Sim's Electrical Companion
Transformer Construction, Types & Calculations
Single Phase Transformers
A transformer is a simple device that changes a primary voltage / current into a secondary voltage / current. The method of achieving this is so simple, a primary source conductor current produces a magnetic fields and is wound around an iron core to increase its subsequent magnetic field. The lines of flux cut across a secondary winding which induces an emf in the secondary circuit. The secondary voltage will be dependant upon the ratio of windings primary and secondary. If there are an equal amount of windings on both sides the transformers output and input will be the same (excluding loses). If the secondary windings are lower the transformer is considered to be a step down transformer. If the secondary windings are greater than the primary windings it is a step up transformer.
Transformers have a ferrous core which increases the magnetic field produced by the primary coil. There are three common types of core construction Shell, Core and Toroid (pictured right). The toroid has the secondary windings wound over/under the primary windings. Shell and core types the windings are clearly separated.
3 Phase Star Connected Systems - WYE Y
In the UK transmission and distribution is mostly conducted using a three phase system. The phase angles are all 120 degrees from each other and the system allows for more than one supply voltage. Commonly for end users 230V line to earth or neutral and 400V across the phases (line to line).
​
Star connected systems are provided for end users where loads may not be balanced. Imbalances can damage the supply system through overloading so imbalances are given a safe alternative path. The path is the earth connected neutral conductor of the system. Often a bonded large metal plate buried beneath the transformer.
Allowing imbalances a route to earth does introduce inefficiency for the sake of safety. To minimise inefficiency in star connected systems at the design level and through control and monitoring load balancing needs to be considered. A good example of load balancing is how homes are supplied from the distribution network. As the supply cable is run up the street the first house is fed from Phase 1 (0 degrees), The second house is fed by phase 2 (120 degrees) and the third is fed from phase 3 (240 degrees). When this is done over a whole street or block the occupation of the homes and use of the available electrical energy will roughly balance out. Imbalances will be lost so balancing systems is important for efficiency and sustainability.
Imbalance scenario:
If there were three houses and house 1 is having a party, the hot tub is on and the sound system is drawing power to the point of overload. House 2 is on holiday and just has a landing light on to deter opportunist thieves. The third house is having an early night with a cup of cocoa and a self help book. The imbalance is quite extreme (on more than one level).
House one is drawing 80 Amps
House two is drawing 8 Amps
Third house is drawing 16 Amps
The imbalanced current being carried down the neutral can be determined with the formula below:
Three Phase Star - Phasor Geometry
Establishing Neutral Currents with Geometry
Star connected systems carry imbalanced loads down the neutral conductor safely to the body of earth. The previous page shows the formula for establishing the neutral current. Historically most of my students who have attempted to use the formula to find the neutral current make an error because of the length of the equation. An alternative method is a phasor diagram. This geometrical method of establishing the neutral current has a much greater success rate, though slightly less accurate, it is sufficient.
Interestingly no matter what order you draw the phases the outcome will always be the same. The distance from the last point to the neutral point will be exactly the same, though the shape will be different!
Three Phase - Delta Connected Systems - Δ
Unlike star connected systems delta systems are reliant upon the phases being balanced which leaves no requirement for a neutral. Delta is used extensively in distribution systems as the loading on each phase is essentially the length of the supply conductors which are all the same. The major benefit of this is reduced cost as there is no need to provide a neutral conductor across the entire national grid. Delta systems are less common in consumers electrical systems but one application for delta is motors. A three phase motor has three windings at 120 degrees from each other of equal length so once in motion the motor has no need for a neutral connection.
​
Delta is susceptible if the phases are not balanced so protective devices will be present to prevent danger or damage from any voltage disturbances or irregularities.
The illustration right shows a delta/star transformer. The P nodes are the primary delta connections and the S nodes are the secondary star connected windings. This kind of transformer is commonly used where transmission is stepped down for distribution to end users. An example pole mounted delta/star transformer is pictured.
Unlike star connected systems delta systems are reliant upon the phases being balanced which leaves no requirement for a neutral. Delta is used extensively in distribution systems as the loading on each phase is essentially the length of the supply conductors which are all the same. The major benefit of this is reduced cost as there is no need to provide a neutral conductor across the entire national grid. Delta systems are less common in consumers electrical systems but one application for delta is motors. A three phase motor has three windings at 120 degrees from each other of equal length so once in motion the motor has no need for a neutral connection.
​
Delta is susceptible if the phases are not balanced so protective devices will be present to prevent danger or damage from any voltage disturbances or irregularities.
The illustration right shows a delta/star transformer. The P nodes are the primary delta connections and the S nodes are the secondary star connected windings. This kind of transformer is commonly used where transmission is stepped down for distribution to end users. An example pole mounted delta/star transformer is pictured.
