Quote:
Originally Posted by Emmalina
I have recently been getting annoyed with a as the start button/display on the Yanmar 3gm30 that are mounted in the charter boats are continually failing so I decided to look into it and I find the relay mounted on this assembly has contacts rated at 3A DC 5A 250vAC now (not having measured it ) the current required by the starter solenoid is well in excess of this. So is this bad design or a scam |
The datasheet for the PE series of relays is available here:
https://www.farnell.com/datasheets/1943544.pdf
While the versions listed in the data sheet have 5A contacts rather than 3A, I would characterize this as a low-specification part with an endurance of only 30,000 operations at 5A. Though nominally sufficient at a 5A load there are better
parts available. Newark has them on clearance for under $2 each which is probably only slightly below the quantity pricing when these were a current production part.
It is important to realize that circuit assemblies in volume production go through a "value engineering" process where alternatives to high-cost
parts are evaluated. Nearly always, this involves substitution of low-cost parts that end up being operated somewhat outside the design envelope. In theory some testing and some engineering is done to determine whether the lower cost part will perform reliably in the particular use case in question. In the case of relay contacts this would involve evaluating the current, the on time, the typical number of operations over the product life, the ambient temperature where the part will operate, and so on. A thorough analysis would set specific
reliability goals (in terms of MTBF for the assembly as a whole) and determine the least costly mix of parts that would still allow the assembly to meet the MTBF. In most cases this would involve destructive testing -- someone would set up a test
environment where the relays would be operated under load, on and off, constantly, until they failed weeks or months later.
Performed well, value engineering produces high quality products at the lowest possible
price, by eliminating expensive parts that don't contribute in a meaningful way to the utility or useful life of the product. Performed sloppily, it results in expensive failures. I suppose there's a point on the continuum where a product has been "value engineered" to the extent that it could best be understood as a scam, but this isn't it. I wouldn't necessarily even hold it up as the dumbest thing a
marine engine manufacturer has done,
electrical design wise.
I would suspect that any decisions made were further clouded by the relatively small space available.
As you appear to have figured out, you can, of course, carve up the PC board and remove the relay, and replace it with something bigger and more reliable. There are some choices on the same footprint that would just drop in, like this 10a relay:
https://www.digikey.com/en/products/...184012/2397919, but they may sit higher.
You could also connect something larger with long enough leads that you can mount the relay somewhere else e.g. on a nearby gunwale or
deck liner. I personally am fond of these still-inexpensive ice cube relays:
https://cdn.automationdirect.com/sta...s/78relays.pdf
I always use the 3 pole ones regardless of my needs which allows me to connect poles in parallel for more current and simplifies my spare parts strategy. Each pole is rated at 15a and 100,000 operations at full load, 10,000,000 operations at no load; in between is in between. It will outlast your engine. There are optional sockets or you can solder straight to the lugs, I've done it both ways.
That will end up being cheaper than replacing the boards the first time, and of course it won't keep failing.
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