RST stands for Reference, Secondary and Tertiary. People also use UVW, XYZ, or ABC for the three-phase systems. For terminal leads, the letters U, V, W represent the coil head, and the letters X, Y, and Z represent the coil end.
Three-phase systems are used to transfer high power to buildings and are capable of powering large electric motors. The wiring is set up in such a way that the power drawn from each phase is equal.
What Does RST Stand For?
R-S-T is the general designation for the three phases of the supply system. U-V-W and X-Y-Z were then used to designate the connections to the windings on motors, generators, etc. The individual windings were U to X, V to Y, and W to Z.
For example, in a wye generator design, X, Y, and Z would be strapped together to produce the neutral point, and the phases R, S, and T would be supplied from the U, V, and W winding terminals.
A-B-C, R-S-T, or U-V-W
The IEC has established a technique for recognizing three-phase winding connections. The preceding is how letters and numerals are employed. The high voltage (HV) terminals are identified by upper-case letters such as A-B-C, R-Y-B, U-V-W, L1-L2-L3.
The low voltage (LV) terminals are identified by lower-case letters such as a-b-c, r-y-b, u-v-w, l1-l2-l3. Each winding has a 1 at the beginning and a 2 at the end. The use of letters and numerals varies by country; check the table below for a general guideline:
|Letters and Numbers Used
|L1 L2 L3 or 1 2 3
|U V W or R S T
|R Y B or A B C
U-V (Without W)
In a single-phase system, U and V (without a W) denote power where it is linked to a piece of equipment, such as a motor. The coming in line is denoted by the letters L1, L2, and L3. So, if it’s a single-phase system, L1 and L2 are required.
|New IEC Color
|Old IEC Color
|Line, single phase
|brown or black
|brown or black
|brown or black
|brown or black
The three-phase system is made up of four wires: three current-carrying conductors and one neutral. The neutral conductor’s cross-section area is half of the live wire. The current in the neutral wire is equal to the total of the three wires’ line currents, and hence equal to 3 times the zero phase sequence components of current.
The three-phase system has various advantages, including the fact that it requires fewer conductors than the single-phase system. It also provides a constant supply to the load. The three-phase system is more efficient and has lower losses.
Types of Connections in Three-Phase System
Three-phase systems are linked using two methods: the star connection and the delta connection. Their in-depth explanation is provided here.
The star connection needs the use of four wires, three-phase conductors, and one neutral conductor. Because it features a neutral point, this form of connection is mostly employed for long-distance transmission. The neutral point conducts the imbalanced current to the ground, bringing the system back into equilibrium.
The star-connected three-phase systems produce two separate voltages, 230 V and 440 V. The voltage between the single-phase and the neutral is 230 volts, while the voltage between the two phases is 440 volts.
There are three wires in the delta connection, and there is no neutral point. The delta connection’s line voltage equals the phase voltage. Delta or Mesh Connection (Δ) Method is also known as a three-phase three-wire system (3-Phase 3 Wire) and is the most popular system for AC power transmission.
1-Phase vs 3-Phase Power
Power is provided to a single-phase power source through two wires known as phase and neutral. Power is provided by three wires in a three-phase power supply (four wires if neutral wire is included). A single-phase supply has a voltage of 230V, but a three-phase supply has a value of 415V.
A single-phase supply requires more wire than a three-phase supply for the same amount of electricity. The efficiency of a three-phase power supply is much better than that of a single-phase power supply, as is the power transmission capabilities. The total network complexity is lower than a four-wire three-phase supply.
Calculating Amps in a 3-phase Circuit
To calculate the amperage, divide the power usage in watts by the line voltage multiplied by the power factor. The power factor in three-phase circuits is the square root of three. If your calculator lacks a square root function, try 1.73 to approximate the square root of 3.
Frequently Asked Questions
Why do we use 1.73 for three-phase?
The usage of the constant 1.732 in a three-phase circuit stems from the fact that not all three phases produce the same amount of power at the same time. The voltage and current in each phase pass through 0 at different times.
RST is often sequenced like YRB or BRY according to the color codes. Normally, the electricity system operates in a three-phase balance. The majority of bulk loads, such as industrial loads, are three-phase balanced loads.