Reference no: EM132723927

Unit 21 Electrical Machines - Higher National Certificate/Diploma in Engineering

Assignment - Electrical Machines

Learning Outcome 1: Assess the constructional features and applications of transformers.

Learning Outcome 2: Analyse the starting methods and applications of three-phase induction motors and synchronous machines.

Learning Outcome 3: Investigate the types of generator available in industry by assessing their practical applications.

Learning Outcome 4: Analyse the operating characteristics of electromagnetic transducers and actuators.

Assignment Brief

Scenario:
As a trainee you are asked by your company to solve the following tasks related to the construction, design and practical experiments of electrical machines. Throughout the term you have studied all theory about construction, design and practical experiments of different electrical machines. In answering any of the tasks you must answer the question in relations to the practical experiments shown during your studies. You must show the practical use of transformers as step up or step down and as an isolation device. You have also to address why induction motors are the most widely used both at home and in industry. Because Synchronous machines are used regularly in power generation, you must analyse the reason of these choices. Transducers and actuators are used widely in a lot of practical applications and you are required to analyse the reasons behind the widespread use of these devices. You must carry out these tasks by the deadline mentioned in this assignment. You must keep in mind that in real life employment your promotion in the company will depend on how well you solved the tasks and the way you present and report the results. All the tasks must be handed in one report within the specified deadlines.

Task 1

a. Describe the C-core and Shell types of transformers including their constructions and operations and the advantages and disadvantages of both types.
b. Explain how transformers are used as step, step down and as isolation machines and what are the main specific applications of these devices in practical situations.
c. A transformer has 500 turns in the primary windings and 100 turns in the secondary side. The resistances in the primary and secondary sides are 0.3 ? and 0.1 ?.respectively and the reactances are 2.0 ? and 0.1 ? respectively.

1. Draw carefully the equivalent circuit and the referred circuit of the transformer naming all elements of both circuits:
2. Discuss the operation of this transformer and illustrate this with diagrams and graphs showing the characteristics of the transformer and its equivalent circuit.
3. Calculate the elements of the equivalent resistance referred to the primary
4. Calculate all elements of the equivalent reactances referred to the primary
5. Calculate all the elements of the equivalent impedance referred to the primary
6. Calculate the phase angle of the impedance
d. A 200 kVA rated transformer has a full-load copper loss of 1.5 kW and an iron loss of 1 kW.
1. Determine the transformer efficiency at the following situations:
i. At full load and 0.85 power factor.
ii. At half full load and 0.85 power factor

2. Determine the maximum efficiency of the transformer at power factor of 0.89. Give your reasons for the value of the efficiency obtained as well as any practical conclusions and requirements in order to improve the efficiency further.

Task 2

a. Analyse and examine how induction motors work and why these motors are the most widely used both at homes and in industry.

b. Analyse and give reasons why three-phase Synchronous generators are the most widely used in power generation including the advantages of these generator over other types of generators.

c. A 400V, 4 Pole, 3 Phase, 50 Hz star connected induction motor has a rotor resistance and reactance/phase = 0.01? and 0.1? respectively and stator resistance and reactance/phase of 0.641? and 1.106 respectively and Xm=26.3. The equivalent circuit is referred to the stator. Determine i) Starting Torque ii) Slip at which maximum torque will occur. iii) Speed at which maximum torque occurs iv) The value of maximum torque
v) Full load torque if full load slip is 4%. Assume ratio of stator to rotor turns as 4.

d. Analyse the different methods of starting induction motors and synchronous machines. Give reasons for choosing these starting methods and whether practical applications will affect these methods and the reasons behind that.

e. Critically evaluate the efficiency of induction and synchronous motors and make recommendations for the use of induction motors in isolation and the synchronous motors in industrial applications.

Task 3

a. Explain the constructions of dc generators (direct current, and excitation) as well as the main types of field excitation of dc machines.
b. Identify a dc generator for a specific application that you choose given reasons for that choice and the advantages and disadvantages of using dc generator in this application.

c. A 3Φ, 16 pole, star connected salient pole alternator has 144 slots with 10 conductors per slot. The alternator is run at 375 rpm. The terminal voltage of the generator found to be 2.657 kV. Determine the frequency of the induced emf and the flux per pole

d. Assess the efficiency of available dc generators and any specific requirements in order to enhance the efficiency of these machines.

Task 4

a. Describe the operations and the main parts of a magnetic actuator and transducer and their practical use as electromagnetic devises

b. Explain fully passive and active transducers as well as magnetic sensors and the different application of these devices.

c. Explain fully solenoid, linear and rotary actuators and describe three types of applications of these devices.

d. The magnetic circuit shown in the figure below has dimensions Ac = Ag= 10 cm2, g =
0.05 cm, lc = 50 cm, and N = 600 turns. Assume the value of the relative permeability, μr = 70,000 for core material. (a) Find the reluctances Rc and Rg. For the condition that the magnetic circuit is operating with Bc = 1.0 T, find (b) the flux and (c) the current i.

e. Justify the selection of a suitable application of your choice of a transducer and its practical use in a specific practical industrial application.

f. Analyse how the efficiency of transducers and actuators in an industrial situation can be improved giving in you answer a practical use of your choices and reasons behind this choice.

Attachment:- Electrical Machines.rar