battery voltage drop when short in starter vs worn brushes

Thread Starter

xchcui

Joined May 12, 2014
208
Hi.

When there is a short in the car-starter, the resistance of the winding decreases so the starter consumes more current and the starter cranks slowly.In that case the voltage drop will be very high
on the battery.
When the brushes are worn or there are bad connections along the starter line, the resistance along the line will increase and the starter will also crank slowly.But in the latter case the current that the starter will draw is decreased(due to the higher resistance) and the voltage drop that i see on the battery will be a small voltage drop,due to the small current(relative)that the starter will consume.
Am i correct?
Thanks.
 

MaxHeadRoom

Joined Jul 18, 2013
23,906
The older starter motors are very high torque, are series field motors and consume a large current at power on, Being series field, they can run with uncontrolled RPM with no load.
The series resistance is very low..
Later versions were PM field with slightly different characteristics.
 

GetDeviceInfo

Joined Jun 7, 2009
1,920
Hi.

When there is a short in the car-starter, the resistance of the winding decreases so the starter consumes more current and the starter cranks slowly.In that case the voltage drop will be very high
on the battery.
When the brushes are worn or there are bad connections along the starter line, the resistance along the line will increase and the starter will also crank slowly.But in the latter case the current that the starter will draw is decreased(due to the higher resistance) and the voltage drop that i see on the battery will be a small voltage drop,due to the small current(relative)that the starter will consume.
Am i correct?
Thanks.
Yes
 

MaxHeadRoom

Joined Jul 18, 2013
23,906
Hi.

When there is a short in the car-starter, the resistance of the winding decreases so the starter consumes more current and the starter cranks slowly.In that case the voltage drop will be very high
on the battery.
When the brushes are worn or there are bad connections along the starter line, the resistance along the line will increase and the starter will also crank slowly.But in the latter case the current that the starter will draw is decreased(due to the higher resistance) and the voltage drop that i see on the battery will be a small voltage drop,due to the small current(relative)that the starter will consume.
Am i correct?
Thanks.
If you experience some kind of abnormal series resitance in a series starter motor, the motor usually will not operate.
 
Last edited:

Thread Starter

xchcui

Joined May 12, 2014
208
Thanks for your replies.
I was asking that,since i know that this kind of dc motors tends to increase
the current when the load is increase(due to changing in the electro magnetic forces that created in the starter when the motor speed is decreased),but
i understand that in the case of high resistance in the line connection
or in the worn brushes,the slow crank of the starter(even that it looks like it works harder)will not cause to draw more current and won't cause sag in the battery voltage.Am i still right?
 

MaxHeadRoom

Joined Jul 18, 2013
23,906
The torque of a series motor is very high due to the very high initial/load current also flows through the field, as opposed to the later PM field version where the field is fixed.
So it depends on what version you are referring to.
 

Thread Starter

xchcui

Joined May 12, 2014
208
The torque of a series motor is very high due to the very high initial/load current also flows through the field, as opposed to the later PM field version where the field is fixed.
So it depends on what version you are referring to.
Yes,the torque of a series motor is very high indeed and i was
referring to the series starters version(not the permanent magnet ones.)
So,according to that starter version,does my description on post #5 correct?
 

MaxHeadRoom

Joined Jul 18, 2013
23,906
There are so many variables it is hard to say definitively, but any resistance occurring in the power to the starter is going to affect it in some way.
 

Thread Starter

xchcui

Joined May 12, 2014
208
Thanks,GetDeviceInfo,for your confirmation,though i was expected
to get more informative answer than only YES.
Thank,Max,for your response,though you didn't relate to my question at
all,while i asked about the different between short in the winding vs worn brushes as refer to the battery voltage drop.
Anyway thanks for your replies.
 

shortbus

Joined Sep 30, 2009
8,952
while i asked about the different between short in the winding vs worn brushes as refer to the battery voltage drop.

Both the short and worn brushes will have an effect of the motor it's self. But voltage drop is more concerned with the health of the battery and the cables leading to the motor. And you seem to be leaving out the starter solenoid, which is used to connect both the motor and the other parts coming from the battery. A bad solenoid(and they do/can go bad) will make a starter motor act like it is suffering from a "voltage drop".

You haven't said why you're thinking you even have a voltage drop?
 

Thread Starter

xchcui

Joined May 12, 2014
208
Hi,shortbus.
My main goal in my question was,in shortly,to verify if the voltage on the battery will sag low when the brushes are worn(creates high resistance)the same as when the starter winding is
in short or it won't sag the battery much.That's all(followed with a little explanation).
Most of the time Max,GetDeviceInfo and other good members give a really
informative and relevant answers,sometimes they are not.
But this is okay,i always appreciate any help and in this forum i got
alot of help on my other questions in my past.
Anyway,I assume that i will probably try to solve this issue in the field.
So thanks again,:)
 
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