Hi - Star Delta, I don't understand it at all

My book says in Star the line current is equal to line voltage divided by impedance times square root 3; in Delta equal to line voltage times square root 3 divided by impedance, giving a 1:3 ratio for respective star and delta line currents.

I see Ohms Law, but not how star impedance is times square root 3, delta voltage times square root 3

Also I have two books each with a diagram of the star delta circuit: one has overload relays before the motor and one after the motor.

These statements and diagrams seem variously contradictory and nonsensical. They don't seem to bear any relation to the actual mechanics of three phase generated electricity and power plant, which may be because I don't understand the actual mechanics of three phase and why we're even talking about square roots and not whole numbers.

For instance: how can line current and phase current be equal in Star, since by Ohms Law line and phase voltage are square root 3 times different? Only if it's the impedance which is respectively determined by square root 3, not the voltage..and isn't the impedance in windings negligible anyway?

Many thanks for any assistance

 

Hi - Star Delta, I don't understand it at all

..and isn't the impedance in windings negligible anyway?

Many thanks for any assistance

Welcome to AAC.

I don‘t understand what you think impedance is, and this could be the source of your confusion. Can you explain your understanding of impedance?

 

Perhaps it's resistance then: I thought it was impedance for AC. The book I'm working from is Practical Marine Engineering Knowledge by Dennis Hall: it gives equations based on Ohms Law - as cited - with impedance for resistance, when explaining how Star start up allows for a third of the current.

 

Hello and welcome to AAC!
I see that you are having a challenge understanding 3 phase analysis, and that is not at all surprising. For starters, have you completely understood everything about DC circuits and circuit analysis? I am asking that because the three phase power lessons arrive about half way through the second semester as part of the AC circuit theory class. But first comes the DC theory and understanding, then into AC theory and the wonderful vector math. And the three phase material is presented AFTER all of the material discussing phase angles and impedance.

So the problem is that the text book section is assuming that you already have been through those two whole semesters and now are fully understanding both DC and AC circuit theory.
Three phase power is not a good starting place. It requires a good foundation to be able to easily grasp it.

 

First of all swap "impedance" for "resistance" until you fully understand 3-phase resistive loads. Then you can start to think about current phase angles and vectors.

Do you understand that a 230V AC phase-to-neutral voltage is a 400V AC phase-to-phase voltage?
In your book, is the load change from star to delta using the same value resistors, but changing how they are connected?

 

Perhaps it's resistance then: I thought it was impedance for AC. The book I'm working from is Practical Marine Engineering Knowledge by Dennis Hall: it gives equations based on Ohms Law - as cited - with impedance for resistance, when explaining how Star start up allows for a third of the current.

You aren’t wrong that impedance can be called “AC resistance” at the simplest level, but impedance is much more complex and so using it to understand what is happening. There are a lot of things you need to understand before you can do the analysis you are trying to do.

It looks like there are at least a couple of people who seem to want to help.

 

Visualize that in this way: In triangle between phases are 380 V, but from each phase to GND is 220 V. In Y between phases are 380, but from each phase to GND 220. Thus, first case the motor coils sees 380, in second case they sees 220. But if both motors have identical power, for example 3*3800 W, then first takes 3*380*10 A and second takes 3*220*17.3 A.
So, now we may declare the identicality - 380*i(triangle)=220*i(Y)=(1/3)*Power.

 

A fair analogy of three phase power would be a three cylinder engine, where one piston reaches the start of the power stroke every 10 degrees. That is the same as the difference between the 3 phases in electrical three phase power.
The rest of the explanation uses vectors and that gets confusing unless you understand vectors.

 

A fair analogy of three phase power would be a three cylinder engine, where one piston reaches the start of the power stroke every 10 degrees. That is the same as the difference between the 3 phases in electrical three phase power.
The rest of the explanation uses vectors and that gets confusing unless you understand vectors.

It is interesting that generator manufacturers seem to prefer 3-cylinder diesel engines.
(by the way, you have a typo - 240° between power strokes in a 3-cylinder engine - sorry for being picky)

 

It is interesting that generator manufacturers seem to prefer 3-cylinder diesel engines.
(by the way, you have a typo - 240° between power strokes in a 3-cylinder engine - sorry for being picky)

I was thinking a 2-stroke engine, and in addition the "2" key on this computer does not always function. And a multiple of 3 makes a lot of sense for directly driving a three phase generator, because both engines and generators have torque ripples.

 

I was thinking a 2-stroke engine, and in addition the "2" key on this computer does not always function. And a multiple of 3 makes a lot of sense for directly driving a three phase generator, because both engines and generators have torque ripples.

3-cylinder 2-stroke: Saab 96s, and East German Wartburgs!
The generators are definitely 4-stroke, but a 2-stroke engine makes a better analogy. An even better analogy is a radial aircraft engine.
I also have a computer with a dodgy “2”, I wonder why I keep getting compiler errors then see “uint3_t”.