Some Phase 6 elecctricial science questions.

Salmonfella

Hey members i dont usually post here regarding phase 6 beacuse i like to do the research and find and study the answers to questions myself,but these couple are really annoying me as i cant seem to find the answes them..thanks ind vance for any help given...


Q1.describe what mesures are taken to dissapate the engergy stored in a relay coil which is switched by a transistor?

O2. a single phase transformer supplies a 2.4kw electric heater at 110 as shown below, if the primary voltage is 230volts calculate the primary current?..


Thanks for any help lads the last one is probaly very easy i just keep confuseing myself by useing worng steps.

dingding

A bit rusty but is No 1 a reverse biased diode to give a circuit for the back emf.

Is no 2. w=vXi therefore w/v = 1 = 2400/230 = 10.43 Amps. As it is a heater you can assume it is a resistive load.

Salmonfella

thanks dingding:D

P.S. i no i spelt electricial wrong:cool:

dingding

And Dingding

Bruthal

dingding wrote: »

And Dingding

m.j.w

Lads could you look at the question I attached and give us an answer if you can. I know the answer to the question is you cant close it because polarity is wrong but what I was wondering is how can you get 475 volts across the switch when there is only one phase and a neutral?

dingding

Don't close the switch you will get a big bang.

What has happened that the two transformers are connected with opposite polarity. To get this to work properly you would need to reverse the polarity of one of the primary windings of one of the transfomers.

m.j.w

dingding wrote: »

Don't close the switch you will get a big bang.

What has happened that the two transformers are connected with opposite polarity. To get this to work properly you would need to reverse the polarity of one of the primary windings of one of the transfomers.

cheers dingding but you didnt read my post did ya

Bruthal

Yea one of the secondaries needs its polarity reversed with respect to the other.

It is in effect connecting the 2 secondaries in series, so adding the voltages together. They will be shorted if the switch is closed.

Reversing one of the secondaries polarities will put them into parallel, showing 0v on V2 (although the load being only on T2 with the switch open might cause a few volts to show), and then the switch can be closed.

Bruthal

m.j.w wrote: »

cheers dingding but you didnt read my post did ya

Its because the 2 transformer secondaries are in series in that diagram (with reference to how the primaries are connected), so at any instant in the phase, they add together. So the RMS is 470v

With the primaries in parallel as they are, but one of the secondaries the opposite way, the CSA of the primary windings are doubled, but the number of turns remains the same. With the secondaries as they are, the number of turns has doubled, so double voltage is the result.

Any suitable load connected to such a setup would go where V2 is connected, without the switch there.

Salmonfella

we had same problem wit this in coll even our lectures could not give a correct answer:eek:

Bruthal

dingding wrote: »

What has happened that the two transformers are connected with opposite polarity. To get this to work properly you would need to reverse the polarity of one of the primary windings of one of the transfomers.

Yea that will do it as well, reverse one primary, or secondary.

Salmonfella

guys in the june 10 paper its same but its 15v on v2 is it safe to close the switch?v1 reading 220v and v3 reading 220v?

dingding

If it is 15 volts it should be safe as the difference is down to the tolerences in the transformets.

dingding

robbie7730 wrote: »

Its because the 2 transformer secondaries are in series in that diagram (with reference to how the primaries are connected), so at any instant in the phase, they add together. So the RMS is 470v

With the primaries in parallel as they are, but one of the secondaries the opposite way, the CSA of the primary windings are doubled, but the number of turns remains the same. With the secondaries as they are, the number of turns has doubled, so double voltage is the result.

Any suitable load connected to such a setup would go where V2 is connected, without the switch there.

Well done Robbie, you put it better than me.

Bruthal

Salmonfella wrote: »

guys in the june 10 paper its same but its 15v on v2 is it safe to close the switch?v1 reading 220v and v3 reading 220v?

Without seeing the diagram, I would think that 15v is because the transformers are correctly connected in parallel (unlike the ones in the post a couple of posts ago), but only one is connected to the load. So there is a 15v difference due to one transformer being loaded possibly with an inductive load.

In that case, closing the switch would be ok. But the diagram would help.

Salmonfella

Thanks dingding

dingding

dingding wrote: »

If it is 15 volts it should be safe as the difference is down to the tolerences in the transformets.

Robbie's explaination is better, it is probably one of the transformers was loaded. You should be able to tell from the drawing.

Rusty at this sort of thing as I got out of electrical and into computers

Bruthal

dingding wrote: »

If it is 15 volts it should be safe as the difference is down to the tolerences in the transformets.

Yea i was thinking inductive load being on one of them before the switch is closed, but the transformer tolerences are a possibility too alright.

dingding

I was thinking at a 5 - 10% tolerence and the voltage variance would have been within this range.

Bruthal

What i would say is, both V1 and V3 read 220v, so both transformers are reading the same. With an inductive load on trandformer 2, it would now be slightly out of phase with transformer 1, and so read a few volts across the open switch between the loaded and unloaded one.

Salmonfella

Its funny how reading your posts how ye explain it so much better the any lecture would thanks lads..haveing greet trouble trying to change adobe reader 8 to allow me to upload:rolleyes:

dingding

No problem Salmonna fella, it is a pleasure.

It is always great to help someone who is enthusastic


Finally getting to use all the stuff I learned in electrical science

dingding

robbie7730 wrote: »

What i would say is, both V1 and V3 read 220v, so both transformers are reading the same. With an inductive load on trandformer 2, it would now be slightly out of phase with transformer 1, and so read a few volts across the open switch between the loaded and unloaded one.

Even better. I think it is time to drop the tolerence theory. They would probably not put something like that into the question. So to summarise Robbies answers.

So it is either an inductive load, causing a phase shift and a voltage difference. or One of the transformers is loaded causing the voltage to drop. The no load voltage would be higher than the loaded voltage.

Bruthal

dingding wrote: »

Even better. I think it is time to drop the tolerence theory. They would probably not put something like that into the question. So to summarise Robbies answers.

So it is either an inductive load, causing a phase shift and a voltage difference. or One of the transformers is loaded causing the voltage to drop. The no load voltage would be higher than the loaded voltage.

Yea even a resistive load might put it a little out of phase, as the transformer winding itself is inductive, so the loaded one would possibly be a little out of phase with the unloaded one. Its an interesting one anyway.

m.j.w

Here is the question here lads. we were told you cant close the switch unless the voltage across it is zero. Said that if you close the switch in this instance the windings will heat up and break down after a period of time. Dunno if that is right or not

Bruthal

m.j.w wrote: »

Here is the question here lads. we were told you cant close the switch unless the voltage across it is zero. Said that if you close the switch in this instance the windings will heat up and break down after a period of time. Dunno if that is right or not

Thats a little different than what was asked last night, so the answer will be a bit different too. The two transformers are 20 volts different, and we can assume this is with no load on either of them by the look of the question.


The question asks the likely affect on the transformers performance if A: A 40kva load is connected, and B: A 80kva load is connected.

What will happen if the switch is closed is, a current will circulate between the two transformers, and the magnitude of current would be the 20v/transformer winding impedence.

This would be added to the load current, and so with the transformers loaded with 40kva shared between them, they will work, as there is 80kva capacity in theory from the two transformers in parallel. But the circulating current would use part of this capacity, so if 80kva load is applied, the transformers will be overloaded by the amount of the circulating current caused by the secondary windings voltage difference.

dingding

robbie7730 wrote: »

m.j.w wrote: »

Here is the question here lads. we were told you cant close the switch unless the voltage across it is zero. Said that if you close the switch in this instance the windings will heat up and break down after a period of time. Dunno if that is right or not

Thats a little different than what was asked last night, so the answer will be a bit different too. The two transformers are 20 volts different, and we can assume this is with no load on either of them by the look of the question.


The question asks the likely affect on the transformers performance if A: A 40kva load is connected, and B: A 80kva load is connected.

What will happen if the switch is closed is, a current will circulate between the two transformers, and the magnitude of current would be the 20v/transformer winding impedence.

This would be added to the load current, and so with the transformers loaded with 40kva shared between them, they will work, as there is 80kva capacity in theory from the two transformers in parallel. But the circulating current would use part of this capacity, so if 80kva load is applied, the transformers will be overloaded by the amount of the circulating current caused by the secondary windings voltage difference.

That reminds me of my days in the board and a situation that could arise if transformers got out of step ie if the tap selector jammed on one transformer the other transformer could race taking more of the load and moving tap. This was a serious condition with currents flowing between the transformers as you had said.

Probably getting beyond the scope of a phase 6 question.

Bruthal

dingding wrote: »

That reminds me of my days in the board and a situation that could arise if transformers got out of step ie if the tap selector jammed on one transformer the other transformer could race taking more of the load and moving tap. This was a serious condition with currents flowing between the transformers as you had said.

Probably getting beyond the scope of a phase 6 question.

Yea i was an apprentice myself there in the late 80`s.

dingding

81 - 85 north west district. Myself.

Bruthal

87 - 91, im only a young fella:D