3 phase motor Question -No neutral


Postby Sparkydelux » Thu May 13, 2010 9:23 pm

Hi,

I was instructed to wire up a 3ph industrial extractor fan , 3ph Spindal, 3ph planer.

Everything was going fine until I came to the final connections where I got a little stumped. (bearing in mind I haven't attempted any 3ph motors in nearly 3 years). I looked at the wires: green/yellow, brown, blue and black.

I was under the assumption Blue was (N), Green/Yellow (E), Brown (Ph), Black (Ph). I was missing a grey(?).

Called up the manufacturers and asked the questions regarding the cables and was told that the 3 colours (Brown, Black, blue) were the phases. No NEUTRAL?

Could some one give me a little 'masterclass' on what I am overlooking?

I thought there needed to be a return ie: Neutral.

I have tried to go over it in my head and the only conclusion I can come up with is that the motor consumes the power it needs there for eliminating the requirement for a return neutral.

Am I completely off the mark? As per usual all your knowledge passed on in always valuable.

Kindest regards,

Sparkydelux
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Postby ericmark » Thu May 13, 2010 11:46 pm

Only single phase motors will have a neutral connected. Even when star wound you don't connect the star point to anything.

You are connecting the 400 volt phase to phase across three windings.

With small motors for 400 volt they are normally star wound so if using an inverter you can move links to delta and run same motor on 230 volt from an inverter.

Larger motors are normally wired delta so that if the start current is too high you can wire it for star/delta start and so reduce start current.

Really big motors tend to be resistor or auto transformer start so does not really matter if star or delta for 400 volt.

But this does mean one has to be careful and read the plate to see how connected. Also of course for overload settings often set to half full load amps with star/delta start according to where the overload is placed.

However often you do need a neutral for the coils of the motor start contactor again care is required as 400v, 230v 110v and 24v are all common and DC and AC coils are not the same some are marked with one voltage DC and another voltage AC so easy to make a mistake.
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