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Name: Lyn N.
Status: other
Grade: 12+
Location: TX
Country: USA
Date: Fall 2013

I have a 1954 MG TF automobile it has a Positive ground battery system. About 10 years ago installed a 1969 MGB Negative ground engine. Kept the TF's positive ground generator, regulator etc. Just finished rewiring the car. It crossed my mind, why does the negative ground starter work connected with a positive ground, it has worked for the past 10 years.

Hi Lyn,

Classic car! But I will avoid the temptation to crack jokes about Lucas electrical systems used on old British cars ("Joseph Lucas; Prince of Darkness", etc.)...... no doubt you have heard them all!

Your starter motor is a series-wound motor, which means that the field coils are connected in series with the armature. It is inherent with a series-wound motor, that, when this type of motor is connected with reverse polarity (as you have done), its direction or rotation remains unchanged. The only way to get a series-would motor to reverse, is to internally reverse the polarity of EITHER the field coils, OR the armature. When BOTH field and armature are reversed in polarity, the direction of rotation remains unchanged.

Many modern starter motors now use a large permanent magnet in place of the field coil. With these starter motors, reversing the polarity of the motor WILL cause the starter to turn in reverse. This is because, when this type of starter is connected in reverse polarity, the armature's polarity is now reversed, but the permanent magnet field's magnetic polarity remains unchanged.

Regards, Bob Wilson

Hi, Lyn

I am an electrical engineer. These sorts of questions are difficult to answer "in general" without working through the details.

If the starter includes a brush-type motor with a permanent magnet field, then I would expect the rotation direction to reverse when you apply the reverse voltage. But if a similar brush type motor had an electromagnet field winding instead of permanent magnet, then I would expect both windings would be reversed. This double reverse would result in the direction of rotation being the same.

Does this help?

Best regards Bob Zwicker

When we say that an automobile has a positive or negative ground, this indicates the side of the battery connected to the chassis, which essentially makes the chassis into an enormous single point ground. By doing this only the polarity not grounded will have to be routed to the point where it is needed via wire. The grounded polarity will already be available on nearly any piece of metal in the car, so in most cases you need add only a single wire to bring power to a new accessory.

I hope that is clear enough to be understandable. The way it applies to your question is that your starter does not care how the + and - polarities reach it, as long as they're connected with the polarities in the right place the starter will operate.

Jerry Gardner

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