phase terminology question 2

[brutus1955]

got into an interesting discussion the other day, and was not able to express myself clearly

the question...

if you have a single phase alternator, 60hz 2 pole 3600rpm
and there are two windings, winding A=120vac, winding B=120vac

how do you refer properly to each winding?
can i use "phase a" and "phase b" even though the two windings are in phase with one another and not seperated by
90,60 or 30 degree's

to use the term "phase" does there need to be a difference in degree's between the two windings?

or how else do you differentiate between the two when discussing a specific winding?

what do you call them?

personally i have always referred to them as "phases" even if there is no seperation in electrical degree's, is that wrong?

thanks

bob g

 

[rbulsara]

Yes, as long as the voltage between your phase a and b is the rated voltage of your single phase gen.

The definition of single phase circuit is that it has two wires with some voltage difference between them, whether the two wires comes from phase and neutral or two different phases of a polyphase system.

Also by definition a single phase circuit cannot have difference of phase angles as there in only one phase.




Rafiq Bulsara

http://www.srengineersct.com

 

[waross]

You may have one winding with a center tap, or you may may have two windings that may be wired in parallel for the lower voltage or series for the higher voltage.
L1, N, L2
T1-T2, T3-T4
or
A=120vac, winding B=120vac works also, just don't call A and B phases.
Take your pick.

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

 

[brutus1955]

so let me get this straight,

i can have phase A and a phase B, only if there is some
shift between them in electrical degree's

if on the other hand, they are both wound on the same poles
and therefore have no shift in electrical degree's they
can no longer be referred to as "phases" but rather
as L1 and L2?
or T1 and T2?

so it is correct to refer to one set of windings a the L1 or T1 winding and the other set as L2 or T2

but do not refer to them as phases unless they are seperated by some set electrical degree's such as 90, 60, 30 or whatever?

i know this may seems as nitpicking, i just got myself into quite a problem trying to explain what happened to the output of an alterator when L1 and L2 (phase A & B) were jumpered and the output instead of being 120 + 120 = 240
was near zero.

what i tried to explain was the polarity of either Phase A
or Phase B was reversed (mismarked) and simply reversing the leads on either would straighten out the problem.

i got into a whole sideways discussion about this being single phase and not 3 phase, so the term "phase" was confusing.

seemed perfectly understandable to me, but as we all know
words have meanings, very specific meanings to some folks
so i don't want to fall into that problem again in the future using the word "phase" improperly.

thanks guys

bob g

 

[waross]

One winding is T1 to T2 and the other is T3 to T4.
This configuration may be connected for either 120 Volts or 240 volts, (Or, if you really want, zero volts, but that is not usually done intentionally. Another name for that connection is a bolted fault. Not good.)

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

 

[wolf39]

brutus1955:

In practice a single phase alternator consists of a three-phase stator with 3 phases, each 120 degrees apart, from which the winding of one phase is omitted. The remaining 2 phases are 120 degrees apart and are connected in series. So the terminal voltage is 1.73205 x phase voltage, in your case 208 volts. The stator is utilized 2/3 of a three-phase unit. Of course, you can omit the second phase also but then you obtain 120 volts and the stator is utilized 1/3 only.

You better not connect the two windings (phases) in parallel as they are 120 degees apart. If you connect the two phases in series, watch the polarity.

Regards

Wolf

http://WWW.HYDROPOWER-CONSULT.COM

 

[RRaghunath]

Interesting explanation (I didn't know that before. Thanks.

 

[itsmoked]

If it's single phase. I would call it "one winding" and "the other winding" which relates to the same phase but connectible in a a couple of ways, as noted previously.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[waross]

Never see that before, wolf39. Below about 20 KVA at 1800 RPM and below about 25 KVA at 3600 RPM generators are wound as a true single phase machine,
In the larger sizes a three phase machine is reconnected i double delta or zig-zag for true single phase. These connections may be connected in parallel for 120 Volt service.
Occasionally two phases of a three phase machine will be used to power a single phase circuit but the third phase is still present in the generator. The few times I have seen this connection, it was done by a worker who was not capable of reconnecting the machine for true single phase.

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

 

[wolf39]

waross:

I must admit that I have no experience in the kVA bracket of generators. My know-how starts in the MVA region and all large 16 2/3 Hz single phase generators in service in Europe for traction purposes (Austrian, German and Swiss Railways) are utilizing generators that are designed the way I described.

Thanks for your information.

Regards

Wolf

http://WWW.HYDROPOWER-CONSULT.COM

 

[waross]

Thank you for some new knowledge, wolf39.
I have done quite a few single phase sets and spent quite a bit of time on manufacturers web sites comparing three phase and single phase plants. Above he sizes I mentioned, 20 or 25 KVA, up to hundreds of KVA it is standard practice to re-rate and reconnect a three phase set for single phase service.
1 Ph KVA = 2/3 3 Ph KVA
1 Ph kW = 83% 3 Ph kW
Often the only changes are replacing the three pole breaker with a two pole breaker and changing the Voltage and current meter switches to a configuration suitable for single phase service.
Can you suggest a reason why traction service uses a phase displacement for single phase? It seems a bit of a waste to use a configuration that cuts the output to 83%. There must be a good reason.
Yours
Bill

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

 

[stevenal]

faq238-777

 

[waross]

Nice FAQ Steven. LPS

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

 

[brutus1955]

ok fella's

the term phases looks to be used only when there exists
a difference in X electrical degree's?

i suspect the proper use should be a difference in location
on the stator in physical X degree's

reason being

i can have a 2 pole machine wound with two coils 120vac each
and it would typically be thought of as a single phase machine, but

what if the machine was wound bifilar, one winding clockwise
and other counterclockwise, therefore there exists 180 degree's of electrical seperation.

so electrical degree's of seperation cannot be the soul determination of whether the unit has phases?

looks like it is placement and physical seperation irregardless of electrical degree's of seperation?

thoughts
bob g

 

[stevenal]

Did you look at the faq? You are describing a single phase machine.

To answer your original question; if you don't have polyphase you have legs rather than phases.

 

[wolf39]

Hi Bill:

As kW and kVA are linked with each other by the power factor, I at first glance don't understand the 83% figure.

Small single phase generators (i.e. in the 20 to 25 kVA bracket) don't require special treatment designwise as the unit mass figure per kVA is very high. Large single phase generators, however, require a massive damper winding to take care of the inverse field. Also, elastic spring devices must be introduced to make sure that the foundation is not overstressed by torque pulsation forces.

You can install, of course, a true single phase winding into a stator core by using only 1/3 of the available slot space. But then the stator core is even less utilized than the two phase design, with both phases connected in series.

What else can we do? Just take a conventional three phase generator with 6 terminals, break the star point and use the three phases independent from each other. Then you have a generator, utilized 100%, with three separate single phase systems. These systems, however, cannot be paralled. If the load distribution between these three phases is well balanced (three phase hydro generators usually are designed for 12% unbalanced load), the damper winding design can be standard.

Regards

Wolf

http://WWW.HYDROPOWER-CONSULT.COM