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BRAC University Department of Electrical & Electronic Engineering

LABORATORY MANUAL

Course No: EEE 224 Course Title: Energy Conversion Lab

List of the Experiments

Serial No 01 02 03 04 05 06 07

Name of the experiment Study of a single-phase transformer and determination of turn ratio And exciting current Determination of transformer parameters and testing of voltage regulation using different loads Determination of starting torque and loading characteristics of a capacitor start motor Determination of the Parameters of a Three Phase Induction Motor No load and loading characteristics of three-phase alternator Alternator Synchronization with infinite bus. To determine the V curve of a Synchronous Motor

BRAC UNIVERSITY EEE 224 Energy Conversion Laboratory Experiment No: 01 Study of a single-phase transformer and determination of turn ratio And exciting current.

Objectives: •

To Study the voltage and current ratios of a transformer.

•

To learn about transformer exciting currents, volt-ampere capacity and short-circuit currents.

Discussion: Transformers are probably the most universally used pieces of equipment in the electrical industry. They range in size from miniature units in transistor radios to huge units, weighing tons, used in central power distributing stations. However, all transformers have the same basic properties which you are about to examine. When mutual induction exists between two coils or windings, a change in current through one induces a voltage in the other. Every transformer has a primary winding and one or more secondary windings. The primary winding receives electrical energy from a power source and couples this energy to the secondary winding by means of a changing magnetic field. The energy appears as an electromotive force across the secondary winding, and if a load is connected to the secondary, the energy is transferred to the load. Thus, electrical energy can be transferred from one circuit to another, with no physical connection between the two. Transformers are indispensable in ac power transmission and distribution, since they can convert electrical power at a given current and voltage into an equivalent power at some other current and voltage. When a transformer is in operation, ac currents flow in its windings and an alternating magnetic field is set up in the iron core. As a result, copper and iron losses are produced which represents active power (watts) and causes the transformer to heat up. Establishing a magnetic field requires receive power (vars) which is drawn from the power line. For these reasons the total power delivered to the primary winding is always slightly larger than the total power delivered by the secondary winding. However, a close approximation used for most transformers is, a) Primary power (watts) = Secondary power (watts) b) Primary volt amperes (VA) = Secondary volt-amperes (VA) c) Primary vars = Secondary vars When the primary voltage is raised beyond its rated value, the iron core (laminations) begins to saturate, and the magnetizing (exciting) current increases rapidly. Transformers are subject to accidental short circuits caused by natural and manmade disasters. The short-circuit currents can be very large and, useless interrupted, will quickly burn out a transformer. It is the purpose of this Laboratory Experiment to show these major points.

(1/1)

Apparatus: 1. 2. 3. 4. 5. 6.

Power Supply Two ac voltmeters ( 0-300V, 0-300V ) Two ac ammeters (1.25A ) One single phase transformer (240V/240V, I1F = 0.8A & I2F = 0.46A) One resistive load Wires & chords

Experimental Setup:

Procedure: a) Voltage Ratio: 1. Connect the circuit as shown in figure 1 2. Turn on the power supply. 3. Gradually increase the voltage by varying the voltage control knob up to rated voltage. (Terminal 1-2) 4. Measure and record the secondary voltageV2.

(1/2)

5. Return the voltage to zero and turn off the power supply. 6. Find the turn ratio by using the formula

a=

V1 V2

b) Current Ratio: 7. 8. 9. 10. 11.

Connect the circuit as shown in figure 2 Repeat step 2 and 3. Gradually increase the current I2 by varying the load impedance and record I1 and I2. Return the voltage to zero and turn off the power supply. Find the turn ratio using the formula

a=

I2 I1

c) Determination of exciting current: 12. Connect the circuit as shown in figure 1 13. Turn on the power supply. 14. Gradually increase the voltage by varying the voltage control knob till the rated voltage is achieved and record I1.

Report: 1. Show all the data in tabular form. 2. Determine the turn ratio (a) of the transformer from the voltage and current readings using the appropriate formula. Discuss the discrepancies, if any. 3. Which method of determining turn ratio is more accurate and why? 4. Why transformers are rated in kVA instead of kW? 5. Draw the vector diagram of a real transformer for resistive, inductive and capacitive load.

(1/3)

BRAC University EEE 224 Energy Conversion Laboratory Experiment No: 02 Determination of transformer parameters and testing of voltage regulation using different loads. Objective: • •

To determine the transformer parameters using short circuit and open circuit test. To study the voltage regulation of the transformer with varying loads

Discussion: The load on a large power transformer in a sub-station will vary from a very small value in the early hours of the morning to a very high value during the heavy peaks of maximum industrial and commercial activity. The transformer secondary voltage will vary somewhat with the load and, because motors and incandescent lamps and heating devices are all quite sensitive to voltage changes, transformer regulation is of considerable importance. The secondary voltage is also dependent upon whether the power factor of the load is leading, lagging or unity. Therefore, it should be known how the transformer will behave when it is located with a capacitive, an inductive or a resistive load. If a transformer were perfect (ideal) its windings would have no resistance. Furthermore, it would require no reactive power (vars) to set up the magnetic field within it. Such a transformer would have perfect regulation under all load conditions and the secondary voltage would remain absolutely constant. But, practical transformers do have winding resistance and they do require reactive power to produce their magnetic fields. The primary and secondary windings possess, therefore, an overall resistance R and an overall reactance X. It is clear that if the primary voltage is held constant, then the secondary voltage will vary with loading because of R and X. An interesting feature arises with a capacitive load, because partial resonance is set up between the capacitance and the reactance X so that the secondary voltage E2 may actually tend to rise as the capacitive load value increases. In transformer parameters are not readily available from the nameplate, they can be approximated from an open-circuit test and a short-circuit test.

Open-circuit test: The purpose of the open-circuit test is to determine the magnetizing reactance X M and the equivalent core-loss resistance Rfe. For safety in testing and instrumentation, the opencircuit test is generally made on the low -voltage side. The connections and instrumentation required for the test are shown in the Figure 1. The primary current is

very low compared to the rated value. Therefore the copper loss in the secondary is zero, and the copper loss in the primary is negligible. Thus, the wattmeter reading is essentially core losses. Applying the rated voltage, the wattmeter, voltmeter and ammeter readings taken during the open-circuit test are P OC, VOC and IOC , respectively, then -1 Xm = VOC /IOC sinφ, where φ = cos (POC / VOCIOC)

ROC = VOC 2 / POC

W core = POC

Short-Circuit Test: The purpose of the short-circuit test is to determine the equivalent resistance, equivalent leakage reactance, and equivalent impedance of the transformer windings. The connections and instrumentation required for the test are shown in the figure b1. The high side of the transformer under test is connected to the supply line and the low voltage side is short circuited. The test provides data that include the effects of primary and secondary resistances and leakage reactances. Furthermore, short circuiting the secondary causes the flux density to be reduced to a very low value, making the core losses insignificant. Thus wattmeter reading is essentially copper losses. The equivalent short-circuit model of the transformer is shown in figure b2. Assuming the wattmeter, voltmeter and ammeter readings taken during the short-circuit test are PSC, VSC and ISC , respectively, then ISC = VSC / Zeq,HS

Zeq,HS = √( Req,HS 2 + Xeq,HS 2)

PSC = ISC2 Req,HS

W cu = PSC

Apparatus: 1. 2. 3. 4. 5. 6. 7.

Power Supply Unit Single phase transformer (1 No.) AC Voltmeter (2 Nos.) AC Ammeter (2 Nos.) Variable Resistive load 1 unit. Variable inductive load 1 unit. Variable Capacitive load 1 unit R

X

E1

E2

IGURE - 1

Figure 3

A I1 0-240V AC Power supply

V

1

3

A I2

V1

V V2

terminal 4 & N

2

5

FIGUREFigure 4- 2

Procedure: A. Voltage regulation test: • Connect the circuit shown in Figure - 3. •

Turn on the power supply.

•

Gradually increase the voltage up to rated voltage.

•

Connect a resistive load on the secondary side of the transformer.

•

Measure Primary voltage (V 1), Primary current (I1), Secondary voltage (V2) and secondary current (I2).

•

Vary the load and record the corresponding I 1, I2, V1 and V2.

•

Return the voltage to zero and turn off the power supply.

•

Repeat all the above steps for inductive and capacitive load.

•

Calculate voltage regulation for (1) Resistive (2) Inductive (3) Capacitive Loading separately.

B. Transformer parameters Determination:

•

Open-Circuit Test: Connect the circuit shown in Figure 1.

•

Turn on the power supply.

•

Gradually increase the voltage up to rated voltage.

•

Measure open circuit voltage (V OC), current (IOC) and power (POC).

•

Return the voltage to zero and turn off the power supply.

•

Determine the open-circuit parameters.

Short-Circuit Test: •

Connect the circuit shown in Figure 2.

•

Turn on the power supply.

•

Gradually increase the voltage so that the rated current flows in the H.T side.

•

Measure short circuit voltage (V SC), current (ISC) and power (PSC).

•

Return the voltage to zero and turn off the power supply.

•

Determine the short-circuit parameters.

Report: 1. Is the voltage regulation negative for capacitive loading? If your answer is yes, explain why? 2. Show the detail calculation of determining transformer parameters. 3. What are core losses and copper losses? 4. Why the determination of voltage regulation is important?

BRAC UNIVERSITY EEE 224 Energy Conversion Laboratory Experiment No: 03

Name of the Experiment: Determination of starting torque and loading characteristics of a capacitor start motor

Objectives: # #

To measure the starting and operating characteristic of the capacitor-start motor. To compare it’s starting and running performance with the split-phase motor.

Discussion: When the split-phase rotating field was described it was stated that the different resistance reactance ratio of the two windings was designed to give the difference in time phase of the currents in the windings necessary to produce a rotating magnetic field. In two phase machines, when the windings are identical but displaced in space by 90o, the ideal time phase displacement of the winding currents is 90o. For both two phase and split phase motors the torque developed at starting can be calculated using the relationship. T= kI1I2Sinα Where k is a machine constant, 11 and 12 are the currents in the windings and α is the angle between the currents. Because of the small magnitude of α in the split phase machine the developed torque is relatively low. It is possible to increase α by adding capacitance in series with the auxiliary winding. If too much capacitance is added, the impedance of the winding is increased to the point that there is an unacceptable reduction in the current which more than offsets the benefit gained from increasing α. The optimum value of C is that where the product of the sine of α and the auxiliary winding current is a maximum. The capacitor and the start winding are disconnected by a centrifugal switch, just as in the case of the standard split-phase motor. Reversing the direction of rotation of a capacitor start motor is the same as in the case of the split-phase motor, that is reverse the connections to the start or to the running winding leads.

Apparatus: 1. 2. 3. 4. 5. 6. 7.

Power Supply Capacitor start motor Electrodynamometer Watt meter (cc 5A ) One AC Ammeter (5A) One AC Voltmeter (220V) Wires & chords

Experimental Setup:

Capacitor Start Motor Watt meter

cc

5A A

Main Winding

pc 240 V AC V 250V

Electro dynamo Meter

Auxiliary Winding

0-240 V DC

Fig: 1 Procedure: (1) Couple the electrodynamometer with capacitor start motor. (2) Connect the Circuit shown in figure 1 (3) Turn on the power supply. (4) Gradually increase the load by varying voltage control knob of the power supply and fill up the table 1

Torque

Amp(A)

Input Power(W)

Speed(RPM)

Output Power(W)

0.2Nm 0.3Nm 0.4Nm O.5Nm 0.6Nm 0.7Nm 0.8Nm 0.9Nm 1.0Nm

Table-1

(5)

Return the voltage to zero and turn off the power supply.

Capacitor Start Motor Electrodyanamometer Main Winding

Auxiliary Winding

0-240 V DC

240 V DC

Fig: 2

(1) (2) (3) (4) (5)

Connect the Circuit shown in figure -2 Turn on the power supply and quickly increase voltage up to 150 volt ac Quickly measure and record the torque Turn of the power supply Calculate the starting torque developed by the motor when supplied with 240 V ac by using following formula

V1 2 V2 2

Report: (1) (2) (3) (4)

Draw all the connection diagram Show all the recorded data in tabular form Plot torque vs speed curve on the graph paper Calculate starting torque at 240 volt

T1 T2

BRAC University EEE 224 Energy Conversion Laboratory Experiment No: 04 Determination of the Parameters of a Three Phase Induction Motor Objective: ฀ To study the construction of Squirrel Cage Induction Motor. ฀ To determine the parameters of the equivalent circuit of a three phase induction motor.

Discussion: The simplest and most widely used rotor for induction motors is the so called squirrel cage rotor from which the squirrel cage induction motor gets its name. The squirrel cage rotor consists of a laminated iron core which is slotted lengthwise around its periphery. Solid bars of copper or aluminium are tightly pressed or embedded into the rotor slots. At both ends of the rotor, short-circulating rings are welded or brazed to the bars to make a solid structure. The short circuited bars, because their resistance is much less than the core, do not have to be specially insulated from the core. In some rotors the bars and end rings are cast as a single integral structure for placement on the core. The short circulating elements actually form short-circuited turns that have high currents induced in them by the stator field flux. Compared to the intricately wound and arranged wound rotor or the armature of the DC motor, the squirrel cage rotor is relatively simple. It is easy to manufacture and is essentially trouble free in actual service. In an assembled squirrel cage induction motor, the periphery of the rotor is separated from the stator by a very small air gap. The width of this air gap, in fact, is as small as mechanical clearance needs will permit. This ensures that the strongest possible electromagnetic induction action will take place. When power is applied to the stator of a practical induction motor, a rotating magnetic field is created. As the field begins to revolve, its flux lines cut the short circuited turns embedded around the surface of the squirrel cage rotor and generate voltages in them by electromagnetic induction. Because this turns are short circuits with very low resistance, the induced voltages cause high current to circulate in the rotor bars. The circulating rotor currents then produce their own strong magnetic field. These local rotor flux fields produce their own magnetic poles, which reacts with the rotating field. Thus the rotor revolves around the main field. The starting torque of the basic squirrel cage induction motor is comparatively low, because at rest the rotor has a relatively large inductive reactance (X L) with respect to its resistance (R). Under these conditions we would expect the rotor current to lag rotor voltage by 90 degrees and that the power factor in the circuit would be low. This means that the motor is inefficient as a load and cannot derive really useful energy for its operation from the power source.

Page 1 of 5

Despite the inefficiency, the torque is developed and the motor begins to turn. As it starts turning, the difference in speed between rotor and rotating field, or slip, goes from maximum of 100 percent to some intermediate value, say 50 percent. As the slip decreases in this manner, the frequency of the voltages induced in the rotor decreases, because the rotating field cuts conductors at a decreased rate; this, in turn, causes the overall inductive reactance in the circuit to decrease. As inductive reactance decreases, the power factor begins to increase. This improvement is reflected as an increase in torque and a subsequent increase in speed. When the slip drops to some value between 2 and 10 percent, the motor speed stabilizes. The stabilization occurs because every tendency for the motor speed to increase to where slip will drop below 2 percent is naturally offset by the fact that, as the rotor approaches within 2 percent of the synchronous speed, the effects of reduced induction overcome the previous tendency to increase the torque as the motor is speeded up from start. Thus, the motor exhibits an automatic speed control characteristic similar to that of the dc shunt motor.

DC Test: The purpose of the DC test is to determine R1. V R DC = DC I DC

For wye connected stator, RDC = 2 R1.wye

Multiply the DC resistance with an appropriate factor (1.6) to get the AC resistance. [to accommodate the skin effect]

Blocked Rotor Test: The blocked rotor test is used to determine X1 and X2. The test is performed by blocking the rotor so that it cannot turn, and measuring the line voltage, line current and three phase power input to the stator. An adjustable AC supply is used to adjust the blocked rotor current to approximately rated current. Since the exiting current (I0) at blocked rotor is considerably less than the rotor current (...