Car engine starter

[WelshRob]

Hello- using this for the first time!
Currently building a diesel engine test bench, and have spec'd a 415Vac -> 14Vdc, 650A(max supply) with 2.5% tap change Transformer with add on rectifier circuit.
During installation, took a no load reading of dc voltage to find 19v dc (using several Digital Multi Meters). Unfortunatlely anything over 14 vdc will cook our starter motors.
Supplier has told me that they eventually tested with an analogue meter, and found 14vdc, due to coil load dropping voltage.
Also informed that with so much as a 12v light bulb connected accross dc output, 14 volts can be observed with a DMM- apparently due to a quirk with the rectifier circuit.
Any ideas? I've heard of voltage drop in transformers, but a lamp on a 650A supply? V puzzled.

 

[IRstuff]

That's certainly possible. Some regulator designs are not configured to regulate a no-load condition. The simplest solution may be to permanently load the output with a resistor equivalent to a light bulb, something like 20 ohms. You can certainly verify the supplier's claims with this.

TTFN

 

[itsmoked]

WelshRob Hi. Sounds like fun.

You will get a drop because as soon as you load that system voltage drops will appear everywhere. The rectifiers, the transformer, etc.

I would guess that if you stick that to a starter it will drop to 14v, for sure.

I would try it on a junk starter. I don't believe a few seconds at 19V would acually hurt a starter anyway. The voltage won't hurt the windings. And damaging a starter thermally takes a while as they live at high temperatures near exhaust sytems and are called on to crank for many seconds on hot days.

Two things to note.
1) A starter normally doesn't see 14V, ever. Since they are such high loads the terminal voltage is usually about 10V during a start.

2) Most diesel starters draw upwards of 1000A. Which is another reason to expect your system's voltage to drop. My diesel draws 1,540A when starting.

 

[WelshRob]

Hello,
thanks for replies, I'm thinking of dropping an analogue coil meter across the terminals, for indication and to drop the voltage (like IR stuff mentioned).
Just a little concerned that the dc voltage will be ok ie dc and not some half converted type, there's also the story of Ford, who had to recall a lot of cars due to suffering a similar problem before
Cheers for the peace of mind (itsmoked) about the starter motor not seeing 14v, fingers crossed.

 

[Warpspeed]

Don't worry about it. As soon as the contactor closes the voltage will drop. The starter will never see the full open circuit voltage anyway.

What can happen is open circuit voltage can rise to the peak crest waveform voltage if there is any slight capacitance present across the output of the rectifier. Under true open circuit conditions even the slightest amount of capacitance can do that. Connecting a digital multimeter with maybe 10 meg ohms input impedance could possibly read nearly the full peak voltage, or some proportion.

Even a few thousand ohms of load will pull it back to the average (not Rms) figure less the rectifier drop. It may look bad, but in practice it will work fine.