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What is impedance?
First comes the simple answer. I'll keep it dead simple, very inelegant but dead simple.
A simple example of impedance
Assume you have available these 4 items on your bench:
(a) A series of eight fresh AA type 1.5 volt cells to create a total of 12 volts supply.
(b) A 12 volt heavy duty automotive battery - fully charged.
(c) a small 12v bulb (globe) of very, very low wattage. and;
(d) a very high wattage automotive high-beam headlight.
Now if we connect the extremely low wattage bulb to the series string of AA cells we would expect all to work well. Similarly if we connect the high wattage, high-beam headlight to the heavy duty automotive battery all will be well. Well for a time anyway. Both of these sets are "sort"of matched together. Light duty to light duty and heavy duty to heavy duty. Now what do you think would happen if we connect the high beam headlightto the series AA cells and conversely the low wattage bulb to the automotive battery?.
In the first case we could imagine the high beam headlight would quickly trash our little tiny AA cells. In the second case our min-wattage bulb would glow quite happily at its rated wattage for quite a long time. Why?, therein lies my expanation of impedance. Consider it!
The heavy duty battery is capable of delivering relatively large amounts of power but the series string is capable of delivering only relatively minimal power. The first is a low impedance sourceand the other, in comparison is a relatively high impedance source.
On the other hand the high beam headlight is capable of consuming relatively large amounts of power but the minature bulb is capable of consuming only minimal amounts of power.
High Impedance loads and Low Impedance loads
Again the first is a low impedance load and the other is high impedance load. If you're keen to apply ohms law you will discover why, research it through the text books.
Imagine a tiny caterpiller chewing on a large blade of grass - no problem plenty to eat there. Now on the other hand imagine a poor cow stuck in a desert with only one similar blade of grass available to eat.
General expression of Impedance The term impedance is a general expression which can be applied to any electrical entity which impedes the flow of current. Thus this expression could be used to denote a resistance, a pure reactance, or as is most likely in the real world, a complex combination of both reactance and resistance.
 In the last two decades alternators have replaced generators in motor vehicles. The reasons are many: output current can be produced at lower rpm, voltage can be more accurately' controlled with solid state regulators, alternators need less maintenance, and they cost less to manufacture.
When modified, auto alternators can provide variable direct current at 0 to 120 volts for battery charging, hot charging, light arc welding, or for running AC-DC appliances and lights. Another simple modification provides AC power to run some transformer-operated appliances. If you know the secrets of its operation and the modifications possible, the small low-cost alternator can become a versatile power plant.
BASIC CONSTRUCTION
The old-fashioned generator contains a wound stater which produces a constant magnetic field in which a revolving coil of wire, called an armature. A commutator on one end of the armature made up of many individual brass segments passes the generated current to the outside world through carbon brushes Because commutator segments must be electrically insulated from one another. They can not be fabricated from a single block of metal. Each commutator segment must be individually attached to the armature shaft. This is a source of mechanical weakness. When the armature is rotated at high RPM, centrifugal force can cause the commutator to explode, throwing segments in all directions.
To prevent explosions, the generator is usually driven at less than engine speed. An auto engine may turn 5000 RPM, but the generator must be geared to a maximum 2500 rpm for safety's sake. As a result, the generator turns so slowly at low engine rpm that it produces little or no current.
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