Re: Electrical dummy

Posted by MarkB_MI on January 30, 2007 at 18:48:10 from (216.234.125.82):

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

Although the cold cranking amps rating is important, it mostly applies to the initial starting torque and becomes less important as soon as the motor starts turning over.

Understand that a DC motor (such as a starter) is also a generator. The voltage generated by a turning motor is called "back electromotive force", or "back EMF". A motor sitting still has zero back EMF. That's true whether there is any voltage applied to it or not. A stopped or stalled starter motor is basically a dead short. So when you connect a battery to it, the battery will source as much current as it can, which is more or less the CCA rating. However, the current through the starter generates torque which will cause the starter to spin as soon as the starter's torque can overcome the internal friction of the engine.

As soon as the starter starts to turn over, it begins to generate back EMF, which opposes the battery's voltage and reduces the current through the starter. If there is no load on the starter motor, its RPMs will increase until the back EMF almost equals the applied voltage and there is almost no current through the starter. But if there is a load applied, then the speed of the starter motor will increase only until its torque equals whatever is required to turn the engine over.

Now if you replace your six volt battery with a 12 volt battery, then the starter can turn much faster before it reaches the equilibrium point where back EMF reduces torque to what is required to turn the engine over.

So, the short answer is this: If you connect a 12 volt battery to a six volt starter, the starter will turn over roughly twice as fast as it does with six volts. And that makes a big difference when starting an engine.

Replies:

Add a Reply Name: E-Mail: Subject: Comments: Optional Link URL: Link Title: Optional Image URL: Email Notification: If you check this box, email will be sent to you whenever someone replies to this message. Your email address must be entered above to receive notification. This notification will be cancelled automatically after 2 weeks.

TRACTOR PARTS	TRACTOR MANUALS

We sell tractor parts!  We have the parts you need to repair your tractor - the right parts. Our low prices and years of research make us your best choice when you need parts. Shop Online Today. [ About Us ]