generator connection wye or delta vs efficiency ? 1

[brutus1955]

ok, let me preface my question

1. a three phase alternator, that can be connected in wye or delta

2. i don't care about frequency, the speed will be fixed at ~5000 rpm whether connected as wye or delta

3. the output voltage needs to be the same whether is it connected as wye or delta

4. the rpm stays the same, but excitation changes to maintain the same voltage whether connected in wye or delta.

now the question

given these criteria above, what if any differences in efficiency might there be with the machine, being connected either in wye or delta.

basically is there an advantage efficiency wise to the alternator given these parameters? now i am concerned with just the alternator itself and not other transmission advantages of one or the other connection type.

it comes to mind that "phase to phase" the wye connected machine has a larger stator resistance than "phase to phase" of the delta connected option.

is this so in practice?

if so would that be a significant contributor to any possible difference in efficiency?


thanks
bob g

http://www.microcogen.info

 

[waross]

It depends.
#1> Maximum voltage is a design issue, not an excitation issue. Magnetic saturation will limit your maximum excitation and maximum usable voltage.
2#> For a given machine, There is a trade off between Volts and Amps. Example, a 9 KVA machine with dual 120 Volt windings rated at 12.5 Amps per winding maximum may be connected as:
Delta, 120 Volts at 43.3 Amps maximum, or 240 Volts at 22 Amps maximum.
Wye, 120:208 Volts at 25 Amps maximum, or 240:415 Volts at 12.5 amps Maximum.
In practice many generators are suitable for about 140 volts per winding so as to be usable at 277:480 Volts. This will boost the rating to about 10.4 KVA.
#3> In practice, the wye connection is preferred as it allows the flexibility of line to neutral loading and facilitates system grounding.
#4> Larger stator resistance. Yes and no. If you are reconnecting a given machine then the stator resistance changes. In practice this happens with generators, but in theory if two machines were designed for the same maximum voltage at the terminals and the same maximum current, the stator resistance may be the same.
For a machine with a rated current of 25 Amps the wye windings must carry 25 Amps. The delta windings need only carry 14.4 Amps and will be wound with smaller gauge wire, offsetting the drop in resistance you will see when reconnecting an existing machine rather than designing from scratch.
Consider a motor with three leads coming out. What electrical tests can you do to determine if it is wye connected or delta connected internally.
The major alternator manufacturers build for the international market. The major gen set manufacturers build for the international market. A machine that will develop 120 Volts at 50 Hz (for 240:415V applications) will develop 144 Volts at 60 Hz.
In practice the same alternator will be supplied for use at 230:400V 50 Hz, 240:415 50 Hz, 120:208V 60 Hz, and 277:480V 60 Hz. It may be connected in delta, usually as a 4 wire delta for 120:240V at 60 Hz. The KVA rating increases to the same value as the 277:480V connection.

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

 

[catserveng]

Great explanation Bill, A Star for you!

 

[waross]

Thank you catserveng.
Yours
Bill

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

 

[brutus1955]

thank you Master Bill

i think i understand what you presented, at least enough so
that i can maybe more closely define my question should there be one.

your direction is much appreciated

bob g

http://www.microcogen.info