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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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27 Jul 2019 15:41 #32155 by PDM
Replied by PDM on topic Help me understand current better
Case in point:
I went to start the ‘78 truck this afternoon, it clicks once and the lights go dim. Doesn’t kill power completely and doesn’t click click click. Voltage on the battery drops from 12.5 to 10 while trying to crank. I check the battery cables, and they are warm. Pulled the starter and it’s locked up tight. Can’t turn it by hand and it heats up a set of light jumper cables quick. It would be easy to throw a battery at this but that wouldn’t have done a bit of good.
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27 Jul 2019 16:29 #32156 by Donut
Replied by Donut on topic Help me understand current better
From the way I've come to understand it, in a normally operating motor circuit the amount of current will always be dictated by the demand of the load. So take a starter and you bench test it, no load on the motor save for spinning the rotor, and there won't be as much amperage as if it were installed and cranking an engine. The higher mechanical resistance that starter needs to overcome to rotate that engine will be seen as a higher current draw from the battery. Inversely, the higher the electrical resistance will have the opposite effect, showing up as a lower current draw.

The same issue (dimming lights, starter click) can be from both high mechanical and high electrical resistance. A low battery will dim the lights a cause a starter click just the same as a seized engine will. Best way to tell is with an amp clamp around the battery and seeing what it's doing.

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28 Jul 2019 01:39 - 28 Jul 2019 01:46 #32175 by Andy.MacFadyen
Replied by Andy.MacFadyen on topic Help me understand current better
Paul did an interesting video on a BMW Mini with slow cranking issues I will look it out and post a link.

With a no-crank or slow crank If you put a voltmeter right across the actual posts of a battery the way falls can help point you in the right direction. An old style analogue volt meter or graphing voltmeter is best for this ---- If when you operate the starter and see a very rapid fall from battery voltage to 6 volts then the fault is almost certainly a dead cell in the battery. If the fault is mechanical ie the engine is tight hydro-locked or defective starter but the battery is heathy the battery voltage will generally drop to 10v and from that point get pulled down gradually.


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Last edit: 28 Jul 2019 01:46 by Andy.MacFadyen.
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