Y or Delta configuration?

Hello all,

We have a motor which can be hooked up either in Y configuration, or Delta configuration. Which configuration will give more start torque, and then the most operational torque, and why in each case?


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Looking on the name plate of the motor has not explained the difference?

Or is this a home work question?

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I would say delta since it might be slightly more efficient (240 vs

208v) but the difference would be m>Hello all,
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Is this a wye start delta run single voltage motor or a dual voltage motor with a voltage ratio of 3^.5?

-=-=- ... Is it ignorance or apathy? I don't know and don't care.

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The 2 configurations will give you the same power, couse the delta config is for lower voltage, let say 220 or 110 volt 3 phase and the Y config is for higer voltage, 380 or 220 volt 3 phase. The are motors that use 660 volt for high (Y) and 380 for low (delta).

Edd wrote:

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| Hiking wrote: |> Hello all, |> |> We have a motor which can be hooked up either in Y configuration, or |> Delta configuration. Which configuration will give more start torque, |> and then the most operational torque, and why in each case? |> |> Thanks. | | Is this a wye start delta run single voltage motor or a dual | voltage motor with a voltage ratio of 3^.5?

A wye start delta run is a different animal than a motor that can run on two voltages at a 1.732:1 ratio.

Available supply voltages at or near that ratio are rare and only in heavy industrial situations. If you have three phase power in either delta or wye configuration, you can connect a motor to A-B-C and not N and run it. There's no need to design a motor for 120, 139, 277, or 347 volt service since you can just design it for 208, 240, 480, and 600 volt service and ignore the fact that the neutral exists. In Europe it's even simpler as if you have three phase, you have 400 volts (380-416). Motors designed for that supply would be designed for 400 volt A-B-C connect.

Only very heavy duty motors where 600 volts isn't enough would have to do other considerations.

As for the wye-start delta-run motors, it's a cheap way to get a lower start voltage. If you have delta service, you could do it with a zig-zag transformer to create a neutral, but it would be cheaper to do a 2:1 anyway.

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Never mind guys, I wasn't so much concerned with which configuration to connect for its own sake, the boys here will do trial-and-error and will wire it from there. The guys here don't care why. For myself, I was only interested in the "why", in understanding the "why"s of this situation.

I remember from school many, many years ago that torque was a result of the phase differential between the outer winding (what do you call this one again?) and the induced EMF in the armature, the greater the difference, the more torque you get. As the armature gains speed and nears synchronization with the rotating EMF in the outer core, the torque is reduced conversely. I understand the EMF in the outer core is not really "rotating", but the effect is of a rotating field... and if I understand this correctly, by increasing the frequency of the outer-core field, one could increase the operational speed of the armature, correct? I suspect, however, that this higher frequency would also result in a reduced starting torque capability for the same motor?

But this is in a Squirrel Cage, single-phase induction motor, I have no idea what the situation is with a 3-phase motor... I expect it's the same operational principle, only difference being that there are three outer-core windings.

What I was really hoping to understand is how these dynamics are different with these two different wiring configuration.

If anyone can explain what happens in this motor/system under these two varying configurations, and has the time and desire to explain it, I am all ears and appreciative.

Can you guess I drove my elect teacher nuts? I have to hand it to him, however, he was extremely knowledgeable about the internal, theoretical method of operation of elect motors... good 'ol Mr. O'Brien. Though he seemed to think it was a waste of time for me to understand elect theory to this level, he did humour me and I did understand a lot of it and it still is a big help... the problem being that as soon as I learned it, I ended up going into another field of work and never used/applied, nor thought of electricity since, for over 12 years, so, unfortunately, forgot a lot, and am just trying to refresh my memory.

If you're in a teaching mood, I'd love to learn/understand this subject a whole lot better. Thanks.

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My recollection is that Y start delt run is normally for high HP motors. The windings are the same but you connect them differently. You start in Y configuration, which draws less current (you don't have full line voltage across the windings), then switch to delta to run. Ths requires a Y-delta starter or connecting multiple starters to do the same. A Y-delta starter probably has the contactors mechanically interlocked, which is better. Y-delta is to reduce the start current on large HP motors - so you don't dim the lights on your block - also may be used for high inertia mechanical loads that take a long time to start.

3 phase squirel cage motors operate the single phase except you don't need a start winding because the 'rotation' of the phases gives a direction for the rotor to turn.

Your description sounds good. The current in each phase produces what appears to be a rotating magnetic field in the stator. This produces a current in the cast aluminum bars in the rotor. The induced current produces a magnetic field that pulls the rotor to catch up with the stator's rotating magnetic field. The greater the difference between the stator field rotation and the rotor rotation, the greater the rotor current and thus torque. The rotor can' rotate as fast as the stator's magnetic field (synchronous speed) because there is no torque at that speed. The difference between the speeds is the "slip". Hope this isn't overkill.



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Thanks Bud,

that seems to confirm that I have the right idea about how the motor works, or is made to work (rotate), but what I'm trying to visualize is the different winding configs (Y as opposed to Delta), and how each works (trying to understand this in the same way as I understand the

1-phase Squirrel Cage induction motor).

Now 'THAT' may be overkill, but if someone wants to tackle it, I'll take the beating.

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visualize is

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Its easy if you draw the delta and wye diagrams showing Line


the resistance between the phases with delta is half what it is with a wye (no center tap to neutral).

Draw the diagrams you will see, assign each winding say a resistance of 5 ohms...for the higher voltage wye connection the resistance doubles giving the same VA for either configuration..

You would use the wye configuation when using *high voltage source on a dual voltage motor... ~delta to wire for the ~lower voltage... torque would be the same for all practical purposes. but thats for a wye with no center tap. only Line

1,2,3 going to the motor, no neutral.

If the wye is center tapped to neutral thats a different issue entirely (4 wires to the motor, L 1,2 3 and a neutral)... thats used for dual voltage soft start configurations, thelower voltage used in the center tapped wye configuration.

when the motor and load gains sufficient speed, the center tap to neutral can be dropped and high voltage put to line

1,2,3... no hot legs allowed etc, (because of the soft start wye configuration) the primary transformer arrangements must be compatible to that use.

There are other options for soft start not involving wye configurations but two separate windings in each phase... series for the soft start, parallel for full bore operation. same voltage in each case.

Check your lines to neutral to see if there is a hot leg. 240

3 phase for instance with no hot leg will read 120 to neutral on each leg... if it has a hot leg, that one will read much higher to neutral (but the same between phases as the others), and shoud be marked with red tape on the leg. You cant use such power in a center tapped wye connected motor...but you can use it in a delta connected motor.. or non center tapped wye motor.

Phil Scott

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Phil Scott

| The rotating field of a 3 phase motor is a true rotating field. That of a | single phase motor has two rotationg components acting in opposite | directions. The 3 phase machine doesn't need special tricks to get started | and has smoother torque than a single phase machine as well as a better | size(weight) to power ratio. | The rotor only sees the rotating field. The torque is dependent on speed as | you indicate and the output power depends on the product of torque and | speed. Consider a 3 phase 208V 1 HP 1760 rpm motor. supplied at 208V and | connected in delta. The torque at rated speed will be the same as that of a | 208V, 1HP 1760rpm motor connected in Y. The line current and voltage will be | the same in both cases but the coil voltage will be higher by a factor of | root(3) in the delta than in the Y. The coil current will be lower in the | delta by the same factor. Total power input will be the same at a given | torque and speed. A delta winding will have more turns per coil by the same | factor so the ampere turns producing the field will be the same in both | machines. The rotor doesn't see the difference. A dual voltage machine is | designed with multiple coils per phase which can be reconnected to either Y | or delta without exceeding coil voltage and current ratings.

If you're connecting the coils L-N at 120 volts each, then I would call that a 120 volt motor. Those would be 120 volt coils.

But it can be a useful motor design to have dual coils so it can be connected with parallel coils L-N on 208Y/120 and series coils L-L on 240 delta. I would call that a 120/240 volt motor, not a 208 volt motor. But it gets around the issues of 208Y/120 systems.

But what about a star-point connection? With 120 volt coils, connect one wire of each to an internal star point, and the other wires to each of the 3 phase lines. The voltage between the star-point and neutral that is not connected should be close to zero, right? So would it even need to be connected?

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