Transformers and Inductors: Assesment of the Critical Infrastructure - Coursework Example

Title: Transformers and Inductors Course Name and Code: Student Name: Lecturer or Lab demonstrator’s name: Date: Table of Contents Table of Contents 2 Aim: 2 Introduction 2 Methodology 3 Simulation, calculation results and discussion 10 Conclusion 16 References 21 Aim: 1. To investigate the speed control of a DC coupled motor – Generator set. By connecting passive and active electronic elements, the speed characteristics of a dc motor was investigated with varying load parameters. Introduction Diverse applications call for different requirements for rotational speed control. To name but a few, they include: crane operations, rolling mills, hoists, machine tools, elevators, locomotive drives and transit systems. In most of these applications, the dc motors are found to be of basic use and of ease of operation owing to the fact that speed control of DC motors above or below the rated speed can easily be achieved. The methods applied to the control of rotational speed for these DC motors stand out to be simpler and of lesser cost compared to those of AC motors. DC motors are proved to be of benefit in machine drives because they exhibit a constant speed during operation. The speed control for DC motors has undergone significant transformation through the past quarter century, with improvement evolving with advancement of modern control system development. [1] The open loop control operation of a DC motor may be unsatisfactory when applied to varied torque. It follows that, in open loop operation, varied torque would also influence speed changes. A closed loop system proves to be of effective in keeping the speed constant, which is achieved by adjusting the terminal voltage of the motor as the load torque changes. With the closed loop control system, in case of increase of the load, the rotational speed decreases momentarily thus causing the speed error to increase. The speed error is fed back from the Tacho-meter which is compared with the reference speed. With increase in speed error the resultant effect is an increase in the control signal which in turn causes an increase in the converter output voltage, i.e. the firing angle is decreased by the control signal for a phase-controlled rectifier, or conversely, the duty ratio is increased if a chopper is used. The converter output voltage increase causes an increase in the armature voltage of the DC motor hence developing more torque that works to restore the rotational speed. Thus, the above system goes through a transient response period until the torque developed matches the load torque applied. Other advantages that encompass the closed-loop system approach in control of the DC motor include the possibility of running the motor at constant torque over a specified range of speed, which is a vital requirement in the application of traction systems. In addition, for the closed loop system, the control system circuit can include protection devices making it ideal to be used for most drive systems at the industries. [1] [2] Methodology Problem definitions In this experiment, a DC motor-generator set was used to evaluate speed control and the effect of different loads connected across the motor output, in which case, the primary load being the generator. The general arrangement for connecting the dc motor and the dc generator was made to follow the design as shown in the figure below in order to obtain a mathematical model of the set up. Figure 1: Dc motor set From the above open loop schematic, the final mathematical model equations were obtained as follows: ………………………………………… (i)  ………………………………………………… (ii) Where: b = Viscous friction J = moment of inertial for the motor  = armature or emf constant. Design and Theory According to the principle of conservation of energy, the relationship existing between mechanical and electrical power is denoted as Tω = EI where mechanical power is the product of torque and angular velocity while electrical power supplied across the motor terminals is the product of the terminal voltage E and current I. Hence from the equation it can be noted that T varies inversely with ω if E is made constant. [3]  …………………………………………………… (iii) Speed control with the open loop operation of the DC motor proved to be unsatisfactory to constant speed control with a varied load torque hence the closed-loop control was adopted. The closed loop control system for the motor control was as shown below in figure 2: Figure 2: Closed loop speed control schematic The control circuit image and connection was captured as shown in the image of figure 3 below: Figure 3: Control electronic circuit board DC Motor – Generator Drive From the design theory of DC motor operation, it is noted that the motor while running produces a counter emf, e that tends to oppose the applied voltage Vapp as shown in figure 1. Since the resultant torque is a function of the supplied current, the rate at which then the current can be controlled is governed by the equation:  ………………………………………………. (iv) Where: L = inductance due to the motor. Thus to counter the effect of back emf, the applied voltage has to be greater than the value of e in magnitude. It is worth noting that the value of e also increases in a linear relationship with the motor speed since flux is constant in the motor. [1] Power Measurement In DC circuits, power is simply a product of the voltage and current. The values of the current and voltage are easily obtained using standard measuring devices: the ammeter for measuring current and voltmeter to measure voltage across a pair of terminals. The voltage across a pair of terminal, denoting the potential difference between the two points of the terminal equals the energy per unit charge required to move electric charge between the two points, while electric current is defined as rate of change of charge (coulombs/second). It follows that power is the product of current and voltage given by the equation:  ……………………………………………. (v) The theory for AC power differs slightly since the phase difference between the voltage and current rules out the use of voltmeter-ammeter method, and only the average power is measured from the voltmeters and ammeters otherwise calibrated in root mean square values. [4] Other forms derived for measuring power, from ohms law, ……………………………………………… (vi) Conversion of watts to horsepower follows that 1hp = 746W. Equipment DC motor – DC Generator unit, as shown in figure 3 below. A DC power supply and a speed control system Various resistive loads: power resistors Cathode Ray Oscilloscope (Tektronix TDS 2002) A Multimeter (HM8012) A DC power supply A bread board Connecting wires Control Electronics Figure 4: Lab connection diagram Procedure In the experiment, an open loop DC motor speed control system, was used and the voltage, speed and efficiency of the DC motor – DC generator unit was measured. The measurement procedure was as follows with a noted speed limitation for the circuit. 1. The DC motor speed-control system was set up using DC Generator unit, voltage controller and a speed meter. The closed-loop system was used to maintain the speed constant by adjusting the motor voltage as the load torque changed. The response of the DC Motor – DC generator was tested for varying loads (using power resistor in the combinations provided. That is 33, 47 and 68 and the output voltage and waveform was captured. 2. The input current and voltage of the DC motor and the output voltage of the DC generator as well as the motor speed without a load using a manually controlled system (manual mode) was measured. Graphs of input voltage against motor speed characteristics and motor speed against output voltage characteristics were plotted. 3. Step 2 was repeated with a load, and similarly the graphs of input voltage against motor speed characteristics, and motor speed against output voltage characteristics were plotted. 4. The power efficiency of the DC motor – generator set was measured and the power flow in the system explained. The power flow diagram was drawn and the efficiency was calculated using the equations (iv) and (v). Software simulation tools and limitations. The simulation software used include Matlab Simulink software and spreadsheet to draw the graph of obtained values. The simulation software Matlab however has a limitation in that it presents the ideal case scenario. Also the use of the software is not as direct as expected and requires some level of expertise which is also time consuming in the bid to learn and understand its use and modelling techniques. Simulation, calculation results and discussion Simulations From figure 1 and using control equations (i) and (ii), the diagram and speed characteristics at no load was as shown in the figure below: Figure 5: open Loop matlab simulation diagram Plotted graph Figure 6: Generated characteristic curves from Matlab Experimental results Measurements without a load/resistance (Manual mode) Vm (Volts) Im(Amps) N(rpm) Vg(Volts) Ig(Amps) Pm(Watts) Pg(Watts) 1.3 1.5 30 0.5 0 2 0 3.2 1.6 320 2.2 0 5 0 4 1.7 465 3.4 0 7 0 5.2 1.8 600 4 0 9.5 0 7 2 900 6 0 11.5 0 8 2 1200 8 0 14 0 9 2 1200 8 0 19 0 10 2.2 1300 9 0 22.2 0 11 2.2 1450 10 0 25 0 12 2.3 1600 11 0 29 0 Resultant oscilloscope diagram Figure 7: Oscilloscope image running the set at No Load Measurements with a load/resistance R(Ohms) N(rpm) Vm(Volts) Im(Amps) Pm(Watts) Vg(Volts) Ig(Amps) Pg(Watts) Efficiency% 0 1600 12 2.2 27 10 0 0 0 47 1575 11.8 2.5 10.5 10 0.2 2.2 20.95238095 23.5 1550 11.8 2.7 32.5 10.5 0.4 4.4 13.53846154 15.67 1500 11.8 2.9 35 10.8 0.6 6.5 18.57142857 Oscilloscope diagram with a load resistance Figure 8: Oscilloscope image with Load connected Graphs 1. Manual mode Figure 9: Input voltage against motor speed graph at no load Figure 10: output voltage against motor speed graph at no load 2. With Load Figure 11: Graph of output voltage against speed at connected load Discussion and analysis Without load, the motor runs at high speed but with significantly low amount of torque. Hence the speed power product reduces and leads to a very low value in terms of efficiency. With the introduction of the load, the supply current increases leading to a high torque, since the speed is maintained at almost a constant value, the ratio of output power to input power narrows and hence indicates a high efficiency. Also, at No load, the input characteristics yield an irregular curve as compared to the output characteristics whose graph as shown above indicates a smooth and straight curve, of better predictability and machine control. Conclusion Comparing the output voltages at no load and during load, for instance at a constant speed of 1600 rpm, it was deduced that on load, the output voltage was slightly higher by a margin of 0.5V. Since for DC power is a product of voltage and current, it was evident that on load there is a better input to output power conversion ratio. The experiment was therefore successful in analyzing the speed torque characteristics and was concluded that it was better to run the motor with a load connected across the output terminal for better power utilization and control. Part II Lab Tutorial Questions Q1.Electrical Safety (a) The purpose of Electrical Safety is to address Griffith University’s legal obligations under the Act IEC/TS 60479-1 (1994-09) (b) How many times Humans are more sensitive to 60-Hz alternating current than to direct current? Men =2.5, women = 2 times (c) Above 100Hz (d) Men: current equal to or above 23mA at 60 Hz and 94mA at 10Hz, Q2. Inductor (a) (b) Flux, Where, .   Wb Flux density, B (c) Inductance,  where I =current   Q3. Energy conversion Conversion from electrical to mechanical energy and vice versa are based upon the principles of the two Faraday’s law: (a) Whenever a conductor cuts a magnetic flux, an emf is induced. This is the first faraday’s law, important in converting mechanical to electrical energy. The magnitude of the induced emf is equal to the rate of change of flux linkages across the conductor. Suppose we have N number of conductors at two different flux linkages  The induced emf will be given by:    (b) Electromechanical Energy conversion system Q4. Transformer (a) N1 = 1000 turns, N2 = 500 turns E1 = 220v Z2 = 5KVA   e2 = 110V (b) Load Impedance, Z Load in KVA,   (c) Load Impedance referred to primary    References [1] "http://hyperphysics.phy-astr.gsu.edu/hbase/electric/shock.html#c1," Georgia State University, 2016. [Online]. [Accessed 8 August 2016]. [2] C. Bussman, "Safety BASICs: Handbook for Electrical Safety," 2001. [3] C. F. Dalziel, "Deleterious Effects of Electric Shock," Electrical accidents and related matters, p. 24, 1961. [4] Carl R,Brenda C, Physics For the Health Sciences, W. B. Saunders, 1985. Read More

Other advantages that encompass the closed-loop system approach in control of the DC motor include the possibility of running the motor at constant torque over a specified range of speed, which is a vital requirement in the application of traction systems. In addition, for the closed loop system, the control system circuit can include protection devices making it ideal to be used for most drive systems at the industries. [1] [2] Methodology Problem definitions In this experiment, a DC motor-generator set was used to evaluate speed control and the effect of different loads connected across the motor output, in which case, the primary load being the generator.

The general arrangement for connecting the dc motor and the dc generator was made to follow the design as shown in the figure below in order to obtain a mathematical model of the set up. Figure 1: Dc motor set From the above open loop schematic, the final mathematical model equations were obtained as follows: ………………………………………… (i)  ………………………………………………… (ii) Where: b = Viscous friction J = moment of inertial for the motor  = armature or emf constant.

Design and Theory According to the principle of conservation of energy, the relationship existing between mechanical and electrical power is denoted as Tω = EI where mechanical power is the product of torque and angular velocity while electrical power supplied across the motor terminals is the product of the terminal voltage E and current I. Hence from the equation it can be noted that T varies inversely with ω if E is made constant. [3]  …………………………………………………… (iii) Speed control with the open loop operation of the DC motor proved to be unsatisfactory to constant speed control with a varied load torque hence the closed-loop control was adopted.

The closed loop control system for the motor control was as shown below in figure 2: Figure 2: Closed loop speed control schematic The control circuit image and connection was captured as shown in the image of figure 3 below: Figure 3: Control electronic circuit board DC Motor – Generator Drive From the design theory of DC motor operation, it is noted that the motor while running produces a counter emf, e that tends to oppose the applied voltage Vapp as shown in figure 1. Since the resultant torque is a function of the supplied current, the rate at which then the current can be controlled is governed by the equation:  ………………………………………………. (iv) Where: L = inductance due to the motor.

Thus to counter the effect of back emf, the applied voltage has to be greater than the value of e in magnitude. It is worth noting that the value of e also increases in a linear relationship with the motor speed since flux is constant in the motor. [1] Power Measurement In DC circuits, power is simply a product of the voltage and current. The values of the current and voltage are easily obtained using standard measuring devices: the ammeter for measuring current and voltmeter to measure voltage across a pair of terminals.

The voltage across a pair of terminal, denoting the potential difference between the two points of the terminal equals the energy per unit charge required to move electric charge between the two points, while electric current is defined as rate of change of charge (coulombs/second). It follows that power is the product of current and voltage given by the equation:  ……………………………………………. (v) The theory for AC power differs slightly since the phase difference between the voltage and current rules out the use of voltmeter-ammeter method, and only the average power is measured from the voltmeters and ammeters otherwise calibrated in root mean square values.

[4] Other forms derived for measuring power, from ohms law, ……………………………………………… (vi) Conversion of watts to horsepower follows that 1hp = 746W.

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