Electromagnetic effects , star / delta motor wiring ?

[JD6400]

I occasionally come across reasonably sized ( not for some of you , I know ! ) star delta motors that have been run in 2 x 3core swa .
Now after looking in to this in the past , it has been suggested that as they tend to be only on motors up to 30kw or so , and as the loading per phase will be near on equal the effects are not worth even considering !?

That said , it is just wrong to do this , is it not ?

What do others think .

 

[D Skelton]

Not really, think about 2 x 3P 3 core SWAs leaving a ferrous DB. The phases are perfectly balanced so no room for eddy currents to appear. Same for YD starting IMHO. The only thing I would say is that I'd like to see them run together.

Someone is bound to find a reg that proves me wrong though

 

[Silly Sausage]

Someone is bound to find a reg that proves me wrong though

<feverously leafing through the BGB with an air of futility>

 

[netblindpaul]

Does BS7671 even apply?

 

[D Skelton]

Why wouldn't it?

 

[JD6400]

I sort of see what you mean , but this is u1,v1,w1 on one leg with u2,v2,w2 on the other , out of the starter .
And although it is not a lot , I normally see the windings differ by an amp or so .
I have been far from top form of late , and just can not seem to get my head around it at the moment .

 

[netblindpaul]

If it's a machine then BS7671 is irrelevant.

 

[JD6400]

If it's a machine then BS7671 is irrelevant.

Normally on older grain dryer fans or large lagoon macerator pumps .

 

[D Skelton]

I sort of see what you mean , but this is u1,v1,w1 on one leg with u2,v2,w2 on the other .
And although it is not a lot , I normally see the windings differ by an amp or so .
I have been far from top form of late , and just can not seem to get my head around it at the moment .

I guessed that mate, my point was that if you can run two 3P motors out of one DB with ferrous material inbetween then the same would apply to running the same 3P motor out of one enclosure provided the phases are equally balanced in each cable ie. you wouldn't want u1, u2 and v1 in one cable and v2, w1 and w2 in the other.

I can't honestly see any problem at all with this set up

 

[netblindpaul]

If the grain dryer fan is part of an assembled machine then BS7671 does not apply.

 

[D Skelton]

If it's a machine then BS7671 is irrelevant.

True, but not if it's the supply to the machine. I just made an assumption however.

The same principles would apply though lol.

 

[netblindpaul]

Well, I don't quite see how the supply to an assembled machine can have cables that are feeding a star delta fed motor.

I'm just trying to make the point that the whole electrical world does not revolve around BS7671.
In fact, it is one of the standards most weakly backed up by statute law.

In fact, really as has been said, there is little issue, from an eddy current standpoint.
Personally I would use a 7 core cable, and never SWA, as it is not suitable for connection to vibrating equipment.
I would be more concerned about the earthing integrity if it was SWA connected to a vibrating machine.

 

[JD6400]

I guessed that mate, my point was that if you can run two 3P motors out of one DB with ferrous material inbetween then the same would apply to running the same 3P motor out of one enclosure provided the phases are equally balanced in each cable ie. you wouldn't want u1, u2 and v1 in one cable and v2, w1 and w2 in the other.

I can't honestly see any problem at all with this set up

I do understand what you are saying , but I don't know why I just can not except it !?
It just feels wrong !
And of course I know that has no standing when it comes down to the crunch , but surely I am not the only one that sometimes thinks the same about some scenarios ?

 

[D Skelton]

Well, I don't quite see how the supply to an assembled machine can have cables that are feeding a star delta fed motor.

Define 'machine' lol.

I'm just trying to make the point that the whole electrical world does not revolve around BS7671.
In fact, it is one of the standards most weakly backed up by statute law.

You are quite right, you make a very good point.

In fact, really as has been said, there is little issue, from an eddy current standpoint.
Personally I would use a 7 core cable, and never SWA, as it is not suitable for connection to vibrating equipment.

I have always favoured flexible conduit for final connection, but I guess it always depends on the application of the motor and any requirements the customer may have. I personally can't see the vibration of any decent motor being bad enough to work loose a decently glanded connection, if the vibration is that bad I would say there is a mechanical issue that needs addressing!

- - - Updated - - -

I do understand what you are saying , but I don't know why I just can not except it !?
It just feels wrong !
And of course I know that has no standing when it comes down to the crunch , but surely I am not the only one that sometimes thinks the same about some scenarios ?

It feels wrong because you would always install using a 7 core cable! lol

 

[JD6400]

More to the point , why do it this way in the first place ?
I mean it is twice the work over just one 7 core !?