AC transformer to power an engine electric starter - Page 1
Forum section: electronics / beginners

author: speedster
author_rank: Newbie
timestamp: February 05, 2014, 11:59:46 pm
content: I have a medium sized AC transformer sitting down and doing nothing. It is a mains step-down transformer, with a center-tap at the secondary side giving two options at 30/15VAC. Each side measures around 11 centimeters, and the thing weights 4~4,5 kilos. The only spec I can verify for sure, is that it used to work inside a 1000Watt UPS, along with two 12V batteries. (By the way, why 2 of them?)So, I had the idea (maybe stupid), to rectify to DC both 15VAC outputs and make an emergency booster for my bike's electric starter, connected in parallel with the small 12Ah Varta AGM battery, which by the way always struggled to crank the engine.The bad thing is that the engine is a twin cylinder with a lot of compression.The good thing is that the mains in my place is 230V.The engine is fuel injected, and fires up really fast. This is good as well.I can't remember the battery's CCA's (Cold Cranking Amps). Magnitude of 100A perhaps? I never managed to buy a set of proper inductive clamps for my multimeter.So, do you guys think this is feasible at all?Note that the battery will always be in place, meaning that the transformer will never take 100% of the load.And if it is feasible, what is the safest and most efficient way to rectify such a big amount of current? Monster diodes? Thyristors? Do I have to worry for the pulsed output? Much smaller brushed dc motors seemed to work with no problem.Thanks!

author: IanB
author_rank: Super Contributor
timestamp: February 06, 2014, 03:42:30 am
content: Quote from: speedster on February 05, 2014, 11:59:46 pmOne thing to mention right away is that if your transformer is center tapped 15 - 0 - 15 V, then you cannot put the two 15 V halves of the secondary in parallel (unless you separate the center tap). As it stands you have a 30 V transformer (or two 15 V transformers in series).Secondly, the best use for a booster is to charge the battery. The battery provides more amps than a booster can (unless it is a really big, really hefty booster).If the battery struggles to turn the starter, have you considered a new battery? By design, the battery can crank the engine (unless it is a crap design).

author: SeanB
author_rank: Super Contributor
timestamp: February 06, 2014, 04:58:15 am
content: Poor cranking is either poor battery, poor battery connections, poor connectors to the starter or a dying starter.But with this transformer it is easy to go get a pair of 50A diodes and connect the transformer in a half bridge and use it as a booster connection. Diodes on a heatsink and only connect for max 1 minute as the transformer will saturate anyway as it is not meant to be powered permanently.

author: speedster
author_rank: Newbie
timestamp: February 06, 2014, 12:12:52 pm
content: QuoteI don't quite understand what you're saying. What I have in mind is illustrated in the attachment.QuoteQuoteIt is indeed a poor design. Small battery, low output alternator, and heavy electric consumption, all contribute to a poor starting efficiency.But this is not the point. I can always plug in the charger, once a week and problem's gone. I just want to be able to have an emergency solution whenever again I come across a flat battery.

author: IanB
author_rank: Super Contributor
timestamp: February 06, 2014, 04:09:14 pm
content: Quote from: speedster on February 06, 2014, 12:12:52 pmThat configuration is OK. It's not strictly connecting the two halves of the winding in parallel since it is only drawing current from one half at a time. It's less efficient, but it works.

author: Fraser
author_rank: Super Contributor
timestamp: February 06, 2014, 04:26:25 pm
content: I saw electronic ignition mentioned....please be careful using a crude unregulated power supply on such a system....you can fry it.