DOL 3 Phase

[Mikey]

A bit confused...

On a 3 Phase DOL motor starter I have seen a few diagrams online and in books etc and they show one side of the coil A1 connected to L1 and then the other end of the coil connected to L3 and obviously going through the control circuit, so the coil is wired phase to phase.... How does that work, and why is the coil not wired from a phase to neutral?

 

[gazj82]

It's a 400v coil instead of 230v

 

[Mikey]

Ah right, ok, so these dont need a neutral?

 

[gazj82]

Correct Contactors should have the coil voltage and type printed on them somewhere, anything from around 12v to 400v is possible in AC or DC so check before you connect.

A lot of contactors can have the coil changed in them as well.

 

[Mikey]

Thanks gaz, now im just a bit confused about the whole current flow business, how can current go from an L1 to an L3 is that not some kind of live to live short? This is obviously somthing I know little about but when searching online or in books about this sort of stuff I cant find any useful info.

 

[gazj82]

Well, rather than do all the work for you, you need to know the phases are not the same, each one is unique, maybe you can work it out from this



Edit: Current follows the same sine wave pattern as the voltage in the above

 

[Lucien Nunes]

3-phase theory can't all be explained in a forum post, study it from the beginning and take the effort to learn the associated maths and eventually all will be clear as day, as to why there's 400V L-L when the L-N voltage is 230.
As for why contactor coils are often 400V rather than 230, with delta-connected loads there's no need for the neutral so it would be a pain to run it just for the sake of the coils. So by convention the coils are 400V, as this will always be available in a 230/400 system whereas 230 might not be. Of course if the control is LV, it's the control transformer primary that's 400V but usually these often have a 230V tap.

 

[gazj82]

I knew I couldn't explain it in a forum post hence the diagram. I remember this being scribbled on a white board in class

 

[Marvo]

....how can current go from an L1 to an L3 is that not some kind of live to live short?

It would only be a 'short' if there was a connection between L1 and L3 without a load. The contactor coil is a 400V load so when it's connected across L1 and L3 only a small current flows through it because it has a resistance.

 

[itselectric]

Lucien, in cases where there are external controls and elv is used then there could be a 12v, 24v or 50v coil. I realise you know it, I am just jogging your memory

 

[Lucien Nunes]

I was working at a location recently where someone had fallen foul of the TP (without N) circuit, with 2A of 230V control load that had to be fed from the same circuit. At least he didn't use a gas pipe to send the current on its way to neutral, as I have seen (and isolated!) elsewhere. A 0-230-400 transformer would have been better than the pyro sheath earth, though.

 

[Lucien Nunes]

I am just jogging your memory

Curious as to why? The coil could be any voltage in which case my explanation about 400V being more universally available than 230V applies not to the coil but to the transformer, as I mentioned. Or perhaps I'm missing your point.

 

[itselectric]

I was referring to coil voltage. but as I said it would most likely be a case where external controls are needed.
I agree on most stand alone starters they are generally 400v or 230v .
I wasn't in any way trying to imply anything different.

 

[itselectric]

I was referring to your post 7# about coil voltages.

 

[itselectric]

cancel

 

[Lucien Nunes]

Ah, I see what's unclear, I should have written - 'if the control is ELV'. Good point.

 

[Mikey]

Thanks for the reply guys much appreciated!