Wye Start Delta Run Theory

Wye Start Delta Run. 

This method is actually reduced voltage but is accomplished by changing the motorphase connections such that a winding that is designed to run with phase voltage equal to line voltage on deltaconnection is wye connected for starting to put less than line voltage on each phase.

Power Circuit Diagram

Visualization of Line and Phase 

(Voltage and Current)

Why it is used in industry?

The star delta starting is an inexpensive two-step method of induction motor starting.The motor designed for delta running is started across full line voltage by connecting the phases in star as shown in diagram.

In direct delta starting:

Starting phase current, Isc=V/Zsc;V= line voltage

Starting line current, Is(delta)= √3Isc

In star starting:

Starting line (phase) current, Is (star)=Isc/√3

Therefore, Is(star)=Is(delta)/3

Hence, Ts(star)/Tfl=((Isc/Ifl)^2*sfl)/3

where Isc =starting phase current (delta), Ifl =Full load phase current (delta), sfl =slip at full load

It is thus seen that star delta starting reduces the starting torque to one third that obtainable by direct delta starting and also the starting line current to one-third.

A star delta starter is much cheaper and is commonly used for both small and medium sized motors.

While 3-phase power allows utilities to deliver more power over smaller, less expensive wires, there are more compelling reasons for using three-phase in the data center—specifically 3-phase Wye.

Why 3-Phase?

To understand electric power in the data center, you need to first understand single- and 3-phase power distribution. Most homes are wired with single-phase that uses one ac voltage delivered over two hot wires and one neutral wire. The voltage across the two hot wires measures 240VAC (for your oven or dryer) and across any hot to neutral measures 120VAC (for everything else).

Most commercial businesses are wired with 3-phase that consists of three ac voltages separated from each other by 120 electrical degrees, or by a third of a cycle. These systems deliver power over three hot wires where the voltage across any two hot wires measures 208VAC.

Another way to look at 3-phase power is as a combination of three single-phase circuits that deliver power in a way that it never falls to zero, meaning that the load is the same at any instant (the concept is easy to grasp when you look at the waveform).

Because the load is constant, 3-phase power is ideal for motors—it eliminates the need for starting capacitors. It also allows for smaller wires (i.e., less copper) and lower voltages for the same power transmission as single-phase, making it less expensive and safer.

Why Wye?

There are two types of circuits used to maintain equal load across the three hot wires in a 3-phase system—Delta and Wye. The Delta configuration has the three phases connected like a triangle, whereas the Wye (or “star”) configuration has all three loads connected at a single neutral point.

Delta systems have four wires—three hot and one ground. Wye systems have five wires—three hot, one neutral and one ground. While both Delta and Wye systems measure 208VAC between any two hot wires, Wye systems also measure 120VAC between any hot wire and neutral. In other words, it’s the neutral wire of the Wye system that allows for providing two different voltages and powering both 3-phase and single-phase devices in the data center.

That’s not to say that Delta doesn’t have its place—we mainly see Delta used for any large motors or heaters that don’t need a neutral. Delta is also used in power transmission because it’s expensive to run a fourth neutral wire all those miles. That’s why distribution transformers are wired as Delta-Wye. This creates the neutral that allows the transformer to deliver power for single-phase loads.

Delta-wired devices can also be fed from a Wye source by simply omitting the neutral. That means that in a data center, a Delta power distribution unit (PDU) can be used when there is only a need for 208VAC, while Wye PDUs are used when there is a need for both 120VAC and 208VAC.

Many of today’s larger blade servers only accept 208VAC because their power requirements can’t be met with 120VAC. However, most data centers still need the flexibility of also being able to power 120VAC devices. So now you know why 3-phase Wye power distribution is the best option for today’s data center.