In this exercise, you observed that when the nominal line voltage is applied to

Question

In this exercise, you observed that when the nominal line voltage is applied to the stator windings of a squirrel-cage induction motor without mechanical load, the rotor turns at approximately the same speed as the rotating magnetic field (synchronous speed). You saw that interchanging any two of the three leads supplying power to the stator windings reverses the phase sequence, and thereby, causes the motor to rotate in the opposite direction. You observed that the motor line currents increase as the mechanical load increases, thus showing that the squirrel-cage induction motor requires more electric heavier loads. You plotted a graph of speed versus torque and used it to determine the nominal, breakdown, and locked-rotor torques of the squirrel-cage induction motor. You also plotted a graph of the speed and observed that the squirrel-cage induction motor draws reactive CONCLUSION power to drive motor reactive power versus power e ac power source to create its magnetic field. Finally, you plotted a graph of the motor line current versus speed and observed that the starting current is many times greater than the nominal line current. REVIEW QUESTIONS 1. The speed of the rotating magnetic field created by three-phase power is called a. no-load speed. b. synchronous speed. c. slip speed. d. nominal speed. The difference between the synchronous speed and the rotation speed of a squirrel-cage induction motor is 2. a. b. c. d, known as slip. always greater than 10%. known as slip torque. always less than 1%. 3. Reactive power is consumed by a squirrel-cage induction motor because it uses three-phase power. it does not require active power. it requires reactive power to create the rotating magnetic field it has a squirrel-cage. b. c. d. 4. Does the speed of a squirrel-cage induction motor increase or decrease when the motor load increases? a. b. c. It increases It decreases. It stays the same because speed is independent of motor load. The speed oscillates around the original value. d. © Festo Didactic 3032900 169

Explanation / Answer

1. It is the synchronous speed.

2. Sync speed = NS

Rotation speed = N

Ns - N = sNs (slip speed where s is the slip)

3. Reactive power is required to create rotating magnetic field.

4. When the load on the motor increases it first decreases the speed and then again increases it so it oscillates around the original vaue.

5. The motor reverses its direction of motion.

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