Study Guide - Downstairs Labs

25% of your Electrical Machines Lab grade is based on your score on the lab final exam. The lab final will be held during exam week in the period preceding the Electrical Machines course final. The exam consists of 33 multiple choice questions based on the material covered in the 12 labs. You will be permitted to take the course final immediately after you complete the lab final.

The following information regarding the downstairs labs may be helpful when preparing for the lab final exam. There will also be questions based on the upstairs labs.

In Lab 1 you looked at a simple series circuit, and studied its behavior. You changed the load from pure resistive (R), to resistive-inductive (LR) and resistive-capacitive (LC) and observed the results on the phasor analyzer. Finally you looked at three-phase power and compared line and phase voltages.

You should be able to:

  • Read and interpret an oscilloscope display.
  • Read and interpret a phasor analyzer display.
  • Understand the relationship between peak and RMS values.
  • Identify leading and lagging values and determine phase shift from an oscilloscope display.
  • Understand the relationship between phase and line voltages.

In Lab 2 you looked at single and three-phase transformers. You hooked up three-phase transformers in three configurations: wye-wye, delta-delta, wye-delta, and delta-wye. For each configuration you looked at the relationship between input and output voltages. In some configurations there was no change but in others there was a step-up or step-down along with a phase shift.

You should be able to:

  • Read and interpret circuit diagrams.
  • Understand the operating principles of single phase transformers.
  • Recall the recall the relationships between line and phase voltage and current in wye and delta circuits. 
  • Recall how different three-phase transformer configurations change (or don’t change) the line voltage and phase angle between the primary and secondary.

In Lab 3 you looked at a DC Generator; first a separately excited generator, then a self-excited shunt generator, and finally a compound generator. You investigated the effect of speed, excitation current and load on the output of the generator.

You should be able to:

  • Describe the main components and principle of operation of a DC generator.
  • Identify shunt, series and compound DC generators by looking at their wiring diagrams.
  • Recall the effects of rotor speed, field excitation, and load on terminal voltage.
  • Recall the relationship between generator load and driving torque. 
In Lab 4 you looked at as three phase squirrel cage induction motor. You produced a graph of motor torque as a function of speed. Then you investigated the behavior of the motor when operated on two phases and on a single phase. Finally, you constructed a single phase capacitor-start motor and observed its operation, and identified the necessity of including a centrifugal switch.

You should be able to:

  • Describe the main components and construction of squirrel-cage and capacitor-start motors.
  • Understand how a rotating magnetic field produces rotor current and torque in an induction motor,
  • Identify on a torque-speed diagram operating points such as: no-load, nominal load, breakdown, and locked rotor.
  • Understand the function of the starting winding and capacitor in a capacitor start motor.
  • Understand the sequence of events that occurs during the starting of a capacitor-start motor.

In Lab 5 you looked at the a synchronous AC Generator. You made graphs of terminal voltage vs. speed, and terminal voltage vs. excitation current. Then you applied pure resistive, inductive and capacitive loads to the generator and observed the terminal voltage as you increased the load while holding the excitation and speed constant.

You should be able to:

  • Describe the main components of an AC synchronous generator.
  • Describe what is necessary for an AC generator to produce a voltage.
  • Recall, qualitatively, the effects of field current, speed, and load on the output voltage of the generator.
  • Define “voltage regulation.”

In Lab 6 you used the same synchronous generator used in the previous lab and paralleled it with the utility grid. You investigated how changes to the prime mover speed control and the excitation current effected the voltage, frequency and load sharing of active and reactive power. Then you used the paralleling simulator to practice paralleling and balancing the load between two identical generators.

You should be able to:

  • Describe the procedure to safely parallel a generator with the bus and balance the loads.
  • Describe how to interpret the information displayed by a synchroscope.
  • Describe qualitatively how changes to the governor setting effects frequency and load supplied by a generator when it is operating alone and when it is paralleled to an identical generator or an infinite grid.
  • Describe qualitatively how changes to the excitation current effects voltage and reactive power of a generator when it is operating alone and when it is paralleled to an identical generator or an infinite grid.