Mystery Box 2

[itsmoked]

This is the power circuit of 29kW on-demand water heater.

It is a single phase unit that has three of these identical circuits in it. I'm trying to ascertain if I can run it on three phases instead.

The top dotted box is the tank with the three elements in it.

Ok gents, what is happening inside the idiot mystery box?

Keith Cress
kcress -

http://www.flaminsystems.com

 

[itsmoked]

We are attempting to get an answer from the maker about 3ph otherwise this whole thing will go off the rails because our [red]52kW generator[/red] will NOT appreciate a single phase load of 29kW...

This is another rail car refurbish.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[roydm]

Rail Car, sounds interesting. I served my apprenticeship with NZ Rail. The passenger cars had a neat old 48VDC generator flat belt driven from the axel with brush-gear that rocked back and forth with change of direction. The armature windings came out and were held under individual screws, one per slot.
How about some details, photo etc.
Roy

 

[waross]

Hi Keith. As you know, I've installed a lot of residential sets, from 14 kW to about 40 or 50 kW, 120/240single phase.
Above about 20 kW, I when I order a single phase set I get a 3 phase set, reconnected and re rated at the factory. Actually most of them still show the three phase ratings and connections as well as the single phase information. The KVA rating drops by 1/3. The power factor goes up to 100%. The engine is left as is (not down sized for the reduced single phase loading), so the sets may be reconnected for 3 phase at full output.
If your 52 kW set is rated for a PF of 0.8, the KVA will be 65.
The single phase KVA will be 43 KVA. At unity PF, your kW rating will now be 43kW.
The common single phase connection used to be zig-zag but some years ago several set makers started using a double delta connection. This is two 120 volt deltas side by side.
Single phase loads are connected to the horizontal side of the vector sketch and the two 120 volt deltas are joined at one corner to give 120:240 volts true single phase with a true neutral and no phase shift. (Or 180 degrees phase shift for those who use "phase shift" to describe reversed polarity).
An explanation of the action of a single phase load on a delta:
Consider three transformers, One single phase and the other two in open delta. The voltage is considered fixed at nameplate rating, and the safe load is determined by the current.
If a load is connected across the open delta, the current must flow through both transformers and the current should not exceed the rated current of the smallest transformer.
If our transformers are rated 25 KVA then the maximum safe load on the open delta side will be 25 KVA.
At first glance the resistance is double that of a single transformer, but the IR voltage drop is more important than the actual resistance. If you vector sketch the resistance, reactance, and impedance voltage drops, You will see that because of the 120 degree phase displacement the voltage drop across the open delta is the same as the voltage drop of a single transformer.
So, two 25 KVA transformers connected open delta on "A" phase and "B" phase may be considered and treated as a 25 KVA virtual transformer on "C" phase.
A single 25 KVA on "C" phase may be connected in parallel with the "Virtual" open delta transformer on "C" phase.
The total safe load on the combination will be; 25 KVA plus 25 KVA equals 50 KVA, or 2/3 or the total arithmetic sum of the original three ratings.
The same holds true for generator connections for KVA ratings.
However, as you know, the kW rating of a gen set is based on the power of the prime mover. With the drop in KVA rating it is possible to run the set with unity power factor loads at the full single phase KVA rating without overloading or stalling the engine.


Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[edison123]

waross.

I don't think a transformer can be equated with a generator so easily delta or not. A severely imbalanced phase loading will heat up the rotor winding due to negative sequence currents. In large generators, the phase load unbalance is restricted to 10% but I am not sure about smaller ones.

 

[waross]

Hi edison 123;
In generator sets of the size that will sometimes be required for a single phase application, (20 KVA to over 50 KVA) such connections and ratings are common from the factory.
Olympian, (owned by Caterpillar) offers a 105 KVA/kW single phase set.
This unit uses the same engine as a three phase 150 KVA set.
This unit uses the generator end as a three phase 165 KVA set.

Onan sets are available as reconnectable for single phase or three phase up to at least 100 KVA.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[itsmoked]

roydm; I am peripherally associated with a private rail repair yard. I'm the go-to-guy on electrical issues. Funny you should mention the generator. This particular car is being refurbished for a historical rating. So we have to update it for basic comfort and safety while leaving everything looking authentic. We have to do insane things like leave the original electrical closet alone but infact wire everything into an alternate modern closet.

All the lighting was previously served by an under slung 32VDC generator and a bank of batteries that would put a submarine to shame. This is no longer allowed so we have to work around it.

Typical cars are like this,(see attach). (I just grabbed this frame off a security web camera {see thread elsewhere} so the quality isn't there.

http://files.engineering.com/getfil...-4e2a-ac40-fa094f0aa307&file=Trainsnatch2.jpg

Bill; Thanks for the explanation. I will need to read it a few more times. We often need 120V, so often we'd use three transformers with the secondaries in wye to give 120 to the neutral and 240 across the lines. If I am understanding your description and apply it to transformers, the open delta would not provide 120V anywhere if I needed 240V also, but would provide 240 with one less transformer?




Keith Cress
kcress -

http://www.flaminsystems.com

 

[davidbeach]

Can't get 120V to neutral from all three phases and 240V line to line. What you can do is have an open delta where one of the transformers, usually the larger of the two, has a center tap; the tap is grounded and called a neutral. Then you have 120V line to neutral from the two ends of that transformer and 240V phase to phase for all three phases. The third phase will measure 208V line to neutral (the hot leg or stinger) but is essentially unusable for any load.

 

[itsmoked]

Roy; Here's one we just had in the yard.

http://www.sierrahotel.com/

Keith Cress
kcress -

http://www.flaminsystems.com

 

[waross]

Hi Keith;
A little explanation. The open delta provides a virtual transformer with the same characteristics as the third transformer. The point of the explanation is that a single phase load across a full delta splits and 1/2 of the current flows through the in phase transformer and 1/2 of the current flows through the two out of phase transformers. You can consider the open delta transformer as a virtual transformer in parallel with the in phase transformer to understand the current division.
Your delta generators are interesting. Are they three lead generators, six lead generators or twelve lead generators?
what portion of your load is three phase and what portion is single phase?

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[ccjersey]

The same voltages as the open delta 3 phase mentioned above are available from a 12 lead generator connected series delta with a neutral brought out from the mid point of one phase.

With some attention to load balance, you could have it all, 120 L-N, 240 L-L single and 3ph without transformers. It's our normal backup generator connection for the open delta 3ph services out in the country. We do load the "B" phase with L-L 240 volt single and 3ph loads. It helps make the generator phase loading closer to even since there's always some L-N loads on the other two phases and none on the stinger.

For the heavy 3 phase loading, our utility normally uses a transformer bank with 2 equal size transformers instead of a small "teaser" for only a small 3 phase load. With these services, the 3 phase motor amperage is commonly a bit unbalanced (6-7%) even though the line voltages are normally very close (less than 1%). A 3 phase motor that's running into it's service factor and runs a lot has a shortened life! Of course proper phase loss monitoring and overload selection gives them a chance, but we still seem to go through too many motors.

 

[waross]

I agree with ccjersey on this connection. Some know it as a four wire delta. Subtract your (balanced) three phase load and reduce the remaining capacity to 2/3 for single phase capacity.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[itsmoked]

I think my mind is going to crack on this one..

1) Series delta? You mean two windings put in series once, again, and again.

2) Then those series pairs put into delta?

3) So corner to corner three corners = 3ph (say) 240V

4) However: You can then run from the center of one of these series pairs for the neutral?

5) Its two corners each providing one of the 120V legs.

6) Across them 1ph 240V?

7) Can you ground that neutral?

8) If yes, the motor's/typical other 3ph loads don't care about this ground?


Is my summation correct?



Oh hey Bill, answers to your previous post:
"[green]Your delta generators are interesting. Are they three lead generators, six lead generators or twelve lead generators?[/green]"

I have no idea. I have been told, "they are only delta".
They are generally Marathons turned by Detroit Diesels. In the 40 to 60kW realm.

"[green]what portion of your load is three phase and what portion is single phase? [/green]"

The big load is the refrigeration required to handle nasty solar loads, thru all glass domes, in the direct sun, in deserts that might be at 130F ambient.

This often represents 3/5ths of the generator capacity as motors.

Then you have an electric range/stove top. It can be about 14kW 1ph.

You will also have lights, fans, TVs, ect. Picture a motor home 90ft long and 9ft wide and two story, with 40 people in it. The typical domestic stuff. Plus water circ, and an a sewer vacuum pump.

In cold realms; 3ph heat that is about 14kW and about 6kW of 1ph base boards.

And in this case a b@st@rd 30kW tankless water heater!

The good news is that the maker came back with, "Oh sure you can hook up those three modules on three different phases".



Keith Cress
kcress -

http://www.flaminsystems.com

 

[roydm]

Keith, thanks for the pictures, that car is a beauty all right. I live just outside Vancouver and see the Amtrak and Rocky Mountaineer go by on a regular basis perhaps your coach will go past one day.
All the talk about 3 phase to single phase alternator connections reminds me of the Aluminium induction furnaces I once worked on, they took 3 phase and converted to single phase 60Hz for the coil with the clever use of transformer and capacitor connection (no DC buss)
Regards
Roy

 

[edison123]

Keith

"Oh sure you can hook up those three modules on three different phases".

That was going to be my next post.

 

[waross]

Hi Kieth. In some parts of the world the four wire delta is a common connection.
Think standard 240 volt delta with 120/240 volt transformers.
This may be either full delta or open delta. One transformer is used to supply 120/240 volt single phase loads. The center point/neutral is grounded. This is not a problem and is closer to the three phase neutral than a corner ground. Some three phase transformers are not happy with this connection, but it works well with either two or three single transformers. It works also with generators.
From time to time I see a large home (Mini mansion) with a three phase panel and possibly a three phase meter. They are all on single phase now, but time was when large air conditioners were only available n three phase. The point is, the single phase portion of the installation is identical to a normal single phase 120/240 service when the third phase is removed. The original three phase compressors have long since died and been replaced with single phase compressors. The supply was usually the 4 wire delta connection with two transformers in open delta.
The most common generators now are 12 lead machines. Each phase will have two 120 volt windings that may be connected in series for 240 V or parallel for 120 V.
This is a little different perspective on basic knowledge that you are familiar with. Your head will stop hurting soon!
Those must be old sets. I believe that Marathon has been a Caterpillar mark for years now. I ran into a lot of old Delco GMs in that size range. Mostly Telco take-outs.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[ccjersey]

The only correction (if it is a correction)to your summary is that we commonly load the B phase with 240 volt single phase loads. Our panels are 3 phase with a mix of single and 3 phase circuits. I know some installations have the 3 phase separated from the single phase and the B phase isn't run into the single phase box, but not here. So we could potentially load the generator very close to it's maximum current with the L-N 120's being the remaining imbalance.

No, the neutral being grounded has no effect on the operation of 3 phase motors.

 

[waross]

The old installations have a three phase panels. When the three A/C and the hree phase power was removed, two bus bars were jumpered together for single phase use.

So we could potentially load the generator very close to it's maximum current with the L-N 120's being the remaining imbalance.
Your single phase current for 240V 1 PH loads will split and "B" phase will supply 50% of the current. "A" phase and "C" phase together will supply 50% of the current. The generator should be derated for the portion of the load that is single phase.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[itsmoked]

Thanks guys, you have difinitively increased my knowledge base on this subject. Now to apply it.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[davidbeach]

So, Keith, now that this one seems resolved, and since you did mention that it is for a rail car application, did you ever resolve the problem in thread238-198694?

 

[itsmoked]

I do not believe so.

You have to realize that this is about 15 different cars that move all over the place and are bought and sold. It is very unlike a stationary facility. It's often frustrating because things like this are sometimes never resolved or they are and you never hear about it.

Same with that other problem. The one where we can't backfeed one car from the other without an instant groaning main breaker trip, but both run fine independently or on the same HEP hookup.

Keith Cress
kcress -

http://www.flaminsystems.com