Alternator question


#1

This is actually a Helicopter Electrical system question:



I have a 14v 60amp Alternator powering a 14v Bus Bar. The Battery is 12v. The manual states that: by design the Alternator requires roughly 3v to produce the electrical magnetic field required inside the Alternator… so if the 12v battery is completely flat (no charge) the Alternator will not be able to supply any electrical power to the SYSTEM even after the engine has been started by some other means (external power)… seems like it should be easy to understand, but I’m not fully grasping the concept.



Why is this so? What does this mean for the operation of the aircraft? Is the alternators charge in some sort of endless loop?


#2

The alternater has windings some of which provide a magnetic field and some of which provide the current. The “rotor” windings provide the magnetic field that induces current in the “stator” windings. The rotor windings need to be energized in order to induce the current. Thus the minimum battery requirement.

It’s get a little more complicated, because although the stator windings are providing power back to the battery (and thus the rotor windings), if you disconnect the battery, the alternator will not produce power. This is because the alternater generates “pulses” of power and if you don’t have a capacitive load (the battery) to hold the voltage between pulses, the rotor will become un-energized “in between” the pulses and therefore no current will be produced.

That’s a gross oversimplification - but it gets the basic idea across


#3

Whether the alternator is in a vehicle or an aircraft they both require the field of the alternator to be excited by a voltage. To start the charging process the battery provides the voltage to the field. When the field in the rotor is excited it creates electricity in the stator windings. The way most alternators are made, the stator has 3 seperate windings that create a 3 phase AC voltage output that ties to the rectifiers built into the alternator. The rectifiers then convert the AC voltage to a pulsing DC output. The DC output is regulated by a regulator circuit that controls the exciter voltage to stay between a specified voltage range and this keeps the alternator output at safe levels to charge the battery. If the regulator senses the output voltage going below the limit it excites the field more and this creates a higher voltage in the stator. The same process goes for too high a voltage, but in reverse.


#4

I have a battery charger home made with an electric motor driving an old alternator that uses an electromechanical relay type voltage regulator. Running the alternator before connecting the output leads to the car battery produces no voltage output but once the alternator’s rotating field is excited by the battery, it countinues to produce voltage even if disconnected from the battery.

The alternator’s field can be excited with a 1-1/2 volt “D” cell battery by momentarily touching the output leads to the battery with corect polarity, of course.

By the way, a vehicle alternator is a 3 phase ac device with full wave rectification so it has no gaps in the output voltage/current waveform as viewed with an oscilloscope. The waveform will have peaks and valleys but will always be positive; never at zero volts/current.

The bottom line is that an alternator requires very little initial current to the field (excitation) to get it to start producing voltage.


#5

It’s pretty simple, really… For a generator or alternator to produce electricity, windings of copper wire have a magnetic field swept past them…This induces a current to flow in the wire. If permanent magnets are used, no electrical power is needed to “excite” the generator. Spin it and it will produce power. But it’s cheaper (and much easier to regulate) if you use electro-magnets to produce the magnetic field needed to produce electricity. Batteries seldom get stone dead, they usually have enough power to establish a small magnetic field in the alternators field windings, and that’s all it takes. Almost instantly, the alternators output will jump up to maximum and quickly restore normal voltage in the system. It takes very little power to “feed” an alternator. Less than 10% of the output is needed to maintain a strong magnetic field. By controlling this small current, the output of the alternator can be regulated.

Try this. Wind 10 or 20 turns of light wire around a nail. Attach the ends of the wire to a sensitive ammeter or voltmeter. Pass a magnet quickly over the coils on the nail and watch the meter needle. This is the principal that ALL generators or alternators operate under.

Until about 1964, most vehicles used generators, not alternators. The difference being a generator produced DC current directly. They were heavy and expensive to build, and they required service. An automotive alternator (as others have correctly stated) produces 3 phase A/C current which is then rectified by 3 big diodes into a fairly smooth DC current to feed into the vehicles electrical system and charge the battery.


#6

to relate this to your post, although i don’t know bupkus about helo’s, i would assume this is a shaft driven alternator? off the main rotor?

as others have mentioned, an alternator will produce NO juice when turned. unless and until it is “excited” by a small amount of juice to get it to produce the required voltage to recharge the battery.

so once the battery goes dead (<3V) even if the shaft is turning the alternator won’t create more juice.

one more reason to NOT fly in a copter as far as i can tell!


#7

Actually, a lot of alternators will self excite if you spin them fast enough. The reason is because of a property called magnetic hysteresis, the magnetic field of an electromagnet does not go all the way to zero when the electric current goes to zero. However, the voltage produced by that small amount of residual magnetism must be more than the .7 volts needed to make a silcon diode conduct forward.
On large alternators used to provide 460 volt 3 phase power, the residual magnetism of the rotor is enough to generate about 30 volts in the main winding without any DC excitation to the rotor. That’s more than enough to give the regulator enough power to start exciting the field.


#8

Well, the others have provided lots of good explanations on how an excited field alternator works, so I won’t repeat that. What does it mean to the operation of the helicopter? Well, I assume you are dealing with a turbo jet engine. I’m pretty sure the engine doesn’t need the elctrical system to run, but there might be some other issues. Clearly, you won’t have any electrical power to the instruments or flight controls. A lot of the flight controls are probably hydraulic, so the beast might still fly, and some instruments might still work, but you wouldn’t want to take this thing up in that condition unless someone was shooting at you.

As others have pointed out, even a brief shot of voltage to the field coils should bootstrap the alternator and then it will self-sustain as long as the engine is turning.


#9

Even piston engine powered helicopters have magnetos to provide ignition so if the battery is dead, you can still fly. Even if the engine does quit, you can glide to the ground with the rotor auto-rotating as long as you keep a small amount of negative collective on the rotor and maintain forward speed. The flywheel effect of the spinning rotor provides enough stored energy to do the final landing flare.


#10

Good point, I forgot about aircraft engines using magnetos. Dual independant ignition systems, too, if I recall correctly.


#11

Remember when you connect up the external power to start the prime mover and flip on the master you will have ‘voltage’ available. The external power will start charging the battery. Once the prime mover starts, the alternator will energize and continue charging the battery and running the avionics, lights, and accessories. If you were to pull the alternator CB, the alternator might not reenergize if the battery had not had enough time to gain sufficient charge or was defective and would not take a charge. The prime mover would continue operating. If this is turbine powered, the ignitor would not be available but the continuous combustion would continue. As I remember, you turn on the ignitor preparatory to departure and landing to forestall flameout at a critical momment.


#12

6 or 8 diodes depending on whether the alternator is wye wound (8) or delta wound (6).