Homework answers / question archive / EN1085 2021/22 Coursework part M (DC Machines)   The coursework components of EN1085 make use of computational engineering tools to help you apply the main techniques and concepts developed in the class

EN1085 2021/22 Coursework part M (DC Machines)   The coursework components of EN1085 make use of computational engineering tools to help you apply the main techniques and concepts developed in the class

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EN1085 2021/22 Coursework part M (DC Machines)

 

The coursework components of EN1085 make use of computational engineering tools to help you apply the main techniques and concepts developed in the class. There will be two coursework components – part M deals with the dc electrical machines topic presented by Dr. Clark, while part P is concerned with problems of complex power and reactive compensation, as covered by Dr. Robson.

 

The aims of the EN1085 coursework components are to:

 

• Gauge your progress an understanding of the materials and concepts covered in class
• Reinforce the concepts of electrical machines and power systems analysis needed for the exam

 

A rough schedule for the coursework delivery is as follows:

 

Week 6: Coursework P set

Week 8: Coursework P due;

Week 9: Coursework part M set

Week 10: Coursework part M due

 

You will deliver two short submissions, each worth 20 marks. This will be scaled to give a 20% contribution to the EN1085 module mark (the other contributions being the 20% test and 60% May/June examination).

 

Report M should be submitted on Learning Central by Friday Week 10 at 23:59

 

If you are unable to submit your report on time, you must clear this with the module leader (Dr. David Clark) prior to submission. Late work (without permission or good reason) will automatically receive a mark of zero.

 

 

Dr David Clark

Room E/3.23

 

 

Coursework – Back-to-back DC Machines

 

As we have seen in class, DC machines may be employed as motors or generators as required by the application. The tasks set herein concern a pair of DC machines, shaft-coupled as shown in figure 1.

 

 

 

Figure 1: Shaft-coupled pair of electrical machines - one acting as motor, the other as generator

Applying a dc voltage to the terminals of the motor will drive a current through its armature winding, developing a rotational torque on the shaft. The motor will spin up and being shaft-coupled the generator will spin at the same shaft speed. The rotating generator armature will develop an emf, which in turn will drive the current in the load. The machines will settle at a steady shaft speed when the torque and power balance in the system is achieved – consider the power flow diagram depicted in figure 2

 

 

 

Figure 2: Power balance in a coupled Motor-Generator system

 

Each machine has its own machine constant k'=kΦ

 and armature resistance Ra

, and experiences a minimum frictional torque Tf

 under no load. These parameters can be captured in an equivalent circuit as given in the Matlab SIMULINK model depicted in figure 3.

 

 

 

Figure 3: Matlab Simulink model of a DC motor-generator pair

 

For a given set of machine, input and load parameters, this model will compute the steady-state power flow condition in the system and evaluate the overall source to load electrical efficiency. Take some time to familiarise yourself with the model and understand what function each component performs.

You have each been allocated a unique randomised set of simulation parameters which should be used to answer the following tasks. All solutions should be supported by equivalent circuit analysis to confirm that the SIMULINK model is giving you sensible answers!  

TASKS

 

Complete each of the following activities with reference to the machine parameters provided on Learning Central. You do not need to prepare a formal report, just answer each question and provide the evidence asked for.

 

(a) Determine the no-load speed of your coupled system in revolutions per minute. Verify this using the machine analysis theory developed in class

[4 marks]

 

(b) For the desired load current specified in the parameter table, what is the input electrical power, the shaft mechanical power and output electrical power of your coupled system. Determine the overall electrical efficiency.

 

[4 marks]

 

(c) Leaving the machine parameters unchanged and adjusting the load current as required, what is the maximum output power of the system and at what load current is maximum power transfer achieved? Give your answers to two significant figures and describe the method used. Verify your result analytically with the help of the power balance diagram in figure 2.

[7 marks]

 

(d)  What is the maximum overall efficiency achievable for your system at the given supply voltage? State your answer as a percentage to three significant figures. Why does peak efficiency not occur at the same load current as maximum power transfer?

 

[5 marks]

 

 

TOTAL [20 Marks]