Electrical theory

[johnboy6083]

heres a link to a page i justfound, with a lot of useful theory, proably more suited to those studying ONC/HNC ect, but useful none the less

Electrical Knowhow | Technical Publications | Schneider Electric UK?

cheers,

John

 

[topquark]

Some interesting material for study there John.

 

[johnboy6083]

i started flickimg thorugh it, and most of it is beyond me, but ill orint some of it out, and have another pop. i much prefer reading things on paper.

Sticky?

 

[topquark]

I'd vote for a sticky. The stuff on EMC is becoming more relevant to sparks who specialise in domestic (no insult should be inferred!). Everyone gets to study about eddy currents and causes, effects and solutions at college these days. Cable design and segregation of cables should also make more sense to folk who have an, better than average, understanding of electromagnetic effects.

 

[Knobhead]

My god you’ve started something now. I spent 2hrs reading just one paper and it only skated over the surface of distribution discrimination. I’ll have to re-read it several time for it to make more sense.

One thing I learned that with CB’s we can have as we know inverse time and instantaneous O/L’s, ever heard of definite time O/L’s? I knew they existed but this is the first time I’ve had a description of them. (You’ll not be fitting one in a CU)

This should be made a sticky.

 

[johnboy6083]

bloody hell Tony. What hope have I got of understanding it then? My hopes of eventually becoming an engineer are slipping away lol

 

[Guest123]

"Stucked"

 

[Knobhead]

bloody hell Tony. What hope have I got of understanding it then? My hopes of eventually becoming an engineer are slipping away lol

Same chance as anyone else. And now you’ve got computers to help. At least with a slide rule you could write formula on the back in pencil before the exam. Bet you can take your books in now, I’m not knocking open book exams because in real life you can open a book if you have a problem at work. Most equipment manufacturers have 24hr help lines to get you out of the mire. It’s now knowing where to find the information and then use your skill to put it in to practice.

 

[Knobhead]

For anyone wanting the calculation for neutral here it is
N = √(A ²+B ²+C ²) - (AxB+AxC+BxC)

There’s a lot more if you go in to vectors, you can work out the neutral phase relationship to phase A. But it’s not a quick and easy calculation, as you have to know the PF of each phase.

Just to muddy the water even more neutral caries 3[SUP]rd[/SUP] harmonic currents. So don’t believe everything you clamp meter tells you.
This is now getting to be a major problem as more VFD’s and IT equipment comes on line. VFD’s and IT add further harmonics to the neutral mainly 5[SUP]th[/SUP] and 7[SUP]th[/SUP]. Where a 3½ core would be used at one time, 4 core is now needed. I recently read of a plant where the neutral had to be 2X phase CSA (the 2.5MVA transformer went “BANG”) eventually.

 

[Knobhead]

For anyone wanting the calculation for neutral here it is
N = √((A ²+B ²+C ²) - (AxB+AxC+BxC))

There’s a lot more if you go in to vectors, you can work out the neutral phase relationship to phase A. But it’s not a quick and easy calculation, as you have to know the PF of each phase.

Just to muddy the water even more neutral caries 3[SUP]rd[/SUP] harmonic currents. So don’t believe everything you clamp meter tells you.
This is now getting to be a major problem as more VFD’s and IT equipment comes on line. VFD’s and IT add further harmonics to the neutral mainly 5[SUP]th[/SUP] and 7[SUP]th[/SUP]. Where a 3½ core would be used at one time, 4 core is now needed. I recently read of a plant where the neutral had to be 2X phase CSA (the 2.5MVA transformer went “BANG”) eventually.

 

[Knobhead]

I don’t know if any of you have ever come across this, a reversed winding in a 3-phase motor. I’ll admit I only came across it twice in 35 years. Off load on a DOL starter the motor will start but make an awful growling noise and soon trip on O/L. on a S/D starter you’ve no chance of it starting.
I’ve found the reversed phase in the past by trial and error. Swap U1 with U2 and try the motor, no good swap the U’s back and reverse the V’s, and so on.

But I came across this method, which is a bit more scientific.

First confirm U, V and W windings, by doing continuity test. Then follow the procedure given below.

1.Connect V and W windings in series. Connect 110V AC to U winding. (Fig 1)
2. Measure the voltage between the V and W series circuit. If the meter reads about 220V, then opposite polarities are connected at the junction of V and W winding. This confirms V & W are correct
3.Connect U and V windings in series. Connect 110V AC to W winding. (Fig 2)
4. Measure the voltage between the U and V series circuit. If the meter reads about 220V, then opposite polarities are connected at the junction of V and W winding. This confirms U & V are correct. At this point if all reading are correct start looking elsewhere for your fault.
4. If the meter reads about 0V. Then same polarities are connected at the junction of V and W winding (V1 and W1 or V2 and W2) (Fig 3 and 4). If this is the case then swap one or the other windings, repeat test 1, this confirmas V and W ore correct.
5. Repeat the same procedure to find U1 and U2, DO NOT swap V and W. (Fig 5)

Please do it safely without touching any terminals during applied voltage.

View attachment 7278View attachment 7279View attachment 7280View attachment 7281View attachment 7282

 

[bomjac]

Tony,

From the darkest recesses of my apprentice days, I seem to recall that you can also work this out by using a 9 volt battery and an old style AVO, in a manner similar to a flick test on CTs. IIRC the same sort of ssetup for each test as you have posted above would be used. Similar principle.

 

[Knobhead]

You've got it, the flick test, I’d forgoten about it.

The company I started with had some ancient test gear, instead of the AVO we would use a galvanometer. That dated from 1905!

 

[Knobhead]

For anyone involved in the maintenance of DC motors you may find this interesting. http://www.reliance.com/prodserv/motgen/c7090/

 

[Knobhead]

Squirrel Cage Motors.

On starting the rotor is stationary so it draws what is called “locked rotor” current. This can be 5 to 8 times the FLC. The current is regulated primarily by the resistance of the winding with a small inductive element. As speed increases the inductance increases while the resistive element remains the same.

View attachment 8281

Running light the motor turns at near it’s synchronous speed causing high inductance in the stator windings and therefore low current. The motor cannot achieve full synchronous speed, as a small amount of “slip” is needed to induce current in to the rotor’s squirrel cage. As load on the shaft is increased slip increases inducing more current in to the squirrel cage, which in turn reduces the inductance of the rotor allowing more current to flow through the windings.

Synchronous speeds for an induction motor is governed by the number of poles in the rotor. This is give by Hz X 60 / No. of poles. So at 50Hz the synchronous speed for a 2 pole motor it will be 3000RPM.