What happens when a star wired balanced device (without a neutral) becomes unbalanced

[netblindpaul]

Re: What happens when a star wired balanced device (without a neutral) becomes unbala

Peter,
Are you perhaps referring to, for example a 3ph motor connected and wired in star rather than delta?

 

[Andy 7671]

Re: What happens when a star wired balanced device (without a neutral) becomes unbala

C@G 236 part 2.."Science and principles"

Failed it first time, rightly so.

Time passes, courses change (2330) and I have to retake by christmas.

Took three months off, slept in my van and went down the city library every day going over and over past papers till I could repeat the equations like the best African Grey, or was it Norwegian blue parrot in the land.

Nearly all my work is commercial,a little on process equipment in very large dairy and terminating the odd motor.

So all this hard won knowledge is atrophying.

Im working with a good maintenance spark and will ask him more on monday

From the tools I have in my head I would say you need the power factor to be as close to unity as possible, less power will be needed and electricity used will be minimised.

This is done either by capacitance or impedance depending on whether the phase angle is leading or lagging.

Now, from Flemming law you can gather rotation of the field, rotation a circle,360 degrees.

3 phases, balanced gives an interval of 120 degrees. 3x120=360 If it is not balanced the phase angle will need to be found to achieve calculation.

This is me trying to remember what I learnt, I am not a maintenance spark, I just thought, if we work through this, by next week we might get somewhere.

Cant believe there is no centre tapped earth, I will ask a few tomorrow, I expect to see puzzled faces.

Any pics...?

 

[Inteificio]

Re: What happens when a star wired balanced device (without a neutral) becomes unbala

Peter,
Are you perhaps referring to, for example a 3ph motor connected and wired in star rather than delta?

Yes I was, but assuming there must be other scenarios where this occurs.

 

[netblindpaul]

Re: What happens when a star wired balanced device (without a neutral) becomes unbala

I'll tell you what Pete, wire up a motor in star and connect the N to the star point, this will explain to you the situation, ensure you have a volt meter & a clamp on ammeter to complete your investigations and then please describe what you find as this I suspect will explain the differences.

 

[Inteificio]

Re: What happens when a star wired balanced device (without a neutral) becomes unbala

Hi Net,

Can you just give me a hint, trying to explain to my boss that I want to connect a neutral to the star point to see what happens would not go down well ;-)

 

[netblindpaul]

Re: What happens when a star wired balanced device (without a neutral) becomes unbala

Just try it and tell us what happens, BTW I thought you were the boss?

 

[Inteificio]

Re: What happens when a star wired balanced device (without a neutral) becomes unbala

me the boss, I wish. Actually his job sucks, but wouldn't say no to the money =-)

 

[netblindpaul]

Re: What happens when a star wired balanced device (without a neutral) becomes unbala

I thought that you had said you were the only electrical design engineer there?
Sorry I obviously misread your earlier post.

 

[RogertheBodger]

Re: What happens when a star wired balanced device (without a neutral) becomes unbala

Hi Peter - is this the kind of thing you mean?

Went to a job a few weeks ago, big house, big extension, problem - no power to sockets, easy?

tested the sockets indicated, 380 volts?
checked the panel, the whole phase 380v L to N WTF.
At the meter, checked the incoming supply - same, 380v, pulled the fuses and found there was a phase down, so I'm totally mystified by now, put the fuses back in, checked the sockets again - 0volts, the original problem.

Anyway, to cut to the end of a long story, after several hours of fruitless searching with the voltage fluctuating between 380v. and 0v. I was standing near the cement mixer when 3 builders came over, two of them grabbed the drum and turned it by hand, the other pushed the start button and off it went, yep, 3 phase mixer running on two phases, putting voltage into the dead phase, got it all sorted in the end, been like that for weeks, mixer running - 380v, mixer not running - 0v.

Star connected motor, running on two phases, how unbalanced can you get? it was a bit hot but how long would it last? - who knows.

Sorry to go on (but that was the short version)

 

[Knobhead]

Re: What happens when a star wired balanced device (without a neutral) becomes unbala

Good job they didn’t turn it the wrong way. A three phase motor will start with just 2 phases. But it won’t be happy. Had they turned it the wrong way the cement would screw it’s way back in to the barrel instead of out when it was tilted.

 

[Inteificio]

Re: What happens when a star wired balanced device (without a neutral) becomes unbala

Thanks Roger,

I like this forum, I learn stuff!