Making a Bench Power Supply to Test Car & Bike Starter Motors at No Load

bridge-rectifiermotorpower supplytransformerups

Does anyone have any information on how much current does a car starter motor draw at no load? It's from a BMW E46 .The motor specs:

Power max: 1.4kW
Voltage: 12VDC
Armature resistance: 0.61 ohm
Brand: Bosch
Model No.: 1740374

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 outside 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. 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 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.

UPS Transformer
Starter Motor

Best Answer

Car Starter motors on large cars do draw a lot of current even at no load .Old pre internet figure is 70 Amps at 12 VDC revving at 7800 rpm .This is from Joseph LUCAS Prince of darkness .The more modern permanant magnet motors will draw less and rev less under no load .Delco units like the lucas units are series wound and behave pretty much the same .Your bridge rectifier is not man enough to do the job .Your transformer will be OK for intermittant use under no load .We did this for a saw bench with rectifier diode aggregate rated at 400 amps .Remember that the start current of the starter motor is many times the run current at no load .I used lots of 100 Volt TO247 shottkies to rectify the DC .You could parallel your bridges to stop them blowing up at start up .You could also use big stud mount diodes .These series wound motors are not happy at 7800 rpm long term,There are stories about solder in the commutator softening in the heat and the copper bars flying apart that I have not seen .If your transformer setup makes less than 12VDC when running the starter motor then it will not turn as fast making your set up more idiot proof .I would check your Armature resistance figure .It seems far too high ,