box".

The engine spins the pulley or gear, allowing
the generator (or alternator) to create power
using magnets and wire and stuff that we
really don't care about.

The power comes out of the big wire from the
generator (or alternator) and is connected to
the aircraft bus or rail. It powers everything
in the airplane.

Through a circuit breaker (or switch (or both))
a little wire (ie not many amps) is connected
from the aircraft bus or rail to the voltage
regulator.

Consider the voltage regulator to be another
black box. It's not a very big one. It has as
input the little wire from the aircraft power,
and a connection to ground. It uses the aircraft
power in two ways (which I think is pretty neat):

1) to power it, and
2) measures it.

The output of the voltage regulator is another
little wire (ie not many amps) which is connected
as the input to the generator (or alternator) and
controls it.

Back to the servo business. When the aircraft
voltage drops (input to regulator) it cranks up
the volts to its output, which is the little wire
controlling the generator (or alternator).

When the aircraft voltage increases, it drops
the output voltage to the alternator (or generator).

That's the 100,000 foot view of an alternator
(or generator) and it's associated voltage regulator.

Basically, the voltage regulator is the brains of the
system, and the alternator (or generator) is the brawn.

Note that the alternator on your car almost certainly
has the voltage regulator internally integrated into it.

It's an aviation thing to have all sorts of extra boxes
hanging about the place (voltage regulator, over-voltage
relay, etc).

[CID]

Trainer Dude, I don't agree. (Surprise)

Without going into another lengthy description that will ruffle more feathers, you need to consider the action of the paralleling circuit in 2 generator systems. That will help explain how the generators are placed on line.

Also, the reset switch is used in systems that use a crow-bar circuit to completely deplete the residual magnetism during shut down. The thought of the engineers who preferred this method was that the generator wouldn't start producing power until there was a operator action. And yes, the RESET action, among other things, flashed the field.

With respect to the old carbon pile regulators, new generator regulators fundamentally don't work much different. The "carbon pile" was simply squeezed or not squeezed by an electromagnet (switched) between two resistance set points that would switch field current back and fourth between two settings just like pretty much any other DC generator regulator.

The carbon pile wasn't quite as fast in this switching action as newer solid state regulators so it was a little easier on the accessory drive shafts but besides that they did the same thing.

[CID]

Keep in mind that if you want to consider servomechanisms to describe a generator, stick to the bang-bang control. Generators (and furnaces for that matter) use that control method to prevent oscillations or under-control.

[Colonel Sanders]

Remember, 100,000 foot view.

This crowd is NOT up for Ordinary Differential Equations
(O.D.E.'s) which are great for understanding RLC circuits,
and associated oscillation/damping behaviour.

http://en.wikipedia.org/wiki/Ordinary_d ... l_equation
http://en.wikipedia.org/wiki/RLC_circui ... d_Response

PS I really like toilets.