DIY bench supply failed when testing starter motors at no load?

MaxHeadRoom

Joined Jul 18, 2013
23,927
There is only one way to accurately measure the stall current of a motor, particularly if it is a brushed motor, that is to apply a known small voltage ahd measure the current with a locked rotor.
If it is a brushed motor, it should be measured in several places and the lowest reading measured.
The resistance is then extrapolated from this measurement.
Max.
 

AlbertHall

Joined Jun 4, 2014
11,608
Lucas did it by putting a thing like a spanner on the shaft with a large spring balance connected to the end. Switch it on and read the current and the balance.
 

Thread Starter

Rahulk70

Joined Dec 16, 2016
521
If you read 0.7 ohms resistance for a starter motor that probably includes the connection resistance for the ohms meter, which is usually about 0.5 ohms for a typical meter. To verify that, put both meter connections on the same motor terminal and read the resistance. Probably it will be the same, or very close.
AND the stall current is actually only at the first instant of power applied, it drops as soon as the armature starts turning. So really you need a fairly fast analog scopeand a current shunt to see it.
I did recheck the resistance reading of the starter and it's correct. I checked my multimeter against some 25W load resistors of different values and the meter shows pretty accurate results. I did some extensive googling and I came across this starter motor data sheet by Lucas India. Though its very close to my motor I think mine draws a bit more current than this unit. But now it makes sense, as you can see the NO LOAD current is pretty high and no wonder my 35Amp rectifier gave its magic smoke! The 50Amp survived though. So, having two high current transformers via two 50A or 100A rectifiers will solve my issue. I have another similar transformer and should get the job done.
 

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Thread Starter

Rahulk70

Joined Dec 16, 2016
521
Higher voltage = smaller wire gauge. That's part of the reason they changed from 6V to 12V all of those years ago.
Yeah, I have read about the reason for the move from 6V to 12V in cars due to the increase on electrical equipments in modern cars. 24V for trucks and buses make sense because of the massive load on the starter from the big engines. But even then the wires are as thick as the ones in a big car because the current is still pretty large of these big vehicles. I think I read in a forum that here they still use 12V in some heavy vehicles when compared to Europe and Asia there they use 24V for the starter. Again I don't really know if this is true anymore today.
 

MisterBill2

Joined Jan 23, 2018
9,834
Yeah, I have read about the reason for the move from 6V to 12V in cars due to the increase on electrical equipments in modern cars. 24V for trucks and buses make sense because of the massive load on the starter from the big engines. But even then the wires are as thick as the ones in a big car because the current is still pretty large of these big vehicles. I think I read in a forum that here they still use 12V in some heavy vehicles when compared to Europe and Asia there they use 24V for the starter. Again I don't really know if this is true anymore today.
R is correct. With the higher voltage comes higher power, not lower current. The world is not running constant power, but rather constantly increasing power requirements. Since losses sre due to current and resistance that means that the voltage gets increased to provide more power at the same current. Just look at the new power lines running along I-94 in Wisconsin. The insulators are 3 feet long. No idea what the voltage is, or will be. I am guessing over 100KV.
 

MisterBill2

Joined Jan 23, 2018
9,834

BobaMosfet

Joined Jul 1, 2009
1,898
Hi everyone,

Does anyone have any info on how much current does a car starter motor draw at no load? The motor specs:

Power max: 1.4kW
Voltage: 12VDC
Armature resistance: 0.7 ohm
Brand: Bosch
Model No.: 1740374

The starter was removed from the engine when tested.

A little history into why I'm trying to find this information.

I'm trying to make a DIY bench power supply to test car and bike motors after rebuilding it. Usually the smaller motors are pretty easy to test as long as their current draw is under 17 Amps.So, a modified ATX supply and Lead acid battery does the job. But testing starter motors is not that easy due to their larger current requirements as the ATX supply can never handle them even at no load. Until recently I used to use a spare car battery to test the starter motor outside the engine after rebuilding or replacing brushes (without no loads). But since the climate I'm living is quite hot during summer (like 113F) the battery goes out pretty quickly in a year and half. So, this power supply would be a great replacement for the battery.

So, for this project I used a transformer from an old center tapped 700VA Tripplite UPS. The output wire of the transformer is a 10AWG which means it should be under 35 Amps I guess. So, I connected the transformer via a KBPC3510 bridge rectifier. It can handle 35A at 1000V & a Imax peak of about 400A for a very short burst as per spec sheet . So connecting the transformer leads to the rectifier and taking the output across the rectifier + and the center tap through a 15000uF 25V cap I was able to obtain 13.21 VDC. So, I tried connecting the starter motor mentioned above without the solenoid and no load to the setup and after 3 seconds the bridge rectifier gave its magic smoke and went out. The transformer wire was also little warm. I have another transformer from a 2000VA UPS and its massive. I think it can handle 55Amps I guess since the wires are 6AWG, but again currently I have only a KBPC5010 rectifier and it can take only 50 Amps almost. Currently I can only test motorbike starters almost without any worries.

Thanks in advance,
R
You need to understand that a starter motor at noload is initially a dead short. A car battery typically dumps up to around 600 or more CCA (Cold Cranking Amps) across that short until the motor starts to turn.
 

shortbus

Joined Sep 30, 2009
8,966
You need to understand that a starter motor at noload is initially a dead short. A car battery typically dumps up to around 600 or more CCA (Cold Cranking Amps) across that short until the motor starts to turn.
But not if the starter is sitting on a bench. It only draws high amperage like that after all of the "slack" in the starter drive is taken up and the flywheel tries to move.
 

be80be

Joined Jul 5, 2008
2,057
There some problems here first off most car starter " Car Starters" start starter solenoids are only 75 amps
Next you can jump start all the cars you want with a welder just one thing hope that the car wasn't made newer then maybe 1970 and has no computer
in it or for that mater any thing that has a computer in it.

When i was kid we started D8 using whatever we could find to jump it with.
But I've seen newer one's kill the computer's they now have in them.
There People who would say that it can't happen but I've seen it happen welding On car with DC welder and Caterpillar D8


Next a Stick Welder will only have about 125 amps output DC at 25 volts that's not going to be nice to a 12 volt Bosch starter.
The TS would be way better to get his hands on a 200 amp charger booster there in the 14 to 16 volts output and can put out the full
200 amps
 

Thread Starter

Rahulk70

Joined Dec 16, 2016
521
You need to understand that a starter motor at noload is initially a dead short. A car battery typically dumps up to around 600 or more CCA (Cold Cranking Amps) across that short until the motor starts to turn.
Yes, I'm aware about the sudden jump in current at the initial settings. But at no load I don't think it would jump that high especially if the solenoid is not engaged then the motor does not have to drive the pinion gear either. Check the reply I have posted for Misterbill2, I've added an attachment from Lucas and it says at no load (motor outside engine) it is about 65A. Also going with a rectifier that is rater 250A and above can handle a sudden current surge for 10ms at above 1100Amps, though I wouldn't turn ON/OFF the starter multiple times in a row in a very short span.
 

Thread Starter

Rahulk70

Joined Dec 16, 2016
521
I don't know how much open circuit is, but I have ran many starters (no load) with just a simple junk battery charger. The starter always spins.
One quick question. Were these chargers those big ones like on wheels specifically having the start function or the small benchtop models with a boost function like 100-150Amp models? Or maybe even a regular charger without boost function but just the regular fast charge 20Amp model?
 

Thread Starter

Rahulk70

Joined Dec 16, 2016
521
There some problems here first off most car starter " Car Starters" start starter solenoids are only 75 amps
Next you can jump start all the cars you want with a welder just one thing hope that the car wasn't made newer then maybe 1970 and has no computer
in it or for that mater any thing that has a computer in it.

When i was kid we started D8 using whatever we could find to jump it with.
But I've seen newer one's kill the computer's they now have in them.
There People who would say that it can't happen but I've seen it happen welding On car with DC welder and Caterpillar D8


Next a Stick Welder will only have about 125 amps output DC at 25 volts that's not going to be nice to a 12 volt Bosch starter.
The TS would be way better to get his hands on a 200 amp charger booster there in the 14 to 16 volts output and can put out the full
200 amps
Exactly, the last thing I want to do is rebuilt a motor and then put double the voltage across it and cause some damage to it. Well even if it survives its life would take a certain amount of hit. Moreover, I wouldn't want to take a chance with someone else
stuff.
 

Tonyr1084

Joined Sep 24, 2015
6,459
I don't know how much open circuit is, but I have ran many starters (no load) with just a simple junk battery charger. The starter always spins.
So have I. However, the starter motor spins up to speed much slower than when hit with the power applied by a fully charged 12 volt car battery. On the bench, powering a starter motor with a charger results in a little roll, but when hit with a battery the starter will jump off the table if not secured.
 

MaxHeadRoom

Joined Jul 18, 2013
23,927
So have I. However, the starter motor spins up to speed much slower than when hit with the power applied by a fully charged 12 volt car battery. On the bench, powering a starter motor with a charger results in a little roll, but when hit with a battery the starter will jump off the table if not secured.
Keep in mind the older ones that were series motors, operated in a runaway condition when off loaded on the bench.
They should never be powered full voltage without a load.
The only thing that limits RPM is the inertia.posed on the armature.
Max.
 
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