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  1. #1
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    Understanding star delta


    Hi guys, this is something that i and many like me seem to struggle with. I wondered if anyone had any good links to a download or even some advice on understanding Star and delta. Iv tried searching online but iv not found much that explains it very well.



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    Re: Understanding star delta

    What do you mean? Do you want to understand the windings and which terminal/wire to connect to what phase or do you just wander why it's used?

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    Re: Understanding star delta

    A bit of everything really Id like to know the difference between star and delta and when the two are most commonly used. Why would you use star over delta and vise versa.

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    Re: Understanding star delta

    Hi Marc,
    Basically it is to reduce the starting current.

    Have a look at the following:

    http://www.lmphotonics.com/m_start.htm#StarDelta

    hope its of use.

    If you look at a motor rating plate you will often see two voltages mention on it....the lower is in Delta connection, the higher is in Star connection.

    Adrian
    Last edited by Electrocoolman; 12-09-2007 at 10:21 PM.

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    Re: Understanding star delta

    From an earlier discussion on the forum.....
    http://www.allshookup.org/specs/hydulic/stardelta.htm

    Some of the links from that thread are no longer valid so I won't point you to it
    Brian - Torquay, Devon, UK
    I have to stop saying "how stupid can you be?" to my co-workers.
    They're starting to take it as a challenge...

    BASIC MAINTENANCE. If it doesn't move and it should then use WD40. If it moves but it shouldn't then use Duct Tape.

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    Re: Understanding star delta

    Hi if you ever read any of my other post you will see I am a bit of a geek when it comes to colating diagrams to help me remember things i dont work on in a day to day basis, so here is what i hold on star / delta, hope it helps in understanding the wiring ethics.
    STAR DELTA START UP PRINCIPLES

    Important that the pause between star contactor switch off and Delta contactor switch is on correct. This is because Star contactor must be reliably quenched before Delta contactor is activated. It is also important that the switch over pause is not too long.

    For 415v Star Connection voltage is effectively reduced to 58% or 240v. The equivalent of 33% that is obtained with Direct Online (DOL) starting.
    If Star connection has sufficient torque to run up to 75% or %80 of full load speed, then the motor can be connected in Delta mode.
    When connected to Delta configuration the phase voltage increases by a ration of V3 or 173%. The phase currents increase by the same ratio. The line current increases three times its value in star connection.
    During transition period of switchover the motor must be free running with little deceleration. While this is happening "Coasting" it may generate a voltage of its own, and on connection to the supply this voltage can randomly add to or subtract from the applied line voltage. This is known as transient current. Only lasting a few milliseconds it causes voltage surges and spikes. Known as a changeover transient.
    Attached Images Attached Images

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    Re: Understanding star delta

    Was really tired last night so didn't have the time to respond yet, but there's no need anymore! Coolments explained it perfect!

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    Re: Understanding star delta

    Hi, thanks for your help, sorrry for the late reply iv been trying to get my head around it. One4 thing that also confuses me is that iv been taught on a single phase relay/compressor that a neutral always goes to the run terminal and if it doesnt then the relay/compressor will just "hum". if that is the case though why on a 3phase do i not need to put a neutral to any of the terminals?

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    Re: Understanding star delta

    The other lives in a three phase motor complete the circuit.

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    Re: Understanding star delta

    Marc,
    When a 3 phase system is 'balanced' as it is in a 3 phase motor, then the sum of the currents at the star point is Zero - i.e. they cancel each other out, and so there is no need to have a neutral connection.
    When in Delta, there is obviously no 'star point', but the currents still sum to zero.
    The best way is to draw your 3 phases on a graph (x axis = time) and then sum them on the vertical, and at any point the total will be zero.
    ECM.

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    Re: Understanding star delta

    Quote Originally Posted by marc5180 View Post
    do i not need to put a neutral to any of the terminals?
    There is no neutral on a three phase motor.
    The three terminals have feeds fed to them that are sequenced at 50hz (50 times a second)(in the UK) because the feed is alternating current.
    So because the alternating curent is sequenced to the motor, the power going to the windings is + and - alternating between the phases 50 times a sec.


    confused yet
    taz.

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    Re: Understanding star delta

    Yes very
    Sorry for lthe late reply, iv been on holiday and just arrived back. Thanks for your help though guys.

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    Re: Understanding star delta

    Hi, thanks for your help, sorrry for the late reply iv been trying to get my head around it. One4 thing that also confuses me is that iv been taught on a single phase relay/Compressor that a neutral always goes to the run terminal and if it doesnt then the relay/Compressor will just "hum". if that is the case though why on a 3phase do i not need to put a neutral to any of the terminals?
    If you are using a neutral, I wonder if you are looking at a 120 volt application. In the US, the neutral provides us a 120 volt power supply which is used for all household receptacle and lighting (low amperage, single phase loads) A delta electrical supply will always have a "high" leg, where the potential to neutral is close to 200 Volts. By the way, again, in the US, a 240 power supply (less common) is the result of a delta transformer, and a 208 power supply (more common) is the result of a Y or star transformer. We occasionally find 120 volt components burning out on new installations because the "high" leg is used. I am really weak on this. Let me find a web page for you to refer to.

    http://polk-burnett.apogee.net/pd/dsaud.asp

    One should never become too old to relearn things.
    Last edited by Dan; 23-09-2007 at 02:02 PM.

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    Re: Understanding star delta

    Quote Originally Posted by marc5180 View Post
    Yes very
    Sorry for lthe late reply, iv been on holiday and just arrived back. Thanks for your help though guys.

    Ah lovely.
    You've been relaxing while weve been holding the fort making sure all the locals can sleep safe and sound in their air conditioned rooms at night.

    Hope you had a good time.

    All the best.

    taz.

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    Re: Understanding star delta

    Quote Originally Posted by marc5180 View Post
    One4 thing that also confuses me is that iv been taught on a single phase relay/compressor that a neutral always goes to the run terminal ?

    I think of three phase like this.

    Imagine the generator turning away back at the power station generating electricity.
    As the generator turns it pushes the electricity out in waves. When the waves reach the electric motor that needs to turn the waves of electricity push the rotor round.
    These waves are 50 times a second so we can't see the staged movement, it's too quick.

    Now the generator is three phase but because it is rotating the electricity can only be pushed down one wire at a time so the phases never short out because the electricity move round them one at a time one after the other.

    Imagine a round about in a park and one person wants to spin it. He would grab a rail and push it, then another as it came round and then yet another.
    Continually grabing and pushing one after the other. Never does he touch more than one rail at a time but the roundabout increases speed.

    Cheers taz.

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    Re: Understanding star delta

    Quote Originally Posted by Dan View Post
    If you are using a neutral, I wonder if you are looking at a 120 volt application. In the US, the neutral provides us a 120 volt power supply which is used for all household receptacle and lighting (low amperage, single phase loads) A delta electrical supply will always have a "high" leg, where the potential to neutral is close to 200 Volts. By the way, again, in the US, a 240 power supply (less common) is the result of a delta transformer, and a 208 power supply (more common) is the result of a Y or star transformer. We occasionally find 120 volt components burning out on new installations because the "high" leg is used. I am really weak on this. Let me find a web page for you to refer to.

    http://polk-burnett.apogee.net/pd/dsaud.asp

    One should never become too old to relearn things.
    Very interesting Dan, but I'm confused as to where you would use the 208 volt supply?

    I can appreciate that the 120v (110V in the UK) is used for safety purposes, and the 240V supply is used in the US for equipment that would draw excessive current on 120V, I also understand 3 phase very well.

    I'm at a loss as to why you would also need 208V?
    I'm back on the Pale

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    Re: Understanding star delta

    Quote Originally Posted by frank View Post
    Very interesting Dan, but I'm confused as to where you would use the 208 volt supply?

    I can appreciate that the 120v (110V in the UK) is used for safety purposes, and the 240V supply is used in the US for equipment that would draw excessive current on 120V, I also understand 3 phase very well.

    I'm at a loss as to why you would also need 208V?
    Hi Frank. 208 is by far the common power supply in commercial and residential buildings. I think it is simply because you have 120 volt potential to the neutral between all three legs. Most all motors of any significant horsepower are wound such as they will operate within the 208-230 volt range. In Florida (my experience is not wide ranging) we see 240 volt (Delta) systems in the more rural areas, and my suspicion is that they are the older distribution systems.

    Being able to balance the 120 volt potential across all three legs is much more efficient than only being able to use it across 2 legs.

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    Re: Understanding star delta

    Sounds a bit like the 480v split phase in the uk that i ocasionally see on farms. The 480v compressors have one start winding and two run windings connected to each of the phases. the phases are 180 degrees out of sinc. as opposed to 120 degrees with 415v three phase. They have a neutral post but will run without it connected.

    Jon

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    Re: Understanding star delta

    Quote Originally Posted by Dan View Post
    Being able to balance the 120 volt potential across all three legs is much more efficient than only being able to use it across 2 legs.
    Dan is exactly right. Although I am not a fan of 208V, in a commercial building it does have a purpose. Most of the loads are 120V, with a water heater and a AC unit connected to the panel.
    Since a 120V circuit cannot be connected to the high leg of the panel, you have to have a bigger service than would be necessary otherwise.
    I work mostly Industrial, and have a low opinion of 208V for operating compressors and the like-no matter what the Motor Dataplate says!

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    Re: Understanding star delta

    Try wikipedia.com. covers most things

    Quote Originally Posted by marc5180 View Post
    Hi guys, this is something that i and many like me seem to struggle with. I wondered if anyone had any good links to a download or even some advice on understanding Star and delta. Iv tried searching online but iv not found much that explains it very well.

  21. #21
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    Re: Understanding star delta

    Frank,
    The 208v in the USA is their 3 phase Line to Line voltage, equivalent to our 415v. Phase to star point - Neutral (and Earth, as the neutral is normally earthed) is 120v (208 / 1.732) NB: 1.732 = sq root of 3 .
    For heavy loads they use 2 phases i.e. 208v in order to reduce the current. As mentioned elsewhere 220-240v equipment will still operate at this voltage satisfactorily.
    You will find that american 220v equipment is supplied through a double pole cb / isolator such that both phases are switched off.

    Adrian

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    Re: Understanding star delta

    Quote Originally Posted by Electrocoolman View Post
    As mentioned elsewhere 220-240v equipment will still operate at this voltage satisfactorily.Adrian
    Not always. I have found many Industrial Installations where the change to 208V caused many problems, New equipment of the Commercial nature is not usually a problem. Particularly in older plants is where problems will occur.

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