EBM 510: Electric Motor Drive Systems Second Semester Examination

Second semester paper for the biomedical engineering degree, sat on Friday the twenty-ninth of January 2021 from eleven to one, question one compulsory at thirty marks and any two others at twenty. Question one defines drive and electric drive, sketches the torque and power limitations in combined armature voltage and field control, works an eight-pole three-phase induction motor on a sixty-hertz supply through synchronous speed in revolutions per second and per minute, the rotor speed at four per cent slip and the rotor frequency at three hundred revolutions per minute, determines the chopper frequency of a series motor on a two hundred and twenty volt supply from a pulse width of two point three eight six milliseconds, a twenty-five ampere current, a hundred volts of back e.m.f. and two tenths of an ohm of combined armature and field resistance, and then draws the block diagram of a separately excited speed control system with tachometer, amplifier gain, moment of inertia and viscous friction and takes its transfer function. The optional questions give two advantages of chopper-based direct current drives over rectifier-controlled devices and find the starting torque of a delta-connected four hundred volt, thirty-six point seven seven five kilowatt, seven hundred and fifty revolution per minute squirrel cage motor direct on line, with a star-delta starter and with a seventy per cent tapping auto-transformer, then work a rectifier-fed separately excited motor on a three-phase full controlled bridge at a forty-five degree delay angle to developed torque, speed, and the delay angle required when the field polarity is reversed. Others list three advantages of induction motors, separate passive from active loads with an example of each, derive the expression relating torque and speed of a cylindrical rotor synchronous motor, and size a single-phase full bridge feeding a one hundred and twenty-five horsepower motor for no-load speed and for rated speed at rated current. The last discusses four-quadrant operation of a direct current motor with a circuit for each quadrant, describes the effect of different rotor resistances on the torque-speed characteristic of an induction motor, and compares controlling an elevator load of two hundred newton metres by varying the supply of a four hundred volt series motor against inserting external armature resistance, with the efficiency of each.