4. Selección de SNPs
4.2. SNPS en regiones reguladoras
4.2.2. Splicing
Hot spot temperature
In any transformer design, there is a place in the winding that the manufacturer believes to be the hottest spot in that transformer. A sensor is placed there and an instantaneous alarm or trip setting is set for that hot spot area.
Heating due to overexitation
Windings temperature and hot spot temperature are the main areas of concern but both transformer oils (and water temperature if it is an oil filled water-cooled type) are usually measured. If it exceeds a pre-set value, it sets off an alarm or initiates circuit breakers to disconnect the supply from the load.
Temperature measurement is done using a Bourdon tube, which actually detects the associated rise in pressure as a result of the high currents.
Figure 3.17: A sketch of Bourdon’s tube, and the meter as we know it
Take note The strain of the material of the tube is magnified by forming the tube into a C shape or even a helix, such 1849 and it was widely adopted because of its
Activity 1
1. What is the purpose of transformers?
2. What is the basic principle of operation of a transformer?
3. Explain the operation of the transformer.
4. Why can a transformer not work on DC?
5. What is meant by a laminated iron core and what is its main function?
6. What do we know about the power rating of transformers?
7. Show by means of neat sketches the difference between a SINGLE three-phase transformer and THREE single-three-phase transformers (Diagrammatic).
8. Name FOUR types of losses that occur in transformers.
9. Name three advantages of three-phase transformers over single-phase transformers.
10. Name two core types commonly used to make transformers.
11. Name the characteristics of a star-star connection.
12. A farmer uses a three-phase delta-star transformer with a windings ratio of 150:3 to reduce a line voltage from 11 kV. He uses both single- and three-phase motors on his farm.
a) Represent the system by means of a neat schematic sketch. Use ONE three-phase motor and ONE single-phase motor to represent the load on your sketch.
b) If the three-phase motor is star connected, determine the voltage across each phase of the motor.
c) This three-phase motor has a rating of 12 kW at a power factor of 0,8 lagging.
d) Calculate the line current supplied at the secondary of the transformer.
e) Calculate the kVA rating of the transformer (assume efficiency is 100%).
The transformer is positioned at the back of a shed on the farm. One day the transformer suddenly starts to get very hot.
f) Name TWO reasons why it may be getting hot.
g) Name TWO ways of reducing this temperature.
13. A balanced three-phase connected load draws 10 A from a
star-delta transformer with a 100% efficiency. The transformer has a transformer ratio of 15:1. The primary winding is connected to a supply voltage of 11 kV/50 Hz. The power factor of the load is 0,9.
a) Draw a fully labelled diagram of the above circuit.
b) Calculate the secondary line voltage.
c) Calculate the total kVA rating of the transformer.
d) Write down the disadvantage of a low power factor on a load.
e) Why will a transformer NEVER have 100% efficiency? Give FOUR reasons.
f) State any TWO methods of how overheating in transformers can be prevented.
14. Name some electrical protection employed on power transformers.
Practical Activity 1
To investigate the relationship between the voltages and currents in a three phase Star-Delta Transformer.
Equipment:
• A clamp on ammeter
• Multimeter with appropriate Voltage scale
• A Star Delta connected transformer using three single phase transformers of the same rating.
Steps:
1. Connect the three transformers in a star delta configuration as shown below.
Star-Delta Three Phase Transformer
Take the following readings and record in the table supplied below:
2. Measure the line and phase voltages on the primary side.
3. Measure the line and phase voltages on the secondary side.
4. Measure the line and phase currents on the primary side. (Use the clamp on ammeter.)
5. Measure the line and phase currents on the secondary side. (Use the clamp on ammeter.)
Connection Meter readings
1. STAR PRIMARY VLINE 1. STAR PRIMARY VPHASE 2. DELTA SECONDARY VLINE 2. DELTA SECONDARY VPHASE 3. STAR PRIMARY ILINE 3. STAR PRIMARY IPHASE 4. DELTA SECONDARY ILINE 4. DELTA SECONDARY IPHASE
L1 L2 L3
L1 L2 L3
Conclusion:
1. What can be observed from the line and phase voltages on the primary side?
2. What can be observed from the line and phase voltages on the secondary side?
3. What can be observed from the line and phase currents on the primary side?
4. What can be observed from the line and phase currents on the secondary side?
Chapter 4 Three-phase motors and starters
Protection Starting methods
A B
Testing Calculations
A B
Applications Three-phase
A B
Introduction
In this chapter, attention will be given to three-phase motors and how these motors can be safely connected to the three-phase supply.
Matters discussed will be the advantages of a three-phase motor compared to a single-phase motor. Basic construction will be covered as well as practical applications. Calculations will include the line and phase voltages, power, power factor, synchronous speed, slip and efficiency.
Specific attention will be given to losses that occur in motors. This refers to electrical (fault finding) as well as mechanical losses.
Once the theory is covered, the students will then learn how to design starter circuits safely (control and main circuits) and connect motors safely to the supply for specific functions.