P and I for star and delta connection of three-phase motors?
Comprehension question about current and voltage in star and delta connection of three-phase motors
I'm trying to understand the differences in current flow between star and delta connections in three-phase motors. I know that in a delta connection, the current Istrang=I_L/root3 and that it's a parallel connection, which means the total resistance is smaller than the smallest individual resistance and therefore the current should be greater than if it were connected in star, even though in a star connection the current I_L=Istr? So let's say the motor has a rated current of 4A, then for a delta connection Istr=I_L/root3 = 2.309A and for a star connection = 4A, but here it's connected in star, so the total resistance is greater.
Additionally, the formula ππ π‘πππ/ππππππππ=1/3 confuses me
The formula says the total power in delta connection is 3 times as large as the total power in star, but when I use the formula P=U*I*root3*cosphi, I get the same result for both circuits and also when calculating Pstr taking into account the different circuits.
I don't understand these connections here…
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On the one hand, star and triangular circuits are not pure parallel circuits, but rather star and triangular circuits.
On the other hand, in your considerations, you neglect the tensions and actual measured values.
It’s very good that you think in strands, because for both types of circuit:
In the case of the star circuit, the strand current is equal to the conductor current, but the strand voltage is smaller by a factor root(3) than the conductor-conductor voltage.
In the triangular circuit, the strand voltage is equal to the conductor-conductor voltage, but for this the strand current is smaller by a factor of root(3).
If you compare the two results, you can see that the strands are the same. Whether the root(3) is now in the denominator of the current or the voltage does not matter.
We want to have the overall performance, both for star and triangle.
Here, too, we see the results are the same. Where is the fact that the power of the motor under triangle is then greater?
This is due to the fact that the conductor current IL, which appears identical above in the equations, is not identical in reality because the conductor current actually measured in the triangular circuit is greater than the conductor current of the star circuit.
For the triangular circuit itself, the triangular conductor current is still larger by a factor of root(3) than the triangular stator current,
but
compared to the star-conductor current, the triangular conductor current is greater by a factor of 3!
You must not forget that the windings in the star receive only 230V, but 400V in the triangle.
More voltage β more current β Much more performance!
In simple numbers to imagine:
If you have an ohmic resistance and double the voltage, then the current also doubles.
Double voltage times double current = 4Γ power.
The star/triangle switching therefore simply selects only between small voltage and large voltage.
The rated current of a motor applies only to rated voltage and a fixed power. In idle, blocked or over/under the rated voltage, the actual current is different.
You must not expect the rated current to adapt to the voltage to keep a rated power constant. If you change a parameter, everything changes, then nothing remains at the same value!