Course - Higher National Diploma in Engineering

Introduction

Unit 21 in Higher National Diploma of Mechanical Engineering dives into the world of electric machines. It explores the fundamental principles behind transformers, motors, generators, and other electrical devices. You'll learn how these machines are constructed and how electromagnetism dictates their operation. By analyzing their characteristics, you'll gain insight into factors like voltage, current, efficiency, and even electromagnetic interference. The unit delves into practical applications, equipping you to identify the different types of generators used in industry and how their design influences their suitability for various tasks.

Assignment - Electric Machines

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Question 1

i) Examine shell and core type transformer options with the customer. In your examination you are to include the design of the physical construction, winding configurations [step up, step down, and isolating], discuss the application for each type of transformer.

Solution: Transformers are widely used by the national grid in order to balance the voltage to the targeted organization of a region. From the source generating station. Large voltage size is generated and transmitted to different sub-stations. These sub-stations are the ones responsible of power distribution in a region like a metropolitan location. When power is from the grid station, under most cases it is stepped up using the step up transformer in order to increase the voltage and reduce the current. While it reaches the station, the voltage is mainly high and have to be reduced in the station using the step-down transformer in order to reduce the high voltage in the transmission lines.

Step-up transformer
Step-up transformers are used in converting the primary voltage to a larger voltage in that way reducing the current. It achieves these by decreasing the current in the output. The windings in the primary core are less compared to the windings in the secondary core. That way the current is in the secondary core is reduced hence increasing the voltage to balance.

The common purpose of the transformers is clear in the distribution grid. When the voltages are transmitted for long distances, the voltage drop may be resulted due to the changes in resistance and the current changes. The transformer is used to rise the voltage that is distributed over long distances.

Step-down transformer
The step down transformers are used in lowering the voltage output when the voltage is high for consumption. In order to reduce the voltage we need to use the step-down voltage to lower the voltages however rise the current ratting. For the step-down transformers the windings in the primary core are more compared to those on the secondary core.

Step down transformers have a high efficiency of about 99% since they are mainly lowering the voltages rather than increasing. These transformers are less expensive compared to the step-up transformers.

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Isolation transformers
These are special case transformers used in blocking the Direct Current. They are used in ensuring that the devices communicating or signals passing are only in the Alternating Current. They are used and mainly customized for specific performance and mostly have a ratio of 1:1 since their main role is to prevent the DC but ensure that AC is working.

ii) Illustrate to the customer the various windings, connections (, how short circuit, and no-load testing is used to determine the losses and overall efficiency of the transformer. Show on an equivalent circuit the iron and copper losses.

Solution: Transformer loses are common when thee transmission is over long distances or depending on the material used. Under common circumstances the iron and copper losses are the common in relation to the material selection. These loses are losses lost due to the material resistance and conduction rating. When the conduction is higher the lower the losses and vice versa.

Short-circuiting results to power loss since there is additional kind of shorter distance for electrical transmission. Hence the power takes the shortest route and does not use the common route due to load. Short circuit are not common and once they happen they result to a complete power loss. For the case of transformers short circuit may be on the windings and these results to loss of power at a very high ratio.

iii) Listed in table 1 are specifications of the transformers that are in stock. Make an assessment on the efficiency of each to provide the customer with the best selections for a single-phase and a three-phase transformer.

Solution:

We were provided with four transformers to make a selection from. Depending on the load applied on the system and other factors, efficiency may vary from one transformer to the other.In our case scenario we are transmitting the voltage to a hotel. A hotel uses more power due to the major machines in the hotel like the machinery in the kitchen machines used in baking and other related machine. These machinery consume a lot of power hence high power is required. Mostly hotels are supplied with a 3 phase transmission line. These is able to serve the purpose of power generation without overloading.

For that reason we have to select a transformer with the highest efficiency.

Transformer A

The open-circuit and short-circuit allows us to calculate the efficiency on the transformer without actually performing a load on it.

where we have that poc is input power on open circuit
psc is input power on short circuit
S is the rating of the power.Rated-power
Efficiency=300*600*0.85/50000=3.06
Transformer B
Efficiency = 500*250 * 0.95/80000
1.48.
Transformer C
Efficiency = 50*75* 0.78/80000
=3.652
Transformer D
Efficiency = 300*400* 0.87/45000=2.32
Question 1 iii B)

Full load efficiency is the best t rely on when selecting the most appropriate transformer to use in the hotel. For that reason we will select the transformers with the highest rating since they will ensure continuous power in the hotel despite overloads. The selection is Transformer A and B

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Question 2

i) Select the best type of motor design to operate the service lift and justify your answer.

Solution: For the lift we consider a number of factors before making the selection of the. The general characteristics of a lift is that we have to set the speed that we want it to run with and these speed will be constant without adjusting it. Additional characteristic is that the load will be varying from time to time since we can add any load on the lift.

For that reason synchronous motor is the best match for the lift. This is because the speed is maintained and is constant. Whenever the load is added it does not change the speed of rotation of the motor.

ii) Analyse the different methods of starting induction motors and synchronous machines and suggest the best type of starter for the service lift motor.

Solution: Different methods of starting induction motor and synchronous machines

Induction and synchronous motors do not start on their own. They rely on something else to get started. Often big induction motors are started on line compared to the others, so when very large motors are started in this manner, the voltage on the supply chains is disrupted due to large starting current surges. To minimize the starting current spike, broad induction motors are started at a lower voltage and only reconnected to maximum supply voltage as they reach close rotational rpm.

Star delta beginning and autotransformer stating are two ways of starting at low voltage. Contactors in the starter conduct the switching operation that connects and disconnects the power supply to the engine. If the current exceeds the motor's rated current, the contactor can trip immediately, disconnecting the motor from the power supply. The stator of a three-phase induction motor receives a three-phase supply, which generates a magnetic field that spins in space all over the stator.

The methods include the following

Pony motors
Pony motors simply implies additional motor which is smaller that can help in starting the larger motor. These motors help in gaining the speed of the actual motor we are starting and once it is started, the larger motor starts rotating as it gains momentum.

DC motors
DC motors are easy to start and once they are started they initiate the motor to start rotating. It is mainly referred to as the coupled dc motor. It excites the larger motor to start operation and with time the excitation is effective and the motor starts rotating.

Using a dumper winding on the motor
These windings are additional of the actual windings on the motor and acts by creating a shorting effect on the motor. Short-circuiting is important in ensuring that the motor starts. Once the shorting is done the motor starts rotating slowly and with time the motor starts well.

iii) The original motor in the service lift had failed after a short time. Critically evaluate the efficiency of the motors that your company has in stock and make a recommendation for a new motor.

Solution: Recommendation for a new motor

The motor currently operating has a number limitations that can be solved by making a replacement of a new one. We have a number of motors in stock and for these kind of service we have two options that we can suggest for the customer. All of them are effective to use depending on the specifications the customer will wish. For the two options the first option might be the best to go with comparing to the other.

The motor selection depends on the kind of machine we are running. For the case of our customer he is operating at a lift which is 2200m above sea level. These implies that the motor should be effective up to this height above the sea level to best math the customer's request.

We select the Motor Option 1 due to a number of reasons. Some of the reasons behind the selection of the motor option 1 is firstly it have a higher horsepower compared to the second option. The horsepower rating is at 30 compared to the second option which is at 25.

Secondly, the power factor is high and goes all way to 0.85 compared to the others which are at 0.8. The other largest advantage is that the motor comes with the thermal protection with it. Hence it have a protection measure while the second option does not have the protection. This is a good move foe selection since there will be an improvement in thermal protection from the machine.

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Question 3 a) You must explain to them the types of generators (DC, induction and synchronous) and their construction.

Solution: Types of generators and their construction

DC generators

DC generators just like any other generator convert the mechanical energy into the form of electrical energy. Some of the components of the DC Generators include the commutator, the armature and the magnet. Some of the other parts like the windings, shafts and the casing ensure operation of the generator (Setyawan, 2020). The shaft is the rotting part and plays one of the vital roles in the generators.
Armature core: This is the cylindrical part that is contains the shaft.
Windings: They are mainly made of the copper material to increase the efficiency.
Commutator: This is the major part in the generator when it comes to conversion. It converts the AC from the windings to DC.
Capacitors: In order to ensure that there is consistency and no fluctuations capacitors are used for that purpose. Solving the issue of fluctuations in the current.

Induction generators
Induction generator does not require any additional components in order to operate as compared to the synchronous generators. They do not need to be added any kind of external components to keep running. Once it is connected to the grid power the voltage is automatically induced into the generator and it starts operating. The induction is similar to the transformer induction.
It consists of the rotors, stators and the magnetic field. These works together into ensuring that the power generation is consistent (Fang, 2017).

The stator: This component is always stationary and does not rotate. It contains the windings around it and the induction takes place here.

Rotor: The rotor part in the induction generator is the rotating part. This part rotates when the generator is generating power to the grid. It is always connected via the shaft to the load component.

Synchronous generators

The synchronous generator runs at a fixed speed even when the load is applied on it. These machine converts the mechanical energy into electrical energy for transmission. The generator can have a 2 pole or a 4 pole depending on the construction and the user specification at the manufacturing. It also contains the stator and the stator windings for the power generation.
Stationary parts: The stationary parts of the synchronous generator include the rotor and the stator. The stator contains the stator windings. These are the components used in generating the power in the synchronous generator.
Power excitation: In order to start working, in most cases there are the excitation component which acts to supply power to run the generator. The component can be easily referred with the excitation voltage.

b) Identify a generator for a specific application based on the available generators in table 4. Justify your selection based on the generator's characteristics.

Solution: Generator for a specific application

Based on the information provided by the customer, the company seems to be using a lot of power. Some machines that consume high power include the drilling machines, pumping and the welding. A generator that can best fit this must have a total capacity which is over the ratings of the machines to avoid overload but at the same time maintain the efficiency.
Getting the total power consumption from the rating will be 3.2+0.6+1.8+0.6= 6.2KW
Based on this we can make a selection of the synchronous generator. This will ensure consistency and continuous generation of power.

c) Access the generators that the company has available as seen in table 4, to make the best possible choice of recommendation for the customer, taking generator efficiency into consideration.

Solution: Recommendation for the customer

The customer is a heavy power user. He have machines that take a lot of power at the same time hence the generator should be able to produce high voltages at a go in order to maintain the operation of the machines. These is also where the efficiency comes in. We have to consider the efficiency of the generator during the selection.

Considering the device that is used the largest power is 3.2, we select a generator with a slightly higher rating. Arc welders are two and each consume 3.2 which implies that the total is 6.4. However if one operate at a time the consumption will be 3.2 fixed. The best selection is generator 2 since the rating is higher compared to the other generators.

The generator is rated 43 Amps and we can get the efficiency from it assuming that one welding machine is being used. It will be 3.2/4*100 = 80%.

FAQ: Electrical Machines

1. Transformers

• Q: What are the constructional features of transformers?
• Q: What are the applications of transformers?

2. Three-Phase Induction Motors and Synchronous Machines

• Q: What are the starting methods for three-phase induction motors?
• Q: What are the applications of three-phase induction motors and synchronous machines?

3. Types of Generators

• Q: What are the different types of generators available, and what are their practical applications?
• Q: Investigate the types of generators available in the industry by assessing their practical application?
• Q: What are some additional generator types based on fuel source?
• Q: How do I choose the right generator for my needs?

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