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Ok I know that on modern day cars that the solenoid also engages the starter motor pinion, but on my early 2000's honda recon 250 the solenoid is separate from the starter motor, so I assumed that on other earlier small engine vehicles it's the same way. Which brings me to my question on bikes and four wheelers, etc that have the solenoid and starter motor separate, would it be possible to switch the solenoid out for a transistor which in turn makes it lighter(I know it's probably only like 3-5 oz, but hey it's a cafe racer)? Thanks in advance for your advice guys.
yes, you could. the reason solenoids are used in that application is that the design is cheaper and more reliable. the back emf coming off a starter motor and especially that when it's switched off is pretty nasty, a really robust solid state design would be fine, but that costs a lot of money, most manufacturers would rather just go with 100 year old tech that's off the shelf
Yea, the solenoid is pretty much just a super heavy duty relay in function.
For example, a GM LS2 small block's starter motor pulls about 300 amps (so almost 4000 watts at 13 volts) when starting the car. Most transistors can't take that kind of current, and any one that could would probably be more expensive than the solenoid.
If you want to save weight, convert to kick only. Or get aluminum rims to replace the heavy steel ones, or get a lighter battery and so on.
You can, however, convert older charging system regulators and rectifiers to modern integrated regulator/rectifier systems that are more efficient and reliable due to modern semiconductor technology. Sparck Moto can hook you up.
So there you go. Odd that it didn't tweak in my brain to suggest going kick only.
That's what I did on my two XS650 motors. Starter motor alone is a pig and I also removed the bendix gears etc in the motor. Pounds lost.
I've got a PMA kit for one of them which will in theory allow me to run without a battery but with li-iron batteries that's hardly an issue these days. My little 7ah battery weighs ounces.
If you want a transistor, you're going to need to go big. Most can't handle that kind of load.
This might be worth some investigation:
http://www.sourcingmap.com/332vdc-380vac-200a-ssr-solid-state-relay-plug-wire-p-391177.html?utm_source=google&utm_medium=froogle&utm_campaign=usfroogle&gclid=COu2h9-iybwCFURsfgodRAkADg
But as mentioned earlier, digital electronics can be pretty picky about EMF, so even though this can handle the load, it might not stand up to the interference caused by electrical motors.
Thanks guys I was asking more to find out why they don't do it. Not as much to lose the weight and etc those were just other reasons for doing it. But thanks that's helpful
paralleling transistors requires a bit of balancing though, as each transistor heats up, the properties, including voltage drop across the junction, change, which can lead to a failure. as far as diodes go, there's also a switching time that needs to be considered, since exceeding the voltage rating even for a microsecond can completely destroy modern transistors.
I spent a little time spec'ing those particular parts, more time than I should have really lol...
The transistors are 12A, 50V, 35W rated. Running three in parallel should give you roughly 105W/36A of capacity. Given that you are dropping about 350mV (give or take) across the transistors, running at 30A (fuse rating on my CB750) you are only burning 18W of power in the transistors. Given that you aren't holding your starter on for 15mins the heating should be minimal. The balancing could be interesting, the transistors are 20% oversized which should mitigate this to a point, or if you were extra concerned you could add a 4th transistor. Other than that I can't think of an easy way to balance them.
The diodes are fast acting schottky diodes rated at 30A/45V, the reverse voltage rating is substantially higher than the system should see (e.g. if you regulator is pushing >45V you have bigger issues..). This is sized based on the rule of thumb that the diode should be the same rating as the motor (e.g. 30A motor = 30A diode).
The starter motor isn't fused. It pulls too much current to be fused. You're going to want to engineer about 120A. They don't use zero gauge wires for nuthin'.
That's awkward.. I'm going to have to update my wiring diagram lol..
That being said 120A seems very high, doing a little bit of fun math, the stock battery is about 10Ah, at 120A that is completely drained in 5mins.. I never timed it but I feel like I've cranked my bike over for more than 5mins at a time. Not trying to be argumentative just trying to make it work in my head.
P.S. Page 18-1 of the manual actually shows the non-fused nature of the starter motor quiet well, unfortunately I only looked at the wiring diagram which doesn't show much.
There is inrush current and sustained current. The inrush is high because the starter motor is stalled at startup and needs to develope enough torque to turn over the engine....Once turning, the motor draws less. The high inrush is needed or the motor won't start turning....
Locked Rotor Amps are as high as 10 times full load running amps.
A solenoid is the smallest, cheapest, most reliable way to switch high current. While a transistor replacement could be made, it would offer no improvement over operation, and could fail shorted more easily than a solenoid.
This mosfet may work better (140A, 50V)
http://www.digikey.ca/product-detail/en/IXTP140P05T/IXTP140P05T-ND/2117344
unfortunately this makes for a more complex circuit to make sure you don't have noise triggering your mosfet, well that an it inverts your start signal
mydlyfryzis, In the long and short of it I do agree the solenoid is the way to go. The transistor solution would only be worth while where you wanted absolute minimum weight, in which case you probably already got rid of the starter... The question does give me something to do while it's cold in my garage tho, so that's a benefit.
For the ultimate lightweight starter-motor-activator, look no further than whatever screwdriver happens to be lying around. In fact, any inanimate metal rod you find near the bike will suffice. Just complete the circuit with gloves on and make sure you have no fuel leaks...
In all seriousness, you ain't gonna gain much by just replacing the solenoid.
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