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I have been studying three-phase systems for the whole course of a subject (on the first year of the university degree). I have finished now, and I know both "Y" (star) or "Delta" (triangle) connections. I have made a lot of computations with them, however I don't know the different applications they have and I would like to know the following in order to increase my knowledge.

I would like to know which one is better (Y or Delta) for different purposes, they must have its advantages and disadvantages, but I have never been told which ones are them. I have tried to do some research on the Internet, but I haven't found in particular a good answer. I have only seen the advantages and disadvantages of Y and Delta motor startup, but I'm thinking more of the "circuit" point of view.

I'm really interested on the subject, but I just seen it from the computational point of view. I would appreciate if someone could explain me a little bit some of the main advantages and disadvantages of using both connections. Thank you.

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    \$\begingroup\$ A couple of simple advantages for delta: in a transformer linking 2 delta wired circuits the secondary will give you all 3 phases out even if one is lost in the primary. Sometimes you don't want to pay for the 4th wire (But there is no magic, you need enough cross section of copper for all the current somewhere). \$\endgroup\$ – Will Dec 26 '14 at 10:29
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    \$\begingroup\$ A simple advantage of star is that sometimes you want a single phase at a lower voltage. Like in the uk where houses are fed with a phase to star point voltage of 230 volts and industrial machinery is fed all 3 phases so can take advantage of 400ish volts from phase to phase. So a star wired distribution system can give you a low voltage single phase and a choice of delta or star connection to big loads \$\endgroup\$ – Will Dec 26 '14 at 10:33
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The two systems have vastly different applications. Yes, there is a lot of crossover between them in some fields, but the two are more suited to certain applications.

Take motors for instance. Delta is far superior for driving motors than star. With delta you can visualize a wave circulating around the triangle, and it's that wave that turns the motor. As the wave moves around the phases it effectively drags the motor around with it. It makes motor design really simple and efficient. Not so with star, where you in essence have to try and combine three single-phase motors in together,

However, when it comes to a situation where you want to spread a load between multiple circuits or devices, and the load on each phase may not be equal (unbalanced system) then a star arrangement has massive advantages. Each branch of the star (phase) is a separate circuit in its own right. The load on each phase is specific to that phase, and they have little influence on each other.

There is also a third arrangement, which is kind of half way between a star and a delta - in this arrangement each delta phase is connected with its own completely separate transformer and there is no common neutral point. This is actually seldom seen much, but I thought I should mention it here anyway. It basically combines both the star arrangement with full isolation, so can have some safety advantages (like having an isolation transformer on a normal single-phase supply) but isn't worth the hassle of a system without a common neutral point.

To clarify what I mean about a wave rotating around a delta, here is a little animation I knocked up:

enter image description here


Note: It's Christmas Day, I'm drunk, and that might all have been complete gibberish for all I know.

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  • \$\begingroup\$ +1, and the final note made me smile. Just curious, I'm not sure I know what you mean by that "wave" in delta and the necessity for 3 single phase motors in star. AC motors work well in star, they just have a different torque/speed curve - less torque, so they usually are "soft started" with star and then switched to delta. \$\endgroup\$ – Mister Mystère Dec 25 '14 at 19:04
  • \$\begingroup\$ With a star each phase is an individual entity. Yes, they all work together, but they aren't the homogeneous item that delta is. As you note, star motors don't have the torque of delta and usually need special treatment to get them started and running. That's not to say they don't have their place, but it's far more common to use the simpler delta method, both for torque and cost. \$\endgroup\$ – Majenko Dec 25 '14 at 19:15
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    \$\begingroup\$ If you replace the triangle of the delta with a circle, and place a single cycle of a sine wave around that circle, then rotate it (change the phase of the sine wave) you can see that wave spin around the circle's axis. \$\endgroup\$ – Majenko Dec 25 '14 at 19:16
  • \$\begingroup\$ Right, I see it now. But isn't that what the magnetic field is doing anyway, regardless of delta or star (only with a different phase at the origin)? I've thought of star as a way to apply phase-neutral voltages to windings, and delta to apply phase-phase. \$\endgroup\$ – Mister Mystère Dec 25 '14 at 19:58
  • \$\begingroup\$ physique.vije.net/TSTI/6_tensions.gif shows phase neutral (V) and phase phase voltages (U) - Star and Delta should therefore have similar torques and currents, related by a constant factor. \$\endgroup\$ – Mister Mystère Dec 25 '14 at 20:03
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Delta is great for balanced three-phase loads and has big advantages in eliminating 3rd harmonics. (You probably covered this in your course.)

One problem with delta is that there is no wye / star point so loads that require a neutral connnection can't be connected. For this reason European domestic power distribution is often three-phase delta at 10 - 20 kV to the local transformer which has a delta primary and wye / star secondary. Each home will be fed from a phase and a neutral connected to the star point and ground.

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You can get the same voltage and same power out of each, with the right winding ratios. The advantages I've seen are usually related to how you want the phases referenced to something else.

One advantage of Y is that you get a way to symmetrically reference all three phases to the same voltage (usually earth). If you've got 480VAC line-to-line three-phase AC, that tells you nothing about how far those voltages are from the metal box your electronics are in. If that box is grounded, but the AC lines are all 10 kV away from ground, bad things will happen to your insulation. Tying your neutral to earth allows you to avoid that, and be 100% sure that all three line are within an acceptable voltage of earth at all times.

Having a neutral can also reduce noise, for similar reasons. If the AC lines can suddenly shift relative to the grounded enclosure, that common-mode noise can couple in through parasitic capacitance and wreak havoc on your control and sensing circuits.

And with a neutral you get an obvious defined neutral path for fault, imbalance, or harmonic currents. Those currents having a definite path back to earth means they can be detected more easily, and thus reacted to.

Delta has no obvious grounding location; the AC lines are generally all floating relative to earth. Now, there are exceptions. I've seen corner-grounded systems where one phase is tied to earth. I've seen a center-tap on one phase that's tied to earth. But I think it would be fair to say those are hacks, trying to add a ground reference to what should be a Y transformer, but isn't for historical reasons.

Why would you want to have no reference to earth? Power transmission over long distances. Ground voltage varies from location to location; you can't just tie ground in one building to ground in another building, or you'll have a ground loop and constant current flow through your neutral/ground conductors. If you're only dealing with transmission, and local grounding is expressly not a factor, delta lets you save money by avoiding stringing an extra cable for no good reason.

So the way I usually see things done in an industrial setting is to run power in a delta configuration all the way to the point of use, then transform to Y to get a local earth reference for the equipment.

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In the heavy duty power world multi (3) phased delta is used when generating electrical power, and is the preffered way to consume large power quantities as well. Phase loads will be balanced, vibrations minimized, cable capasity maximized, ...

Outside the power world single phase is preffered for convenince.

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From a power system angle, the two are quite different when it comes to protection. For example, in a delta arrangement, detecting earth faults isn't as straight forward (unless you also have earthing transformers). The fact that delta may have no earth reference can be both an advantage or disadvantage depending on your situation (an earth fault won't cause fault current to flow for example). Transformer configurations are typically chosen to fit in with existing infrastructure; you might need a certain vector group tranny to match up two existing networks, and different vector shifts are achieved using different combinations of delta and star.

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  • \$\begingroup\$ "Delta typically has no earth reference" - in the power systems I work with, delta systems are always grounded through earthing transformers. I have never seen a completely un-grounded delta system. \$\endgroup\$ – Li-aung Yip May 14 '15 at 10:01
  • \$\begingroup\$ Yea I think 'typically' was wrong here - I was thinking of industrial applications where there some older ungrounded delta systems around. I've never seen one in any of the distribution/transmission systems I've worked on either. \$\endgroup\$ – Raggles May 27 '15 at 19:41
  • \$\begingroup\$ I don't recommend ungrounded delta systems, but I have seen and worked with a lot of them. I believe the case is very strong for a Y-system with the center-point grounded, even though it is somewhat more expensive. Further, although a Y-system can be operated as a 3-wire system, there are operational advantages (ground-fault sensitivity, for example) to running all 4 wires. System reliability will pay for the extra costs of choosing the grounded Y AND of running the 4th wire. \$\endgroup\$ – Tom Johnson Feb 27 '18 at 20:33
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In delta connection takes higher current but in star connection current is one-third of delta connection so to protect motors from high starting current motor are connected in star during starting. But torque is directly porpotional to square of the voltage so to have better performace of induction motor it needed to be connected in delta. So nomally star delta starter is used for induction motor.

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If one primary of a delta/delta transformer fails the secondary will still have all three phases. Just the nature of a triangle. A property often exploited to reduce the transformer cost by 1/3 on small installations.(called open delta in the USA) enter image description here

Floating (no ground reference) Delta is often found on ships, they use detectors and monitors rather than traditional fuses to handle faults to the hull("ground"). For several reasons, they don't want a ground short to pass excess current through certain parts of the hull, nor stop the motors during critical maneuvers in high seas. But the biggest factor likely has to due with galvanic corrosion as many larger vessels rely on actively applied voltage and current to make the hull the cathode in the voltaic pile that is corrosion.Having a multi megawatt propulsion circuit grounded to the hull would make this a challenge for sure.

The delta circuit voltage relative to the hull can be kept in reasonable limits with simple electronics as any major drift is basically the current equivalent of a static charge and the excess can be balance/drained with some moderately high impedance resistors, zener diodes, and the like attached between the hull and all three phases.

Also, back on land, if put a delta/delta in parallel with a wye/delta transformer(or wye/wye delta/wye) you rotate the phases and end up with a 6-phase system, which is used for some large industrial purposes, both very large motors (smoother torque and more reasonable amperage on each phase) and before rectifying to DC (less ripple). Disadvantage vs 3phase is the need for twice the parts and pieces and complexity of connections, with 3 all connection patterns yield either smooth clockwise abc or CCW cba rotation. With 6 phases you can get them out of order so rather than abcdef or fedcba, you might accidentally skip forward and back in the list like adcfeb, which would likely would just twitch and not turn at all or be very rough and inefficient.

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  • \$\begingroup\$ I think your first paragraph is erroneous. If you lose a phase on the input of a delta/delta transformer you are effectively single-phasing it. All three windings will be in-phase or anti-phase with V/2 applied to the series connected coils. \$\endgroup\$ – Transistor Jan 7 '18 at 13:22
  • \$\begingroup\$ Through the mid 20th century power companies routinely used only 2 transformers phases to supply 4-wire 240/208/120 open-delta for small commercial customers that needed a both 120/240 single phase and some 240v 3phase. The single 208 leg was not considered stable(with loads on other legs) and only infrequently used. \$\endgroup\$ – Max Power Jan 7 '18 at 13:35
  • \$\begingroup\$ Can you post a link to a diagram or article? I can't see how it can work. You might also mention what country used this approach. With those voltages it sounds like North America. \$\endgroup\$ – Transistor Jan 7 '18 at 13:37
  • \$\begingroup\$ OK. Those all still have three phases but have removed a winding or coil. Your first sentence wasn't clear to me on first reading. If you embed the image 2(b) from that article (and give the source reference) your meaning will be very clear. \$\endgroup\$ – Transistor Jan 7 '18 at 13:53
  • \$\begingroup\$ yourelectrichome.com/2011/05/… If you centertap one of the two secondary windings you will have two 120 and a 208 "wild leg" from the center tap to each corner. This is in the USA but is likely used in any place with 3 phase supply. (it simply saves the cost of one transformer) another variation ties a corner to ground and has no center tap. It is often used for remote small loads like irrigation pumps and pivot motors on farms. \$\endgroup\$ – Max Power Jan 7 '18 at 13:56
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Get rid of wiring procedure (there are thousands link and pics on the internet) . Delta is used generally and get the best efficiency, yet only one simple thing is because the bigger motor is required much higher current and with these amount of the current can fail some components (inside or outside)the systems . Then the star wiring is needed to use at the initial. So,sometime we call soft start.

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