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What's the difference between a relay and a solenoid? Aren't they both electro-magnetic switches?
It occurs to me that the solenoids I can think of are for starters and therefore momentary, and relays for headlights or the like are toggle (but that's more a function of the switch they are attached to). Is this the difference?
Can I use a 150amp 12v Single pole normally open relay in place of the starter solenoid? Is it the same thing?
The solenoid would be pulled in with a coil that has voltage introduced on it. Why not just go with a solenoid like the stock application? In process controls we use both relays and solenoids and both are reliable.
I've always thought of solenoids as electro-mechanical devices. They use electrical signals to control the motion of something, typically in an on/off sense. Like the electronically controlled valves for faucets and such. I think they used to call starter relays solenoids because they were electrically controlled plunger type switches to handle the large current of a starter.
Relays are typically still electro-mechanical in nature, but are specifically for switching electricity. I think they are just referred to as starter relays now. I think the solenoid term for starters is just a carry-over from the old days.
A starter solenoid and a starter relay are the same thing. It's possible they are designed differently, but they serve the same purpose.
I think they used to call starter relays solenoids because they were electrically controlled plunger type switches to handle the large current of a starter.
...Relays are typically still electro-mechanical in nature, but are specifically for switching electricity. I think they are just referred to as starter relays now. I think the solenoid term for starters is just a carry-over from the old days...
A lot of them still are mechanical in nature. The One off my 2006 R6 jumps in your hand and snaps loudly just like the one out of my '62 Ford.
What Deviant is talking about is the kind that are mounted to the starter. I don't know if there are motorcycles that have these or not, I've never seen one.
Something like this:
https://www.waytekwire.com/item/75671/Picker-PC7150-1C-C-12C-N-X-150A-Mini-ISO-Relay-/
would be nicer to package in my little electrical box than this:
http://ecx.images-amazon.com/images/I/41qBDucF3yL._SY300_.jpg
A solenoid and a relay are basically the same. Both are relays and as such both make contact while low level power is applied to one side so that high power can flow on the switched side. In both cases, a magnet is energized and it causes a plunger to move which in turn connects the other set of contacts.
The overage relay can handle a fairly heavy load but a starter load is really high, so a starter relay is pretty heavy duty and you will hear it clink/click.
Now the fun part is that almost any starter relay for almost any vehicle will work on almost any other vehicle. OK, so that's a little broad brush but you can probably fit and R1 or 929 or GSXR starter relay to a CB350 or any of our old bikes. Just choose one that is compact and clean and fused and can be bolted where you want it to go.
Do not be tempted to use a small micro relay of the sort that works perfectly for headlamps - it cannot handle the current draw of a starter. And yes, there are relays that could handle that sort of power but why go there when starter relays are so cheap and available.
Speaking from an electrical perspective solenoids are just relays that can handle substantially more current. Your normal relays top out at around 50A (off the top of my head, I haven't validated this in any way shape or form) where I've seen solenoids up to 4000A (3 phase 600V solenoid actuated contactor really...).
Your starter isn't fused because it drawings something in the range of 150A on a bike. You can actually measure the resistance of your starter and guestimate the inrush current.
That's really high level but hopefully it helps some.
Actually, measuring the resistance of the starter won't work. Motors work on a difference principle than Ohm's Law. Can't remember the details, but I'll see if I can find something.
Generally speaking, relays and solenoid are more-or-less the same thing. Solenoids usually handle a lot more current, however. Relays usually handle anywhere from 10A to 50A. Solenoids can handle in excess of 600A in an automotive application.
The major point to the function of a solenoid is that it's exactly the same as a transistor in this application. It amplifies the current. A relay is more like a door and lets current through. I don't think they're interchangeable.
Actually, measuring the resistance of the starter won't work. Motors work on a difference principle than Ohm's Law. Can't remember the details, but I'll see if I can find something.
For the initial in rush the resistance and inductance of the motor that will set the current. As the motor starts to build a magnetic field (spins) it generates a back EMF. The back EMF can be thought of as effectively lowering the voltage drop across the motor. It is still Ohm's law but everything is a function of time and excessively complicated...
All that gobalty goop said, the solenoid needs to be able to take that in rush with out releasing any of its magic smoke and a normal relay isn't going to work.
The only thing I'll add to this is look at BMW and Guzzi starters. These are actual mechanical plunger solenoids. And they draw somewhere in the neighborhood of 250A. They're beefy, and no way would a standard relay cut it, even if it didn't need the mechanical plunger portion.
If all you need is a heavy duty relay there are many choices, including early Ford solenoids. They are compact and cheap. However, if you need both the mechanical movement to engage the drive pinion while simultaneously completing the high current connection to the starter then your options are limited to OEM style. This is old school automotive/truck style. I believe most motorcycle applications are using solenoid style relays to provide high currents to the starter, and a separate mechanism like a sprag clutch, or helix on the starter motor shaft for mechanical engagement.
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