Delta to Permanent Star Conversion

The efficiency of any machine = Output/Input = Output/(Output+Losses)

Thus the efficiency of a machine is dependant on its losses.

In an electrical machine the losses are

- Friction Losses (depends on speed)

- Rotational loss (wind drag, fan drag etc) - depends on speed

- Noise and Vibration losses

- Copper losses ( dependant on current and hence Load)

-Iron Losses (depends on voltage).

With the first three almost constant now, by basic maxima and minima (calculus) the efficiency of the motor is maximum when the Copper and Iron losses are equal.

Obviously this is planned to be the condition on full load.

As the load goes down, the Copper loasses go down and iron losses comparatively go up.

To get a balance back, the voltage across the windings must be reduced (reducing the Iron loss) but to have the same power now the copper losses go up (more current).

This is achieved by the Y connection.

In the permanent Y connection you don;t switch over from Y to Δ (which is done for increasing the full load torque from increasing the starting torque). n this the system is run at the Y through out. Here though the full load torque is reduced, the motor will not stall, since the full load will never be demanded from it.

Further to your explanation:-

In a Delta config you apply the phase voltage over each winding individually, in "Y" you apply the phase voltage over two windings in series, therefore double the impedance, this cuts cuts current usage as well as torque....

Excuse me

In USA we call line (phase to phase) voltage delivered by at least three wires (for each of three phases).

And in Delta those voltages are across each of three windings.

Applying line voltage (e.g. 480 V) in Star or Wye connection to two windings (in real we have to connect all three wires to three motor terminals) creates phase voltage (e.g. 277 V) across one winding The nature of three phase systems is that there is "phase shift of " 120 electrical degrees between phase voltages. AC Circuits -101_ Experiments in three-phase systems with an oscilloscope.

In Europe we had 380/220V (motors) and 400/231V (secondary transformer's windings supplying low voltage distribution system). In both systems about 5% is for voltage drop.

By changing your connection from delta to wye(star), the voltage will be reduced to 1/sqrt(3) or 58%. Since the torque (and therefore the horsepower) varies with the square of the voltage, the torque(horsepower) will be reduced to 58% x 58% or 33%. The effective hp of the motor will be reduced to 33% of the original hp. This will significantly reduce the motor energy consumption but then you can only load it at 33% of its original hp. Beyond that will be overloading.

If you are sure that the motor load is less always than 33% of the original hp, then converting it to wye from delta would be feasible.

Hi,

As sb and myn1996 both mentioned it is feasible to permanantly connect in star, and I would actually reduce the motor size rather than run it in star. Motor are designed at specified voltages and are more efficient running at these voltages and loaded (obviously less than full load) and giving better savings.

regards,

Trevor.

Open motor's terminal box.

You may do reconnection only if each of three phase windings have both terminal accessible:

For example: T1 & T4 / T2-T5 / T3-T6 (USA standard)

1. Short T4-T5-T6

2. Connect power supplying wires: L1 to T1 , L2 to T2 and L3 to T3

3. Use only one of existing (or install a new one) contactor at the Power Supply Unit (el-cabinet, control box?) to control On-Off your motor.

In Europe and some other countries you should see terminals marked : Beginning windings U-V-W and ends X-Y-Z. Do the same; short (bridge) X-Y-Z and connect power from contactor to U-V-W.

Is this what you ask for?

Why dont u install a veriable Frequency drive & reduce RPM, it will help u to save huge power, Payback will be very fast

The author's idea is the simplest and, of course, realistic in such small required mechanical load. I believe he needs only a diagram of necessary connections (re-connections). And this is easier to do on drawing w/o hundreds of words.

The OP asked for cause and not method

You are right.

let me do corrections in the simplest way:

Comparing currents in Delta (100%) in Star or Wye connection current is lower P = 3*V ph*I*cos phi .Lower current ->less power needed. Result (cause?) = saving on kWh.

To show full physics (electromagnetic motor behavior vs mechanic power on shaft) read Electric Machinery -101 text books.Or Search this topic.

For practical / application aooroach do experiments - measure.