Scenario: As a practicing electrical, electronic and communication or mechanical engineer in the industry, you may be expected to use your engineering background to perform a range of engineering tasks relating to a range of sub-disciplines in electrical, electronics, mechatronics, and communication systems. They may include, analysis and selection of a power transformer, power generator, electric motor, sensors, and actuators in a range of practical scenarios such as the tasks given herein.

 

Parameters (Per-phase circuit parameter)	Symbol	Value
Primary resistance
Secondary resistance
Primary leakage reactance
Secondary leakage reactance
Magnetizing reactance
Core-loss resistance

Activity

Task Part 1

 

A 3-phase power transformer (80 kVA, 11000V/415V, 50 Hz) serves a balanced linear 3-phase load. When the transformer is operating at the rated capacity, load power is measured to be 74.8 kW. Because load power factor measurements are not available, you are tasked with estimating the load power factor. Following performance indicators of the transformer are available: voltage regulation , full-load power efficiency . 

Utility side voltage can reasonably be assumed to be constant at 11000 V.

You have been tasked with comparing the above transformer (from Alpha TF Ltd.) with an 80 kVA, 11000V/415V, 50 Hz, 3-phase transformer from a different supplier, Beta TF Ltd. in terms of percentage voltage regulation and full-load power efficiency. While operating at the rated capacity serving a 3-phase balanced linear load, transformer terminal voltage is measured to be 402 V. Load power factor is 0.82 inductive. Single phase equivalent circuit parameters of this 3-phase transformer are given bellow.

 

The nameplate of the 80 kVA, 11000 V/415 V, 50 Hz, 3-phase transformer states vector: Dyn. Further, the winding connection is depicted in the handbook as shown in Fig. T1.1. You are supposed to discuss the methods of connections in the primary and secondary sides of this 3-phase transformer with the electrical safety expert of your workplace. 

 

Fig. T1.1: Dyn winding connections in the 3-phase transformer 

 

Electrical machining facilities sometimes are in a poor electrical network environment. The transformer in Fig. T1.2 is proposed to provide additional safety to the users of such machines that will be used in the newly built facility. You are required to identify the type of this transformer and explain the nameplate data. Primary: Standard primary winding 0-230-400 VAC with a +/- 15V adjustment possibility around 0; Secondary: 2 115 VAC.   Further, you are asked to explain the electrical safety team, using suitable clearly labelled diagrams, on how the user electrical protection is different when the machine is used with and without the transformer in Fig.T1.2.     Since many of the technical staff is not familiar with the type of transformer in Fig. T1.2, you are also required to briefly explain the other typical applications of the type of transformer in Fig. T1.2.         Fig. T1.2: The transformer proposed for galvanic isolation for the machines in poor electrical environment   It is observed that the devices shown in Fig. T1.3 is inserted through the neutral wire in the load side and the output is sent to the metering panels. The 3-phase load consists of two components as follows. Load voltage is 400 V, 50 Hz.
		Phase	Power (kW)	Power factor
	Load component 1	3-phase motive load	70	0.75 lagging
	Load component 2	1-phase resistive load	7	0.80 lagging
	Discuss the applications and the working principles of the device in Fig. T1.3. What is the current signal generated by the device in Fig. T1.3 when the single-phase load is OFF? What is the current signal generated by the device in Fig. T1.3 when the single-phase load is ON?

 

 

Fig. T1.3: The device inserted through the neutral wire [Courtesy: Schneider Electric] 

 

Task Part 2

In a proposed design for an electric automobile, the shaft of a 4-pole, 3-phase, induction motor is connected directly to the drive axle; in other words, there is no mechanical transmission. The outside diameter of the tires is 20 inches. Instead of a gear box, a variable speed drive (VSD) is used. The VSD here is a variable frequency drive (VFD) that produces a variable frequency variable voltage 3-phase AC supply to the motor from a 48 V battery pack. The vehicle, including the batteries and occupants, has a mass of 1000 kg. The power efficiency of the VFD is 95%. Power efficiency of the motor may be assumed to be 90% at all speeds. 

 

Fig. T2.1: A low-cost electric car [Courtesy: pinterest.com]

 

1. Assuming negligible slip, find the range of frequencies needed for speeds ranging from 5 to 70 mph
2. Find the current taken from the battery pack as a function of time while accelerating from 0 to 40 mph uniformly in 10 seconds. Ignore the aerodynamic and road friction.
3. Selection of the motor type is part of the design task. You are supposed to analyse the different types of electric motors commercially available in terms of basic construction, power range, maintenance cost, and applications. You are also expected to assess the other motor types for this electric automobile system application with a special discussion on the speed control aspects.    
4. One of your colleagues argues that induction and synchronous motors should not be selected because they often require special starting methods. You may want to analyze at least 3 commonly used ac motor starting methods using suitable diagrams and torque-speed curves. Giving reasons, recommend a method that can be used both as the starter as well as a variable speed drive (VSD) of the induction and synchronous motors.

 

5. One of your colleagues argues that synchronous motor has much superior speed regulation than induction motor, and hence if a synchronous motor is used, the automobile speed will not drop drastically while climbing an inclined road. Discuss the merits of this argument by the use of the torque-speed curves. 
6. One of the colleagues argues that a permanent magnet synchronous motor (also called brushless dc motor-BLDCM) is the ideal option for this electric automobile application. Select an induction motor that meets the design requirements from the market and compare with a BLDC motor with similar specifications in the market in terms of speed regulation, power efficiency, power factor, ease of maintenance, and the cost. 

 

Task Part 3

 

Conventional diesel engine-generator sets are used as backup electricity generators at your workplace. A new 100 kVA generator is required to supply the load created by a proposed expansion plan of the factory. An inverter type diesel generator is proposed instead of conventional type. Comparison between two types is given in the following table.

 

    conventional     Inverter type Power rating          100 kVA     100 kVA

                     Fuel consumption                                                  3.5 kWh/liter                      4.4 kWh/liter

                  THD                                                                              15% 1%

                     Voltage regulation                                                         5% 1%

Frequency regulation under inrush 3-5%    0% loads

   

Rated load of the extended facility is supposed to be 100 kVA with 0.8 power factor. This load is supposed to operate 5 days per week and 8 hours a day. You are requested to make a report (maximum of 2 pages) identifying the construction and technologies used in the conventional type and inverter type diesel generators. It is important to relate the identified technologies to the performance characteristics outlined in the above table. 

Inverter type is generally more expensive. You are expected to estimate the price difference between the two types required to ensure that the payback period of the extra investment is no more than 3 years. Assume 20 working days per month on average. 

 

Low workplace noise and power quality are of serious concern at the extended facility. You are expected to select the type of the generator also explaining the comparison based on the characteristics and the technologies involved in the two types.       

 

A printing machine is supplied by a conventional diesel generator. However, during the operation, the engine-generator set vibrates rapidly. Recorded output frequency is found to be chattering fast around the rated value. It is also noted that the printing machine consists of certain electric heating loads that turn on and off quite rapidly. 

 

1. Theoretically explain this scenario giving the reasons why the encountered problems happen

 

2. It is recommended that the conventional generator be replaced by an inverter type diesel generator of the same capacity to rectify the two problems; namely, mechanical vibrations of the engine-gen set and frequency chattering of the output. Your opinion is sought with clear discussion of the problem in the light of the technologies used in the inverter type generator. Frequency regulation curves from a proposed supplier are given in Fig. T3.1. Both types have a speed governor and AVR. 

  

 

Fig.T3.1: Frequency regulation (www.elemax.jp/products_iv.html)

 

Task Part 4

A magflow meter has been recommended to measure the liquid flowrate in your workplace application that requires high precision measurements. Two options have been put forward. The cheaper option does not have a battery level indicator. Other option has but it is found to be expensive. One of your colleagues argues that magflow maters are active transducers and hence the dc supply to the mater has to be regulated at a constant value or the flowrate measurements will be erroneous. Further, it is also emphasized that a battery charge level indicator is a must in this device. Do you agree? Explain giving reasons. You are encouraged to use a clearly labelled schematic diagram to explain the principle of operation of the magflow meter to support your reasoning.

At the workplace liquid flow system application, malfunctioning of a solenoid valve is believed to be due to weaker solenoid force. To encounter higher line pressure and larger orifice, it is recommended to replace the existing direct acting solenoid with an internally piloted solenoid valve. You are asked by senior management to analyse the operation of the two types and review the above recommendation. It is always advisable to use a clearly labelled schematic diagrams to explain the principles of operation in support of your review.

  The vibration sensor in the vibration monitoring system of the engine-gen set (in Task Part 3) is found to be an electromagnetic type. It is stated that EMI may be significant in the environment and hence a sensor type with better EMI resistance should replace the electromagnetic type sensor. You are asked to compare the electromagnetic vibration sensor and piezoelectric vibration sensor types in terms of EMI resistance, off-axis sensitivity, mounting orientation and mechanical durability, and hence make recommendations to improve the operating efficiency of the vibration monitoring unit. You are encouraged to use a clearly labelled schematic diagrams to explain the principles of operation of the sensors in support of your arguments

 

 

Learning Outcomes and Assessment Criteria
Learning Outcome	Pass	Merit	Distinction
LO1: Assess the constructional features and applications of transformers	P1 Examine the types of transformers available P2 Discuss suitable applications for available transformers P3 Discuss the different methods of connections available for three-phase Transformers	M1  Illustrate the operation of the transformer, considering the equivalent circuit	D1  Assess the efficiency of a number of available transformers and make a recommendation for an actual operational requirement
LO2: Analyse the starting methods and applications of the threephase induction motors and synchronous machines	P4 Analyse the types of electrical motors available, discussing suitable applications   P5 Analyse the different methods of starting induction motors and synchronous machines	M2 Justify the selection of a motor for a specific industrial application	D2 Critically evaluate the efficiency of a number of available motors and make a recommendation for a specified operational requirement
LO3: Investigate the types of generators available in the industry by assessing their practical application	P6 Explain the types and construction of generators   P7 Identify a generator for a specific application, considering their characteristics	M3 Justify the application of a specific type of generator	D3  Assess the efficiency of a number of available generators and make a recommendation for a specified operational requirement
LO4: Analyse the operating characteristics of electromagnetic transducers and actuators	P8 Analyse the operation, types and uses of electromotive transducers and actuators, examining features that support their suitability for specific applications	M4 Justify the selection of suitable transducers for specific industrial Applications	D4  Analyse the practical application of transducers and actuators in an industrial situation and make recommendations to improve the operating efficiency of the units in use