EELE 354 Lab Assignment 11: DC and AC Motors

Transcription

1 EELE 354 Lab Assignment 11: DC and AC Motors

2 EELE 354 Lab Assignment 11 1 Lab Overview: Underneath your workbench, there are three electromechanical machines: a three-phase induction machine, a three-phase synchronous machine, and a DC machine. These three machines share a common shaft, and collectively they are referred to as a motor-generator (MG) set. In this two part experiment, you will work with the DC and the induction machines operating in motor mode (i.e. electric power delivered to produce mechanical power), and use the synchronous machine in generator mode (i.e. mechanical power used to produce electricity) to provide a load. Objectives: The objectives of this laboratory assignment are: Increase familiarity with the operation of DC and induction motors. Study the implications of varying the input (e.g. voltage, current, and frequency) and output (load demand) parameters of DC and induction motors. Pre-lab Assignment: Read through the entire lab assignment. There is no additional pre-lab work.

3 EELE 354 Lab Assignment 11 2 Lab Experiment I: DC Motor A traditional DC machine has two sets of electromagnet windings: the armature windings on the rotor (rotating component) and the field windings on the stator (stationary component). When DC current flows through the two windings, a torque is produced on the rotor conductors as a result of the interaction of the magnetic fields associated with the two windings. This torque causes the rotor to turn. As it does a commutator & conducting brushes allow the current in the armature winding to periodically switch direction when appropriate so that the torque on the rotor, and thus the direction of rotation of the rotor, is always in the same direction. The schematic diagram below shows the connection of a variable-speed DC motor drive to a three-phase AC voltage supply. The DC-drive in this diagram serves two purposes. First, it rectifies the AC supply voltage into DC-voltage for application to the DC motor windings. Second, it allows variability in the DC-voltage output, which as you will see in your experiment, allows variability in the rotational speed of the motor. It should also be noted, that the field winding is externally excited by a separate power source. In class, we have discussed series, shunt, and compound connected motors. An externally excited winding is somewhat similar to a shunt connected motor, as the field current and rotor current are independent. Figure 1: Variable speed DC motor drive schematic. 1. At one of the lab benches, your instructor has set up the DC motor drive system as shown above. Connected between the DC drive and the armature terminals of the DC machine, your instructor will have inserted the voltmeter portion of the power analyzer to measure the DC voltage output of the DC drive, which can vary from 0 V to the rated motor voltage.

4 EELE 354 Lab Assignment Note and record the nameplate ratings of the DC machine: 3. No-load testing: Your instructor will lead you through a series of tests. You are to complete the table below and answer the questions asked. Start by calculating the DC drive output voltage levels required to correspond to the % rated voltage dictated by the table. Your instructor will then adjust the output voltage of the drive to closely match these voltages; record the voltage and the RPM speed of the motor shaft. % Rated Voltage Calculated Armature Voltage (V) Measured Armature Voltage (V) Measured Rotor Speed (RPM) 5. In the figure provided below, plot a curve of the motor speed as a function of armature voltage using the recorded data.

5 EELE 354 Lab Assignment Rotation Direction Test: Your instructor will restart the motor. Note the direction of rotation of the motor shaft as viewed from the end of the shaft. Clockwise or counterclockwise? 7. Your instructor will stop the motor, interchange the connections at the terminals of the armature winding, and restart the motor. Note the direction of rotation again. Clockwise or counterclockwise? 8. Variable Load Test: Your instructor will stop the motor and load the synchronous generator (the middle machine under your workbench) coupled to the shaft of the DC motor with a three-phase Y-connected load. By doing so, your instructor has now effectively set the synchronous generator to act as a load for the DC motor. That is, by adjusting the amount of electrical power generated by the synchronous machine, your instructor can control the amount of mechanical power (speed and torque) required of the DC motor. In steps, your instructor will increase the load current by decreasing the three-phase resistance. Record the load line current, the DC motor armature current, and the motor s speed in accordance with the table below. Resistance (Ω) Load Line Current (A) Armature Line Current (A) Rotor Speed (RPM) 9. In the figure on the next page, plot two curves. The first should show the armature current as a function of load current (use the left vertical axis for armature current values). The second should show the rotor speed as a function of load current (use the right vertical axis).

6 EELE 354 Lab Assignment In the space below, comment on the results of the DC motor experiment. In particular, comment on the relationship between no-load motor speed and the motor s input voltage, the direction of motor rotation and winding connections, and the relationship between load, motor current, and speed.

7 EELE 354 Lab Assignment 11 6 Lab Experiment II: Induction Motor An induction motor also has two windings; the stator winding and the rotor winding (either wound-wire or squirrel-cage bar conductor type). AC voltage is applied to the stator windings resulting in an AC current flow. As discussed in class, this AC current flow results in a rotating magnetic field generated by the stator electromagnets. Unlike the DC motor, no external excitation is applied to the rotor winding. Instead, the motor operates through electromagnetic induction. That is, as the stator s magnetic field rotates around the rotor, which spins at a speed less than that of the magnetic field, a voltage is induced in the rotor winding. This causes a current to flow in the the short-circuited rotor windings. A resulting force occurs on the rotor windings due to the interaction of these conducting windings and the stator s magnetic field. This force on the conductors corresponds to a torque on the rotor causing it to spin. The schematic diagram below shows the connection of a variable-frequency induction motor drive to a three-phase AC voltage supply. The variable-frequency drive in this diagram uses power electronics to convert a fixed-frequency, fixed-voltage input (60 Hz, 120/208 V) to a variable-frequency, variable-voltage output. This allows variability in the speed and strength of the rotating stator magnetic field, and thus variability in the rotational speed of the rotor. Figure 2: Variable frequency induction motor drive schematic. 1. At another of the lab benches, your instructor has set up the induction motor drive system as shown above. Connected between the variable frequency drive and the induction motor windings, your instructor will have inserted a power analyzer to measure the AC voltage, current, and frequency applied to the motor. Your instructor will lead you through a series of tests.

8 EELE 354 Lab Assignment Note and record the nameplate ratings of the induction machine. 3. Recall from class that the rotor of the induction motor spins at a frequency slightly less than that of the synchronous speed of the rotating magnetic field. Also recall from class, that the relationship between the number of poles per phase in a threephase induction motor and its synchronous speed is given by, n = 120f P where n is the synchronous speed in RPM, f is the excitation frequency in Hz., and P is the number of stator poles per phase. Using the above two pieces of knowledge, based on the rated speed of the motor (this is the rotor speed under rated load), determine the number of poles per phase in the motor. 4. Rotation Direction Test: Your instructor will start the motor. Note the direction of rotation of the motor shaft as viewed from the end of the shaft. Clockwise or counterclockwise? 5. Your instructor will stop the motor and interchange the connections at the terminals of the induction machine windings. Again, note the direction of rotation again. Clockwise or counterclockwise?

9 EELE 354 Lab Assignment Variable Load Test: Your instructor will load the synchronous generator as had been done when testing the DC-motor in Part I. With the excitation frequency set to 60 Hz., he will run the motor under no load conditions, i.e. no resistors connected. Record the speed and the induction motor current in the table below. Then, your instructor maximize the load current so as to load the induction motor to the maximum amount capable with the synchronous machine. Record the motor speed in the table below and induction motor current in the table below. Motor Armature Voltage (V) Measured Armature Current (A) Measured Rotor Speed (RPM) No-load High-load 7. In the space below, comment on the results of the induction motor experiment. In particular, comment on the relationship the direction of motor rotation and winding connections, and the relationship between load and rotor speed. Name and initial of lab partners: Lab Partner 1: Lab Partner 2: Lab Partner 3: