Motor starter circuit

qitara

Joined Jan 18, 2013
112
Hi

This is a simple automotive motor starter circuit, the problem is that i am not able to comprehend how the solenoid coil will still have a voltage after the solenoid contact has closed ?. every motor starter i have checked works the same way, the negative terminal of the solenoid is connected to the positive terminal on the motor. The current travels from the positive terminal on the solenoid out trough the negative terminal and in trough the positive terminal on the motor.

Sorry if i haven't made it clear enough

bwilliams60

Joined Nov 18, 2012
1,430
You are missing part of the puzzle. A starter solenoid will have two coils in it. The first is the pull-in coil which is energized when you turn the key to the start position. The magnetic field will pull in a plunger and when it reaches a second point, the hold-in coil will become energized and the pull-in coil will be released. This hold-in coil will hold the plunger in place until you release the key. If you want a more detailed explanation, I can give you that as well. Not sure how much information you require.

faley

Joined Aug 30, 2014
88
I suspect you may be misinterpreting that which you can see. I've attached a diagram of an '05 Chevy starter circuit for your perusal.

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bwilliams60

Joined Nov 18, 2012
1,430
Here is the long version. On the outside of a solenoid, you will find three or four terminals depending on ground orientation. The small terminal which comes from your ignition switch, we will call the "S" terminal. The opposite side of the solenoid may have a second small terminal and is used to ground the solenoid. You are working on automotive so we will deal with three terminals which will exclude this ground terminal. The two large terminals are B+(battery) and the M(motor) terminal. Current flow is as follows. When the ignition key is turned to the start position, current flows to the "S" terminal. from there it travels through the pull-in and hold-in coils. The hold-in coil is case grounded 9in this scenario) and allows the hold-in winding to energize. The pull-in winding current passes through its windings (made of a much larger wire gauge) and finds it's way to the "M" terminal where it passes through the field coil, brushes, armature and then to ground. This coil will pull-in the plunger attached to your starter drive. When the plunger (which contains a contact disc) connects the two larger terminals together (B & M), the potential difference between the "S" terminal and the "M terminal is 0 volts. The pull-in coil now drops out and the hold in coil (which draws much less amperage) takes over and keeps B&M connected until you release the key switch.