KD5067 Power, Machines and Renewable Energy

INSTRUCTIONS

Format

• to that provided in the lab sheets. Present your results for both Y and  connections of the stator windings. (LO2, LO3: 20 %) 3) Using Excel, Matlab, or other suitable software, plot the following operating characteristics of the motor as functions of load torque: rotor speed, stator line current, power factor, input power, and slip. For each performance metric, present two curves on the same figure: one for the Y-connected stator windings and one for the Δ-connected configuration. (LO2, LO3: 20 %) 4) The induction motor under consideration is driving a constant-torque load (TL). The torque–speed characteristic gives two possible steady-state operating points, as illustrated in Figure T4. Which operating point is stable, and why? Explain your reasoning. (LO1: 5 %) Page 2 of 5 Coursework 2 Page 3 of 5 TORQUE (Nm) 0 NS SPEED (r.p.m.) C TL=K B Figure T4 5) An inverter-fed, 230V, 50 Hz, 1425 rev/min, 3-phase, Δ-connected wound rotor induction motor with constant V/f control has a stator to rotor turns ratio of 2:1. The per phase parameters at standstill are: Stator winding: Resistance = 0.02 Ω, Leakage reactance = 0.1 Ω. Rotor winding: Resistance = 0.005 Ω, Leakage reactance = 0.025 Ω. Using the approximate equivalent circuit of the motor (i.e. ignoring magnetising reactance and iron loss resistance) calculate: (a) The slip, the line current, the mechanical power, and the electromagnetic torque. (LO1: 6%) (b) The maximum torque and the starting torque per ampere normalised to their rated values (i.e. at 50 Hz) if the stator frequency is reduced to 20 Hz. (LO1: 7%) 6) A separately excited DC motor rated at 40 A and 200 V is supplied from a conventional 230 V, 50 Hz AC mains source via a single-phase thyristor rectifier. The rotor winding has a resistance of 0.25 Ω. (a) Calculate the armature voltage required to reduce the speed of the fully loaded motor by 30%. Then determine the rotor voltage needed to achieve the same speed reduction at 80% of the rated load torque. (LO1: 6%) (b) What is the maximum permissible armature voltage at start-up to prevent the motor from being overloaded? (LO1: 3%) (c) Can this motor be used in high-speed applications? If so, describe how a 20% speed increase of the unloaded motor could be achieved. (LO1: 3%) 7) Prepare a Microsoft® PowerPoint-based audio/video presentation of up to 15 minutes that discusses the use of induction generators in wind-power applications. The presentation should outline, but not limited to, the following aspects: operating principles, key performance characteristics, advantages and limitations, and typical system configurations used in wind-energy conversion. (LO2: 20%) Coursework 2 Page 4 of 5 ASSESSMENT Tasks (LOs, BTLs) Criteria Weighting Task 1 (LO1, L1) (a) The main advantages of variable voltage starting, targeting medium-to-large induction motors (3%)
• (b) At least one type of electro-mechanical and power electronics starters for induction motors clearly stated (2%)
• Relative advantages and limitations (2%)
• (c) The main disadvantage of fixed (line) frequency starting (3%). 10% Task 2 (LO2, LO3, L3) Clarity and accuracy of measured and predicted performance metrics with results separately tabulated for Y and Delta connected machine including the: motor line current (2%)
• motor electrical power (2%)
• motor power factor (2%)
• motor load torque (2%)
• motor speed (2%)