Scott R. Smith ENEE473 Lab 12: Motor Speed Control May 4, 2007 Purpose: Since the frequency can be directly controlled on the synchronous machine (SM) by controlling prime mover speed, it can be used to control induction machine (IM) speed. The DC Dynamometer (DCD) will act as the prime mover for the SM. The electrical frequency of a machine is related to the speed of rotation by the following formula. ρfn120=(1) Here the electrical frequency of the IM input will be varied by the SM. Both machines have four poles, so equation 1 simplifies to the following. fn30=(2) Devices and Instrumentation: DC Dynamometer– The DCD is a direct current machine. It can be mechanically coupled to the IM using plastic pins and collars. The DCD can be used to measure torque, lock the rotor, be a prime mover, and act as a mechanical load. The rating of the DCD is 1kW. Multimeter– The multimeter will mainly be used to check actual resistance values and measure voltages. Oscilloscope– An oscilloscope can be used to measure frequency, phase sequence, and phase difference among many other things. When connecting the oscilloscope to a floating AC source, make sure not to connect the neutral to the ground on the oscilloscope because this presents an opportunity for the oscilloscope to sink current on the ground wire and it is not designed to do so. Three-Phase Induction Machine– The IM is very similar to a transformer with the secondary winding shorted. I am assuming that the induction machine is viewed as a balanced Y load. The stator has three-phase windings distributed in space and time by 120 degrees. When the three-phase windings are excited from a three-phase source a rotating magnetic field is created. The rotating field interacts with the rotor by inducing a voltage across the bars in the rotor. The induced voltage creates current in the rotor which lags the voltage applied to the stator. The rotor current produces a rotor reaction magnetic field which interacts with the stator magnetic field. The rotor field continuously tries to align with the stator field. Torque is produced because the rotor reaction field always lags the stator field. The ratings of the machine are 1kW, 208V, 6.2A, and 1770RPM.
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