View Full Version : C 172 electrical system

20th Dec 2001, 03:28
Hello all..its been a long while since my CFI
and in it we learned the complexities and simplicities of the electrical system on the cessna 172,now my question, how exactly does the alternator convert the direct current eminating from the battery. Also if possible, what other hidden things should i know about the electrical system.

The more I know, the more I dont know!

Squawk 8888
20th Dec 2001, 05:28
Think of it as an AC generator without the permanent magnet. In an AC generator, the moving coil is connected to the brushes such that the same end of the coil is always contacting the same brush. Since the current flow through the coil reverses with each half-rotation, the result is an AC current at a frequency equal to the generator's RPM.

Now take away the permanent magnet and replace it with an electromagnet. This is the field coil. When DC is applied to the field coil, you have the magnetic field needed to generate the AC power.

The main advantage of an alternator over a generator is that it produces more power at low RPMs, though I'm not sure why. The biggest drawback is that it will not generate any power at all unless there is power in the field coil, so hand-starting with a dead battery will not charge the battery once the engine's running- you need an electrical source to get it going.

20th Dec 2001, 10:08
A battery too flat to start the engine often has enough juice to get the alternator charging after a hand start. If the ammeter shows the battery charging, you're in business. If it's discharging, you have to shut down and get a boost.

Don D Cake
20th Dec 2001, 16:24
Here's a very good explanation of the workings of an alternator....

<a href="http://www.vtr.org/maintain/alternator-overview.html" target="_blank">here</a>

<img src="smile.gif" border="0">

Squawk 8888
20th Dec 2001, 22:23
Great article, Don. Looks like I had it backwards- I thought the field current was applied to the stator.

Am I correct in assuming that using the battery (via the regulator) to boost the field current is the reason it produces more power at low speeds than a generator would?

Don D Cake
21st Dec 2001, 17:25
I believe, and I stand to be corrected, that a generator that has field coils that rotate, generating coils that are stationary and rectifies it's output using diodes is known as an alternator. A generator that has stationary field coils, rotating generating coils and rectifies it's output by commutation is known as a generator - still with me?

I can't see any reason for similarly rated generators and alternators to have different current outputs at low rpm. Once an alternator/generator is producing current, the device is self sustaining ie it produces it's own field current without the need for a battery (although the battery is not needed to produce field current don't even think about removing it as it forms an integral part of the regulating circuit. If you take the battery away when the alternator is running you can literally blow things up).

However consider the following....

The problem with commutation, which was all you could do before diodes were invented, is that the brushes and commutator can arc and overheat easily. High current output requires a large mechanical design size which is why size for size alternators have a higher output that generators. The idea that generators have poor output at low rpms could stem from the fact that size restriction meant that generators were rated with very little, if any, headroom ie barely adequate rating at high rpm would result in below adequate rating at low rpm.

A type of generator that would certainly suffer at low rpms would be one incorporating permanent magnets rather that electro-magnets. The output of this generator can only be controlled by the speed of rotation. Any excess current has to be shunted to ground via a big resistor. This is inefficient and hot! In a situation that involved high rpms and low load requirement (fully charged battery and no auxiliary equipment switched on) the regulator would get very hot, so the generator would be rated with no headroom. At low rpms with high load requirement, the output of the generator would be insufficient and the battery would discharge. I believe it was quite common in the good old days for cars in a traffic jam in winter to discharge their batteries fairly quickly.

My had hurts now, I'm going to the pub for an Xmas drink....

Squawk 8888
22nd Dec 2001, 01:18
That's sorta what I was getting at- if the field comes from a coil instead of a permanent magnet, then the power output can be boosted by (1) spinning it faster and (2) boosting the field current. Therefore in a high-rpm, low-load scenario cutting back the field current can reduce the output. One other advantage to that- reducing the field current reduces the mechanical load, giving more horsepower to that spinny thingie up in the front.

24th Dec 2001, 16:16
Alternators run at far higher r.p.m.'s than generators,the alternator on the average light aircraft rotates at 10000 rpm when the engine is at 2700 rpm, hence it will give a respectable output at low engine rpm. If you run the average generator at this rpm the commutator would explode.An alternator has slip rings which can stand these high rotational speeds.

Merry Christmas

Squawk 8888
25th Dec 2001, 23:41
Good point- with high voltage, high current scenarios a lot of the power would be lost to arcing in the commutator. Continuous contact with a slip ring wouldn't have that problem.

26th Dec 2001, 00:01
Get somebody else to: -fix it
-replace it
-handprop it
-or break it completely

26th Dec 2001, 09:51
Another disadvantage of a system employing brushes would be it's inherent susceptibility to arcing at higher altitudes.