Star-Delta Connections ...

http://www.google.com/search?q=delta+vs.+wye

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on reading a couple, there are pros and cons for both......depends mainly on what you are doing with the power

The primary use of delta is for long-distance, high voltage distribution, 'cause it takes less wire.

Some industrial plants may have large, delta-circuit, motors which run on relatively high voltages (actually called medium voltages: 2400 - 7200) and are fed directly from distribution mains (through disconnects, motor control centers etc.), the frames of which are grounded. Normally, from medium voltages, delta - wye transformers are used to provide lower voltages, such as 480/240/277 or 120/208, and the center of the wye is grounded.

There's lots to learn about this . . .

Star or Delta connection is not so different in low voltage area. In the viewpoint of safety, Star connection can detect ground fault easily. In case of Delta connection, it is hard to detect ground fault.

In addition, high-voltage and high-power area of power distribution system, Star connection has some major point. For example, in transformer one side of coil of each phase can connect to ground potential. It means that the insulation strength of transformer can be reduced compared with Delta connection . It realize cost down of transformer. Delta connected coil need same insulation strength on both side of coil, so it need more cost for insulation. But in Star connection, harmonics problem will appear. In case of Delta connection, harmonics current is circulating in delta-connected coils and not appear in distribution line. If the harmonic current was there, it will be a cause of over heat of lead-phase capacitor, lag-phase inductance and transformaer.

I'm not a hands on man, but the question remines me of the star/delta starter switch that we used to start electric motors, In that case I think the advantage to use this sytem was starting torque. Parhaps someone with practical experience may correct me.

Star/Delta, with reference to medium to large motors using Star/Delta takes less load/current for starting of motors, this is useful on small ships tugs ect as you dont get a large drop in voltage.

...by starting the motor in Star, and switching to Delta as the motor runs up to speed.

You are right; star-delta is all about satarting torque. Motor over 5 HP should not be started in delta becaus of the high current take which can be a lot higher then the actual current rating of the motor when run on full load. Manual star-delta switching gear is still used although timer controlled switching transition is more popular. If you start, say, 5 HP, 3 phase motor in star, it will gradually build its speed within seconds (all depends on the HP of the motor) to the point where delta connection takes over. Soft starters are obviously a better option these days although they cost a lot more.

I have designed motor controls for multiple interlocked navy vaneaxial ventilation fans to maintain negative and positive air pressures in designated compartments in nuclear facilities. All were large 3-phase 450 volt delta motors and were started with across the line starters. Having 600 percent starting current was no big deal. You just size your distribution system and starter heater coils accordingly.

It's more related in the reduction if starting curent. During the starting of a motor the back emf will be less and the starting current will be 5-7 times the rated current there are chances of burning of winding of motor. so while starting we apply reduced voltage so as resulting reduced current. The sarting torque will also gets reduced as we are applying less voltage. Once after delta connection established, it will be able to develope enough torque.The main advantage of using this is for safe starting of higher rating motors.

Dear Shobha

one funda is not clear to me. When connected in star , line current is equal to phase current ( I ph= I L). So when motor is connected in star, full line line current will pass through winding. So how current reduction is happened is star connected winding?

Regards

If you have 3 resistors in a star connection , you can work out a resistor sizing to give the equivalent delta. This enables the lecturer to mark papers and other stuff.

I only looked in here because I don't know either !

variable resistors unit is call reostart to manually control starting current of the motor.

kim

usa

I hate u kim, I want to screw u

STAR&DELTA CONECTED RESISTENCE EQULENT RESITANCE

They both have advantages and disadvantages. They both can be ungrounded, but generally DELTA is found ungrounded.

In Ungrounded Deltas, the first fault does not shut down the system, but can have high Transient overvoltages that are not easily detected, ground faults are hard to locate. You usually find deltas at the connection of a tranformer bank.

Grounded WYE have the disadvantage of partial shutdown at first ground fault, but system potentials are limited to safe values and the location of a ground fault is immediately indicated. Wye connection improves service reliability, you can add ground fault and provides two voltage levels on the same system.

Hope this helps

A contact of mine intends to wire this boat as follows:

A 16kva 3 phase generator providing power to a 3 phase battery charger using Delta wiring. He has only three wires from the generator A B & C phase feeding into a 3 phase distribution box. The three output cables from the distribution box are armour shielded and run in ther own trunking, each output is protected by a suitable 3 phase CB. No earth is used.

He then wants to use the 24V batteries that the charger is charging, to feed a single phase sine wave inverter. He wants to take a 115V output off each side of a centre-tapped transformer to achieve 230V without any earth connection.

He is adamant that this is safe as if one of the 115V wires touches the steel boat hull, there is no return path ??

This is a little out of the scope of my normal work, but it sounds dodgy to me.

Is either of these practices safe on a boat ??

It seems to me that the way he wants to wire the boat is safe so long as no fault to "ground" -ship's hull-is present on the vessel.

However if an undetected fault is present and someone steps ashore they complete a circuit when one point of their body is touching the boat and the other part is grounded.

no they wouldnt. If they stepped ashore while one end of the winding was connected to the hull by a fault, they are simply bridging the connection from the "live" hull to the shore in parallel with the sea water, no return path to the other end of the inverter and hence no circuit, no current flow, no PD....

A shock from an inverter output can only be gotten if 1. one end is "earthed" on purpose then a user touches the other "live" end and makes a path to the other end via the hull with their body. Or 2. if they touch both output terminals at the same time. Thats why safety isolating transformers should not be earthed since both terminals have to be touched for a shock to occur.

AND - (same person here as last post)

to the OP asking about step by step, when the star is made, there are 2 windings across each phase pair, so each single winding receives half voltage (roughly) - draw the star with three resistors and youll see what i mean, that allows the motor to start against a heavy load and not draw too much current, since 2 windings are in phase at the same time and same polarity so two fields are pulling the rotor round at any one time, starting is reliable and current demand is low.

When the delta connects and the star is broken, its now three resistors in a triangle so each winding now receives full voltage, but only one winding is generating a peak field and pulling the rotor, since each winding is now independent and follows the phase sequence 120 degrees apart, and each single winding will draw all the current required to move the load, but since the motor is already moving and has inertia, the field required to maintain speed is much less so one winding pulling at a time is fine.

Motors draw more current started in delta, less in star, but high speed momentum and resistance to load change is poor in star as the winding voltages are halved, so for speed consistency and good flywheel effect on varying loads, delta gives better running smoothness and torque once full speed is achieved.

well! Delta circuit also has some advantages over star connections. Delta circuit provides a closed path to the third harmonics and eliminates the same for the external supply and thus subsides the possibility of maximum damage to the power wave..

Delta circuit also provides closed path to zero sequence current which has proved to cause overheating in transformers in the absence of any delta circuit.....

In early days way back to 1960, when cost was the matter of second consideration, delta circuits were used but now a days taking the economy in to consideration Star circuits are being used.