So what's the odds?

[Chrispy]

As a few here may know I have been moving workshops, well for the last few weeks my electrician and myself have wired up 20 three phase machines some quite new but a lot old with just three red wires so it's a 50/50 chance that the machine turns the correct way first time, now so far there is no mains electricity supply hopefully that's being connected next week so what do you think? How many will need rewireing?

 

[marcros]

10!

 

[Chrispy]

Sorry 21 forgot the new roller door.

 

[JSW]

21!

 

[ED65]

Is this like the probability of buttered toast landing face-down? If so IME chances are not 50:50 :lol:

 

[JSW]

Is this like the probability of buttered toast landing face-down? If so IME chances are not 50:50 :lol:

Exactly, and as we all know, if the toast did land right-side-up, it's because you buttered the wrong side

 

[sunnybob]

You have three live, it only needs one to be wrong to make it go backwards, so the odds arent 50/50, theyre 33%

with odds of 1 in 3 being wrong, you would be very unlucky to score a maximum, so taking a third of a third and rounding, i go with 4.

I'm sending my banking details for you to pay the million quid into under a seperate email.

 

[bussy]

Hi Chris
Be careful when trying the roller doors as if that one is wired in reverse the whole of the shutter may wind itself off, suggest you have the door half open so you have time to hit the stop button.
Marty

 

[Chrispy]

Hi Chris
Be careful when trying the roller doors as if that one is wired in reverse the whole of the shutter may wind itself off, suggest you have the door half open so you have time to hit the stop button.
Marty

Good tip, thanks.

 

[Chrispy]

You have three live, it only needs one to be wrong to make it go backwards, so the odds arent 50/50, theyre 33%

with odds of 1 in 3 being wrong, you would be very unlucky to score a maximum, so taking a third of a third and rounding, i go with 4.

I'm sending my banking details for you to pay the million quid into under a seperate email.

Bob not sure how to get only one wire wrong maybe I've been doing it all wrong.
Ps can't find bank details.

 

[TopCat 32]

All of them , if your luck is like mine, its called sods law, good luck

 

[sunnybob]

To reverse the direction on a three phase motor, you only have to swap any two of the three lives.
So although you are moving two wires, its only one swap.

I'll get onto my nigerian bank to see what the hold up is with the bank details. those princes, cant do a days work to save their fortunes.

 

[Neil S]

The chances of picking the first wire correctly is 1 in 3.
The chances of picking the second wire correctly is 1 in 2
the chances of picking the third wire correctly is 1 in 1

The Chances of picking all wires correctly is 1/3 x 1/2 x 1/1 which is 1/6

So you'll be lucky if you get more than 1 in 6 right from 21 machines that's 3 to 4 machines. So if you have a lot of rewiring to do then don't feel bad as the odds were stacked against you.

If none of the installation is wired to the mains yet i.e. completely isolated you could wire in a simple battery to the neutral and one red wire then check all the other outlets with a bulb or multi meter and label all the common reds with a number say 1 then do the same for the other two red wires. That way once its connected to the mains and you have sorted out which is the correct phase order you can instantly rewire all of the remaining machines correctly.

You must not attempt this however if there is ANY chance of ANY of the existing wires being connected to any part of the mains including the neutral and earth.

-Neil

 

[Limey Lurker]

51 years ago, I and a workmate wired in 19 3-phase machines. They ALL went the correct way first time.

 

[MusicMan]

Neil

But you can rotate the three wires rigidly around into the 3 positions, so 1 in 6 goes to 3 in 6 or 1 in 2.

Alternatively: There isn't only one correct choice for the first wire, any choice will do. Once you have fixed one wire there are two ways of connecting the other two, one right and one wrong. Hence again a 1 in 2 chance of being right in a random selection.

So if you do it randomly a few dozen times, the average will be 10.5 right. Which doesn't tell you anything about the result of a single trial!

Keith

 

[Neil S]

Doh!
You're right. I hadn't allowed in my calculations that as long as the phases occur in the right order then the motor will run. My calculations were only valid if you needed to get the phases in the same fixed order Red, Yellow, Blue (say).

-Neil

 

[Mr T]

Ptobsbility theory did my head in at school, thankfully I haven't had to use it in the last 50 years!

Chrid

 

[MikeG.]

We're shooting in the dark. There are 21 machines, but some unspecified number have modern wiring. So, the best we can say is some percentage (is it half?) of the 3-red-wire ones will likely be wrong first time. Until we know how many of them there are, the million dollar prize is safe (ish).

 

[Geoff_S]

Just stand on the other side if it's wrong

 

[Fitzroy]

You lot distract me at times. Credit to google for most of the details, but considering the wiring as a coin toss 50:50 on correct or incorrect. Several different conclusions can be reached. This is based on the assumption all machines have 3 red wires, and you can test them and reverse the main feed wires if you have more than 1/2 the machines wired incorrect.
- If you had to pick a specific number you would pick 10 machines to rewire with a 34% chance of being correct.
- If you had to pick a number of at least machines to rewire you'd say 4, with a 99.9% chance of being correct. However that's effectively picking 7 out of 10 options which feels a bit cheeky!
- In reality the odds of having to rewire 8, 9 or 10 machines are not very different 5:1, 4:1 and 3:1 and I'd not wager anything on picking the correct horse amongst these.
- Limey's pal 51yrs ago had a 36 in 1million chance to get them all correct, hope he played the lottery that week also!

F.