Drive Speed Voltage and Power Strength

The voltage output of your regulator is directly related to the drive speed and the actual shaft speed of your alternator. Even though your drive speed may be fixed, you can vary the speed of your alternator (shaft speed ) by changing pulley sizes.

With equal size pulleys, the speed of your alternator will equal the speed of your drive motor (assuming you are not losing speed through a slipping belt). By increasing the size of the drive pulley, you increase the speed of the alternator. If the drive pulley is twice the size of the alternator pulley, the alternator will spin twice as fast as the drive pulley. The formula to calculate pulley sizes and drive speeds is:

To Calculate Alternator Speed

Alternator Speed (RPM) = RPM of Drive Motor x Diameter of Drive Pulley

Diameter of Alternator Pulley

For example Assume motor drive is spinning at 3000 RPM with a drive pulley of 10 cm and alternator pulley of 6 cm diameter, then

Alternator Speed will be = 3000 x 10

Therefore the speed of the Alternator6 = 5000 RPM To calculate the required pulley size when you want a specific alternator speed is as follows: For example the upper safety limit on an alternator is 14000 RPM, so let's assume you have a drive motor capable of 3000 RPM, but you want to drive the alternator at its maximum of 14000 RPM. Usually it is more convenient to change the drive motor pulley rather than the alternator pulley, so with an alternator pulley of 6 cm diameter and a drive motor spinning at 3000 RPM, you now want to know what sized drive pulley to use in order to spin the alternator at 14000 RPM. Here is the formula.

Pulley diameter of Drive Motor = Alienator RPM .l.i^M.tiLt£I_fli^ile.rii()tor Pulley

RPM of Drive Motor

Using the above example:

Pulley diameter of Drive Motor = ¡4000 x 6


Required diameter of Pulley = 28 cm.

Note: Inches can be used instead of centimetres - it makes no difference as long as you do not mix the two. In other words all measurements must either be in inches, or they must all be in centimetres.

Calculation of Power Strength, Amperage and Volts

You will often want to know the exact current draw on certain tools and appliances in order to calculate the required output of your battery bank or alternator. There is no point in using excess fuel and overdriving your alternator if you are not going to use the power.

For example, let's assume you have geared your alternator to put out 35 amps at 100 volts, you then want to know how many lights or appliances you can operate off this. The formula is :

watts - amps x volts

In the above example:

Output in watts = 35 x 100 = 3500 This means you have a power output capable of supplying 3500 Wai t* You will now want to know what this actually means in real terms. To work this out, read the specifications stamped on the appliances, then calculate as follows:

For example: One 100W light bulb consumes 100 watts of power which is quite straightforward. However, most appliances are marked in amps, e.g.: most 10mm drills are around 1.5 amps at 240 volts. This works out as:

Watts = 1.5 x 240 = 360 watts Therefore, the power consumption of the drill is 360 watts. (The number of watts is a direct measurement • of energy at actual motor power.) For the purpose of comparison, the 12 volt Bullcraft drill, 10mm (3/8") draws 25 amps at 12 volts for a rating of 300 watts.

E.g.: Watts = 25* amps x 12* volts = 300 watts (* These figures are stamped on the drill) There are occasions when you know the watts, but want to calculate the amps. This may be to assess wiring size, fuse size, etc.

In this case the formula is :

y Volts

Using the above Bullcraft drill example:

Amps = 300 watts = 25 amps 12 volts

Note : This formula is important because high amps require thicker wiring to carry it. If the wiring is too light then the current flow is restricted and heat quickly builds up burning the wiring out and can ignite buildings. This heat build up can be easily demonstrated by using light jumper leads to start a car with a dead flat battery. These light jumper leads (or booster leads as they are often called) are usually only rated at about 25 to 50 amps. Most starter motors draw well over twice that amount. The result is the starter motor cannot get enough amps through the thin wire in the leads, so it turns over very sluggishly, then the leads quickly overheat to the extent that the plastic insulation melts within a very short while.

As a general rule all tools and appliances operating on }ow voltages require higher amperages to get the same performance, and as a result heavier wiring must be used to carry the extra amps. If the wire heats up, or power loss is significant then heavier wiring is required.

To assist you in compiling your projected power requirements for your home, we have listed the average power consumption of average-sized appliances :

100 watt light bulb


Vacuum cleaner

400 W

Frying pan

1200 W

Washing machine

500 W


300 W

Water heater

2000 W

Solid state stereo

60 W

Room heater

1000 W

Colour television

350 W


600 W


1000 W


50 W


12000 W


80 W


800 W

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