Three-Phase Induction Motors - Questions and Instructions

1025ODLN Electrical Machines

Answered

Question:

Three-Phase Induction Motors

Instructions

This assignment forms part of the formal assessment for this module. If you fail to reach the required standard for the assignment then you will be allowed to resubmit but a resubmission will only be eligible for a Pass grade, not a Merit or Distinction.You should therefore not submit the assignment until you are reasonably sure that you have completed it successfully. Seek your tutor's advice if unsure.

2. You will need to use some simple drawing skills to attempt this TMA effectively. You will need graph paper, please download a couple of sheets and draw clearly on them.

3. Ensure that you indicate the number of the question you are answering.

4. Make a copy of your answers before submitting the assignment.

5. Complete all details on the front page of this TMA and return it with the completed assignment including supporting calculations where appropriate. The preferred submission is via your TUOL(E) Blackboard account:

Answer These Below Questions:

1) This question is about synchronous induction motors on infinite busbars. The exciter current induces a back emf (E) and the magnitude and angle of the current will depend on E’s magnitude.

a) Look at this phasor diagram in FIG 1, copy it and identify the power factor and the load angle by circling it on the figure.

b) What can be said about the generated EMF if there is no load?

c)Describe the significance of IXs

d)Draw two more phasor diagrams of the machine operating at constant load

i) with power factor unity

ii) power factor lagging

e) Describe what happens to the stator current of the synchronous motor as the excitation current changes.

2) This question uses construction skills to determine the machine characteristics. A three-phase delta-connected synchronous induction motor is connected to a 415 volt, 50 Hz supply. The winding resistance is negligible and the synchronous reactance per phase is 4 ohms. The input power is 25 kW and the motor has a leading power factor of 0.8.

a) find

i)Calculate phase current

ii)Calculate the stator reactive voltage drop IXs

b) Draw the phasor diagram for this machine to scale (use graph paper)

i)draw the generated voltage V

ii)use the IXs phasor and the V phasor to construct the phasor for E.

iii) measure or calculate the magnitude of E

iv) By construction and/or by calculation find the load angle.

c) If the excitation and, as a consequence the generated emf, is reduced by 10%:

i) Calculate the new value of the generated emf E’

ii) Construct the new phasor forE’’ on the same diagram, using the same value for reactive volt drop

iii) Calculate or measure the load angle

iv) Draw the new stator current phasor from the phasor diagram, measure or calculate the new power factor

d) Comment very briefly on the effect that reducing the excitation has on the power factor and the load angle.

3) This question is about induction motors. A wound rotor, induction motor has a rotor resistance of 0.6 ohms per phase and a rotor leakage reactance of 8 ohms.

a)What is the operating speed (rpm) of this 2-pole induction motor fed from a 50 Hz supply if the slip is 5%?

b) If maximum torque occurs when R2 = sX2, what is the slip at maximum torque?

c)What resistance must be added to the rotor circuit to give maximum starting torque?

4) This question is about the standard tests for determining the characteristics of a motor. The motor’s characteristics are tested using a two-stage process. The no load test and the locked rotor test. The results will populate the equivalent circuit shown in figure 2.