1. Current transformers (CTs)
Measure/Monitor the current in a line
Isolate the metering/relay equipment connected to the secondary
High-precision transformers in which the ratio of primary to secondary current is a known constant.
The primary is connected in series with the line. The nominal secondary current is usually 5 A irrespective of the primary current rating.
Precaution; never open the secondary circuit of a CT while current is flowing . If the secondary is accidentally opened, the primary current unchanged. flux changes at an extremely high rate, inducing voltage peaks of several hundred volts across secondary. Operator could easily receive a bad shock.
Lecture 10
Electro Mechanical System

2. Toroidal current transformers
When the line current exceeds 100 A, we can sometimes use a toroidal current transformer.
It consists of a laminated ring shaped core that carries the secondary winding.
The primary is composed of a single conductor that simply passes through the center of the ring.
Toroidal CT's are simple & inexpensive are widely used in low voltage and medium voltage indoor installations.
Incorporated in circuit-breaker to monitor the line current. If the current exceeds a predetermined limit, the CT causes the them to trip.
Lecture 10
Electro Mechanical System

3. Variable autotransformer
Variable auto transformer is used when we wish to obtain a variable ac voltage
Single layer winding wound uniformly on iron core
A movable carbon brush in sliding contact with winding serves as variable tap
Input voltage E1 is usually fixed to 90% tap on winding enabling E2 to vary between 0 to 110%
Variable transformers are efficient and provide good voltage regulation
Lecture 10
Electro Mechanical System

4. Induction heating transformers
High-power induction furnaces also use transformer principle to produce high-quality steel and other alloys.
High-frequency 500 Hz source is connected to coil that surrounds a large crucible of molten iron.
The coil is the primary, and the molten iron acts like a single secondary turn, short-circuited upon itself.
It carries a very large secondary current. This current provides the energy that keeps the iron in a liquid state, melting other scrap metal as it is added to the pool.
Lecture 10
Electro Mechanical System

5. Induction heating transformers
Channel Furnace
Transformer having a laminated iron core is made to link with a channel of molten iron.
The channel is a ceramic pipe that is fitted to the bottom of the crucible.
The primary coil is excited by a 60 Hz source, and the secondary current I2 flows in the liquid channel and through the molten iron in the crucible.
The channel is equivalent to a single turn short circuited on itself.
Lecture 10
Electro Mechanical System

6. Chapter 12 Three-Phase Transformers
Lecture 10
Electro Mechanical System

7. Electro Mechanical System
Introduction
In the operation of power systems, transformers are required to change the voltage levels throughout the network.
All major power generation and distribution system use three phase ac system.
To make three-Phase one option is to connect Single-phase transformers into a 3-phase bank.
3-phase unit have three primary windings and three secondary windings on a 3-legged core.
using a bank of three single phase transformers, the windings may be connected in a variety of ways:
The primary side may be connected in a wye or delta.
The secondary side may be connected in a wye or delta.
Lecture 10
Electro Mechanical System

8. Electro Mechanical System
Transformer Banks
3-phase transformer banks can be analyzed using simplifying assumptions
Excitation currents are negligible.
Winding impedances are negligible.
Apparent input power equals the apparent output power.
To make three-Phase one option is to connect Single-phase transformers into a 3-phase bank.
Retain all basic single-phase properties.
Individual voltage and current transformations are based on the single phase turns ratio.
3-phase transformer banks can introduce a phase shift between the three-phase primary and the three-phase secondary.
Function of the primary and secondary winding connections.
Lecture 10
Electro Mechanical System