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Help me understand current better

  • Tutti57
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10 Jul 2019 21:56 #31699 by Tutti57
Help me understand current better was created by Tutti57
I've read training material that will say something like, perform a current draw test on a starter to determine if it is bad.

If the starter is bad, would the current be high? Does it depend on the type of failure?

I'm thinking, if a starter is freewheeling, there is low resistance, meaning high current, but with low mechanical resistance, wouldn't it take less power to run, so maybe low amps?

What if you stuck a stick in an electric fan, causing mechanical resistance?

Thanks!

I think my confusion stems from the term current draw and my thinking of mechanical resistance impact a circuit.

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11 Jul 2019 07:41 #31700 by PDM
Replied by PDM on topic Help me understand current better
At a given voltage, a higher physical / mechanical resistance will cause the motor to slow and draw MORE current. This is what makes a relative compression test work. What actually happens is the magnetic field increases with higher motor speed, and the magnetic field resists current flow. Current through a stalled motor is very high because there is no magnetic field and it is essentially a shorted circuit.

As electrical resistance in the motor increases (such as corrosion), the motor will slow and draw LESS current.

Current draw will also increase as voltage drops.

Now, with an amp clamp on a scope, you can actually see the current as it passes through the brushes to each commutator bar. This is a great bench test, but doesn't seem to work on the car because of so many other things going on mechanically and electrically during cranking

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11 Jul 2019 13:45 - 11 Jul 2019 13:47 #31703 by Andy.MacFadyen
Replied by Andy.MacFadyen on topic Help me understand current better
As above electric motors are funny things if the motor is just starting to turn or is over loaded the current goes through the roof. When I was a lad my town still had electric trams and when the driver started the tram going there was a huge variable resistor in series between the electrical supply and the electric motor to control the starting current.
A bad starter motor can either show too high a current or sometimes too low or show missing hump when displayed on the oscilloscope.
With starter motors that have done a big mileage you can sometimes see excess current due to internat carbon dust build up.

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Last edit: 11 Jul 2019 13:47 by Andy.MacFadyen.

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12 Jul 2019 19:40 #31740 by Tutti57
Replied by Tutti57 on topic Help me understand current better
Great replies guys. Makes sense.

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13 Jul 2019 04:50 - 13 Jul 2019 04:51 #31744 by Andy.MacFadyen
Replied by Andy.MacFadyen on topic Help me understand current better
One other thing that can cause high starter current and slow cranking particulary on manual transmissions is wear in the crankshaft thrust bearing.causing additional friction. This usually shows up as non or very slow cranking when the clutchis depressed. We used to se this a lot on British cars built before the mid 1970s

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Last edit: 13 Jul 2019 04:51 by Andy.MacFadyen.

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