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 !?

 

[netblindpaul]

Define 'machine' lol.

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!

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

Ok, the definition of "machinery" electrical equipment is easy, covered by BS EN 60204-1, and not BS7671.

If it is an assembly of linked components and is not part of the fixed electrical installation then it's machinery.

I do sometimes use flexible conduit.
However, what do, you, put "inside" the flexible conduit?
6491 type wire?
If so, again not suitable for connection to vibrating equipment.

Any rotating device vibrates.

Oh & I think it was me who said 7 core not the OP.

 

[D Skelton]

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 !?

This is the obvious issue that you can't get your head around. I'd like to think that any decent spark would use one cable or one method of containment over two cables, still, I can't see an issue regs wise or principle wise with the latter of those two options.

 

[netblindpaul]

Not even the unsuitability of the wiring materials chosen?

 

[JD6400]

Ok, the definition of "machinery" electrical equipment is easy, covered by BS EN 60204-1, and not BS7671.

If it is an assembly of linked components and is not part of the fixed electrical installation then it's machinery.

I do sometimes use flexible conduit.
However, what do, you, put "inside" the flexible conduit?
6491 type wire?
If so, again not suitable for connection to vibrating equipment.

Any rotating device vibrates.

Oh & I think it was me who said 7 core not the OP.

Our standard set up on a dryer would be to come from the control panel in 7c swa (with a 2c controls swa for the isolator indicator switch if applicable with the control panel spec) on tray up to a 6 pole isolator then out in to the dryer in armoured flexible conduit on swivel top hats with what I call controls cable ( fine AWG ) in that with all terminations crimped with which ever one is appropriate for the motor .

 

[netblindpaul]

Sounds nearer the mark than SWA TBH.

The twin x 3C swa still may well (probably) need a local isolator, so it is a tonne of extra work.

 

[D Skelton]

Ok, the definition of "machinery" electrical equipment is easy, covered by BS EN 60204-1, and not BS7671.

If it is an assembly of linked components and is not part of the fixed electrical installation then it's machinery.

There are bound to be grey areas here surely between machine and fixed wiring!

I do sometimes use flexible conduit.
However, what do, you, put "inside" the flexible conduit?
6491 type wire?

6231 obviously

Oh & I think it was me who said 7 core not the OP.

huh?

Edit: Just thought I'd better point out that I'm not condoning the practice of using two seperate SWA cables to feed a motor, I'm just saying that there isn't anything technically wrong with it IMHO.

 

[netblindpaul]

Well D Skelton I disagree, & I bet you I can find regs in BS7671 & other standards that say that it is unacceptable, however, I can't be bothered to waste my time in proving something so obvious as to use fixed wiring materials on a vibrating machine.

You have obviously never been involved with vibration analysis and studied rotating machines.

 

[D Skelton]

Well D Skelton I disagree, & I bet you I can find regs in BS7671 & other standards that say that it is unacceptable, however, I can't be bothered to waste my time in proving something so obvious as to use fixed wiring materials on a vibrating machine.

Fair enough, nothing crystal clear in BS 7671 as far as I'm aware, 522.7 goes on about cables/wiring being 'secure' when connected to vibrating equipment, nothing about exactly what types are suitable. You don't have to prove anything to me mate, I don't doubt your credentials which is why I find your slightly haughty tone a little confusing :s I'm here for a discussion, not an argument and I'm more than happy to learn from those who are more experienced than I am.

You have obviously never been involved with vibration analysis and studied rotating machines.

I know a little but no, I haven't been involved with vibration analysis.

I never said it was a good idea, I just couldn't see anything technically wrong with it. As I said previously, it all depends on the application. If a motor has been in service for 20 years and the SWA glands connected to it are as secure as when they were first fitted then what's the bother, same as if the final terminations are still secure? You'd expect regular maintenance on motors and machinery anyway so the risk of connections coming loose are slim and bad vibration issues would surely be picked up and addressed before they became too much of an issue.

Like I said, I wouldn't choose to wire a motor in this this way, ever, but would I condemn one if I found it wired using 2 x SWA cables and everything was still secure? No, probably not. However, if you could find me some regulation that says otherwise or describe to me with my limited knowledge of vibration analysis exactly why a secure SWA connection is still a problem then I will obviously have to take this into account and reform my opinion/knowledge on the subject.