How much current does car starter motor draw without any load?

[HVManiac]

Hello everyone,

Does anyone have any information 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 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 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

 

[telectrix]

rig a startermotor up to a good battery and clamp meter (D.C.clamp meter) the current.

 

[R-fur]

As Tel says, in addition fix the motor down to stop it jumping about. In theory you shouldn't run a series wound motor without a load as there is nothing to limit the speed but loads of people test starters by connecting them to a battery, just be careful.

 

[static zap]

As Tel says, in addition fix the motor down to stop it jumping about. In theory you shouldn't run a series wound motor without a load as there is nothing to limit the speed but loads of people test starters by connecting them to a battery, just be careful.

--Fix it to something --
Bearing the run away speed issue in mind.. I did one underwater .. -- Larking about as a teenager !
..It survived - came out cleaner ...

 

[HVManiac]

rig a startermotor up to a good battery and clamp meter (D.C.clamp meter) the current.

Unfortunately the clamp meter I have only works with AC. Looks like I'll need to get a DC one with high current rating. I'll have to wait till the lockdown is eased.
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As Tel says, in addition fix the motor down to stop it jumping about. In theory you shouldn't run a series wound motor without a load as there is nothing to limit the speed but loads of people test starters by connecting them to a battery, just be careful.

HI,

This isn't series wound motor. This is a Permanent Magnet model. I had opened it to clean it up, clean the commutator and replace the brushes. So, technically it should draw lesser current compared to series wound starters. Those ones needed 115A+ right from beginning to start even at no load I heard from folks.
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--Fix it to something --
Bearing the run away speed issue in mind.. I did one underwater .. -- Larking about as a teenager !
..It survived - came out cleaner ...

That kinda remained me of the underwater motor break in technique which I used to do as a kid for my brushed RC car motors. Unfortunately I cannot do that for this starter motor as I have to return it to the person after rebuild/repair .

 

[Wilko]

Hi - to estimate the starter motor current draw unloaded - I‘d try a car battery, two heavy leads and a D.C. voltmeter. Wire it up and connect the Voltmeter across one of the supply leads and measure the voltage drop when the motor is running. Then disconnect the battery and measure the resistance of the lead between those two points you were just measuring the voltage drop. Apply Ohm’s Law, I=V/R and hopefully you get some useful info .