loss of phase... what if question

[Electric-Light]

Transformer is 3kVA
Light bulb is 120v 500W each.
One of the primary wire is disconnected.
Suppose primary is fed 3phase 3W 480v
Secondary is 208Y120
What would happen to voltage seen by each of the light bulb? Would some of them shut off completely or just dim?

Assume there are no motors parallel with the primary or secondary of the transformer that can act as a rotary phase converter.

 

[winnie]

I would expect one phase to stay at full voltage and operate normally.

I would expect 2 phases to drop to 1/2 normal voltage. What the lights do in that case will depend upon their specific characteristics.

I would expect incandescent lamps to dim quite a bit but continue to function.

I would expect fluorescent lamps to either go out or stay at full brightness, depending upon the characteristics of the ballast/driver.

-Jon

 

[Electric-Light]

I would expect one phase to stay at full voltage and operate normally.

I would expect 2 phases to drop to 1/2 normal voltage. What the lights do in that case will depend upon their specific characteristics.

I would expect incandescent lamps to dim quite a bit but continue to function.

I would expect fluorescent lamps to either go out or stay at full brightness, depending upon the characteristics of the ballast/driver.

-Jon

for the purpose of this question, please assume incandescent lamps.
With the loss of A, you only have BC instead of AB, BC and CA as it would be normally, so you're effectively having single phase 480v going into the transformer.

With that said... how does the secondary respond.. and does it make a huge difference if it's a single core, three coil design vs three separate transformers?

What sort of phase relationship do the three A-N, B-N and C-N have when the primary is fed single phase?

 

[winnie]

With the loss of A, then you still have the AB and CA coils energized, just in series connected to the B and C supply legs.

If you had three separate transformers making up your bank, then with _balanced loading_ I would expect equal voltage division between the AB and CA circuits, with the two associated secondary coils at half normal output voltage. The secondary coils would all be in phase or inverted relative to the other secondary coils.

With non-balanced loading on the secondary (with separate transformers), the voltage on the AB and CA circuits would change, in similar fashion to voltage division on a split phase system where the neutral is lost.

I will need to think more on what happens with a common core, and if this tends to balance out the voltages or not.

Even with only one primary coil energized on a common core transformer, you would get voltage induced in all of the secondary coils, so my hunch is that this would tend to balance out the voltages seen with non-balanced loading.

-Jon

 

[Tony S]

For say blue failed for the primary.

If you were to measure across the secondary YN connections you will get:
Red ? neutral = normal O/P voltage
Yellow ? neutral = .5 normal O/P voltage
Blue ? neutral = .5 normal O/P voltage
Red ? yellow = normal O/P voltage
Yellow ? blue = zero voltage
Blue ? red = zero voltage

 

[junkhound]

Yellow ? blue = zero voltage
Blue ? red = zero voltage

Look at the diagram in the original post and see if you do not want to revise that answer <G>

 

[Electric-Light]

B-C = 480v
C-A-B = 480v

So,what would secondary voltages be for:
N-A, N-B, N-C
and
AB, BC and CA?

 

[Tony S]

Considering the original drawing isn?t there now and there is no mention of the affected phase in the original script I stand by my answer. ?Assuming blue has failed.?

 

[Smart $]

B-C = 480v
C-A-B = 480v

So,what would secondary voltages be for:
N-A, N-B, N-C
and
AB, BC and CA?

Assuming secondary A,B,C is powered by primary CA,AB,BC respectively, then
NA = 60
NB = 60
NC = 120
AB = 360
BC = 60
CA = 240

 

[junkhound]

.

Hmm, did a 'reply with quotes' to the OP and that puts the OP diagram right up front, as above.

Primary point on xfmr with open wire is 240 Vac to each of the other phase connections on the transformer and both are t;he same phase which is 180 out of phase with the connected phase pair.

Secondaries: One phase is 120V, the other 2 are in phase 60 V also 180 deg out of phase with the 120V phase.

 

[mivey]

Assuming secondary A,B,C is powered by primary CA,AB,BC respectively, then
NA = 60
NB = 60
NC = 120
AB = 360
BC = 60
CA = 240

With primary supply of
AN = 277<0d
BN = 277<-120d
CN = 277<120d
AB = 480<30d
BC = 480<-90d
CA = 480<150d

then losing supply A so we get the following at the primary windings:

AC = 240<-90d
BA = 240<-90d
CB = 480<90d

the secondaries become:
AN = 60<90d
BN = 60<-90d
CN = 120<90d
AB = 0
BC = 180<-90d
CA = 180<90d

please double-check to see if I mis-crunched a number

 

[Smart $]

With primary supply of
AN = 277<0d
BN = 277<-120d
CN = 277<120d
AB = 480<30d
BC = 480<-90d
CA = 480<150d

then losing supply A so we get the following at the primary windings:

AC = 240<-90d
BA = 240<-90d
CB = 480<90d

the secondaries become:
AN = 60<90d
BN = 60<-90d
CN = 120<90d
AB = 0
BC = 180<-90d
CA = 180<90d

please double-check to see if I mis-crunched a number

Actually, I had to recheck mine, too, and I see where I made a mistake! Your magnitudes are good but three angles are off.

AN = 60<-90d
BN = 60<-90d
CN = 120<90d
AB = 0
BC = 180<90d
CA = 180<-90d

 

[junkhound]

ah, good, someone finally agrees with my answer <G>

Or, are we both wrong ?

 

[mivey]

Actually, I had to recheck mine, too, and I see where I made a mistake! Your magnitudes are good but three angles are off.

AN = 60<-90d
BN = 60<-90d
CN = 120<90d
AB = 0
BC = 180<90d
CA = 180<-90d

Could be. I don't feel like re-checking.

 

[mivey]

ah, good, someone finally agrees with my answer <G>

Or, are we both wrong ?

Could be either. I'm too tired to check again.

 

[iwire]

Loss of phase, what happens?

Putting aside the boring math.



1) I make money.

2) I find it enjoyable to walk into a large retail store with a lost phase and watch the light show that happens with the T-8 lighting, fixtures randomly lighting and going out. It can be quite impressive.

3) When it's all over I usually have contactor coils to replace.

 

[junkhound]

randomly lighting and going out

If you watch for a few minutes you will see the pattern, not random (but you knew that)

 

[Electric-Light]

With primary supply of
AN = 277<0d
BN = 277<-120d
CN = 277<120d
AB = 480<30d
BC = 480<-90d
CA = 480<150d

then losing supply A so we get the following at the primary windings:

The primary, as shown is fed 3PH3W, no neutral feed.
So, loss of A feed means you only have one phase.

 

[Smart $]

The primary, as shown is fed 3PH3W, no neutral feed.
So, loss of A feed means you only have one phase.

Yes... but we still have three primary windings connected to the two "good" lines. The two windings with connection to the lost line still have the connection, so each connects to one "good" line through the other, which means those two windings will see half the voltage between the two "good" lines.

 

[Electric-Light]

Yes... but we still have three primary windings connected to the two "good" lines. The two windings with connection to the lost line still have the connection, so each connects to one "good" line through the other, which means those two windings will see half the voltage between the two "good" lines.

But there would be no 120deg separation, right?