Ideal transformer
An ideal transformer is shown in the fi gure below. Current passing through the primary coil creates a magnetic fi eld. Th e primary and secondary coils are wrapped around a core of very high magnetic permeability, such as iron, so that most of the magnetic fl ux passes through both the primary and secondary coils. An ideal transformer would have no losses, and would therefore be 100% effi cient.
Figure 4.9:
Solid core transformer
Figure 4.10:
Toroidal cores
Figure 4.11: Core type transformer
Due to the high cost of distributing electricity at low voltage and high current levels, transformers fulfi l a most important role in electrical distribution systems.
Electricity suppliers (e.g. Eskom) distribute electricity over large areas using high voltages, commonly called transmission voltages. Transmission voltages are normally in the 35,000 volt to 50,000 volt range. Remember that volts times amps equals watts, and that wires are sized based upon their ability to carry amps.
High voltage allows the electricity suppliers to use small sizes of wire to transmit high levels of power, or watts. Transmission lines can be recognized because they are supported by the very large steel towers that are seen around utility powerplants and substations. As this electricity gets closer to its point of use, it is converted, through the use of transformers, to a lower voltage, normally called distribution voltage. Distribution voltages range from 2,400 to 25,000 volts depending upon the electricity suppliers. Distribution lines are the ones that feed the polemount and padmount transformers located closest to your home or place of business. Th ese transformers convert the distribution voltages to what we call utilization voltages.
Utilization voltages are normally below 600 volts and are either single-phase or three-phase and are utilized for operating equipment, from the light bulbs and vacuum cleaners in our homes to the electric motors and elevators where we work.
Autotransformers
An autotransformer has only a single winding, which is tapped at some point along the winding. AC or pulsed voltage is applied across a portion of the winding, and a higher (or lower) voltage is produced across another portion of the same winding.
For voltage ratios not exceeding about 3:1, an autotransformer is less costly, lighter, smaller and more effi cient than a two-winding transformer of a similar rating.
By exposing part of the winding coils and making the secondary connection through a sliding brush, an autotransformer with a variable turns ratio can be obtained, allowing for very small increments of voltage.
Figure 4.12:
Centre-tapped transformer
A centre-tap is a connection made to a point halfway along a winding of a transformer. Volts tapped (VCT) describes the voltage output of a centre-tapped transformer. For example: a 24 VCT transformer will measure 24 VAC across the outer two taps (winding as a whole), and 12 VAC from each outer tap to the centre-tap (half-winding). Th ese two 12 VAC supplies are 180 degrees out of phase with each other, thus making it easy to derive positive and negative 12 volt DC power supplies from them.
Figure 4.14: Centre-tap transformer
In a rectifi er, a centre-tapped transformer and two diodes can form a full-wave rectifi er that allows both half-cycles of the AC waveform to contribute to the direct current, making it smoother than a half-wave rectifi er.
Figure 4.15: A centre-tap transformer used in a rectifi er circuit Current transformer (CT)
A current transformer (CT) is a measurement device designed to provide a current in its secondary coil proportional to the current fl owing in its primary coil. Current transformers are commonly used in metering and protective relays in the electrical power industry where they allow safe measurement of large currents, oft en in the presence of high voltages. Th e current transformer safely isolates measurement and control circuitry from the high voltages typically present on the circuit being measured.
Th e CT is typically described by its current ratio from primary to secondary. For example, a 4000:5 CT would provide an output current of 5 amperes when the primary was passing 4000 amperes. Care must be taken not to disconnect the secondary winding from its load while current fl ows in the
primary, as this will produce a dangerously high voltage across the open secondary and may permanently aff ect the accuracy of the transformer.
An instrument that uses a CT is a clamp meter. In electrical and electronic engineering, a current clamp or current probe is an electrical device that has two jaws which open to allow clamping around an electrical conductor. Th is allows properties of the electric current in the conductor to be measured, without having to make physical contact with it, or having to disconnect it for insertion through the probe.
Take note
Figure 4.17: Connecting a current transformer (CT) to a circuit Voltage transformers (VT)
A voltage transformer (VT) or potential transformer (PT) is another type of instrument transformer used for metering and protection in high-voltage circuits.
Th ese transformers are designed to present negligible load to the supply being measured and to have a precise voltage ratio to accurately step down high voltages so that metering and protective relay equipment can be operated at a lower
potential. Typically the secondary of a voltage transformer is rated for 69 V or 120 V at rated primary voltage, to match the input ratings of protective relays.
Figure 4.18: Connecting a potential (voltage) transformer (PT) to a circuit