Horsepower: 746 Watts = the amount of energy in 1 horsepower
There are only 2 formulas that you need to know and understand, in order to
solve most common electrical problems on small power equipment.
The first is Ohms Law: E = IR.
This says that Voltage = Current X Resistance.
Conversely, you can Find I with I = E/R and R = E/I.
The second is the Power formula: P = EI.
This says that Power in Watts = Voltage X Current (Note:
current is often called amperage). Conversely, E = P/I
or I = P/E and P
P = I2R.
Now, let's apply these formulas to a couple of tractor examples to see how
Example 1:Your tractor has two 12 volt headlights rated at
36 watts each, connected in parallel. Your tractor has a 6 amp charging
system. You would like to add two more lights, can you do this with the
charging system you have? There are two ways to solve this.
1st Method, using the Power formula: P,
orPower = 12volt system, so
[12 Volts] X I [Current 6
amps.] = 72 Watts. This is the capability of the charging system. Two
lamps in parallel with 36 watts each = 72 watts. No extra power is
available for more lights. Connecting more light would result in the battery
not staying charged and likely "smoke" the rectifier or stator in
the charging system.
2nd Method, using Ohm's Law: Your lights are rated at 36
Watts. Current available, I=P [36W] /E [12V],
so Current, or I = 3 amps each. Two lamps, at 3 amps each, equals 6 amps,
the total capacity of the charging system.
Example 2: Your mower has an electric clutch with 4 ohms
resistance and the two lights above. You are looking at a replacement
engine. What size alternator does it need? To solve this we need
to use Ohms Law to find how much current the clutch draws. You have a 12
volt system, so:
I, Current =E [12 Volts] / R [4
ohms] = 3 Amps
You would need a charging system that would provide
current to charge the battery after starting, plus 3 Amps for the clutch, plus
6 Amps for the lights or more than a 9 Amp charging system. In this case
an engine with a 6 amp alternator would not be a deal even if it was cheap, as
it would need to be converted to a different charging system.
Next I will go to some basic circuits, and explain how resistance, voltage
and current interact in a circuit. Once you understand these principles,
how you would use a voltmeter to locate troubleshoot wiring problems in you
tractor, or actually any electric circuit.
First we need to know some common symbols and what they are. Then later we
can use them to draw a diagram and analyze how it works.
These symbols are used to create schematic diagrams, these show electrically how
a circuit works. Some industrial and other diagrams use some variation of these
symbols. In the next article we will look at series and parallel
circuits and how they work, using some of these symbols.