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

Thread Starter

Rahulk70

Joined Dec 16, 2016
504
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
 

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Tonyr1084

Joined Sep 24, 2015
4,535
Don't know if I have a direct answer to your question; but I'm thinking why not replace the car battery with a super capacitor type battery. Won't be bothered by the heat and can be charged up for testing fairly quickly, say maybe just a few minutes on a charger. I've seen them on YouTube starting cars - so they can definitely handle a "No-Load" condition. And you can probably use the ATX to charge the caps. Of course, it'll be strictly 12 volts, but most car batteries on average are probably around 12.2 or 12.3 volts. 12.6 volts would be a new battery fully charged and resting. I have a PS that is 13.8 volts at 19 amps from Radio Shict. That'd probably charge the caps to the same voltage a car battery would see during normal engine running conditions.

Here's a seven minute YouTube video:
 

Wolframore

Joined Jan 21, 2019
1,748
1.4 kW @ 12V is 116.7 A. Some may even need more, I’m unsure whether this max power rating is the stall rating. I would say higher the better. 150-200A as a minimum for no load. It will not be inexpensive. I wonder if a car jump starter/charger might be more economic solution.
 

be80be

Joined Jul 5, 2008
2,042
Most tester use 200 amp chargers to test starters or as been said just use a car battery lot cheaper then a good 200 amp charger with boost setting.

 

AlbertHall

Joined Jun 4, 2014
9,873
I used to repair some large Sorensen power supplies, one of which was 20V @1000A. This is what Lucas used to test starter motors but that testing included stall current. I don't remember what the no load current was (that was also tested).

The initial motor current will be the stall current briefly.
 

bwilliams60

Joined Nov 18, 2012
1,315
I used to rebuild starters and alternators for a living and as @Wolframore said, you are looking in the area of 115-200 amps depending on the starter. We had a test bench that we picked up fairly cheap which had a 850CCA deep cycle battery in it and because we were doing alternators as well, it was always charged. If you don' t want to go that way, a battery booster pack would be the best way to go. It charges quickly and can do several cycles of starter tests before needing a recharge. We use them at school for starter rebuilding. Supercaps will not work as they discharge rapidly and need to be backed up by a battery unless you have several supercaps in a bank which can be fairly costly. Good to see people still rebuilding.
 

shortbus

Joined Sep 30, 2009
7,688
I used to rebuild starters and alternators for a living and as @Wolframore said, you are looking in the area of 115-200 amps depending on the starter
Hi, that was for wound field starteds though wasn't it? From pictures of the starter online, it looks like this is a PM starter, so the amp draw could be lower, The starter drive solenoid looks almost as big as the starter motor in the photos of it.

Removing that solenoid could also lower the power needed to test just the motor itself too, on the bench. The motor will work without the drive pulling in.
 

bwilliams60

Joined Nov 18, 2012
1,315
You are correct @shortbus. For some reason, all pictures dont come up on a tablet so I didnt see any pics associated witht he story. I will open this up on PC. If it is a PMGR, they will pull 60-100 amps on a bench. I prefer to test with the solenoid to get the whole picture but to each his own. Either way I think he is going to a battery or battery pack. Power supplies with that much punch arent cheap.
 
The initial motor current will be the stall current briefly.

Yep. At the time the motor starts, you just have winding resistance with no inductance. Back to basics v(t) = Ldi/dt; di/dt is through the roof. The inductor opposes the change in current.

Deep cycle batteries have a higher internal resistance than automotive batteries. Once the automobile battery looses it
's charge (deep cycles), it starts dying. A deep cycle battery has less "cold cranking amps" or more internal resistance.
 

MaxHeadRoom

Joined Jul 18, 2013
20,700
Although these are DC and like just about all DC motors, the only way the current is limited is when the generated EMF starts to equal the applied voltage.
The older ones are wound field - series motors and have extremely high starting torque due to the very low resistance of both armature and field, Being a series motor, you should not run on the bench as they can start to run away up to very high rpm without load.
Max..
 

Tonyr1084

Joined Sep 24, 2015
4,535
Actually I had in mind one of those 40 amp jump starters like the OLD one I have in my garage. Alone it can't start a car, but with a weak battery to assist - it usually gets the job done.

One thing I keep wondering about is a DC motor without any load; how can it draw such a high current? Well, maybe it does at first. But to be honest, I'm not the motor expert here, so maybe I don't know what I'm talking about at all.

I've bench tested a motor cycle starter, gear reduction type with a PM field. While it wanted to jump off the bench, it drew a spark when connecting it, but I ran a small gauge wire - probably 12 gauge stranded and the wire - I don't recall it getting hot. But then again, it was merely a momentary test.
 

MaxHeadRoom

Joined Jul 18, 2013
20,700
One thing I keep wondering about is a DC motor without any load; how can it draw such a high current? Well, maybe it does at first.
The current is virtually limited by the resistance of the armature (DC motor) . Or both armature and field in a series motor.
BTW, when the original series motor starter was introduced for use as an automobile starter, there were many that doubted it would last for any more than a reletively few starts!
Max.
 

Thread Starter

Rahulk70

Joined Dec 16, 2016
504
Actually I had in mind one of those 40 amp jump starters like the OLD one I have in my garage. Alone it can't start a car, but with a weak battery to assist - it usually gets the job done.

One thing I keep wondering about is a DC motor without any load; how can it draw such a high current? Well, maybe it does at first. But to be honest, I'm not the motor expert here, so maybe I don't know what I'm talking about at all.

I've bench tested a motor cycle starter, gear reduction type with a PM field. While it wanted to jump off the bench, it drew a spark when connecting it, but I ran a small gauge wire - probably 12 gauge stranded and the wire - I don't recall it getting hot. But then again, it was merely a momentary test.
I did see this video a couple of years back. It’s a fairly good setup. But in the clip he says the supercap pack cost him very little. I suspect its one of those fake ones. The genuine ones here in US cost like $18+ per pieces. Building a pack would be too quite expensive. A car jump start looks more economical but as I mentioned in the original post, in summers the extreme heat kills the battery in no time. Plus I have a daytime job so, rebuilding and repairing auto electrical is more like hobby + second job. So, if I don’t use the battery in a while I noticed it’s life gets affected pretty badly and its just a matter of time it goes bad. Again jump start packs as you mentioned are useful only fun conjunction with a car battery. Well it may work for bike starters as they draw under 30Amps mostly.

Well the back e.m.f also could be quite high initially I guess which causes the sudden spike in current.
 

Thread Starter

Rahulk70

Joined Dec 16, 2016
504
1.4 kW @ 12V is 116.7 A. Some may even need more, I’m unsure whether this max power rating is the stall rating. I would say higher the better. 150-200A as a minimum for no load. It will not be inexpensive. I wonder if a car jump starter/charger might be more economic solution.
Car jump starters need a car battery in conjunction to boost the current or the jump pack could get damaged. I do have an old massive bench top 1960s Lucas Heavy Duty car battery which has a 100Amp boost function. It was initially used at a industrial facility to charge their Detroit Diesel generator batteries that very very large. The person who gave it to me said that the boost function should never be used alone without a battery to start a car. I don't want to risk damaging the selenium rectifier in it as its very difficult to replace and it cost a lot too.
 
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