Re: Electrical dummy

Posted by KEB on January 30, 2007 at 17:22:52 from (192.31.106.34):

In Reply to: Electrical dummy posted by B-maniac on January 30, 2007 at 14:59:03:

B-maniac. The direct answer to your question is no. There are several reasons.

The strength of the magnetic field generated by an electromagnet is proportional to the number of windings times the current through the windings. Inside the starter, the torque generated by the starter is a function of the magnetic field strength which it turn is a function of the number of turns of wire in the windings times the current flowing through the wire.

In a motor, the size of the wire represents a compromise. If I use a larger wire, I can get more current through it for a given voltage, but I can't have as many turns as if I used a smaller wire. For any given voltage and motor size, there's an optimum compromise between wire size and current.

A starter designed for 12 volts draws less current than a starter designed for 6 volts. Because a 12 volt battery have more voltage to force current through the motor windings, a 12 volt starter can use smaller wire and end up with the same magnetic field strength (which translates to torque) with less current. That's why starter cables for a 12 volts system are smaller than cables for a 6 volt system; the 12 volt starter draws less current and can therefore use a smaller cable.

Remember that CCA is the maximum current the battery can supply, not necessarily the current it can force to flow through the starter. A larger CCA battery will have more reserve capability, i.e. be able to turn the starter for a longer time, but won't necessarily be able to turn the starter any faster unless the current drawn by the starter is close to or above the CCA rating.

An example of this is a diesel truck, which uses either one really large 12 volt battery or two smaller ones in parallel to provide enough current to turn over the starter. If I were to take the truck battery and connect it to a Toyota, the Toyota starter wouldn't turn over any faster, since the maximum current is limited by the starter, not by the battery (the starter draws less current than the battery can supply). On the other hand, if I were to take the Toyota battery and try to start the diesel with it, the current would be limited by the capacity of the battery and the diesel would turn over slowly or not at all (the starter wants to draw more current than the battery can supply).

Clear as mud, right.

Keith

Replies:

• Re: Electrical dummy Raleigh 18:16:23 01/30/07 (0)

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