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Chapter 4.5 - Electric Motors and Variable Speed Drives

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Selected questions Chapter 4.5: Electric Motors, Variable Speed Drives Short type questions 1. The resistance of a motor stator winding at 30 C is 0.264 ohms per phase. What will be the resistance of the stator winding per phase when the motor winding temperature is 100 C? Ans 2. Stator resistance at 100 C = R1 x [(235 + t1] / (235 + t2)] = 0.264 x [(235 + 100) / (235 + 30)] = 0.334 ohms per phase Calculate the full load power factor of a motor with following operating details: Output power Efficiency Voltage (V) Full load current (Ifl) = 30 kW = 85% = 415 V = 58 A Ans. Full load power factor = [Input power / (1.732 x V x Ifl)] x 1000 Input power = output power / efficiency = 30/0.85 = 35.3 kW Full load power factor 3. Calculate the percentage loading of a motor using the following data: Ans. Synchronous speed in rpm Nameplate full load speed Measured speed in rpm Nameplate rated power % Loading = 4. = [35.3 / (1.732 x 415 x 58)] x 1000 = 0.847 = 1500 at 50 HZ operating frequency. = 1440 = 1475 = 11 kW 1500 1475 *100% = 41.6% 1500 1440 Calculate the rotor I2 R losses using the following data:? Slip Stator input Stator I2R losses Core loss Ans. Rotor I2R losses = 4% = 3000 W = 100 W = 50 W = Slip x (Stator input stator I2R losses - Core loss) = 0.04 x (3000 100 50) = 114 W 5. What are the two main advantages of application of a variable frequency drive in an induction motor other than accurate speed regulation? Ans. Two of the main advantages are (i) (ii) 6. High operating power factor Energy savings during part load operation for longer duration of time. Why the accuracy of the slip method is limited? The largest uncertainty relates to the accuracy with which manufacturers report the nameplate fullload speed. Manufacturers generally round their reported full-load speed values to some multiple of 5 rpm. While 5 rpm is but a small percent of the full-load speed and may be considered as insignificant, the slip method relies on the difference between full-load nameplate and synchronous speeds. Given a 40 rpm correct slip, a seemingly minor 5 rpm disparity causes a 12% change in calculated load. 4.5 - Electric motors (table format) 29

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