Faculty of Engineering and Information Technology. Lab 3 Transformers

Transcription

1 Faculty of Engineering and Information Technology Subject: 485 Fundamentals of Electrical Engineering Assessment umber: 3 Assessment Title: Lab 3 Transformers Tutorial Group: Students ame(s) and umber(s) Student umber Family ame First ame Declaration of Originality: The work contained in this assignment, other than that specifically attributed to another source, is that of the author(s). It is recognised that, should this declaration be found to be false, disciplinary action could be taken and the assignments of all students involved will be given zero marks. In the statement below, I have indicated the extent to which I have collaborated with other students, whom I have named. Statement of Collaboration: Marks Lab work / Analysis / Questions / Signature(s) TOTAL /5 key Office use only Assessment Submission Receipt Assessment Title: Student s ame: Date Submitted: Tutor Signature: Lab 3 Transformers

2

3 L3. Lab 3 Transformers Open-circuit and short-circuit tests. Resistance. Leakage and magnetising reactance. Introduction The measurement of equivalent circuit parameters is an important concept in electrical engineering. We will measure the equivalent circuit of a single-phase transformer. Objectives. To observe safe operating procedures when working with a power transformer.. To measure AC voltage, current and power at mains frequency and amplitude (50 Hz, 40 V). 3. To measure the equivalent circuit parameters of a power transformer. 4. To calculate the transformer performance with a resistor and capacitor load and compare with measured performance.

4 L3. Equipment single-phase 40 V, 8A autotransformer Warburton Franki Variac, single-phase 40 V : 0 V,.5 kva, 50 Hz isolating transformer Standard Waygood DC/AC current probe Prova Model 5 digital multimeter three-phase resistive load, 0 Ω per phase three-phase capacitive load, 60 μf per phase AC voltmeter / ammeters YEW clip-on power quality clamp meter Fluke 345 x0 voltage probe Safety Cat. B lab Warning! This is a Category B laboratory experiment. Please adhere to the Category B safety guidelines (issued separately). Remember:. Choose suitable METER SCALES and WID DOW and SWITCH OFF the supply Variac when making circuit connections.. Ensure equipment is earthed.

5 L3.3 Theory. Winding Arrangement There are several windings on the iron core. The ends of each winding are brought out to terminals and therefore more than one transformer connection can be made. The primary and secondary windings are concentric. A diagram is shown below: Rating:.5 kva or 6 A, 40 V a 5 / 0 A 5 / 45 a 4 A 4 / 360 a 3 / 0 A 3 / 40 a / 60 A / 08 a secondary A primary E Figure 3. Single-phase transformer windings

6 L3.4. Magnetic Circuit The transformer is a core-type transformer with concentric windings on one limb. The primary (the high voltage exciting winding) is on the outside: primary a b secondary l Figure 3. Single-phase transformer construction The magnetic equivalent circuit is: a b R a R b l R l F F Figure 3.3 Single-phase transformer magnetic equivalent circuit

7 L Electric Circuit The electric equivalent circuit is: R L l i i' i L l R i e i c i m v v R c e L m e ideal Figure 3.4 Single-phase transformer magnetic equivalent circuit

8 L3.6 Lab Work [3 marks] I DC Resistance. The resistance of the windings is very low. Therefore, we need to measure the resistance of the leads used to take the measurement, and subtract this from the recorded value for the windings: R lead A 5 a 5 DMM lead resistance R lead A a { } short a 3 to a 4 Transformer 45 : 40 Figure 3.5 Measurement of winding resistance for a transformer. Using the DMM, measure the DC resistance of the two leads used to take the measurement (plug one lead into the other): R leads Rlead Rlead 3. Using the DMM, measure the DC resistance of the primary 45 V winding and the secondary 40 V winding (short a 3 to a 4 ). Remember to correct for the lead resistance. R R 4. Calculate the transformer s resistance (referred to the primary), when : R DC R R

9 L3.7 II Short-Circuit Test. Do not connect the supply or turn on the power until circuit connections are checked by a lab tutor.. Wire up the circuit shown below the turns ratio is 45:40. Clamp Meter I sc A 5 a 5 The Fluke clamp meter is used to measure the source voltage and any currents. 30 V 50 Hz (GPO) V sc P sc FLUKE A a 4 a 3 short { a 3 to a 4 } ote the earth connection! a Variac Earth 45 : 40 Transformer Figure 3.6 Short-circuit test circuit for a transformer 3. The Variac makes the earth connection from the GPO available at a green terminal. Ensure that the earth (frame) of the transformer is connected to the earth terminal of the Variac. 4. The clamp meter has an arrow (moulded into the plastic) that indicates the assumed polarity of the current. Ensure that the positive current (from left to right in the red lead in the diagram above) is passed through the clamp in alignment with the arrow. 5. After the circuit has been checked, turn on the Variac and bring up the voltage SLOWLY until the current I 5 sc A RMS.

10 L Measure voltage, current and power on the primary side of the transformer: Primary voltage: V sc Primary current: I sc Primary power: P sc 7. Calculate the transformer s parameters (referred to the primary): Psc R eq I sc Z eq V I sc sc X Z R eq eq eq 8. Wind down and switch off the Variac.

11 L3.9 III Open-Circuit Test. Do not connect the supply or turn on the power until circuit connections are checked by a lab tutor.. Wire up the circuit shown below the transformer turns ratio is 40:60. ote the earth connection! DSO Clamp Meter I oc A 3 a x0 probe 30 V 50 Hz (GPO) V oc Poc FLUKE v A a Current Probe PROVA Variac Earth 40 : 60 Transformer Figure 3.7 Open-circuit test circuit for a transformer 3. Set the current probe s range to 400 ma and press the ZERO button. ote that the 400 ma range gives mv / ma at the output. 4. On the DSOs Channel menu, push the Probe softkey. Set the Units to Amps. 5. Trigger the DSO using Ext-Line. 6. Push the Acquire key (in the Waveform section) and choose Averaging to reduce the noise on the display.

12 L After the circuit has been checked, turn on the Variac and bring up the voltage SLOWLY in steps until the voltage V 00 oc V RMS. Using the Quick Measure feature of the DSO, measure the RMS value of the primary current and tabulate below: V oc (V) I oc (ma) 8. Plot V ~ I. oc oc Open-circuit V - I characteristic V oc (V) I oc (ma)

13 L3. 9. For V 00 oc V RMS, accurately sketch the waveforms of v and i oc, noting both horizontal and vertical scales. Open-circuit test waveforms Remember: the Channel scale on the DSO is the voltage produced by the current probe, so you will have to scale the voltage to get the current. 0. For V 00 oc V RMS, use the DSO cursors to measure the phase angle between v oc and i oc (use zero crossings as reference): Phase angle:

14 L3.. Using the clamp meter, measure the power factor: cos Poc V I oc oc Hence determine: Phase angle: Comment on the accuracy of the DSO and clamp meter in measuring the phase angle. Which measurement is more useful, and why?

15 L3.3. Calculate the transformer s parameters (referred to the primary): R c I oc Voc cos X m I oc Voc sin 3. Wind down and switch off the Variac. Turn off the PROVA In the model of the transformer, show the element values: R L l i i' i L l R i e i c i m v v R c e L m e ideal R L l R c L m L l R

16 L3.4 IV Load Test with Resistor. Do not connect the supply or turn on the power until circuit connections are checked by a lab tutor.. Wire up the circuit shown below the turns ratio is 45:40. The V and A meters are YEW meters on appropriate settings. ote the earth connection! 30 V 50 Hz (GPO) V P I P P FLUKE A 5 A a 5 a 4 a 3 V S V A YEW 300 V I S YEW 3 A 0 a Variac Earth 45 : 40 Transformer Resistor Bank Figure 3.8 Load test circuit for a transformer 3. After the circuit has been checked, turn on the Variac and bring up the voltage SLOWLY until the primary voltage is 40 V. 4. Measure voltage, current and power on the primary side of the transformer: Primary voltage: Primary current: Primary power: V P I P P 5. Measure voltage and current on the secondary side of the transformer: Secondary voltage: Secondary current: V S I S 6. Wind down and switch off the Variac.

17 L3.5 IV Load Test with Capacitor. Do not connect the supply or turn on the power until circuit connections are checked by a lab tutor.. Wire up the circuit shown below the turns ratio is 45: V 50 Hz (GPO) V P I P P FLUKE A 5 A a 5 a 4 a 3 V S V A YEW 300 V I S YEW 3 A 30 F The V and A meters are YEW meters on appropriate settings. ote the earth connection! a Variac Earth 45 : 40 Transformer Capacitor Bank Figure 3.9 Load test circuit for a transformer 3. After the circuit has been checked, turn on the Variac and bring up the voltage SLOWLY until the primary voltage is 40 V. 4. Measure voltage, current and power on the primary side of the transformer: Primary voltage: Primary current: Primary power: V P I P P 5. Measure voltage and current on the secondary side of the transformer: Secondary voltage: Secondary current: V S I S 6. Wind down and switch off the Variac.

18 L3.6 Questions [ marks] Encircle the correct answer, cross out the wrong answers. [one or none correct] All questions are worth 0. marks each. (i) i i' ideal i m e i A transformer model. v L m e v R The relations are: (a) (b) (c) v e i v i, m, e i i i R R,, i,, (ii) The magnetising reactance of a transformer is: e i e R (a) negligible (b) linear (c) nonlinear (iii) A lossless transformer has 5000, 00. The mean length of the core is 50 mm and its c.s.a. is 00 mm 4. r 0 4. The secondary self and primary to secondary mutual inductances are respectively: 3 (a) 50 H, H (b) 50 H, H (c).5 H, 0 5 H (iv) An ideal transformer is rated as 40 V : V. It supplies a load with 500 ma at 6 V (RMS AC). The primary current is: R R R (v) (a) 5 ma (b) 0 A (c) 50 ma 30 V 70 W 0.45 A Short-circuit test on a real transformer. R eq real (a) 346 (b) 756 (c) 376

19 L3.7 (vi) 00:5 ideal F The primary equivalent of the capacitive load is: (a) 400 F (b) 50 nf (c).5 nf (vii) A lossless transformer is to operate from a 30 V, 50 Hz supply and the maximum flux density in the core must not exceed.4 T. The minimum core cross-section for a 00 turn primary coil is: (a) m (b) m (c) m (viii) 5 To achieve maximum power transfer to the 8 load, the turns ratio should be: ideal (a) (b) (c).65 (ix) i i = 0 v v O.C. In the transformer shown, the mutual inductance is: (a) 0 (since i 0 ) (b) L d dt (c) L (x) The leakage reactance of a transformer is: (a) negligible (b) linear (c) nonlinear

20 L3.8 Report Only submit OE report per lab group. Complete the assignment cover sheet. Ensure you have completed:. Lab Work waveforms, readings, calculations.. Analysis explanations where required. 3. Questions multiple choice questions. The lab report is due at the time stated in the Learning Guide. You should hand the report directly to your tutor.