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EE 332 Lab Manual Lab 4 3-Phase Synchronous Machine Lab 4: 3–Phase Synchronous Machine Goals induces a voltage E in the armature circuit. Note that E is an ac phasor quantity, fully To review power factor principles described as Review and familiarisation with three– E Ee ˆ j t (4.1) phase systems The rms magnitude of the induced voltage, E, To understand the modes of operation of is often referred to as the “excitation” and the synchronous machines phase angle,, is referred to as the “load To understand how synchronous machines angle”. The excitation of the armature is often may be connected to the power system dealt with as an independent variable. In reality, it is a function of the field winding, Introduction field circuit design and speed of rotation. The steady-state behaviour of a synchronous This lab will consider the operation of machine may be described using the per-phase synchronous machines as both motors and equivalent circuit models shown in Fig. 4-1 generators. You will investigate the ability of and Fig. 4-2. Notice that the only difference synchronous machines to consume or supply between the models is the definition of the VARs to the power system, independent from direction of positive current flow: out of a the direction of power flow. Limits on the generator and into a motor. The diagrams performance of the machines will also be represent one phase of the machine, with V, E explored. Note that the theory refers to round- being phase voltages, Ra the armature rotor machines; saliency is beyond the scope of resistance and XS the synchronous reactance. the course. Theory Synchronous machines are similar to DC machines in that: they have a field winding and an armature winding; a dc current is passed through the field winding to produce flux; the Fig. 4-1 Synchronous generator per-phase equivalent flux from the field winding induces a voltage circuit model in the armature winding. However, unlike DC machines: the field winding is on the rotor and the armature winding is on the stator; the armature winding is an ac winding. Armature Circuit For the majority of the analysis of synchronous machine behaviour, we are only concerned Fig. 4-2 Synchronous motor per-phase equivalent with the armature circuit. We assume that there circuit model is a field winding that produces a flux which Revised Winter 2008 4-1 EE 332 Lab Manual Lab 4 3-Phase Synchronous Machine In many machines, it is reasonable to assume having zero phase angle, and the current, that XS >> Ra and resistance can be neglected. synchronous reactance voltage drop and In this case, Fig. 4-1 and Fig. 4-2 can be excitation voltage are drawn relative to the replaced with Fig. 4-3 and Fig. 4-4 terminal voltage. If positive power flows in the same direction as positive current then the phase angle of current relative to terminal voltage, , must be in the range: 90 90 (4.4) Constructing a phasor diagram, it becomes clear that E leads V in a generator, and Fig. 4-3 Simple generator per-phase circuit E lags V in a motor The four basic shapes of phasor diagram for the circuits in Fig. 4-3 and Fig. 4-4 are shown in Fig. 4-5, Fig. 4-6, Fig. 4-7 and Fig. 4-8. It can be seen that the terminal power factor is a function of the magnitude of the excitation. If |E|>|V| in a generator, then for (4.2) to hold, the power factor must be lagging. Similarly, if Fig. 4-4 Simple motor per phase circuit |E|<|V| then the power factor must be leading. Generating vs. Motoring Viewing the circuit models, the only obvious distinction between motoring and generating is the defined direction for positive current. Writing down the voltage loop equation for a generator from Fig. 4-3 results in (4.2). E V jI a X S (4.2) a Writing down the voltage loop for a motor from Fig. 4-4 results in (4.3). V E jI a X S (4.3) Fig. 4-5 Over-excited generator. |E| >|V|, lagging power factor In the physical system, the distinction between motor and generator is related to power flow. If power flows from the mechanical system, through the machine to the electrical system, the machine is operating as a generator. If power flows from the electrical system, through the machine to the mechanical system, the machine is operating as a motor. To help a understand the operation of a synchronous machine, it is usual to use phasor diagrams. Fig. 4-6 Under-excited generator. |E|<|V|, leading The terminal voltage of the phase is defined as power factor Revised Winter 2008 4-2 EE 332 Lab Manual Lab 4 3-Phase Synchronous Machine development for military applications. In typical synchronous generators and medium- large size synchronous motors, the rotor field a is created using a DC field winding. Unlike a DC machine field winding, the field winding is rotating at synchronous speed; this creates the difficulty of how to create the rotor DC field current. Two possible options are available: 1. Use slip rings and carbon brushes to supply the DC current from an external source. Fig. 4-7 Over-excited motor. |E|>|V|, leading power 2. Generate an ac voltage on the shaft by factor using an additional stationary field winding and rotating conductors. Then rectify the induced ac voltage over the rotating conductors to DC, with diodes mounted on the rotor. No slip rings and brushes are needed for this case. a The first option is often popular as the initial capital cost is lower. This is offset by higher Fig. 4-8 Under-excited motor. |E|<|V|, lagging power operating costs as maintenance is required on factor the brushes and slip rings. The second option is often used in larger machines. A small ac For a motor, it can be seen that for (4.3) to generator called a brushless exciter has its hold, an over-excited generator must have a armature circuit mounted on the shaft of the leading power factor. From these observations, rotor, and its dc field circuit mounted on the and knowledge of a typical power system, one stator. The generated AC current is then can state that the majority of synchronous rectified directly on the rotor using a 3-phase machines will be operated with |E|>|V|. (Most rectifier. loads on the power system are lagging, so In this lab manual If represents the DC currents generators must generate the VARs required in different windings depending on the type of by the system, also operating at lagging power exciter. For synchronous machines with factor. Most times when a synchronous motor brushes, If represents the rotor field current is used, it is operated at leading power factor to supplied directly from an external source. For help improve the overall power factor at a brushless synchronous machines, If represents site.) Note that in this lab, we will assume that the DC current in the field circuit mounted on the supply is an infinite bus: |V| is constant, f is the stator. constant. Field Circuit Torque and Power Equations to approximate power and torque The field of synchronous machine can be can be found from the simplified phasor created either by a winding or a permanent diagram. It can be seen in Fig. 4-9 that the load magnet. Permanent magnet machines have angle and power factor angle are related by typically been small motors, although large multi-MW machines are currently under E sin I a X S cos (4.5) Revised Winter 2008 4-3 EE 332 Lab Manual Lab 4 3-Phase Synchronous Machine It can be seen in Fig. 4-10 that for a generator. Q 3VI sin (4.12) 3VE 3V 2 Q cos (4.13) XS XS For a motor, due to the change in direction of a positive current, 3V 2 3VE Q cos (4.14) XS XS Fig. 4-9 Phasor diagram angles Re-plotting the data from Fig. 4-10 to plot Using the definition for voltamps and power in power on the positive x-axis and VARs on the a 3-phase system: positive y-axis, it is possible to plot the S 3VI (4.6) operating chart of a synchronous machine, P S cos (4.7) shown in Fig. 4-11. where V and I represent phase quantities. Substituting (4.5) into (4.6), (4.7) gives 3VE P sin (4.8) XS Using the mechanical power equation Pmech m (4.9) and noting that in a synchronous machine mechanical speed equals synchronous speed, 4 f m s (4.10) p results in the torque equation 3VE sin (4.11) X S s Note that f is frequency in Hz, p is the number Fig. 4-11 Synchronous machine operating chart of poles in the machine. Using (4.8) the phasor diagram of Fig. 4-9 can be scaled to give A machine can operate anywhere within the power and VARs, shown in Fig. 4-10. following limits: 1. Stator heating limit; Smax. Set by the maximum allowable armature current 2. Rotor heating limit; Emax. Set by the maximum allowable field winding current. 3. Prime mover limit; Pmax. The maximum power available from the mechanical system. 4. Static stability limit, =90. The limiting case of (4.8). Fig. 4-10 Phasor Diagram Power Revised Winter 2008 4-4 EE 332 Lab Manual Lab 4 3-Phase Synchronous Machine Finally, the case where the armature resistance is not negligible must be considered. In this case, the terminal electrical power is given by Pt 3VI cos (4.15) Pt 3VLL I L cos and the armature losses are given by Pscl 3I a2 Ra (4.16) The relationship between electrical terminal Fig. 4-12 Synchronization circuit power and the mechanical power depends on whether the machine is a motor or generator. For a generator Depending on the state of the lamps, Pmech Pt Pscl (4.17) differences in the voltage conditions across the contacts can be determined. For a motor The possible voltage conditions and lamp Pmech Pt Pscl (4.18) states are listed below. Torque can be found from the mechanical 1. All conditions are met. The lamps will power using (4.9). flash in union at a very low frequency equal to the difference between the generator and system frequencies. 2. Phase sequencing is incorrect. The lamps Parallel Operation of will flash at different times. This can be Synchronous Generators corrected by interchanging two of the machine phases. The majority of synchronous generators are 3. There is a big difference between the operated in parallel with other generators. generator and system frequencies. The Most generators are connected to a power grid. lamps will flash in unison at a high Exceptions to grid connection include frequency. In this case, adjust the speed of emergency generators and remote sites the generator to reduce the frequency of the (including communities in the north, logging lamps. and drilling camps), though these situations 4. Voltage magnitudes are different. The often have a number of generators in parallel. lamps will glow steadily. The brightness of When connecting generators to a system with the lamps is a function of the voltage an existing voltage certain conditions must be difference. Increase or decrease the fulfilled: excitation to reduce the intensity of the 1. The RMS line voltages must be equal. lamps. 2. The phase sequences should be consistent. 5. Phase angles are not equal. The lamps will 3. The connection must be made at or near an glow steadily. It is unlikely that the instant when the two voltages are in phase. generator and system will have exactly the 4. The generator frequency must be slightly same frequency and constant phase angle higher or lower than the system frequency. error. The simplest method to meet these conditions is to use synchronization lamps connected across the contactor, as shown in Fig. 4-12. Revised Winter 2008 4-5 EE 332 Lab Manual Lab 4 3-Phase Synchronous Machine Starting a Synchronous Motor Synchronous machines only produce torque at synchronous speed. Generators are usually brought to synchronous speed by the mechanical system, but starting motors is more challenging. Many synchronous motors are constructed with an induction cage on the rotor. This allows the motor to start from standstill and accelerate to synchronous speed. Once the motor is at synchronous speed, the field winding can be turned on and the machine acts like a synchronous motor. Other options include placing a small induction motor on the same shaft as the synchronous Fig. 4-13 Synchronous machine open and short machine. The induction machine accelerates circuit test data. the synchronous machine to synchronous of short circuit armature current with field speed with no load attached. Once at current is recorded. Typical plots of o/c voltage synchronous speed, the synchronous machine and s/c current are shown in Fig. 4-13. is energised and a load applied via a Open circuit phase voltage (also the excitation) mechanical clutch. A final option involves is given by using a variable frequency supply to slowly accelerate the machine to the desired operating E 2 N fˆ c (4.19) synchronous speed. At lower field current levels, the field current- flux relationship is linear and terminal voltage rises linearly with field current. As the field Open and Short Circuit Tests current increases during the open circuit test, The accurate determination of synchronous saturation occurs and the incremental rate of reactance is non-trivial. Synchronous reactance change of voltage with field current is reduced. depends not only on field current, but also load During the short circuit test, the flux produced angle, armature current and armature by the armature winding acts against the field reactance. However, if we assume that it is winding flux, reducing the total flux in the possible to linearize the system about the rated machine and preventing saturation. As a result, voltage, and that armature reaction always short circuit armature current increases linearly opposes the rotor magnetic field, then it is with field current. Now, during the short possible to derive an estimate of the circuit, test, the per phase equivalent circuit synchronous reactance based on open circuit model may be drawn as in Fig. 4-14. and short circuit tests. During an open circuit test, the machine operates at synchronous speed and the variation of line-line voltage with field current is recorded. During a short circuit test, the terminals are short circuited and the variation Fig. 4-14 Short Circuit Revised Winter 2008 4-6 EE 332 Lab Manual Lab 4 3-Phase Synchronous Machine It can be seen that controlled using a variable speed drive to E obtain the required torque and speed. XS (4.20) I a sc More strictly: Lab Procedure Eoc I f XS I f (4.21) Safety I a sc I f Re-read the safety rules from lab 1. It will A plot of synchronous reactance as a function only take a minute and could save you of field current is shown on the graph in Fig. from serious injury. 4-13. It can be seen that even using these Make sure that your circuits are always simple assumptions synchronous reactance is checked before energizing the circuit nonlinear. An approximation is possible if the system is linearized about the rated terminal This lab requires the on-bench generation voltage for the machine. To find XSsat as shown of ac and dc voltages. Remember that the in Fig. 4-13, the following procedure is used: generated voltages are not protected and exercise extreme caution when creating 1. Find the field current that will produce your circuit and carrying out your rated open circuit terminal voltage, Ifr measurements. 2. Find the short circuit armature current Iafr that corresponds to the field current Ifr. Open Circuit Test 3. Find the reactance given by V In this test you will use a Fluke 25 as an X Ssat rated (4.22) I a fr ammeter (10A connection) to measure DC field current, a Fluke 87 to measure speed and It should be noted that this reactance is an a Fluke 43B power analyzer to measure ac approximation. It is based on the assumption quantities. Connect the circuit for the DC that armature reaction opposes rotor flux. As a machine panel and synchronous machine panel result, the approximation should be best when as shown in Fig. 4-15 and Fig. 4-16. On the generating with a lagging power factor. DC machine panel, set the DC drive speed control to zero (counter clockwise) and current Lab equipment control to maximum (clockwise). Some of the synchronous machines in the lab have a nameplate that states star (wye) connection; others have a nameplate that states delta connection. However, all machines used in this lab are wired with a wye connection. Most of the machines sets in the lab are 4-pole machines, with synchronous speed of 1800 rpm. In this lab, the DC machine is used to drive the synchronous machine when generating and load the synchronous machine when motoring. When generating, the DC machine is . Fig. 4-15 DC panel wiring for open circuit test Revised Winter 2008 4-7 EE 332 Lab Manual Lab 4 3-Phase Synchronous Machine slowly reduce the speed and make sure that the field current remains constant throughout this test. Record the speed and open circuit line- line voltages at 200rpm steps from 1800 rpm to 200rpm. Turn the synchronous machine field winding variac to zero, reduce the DC drive speed to zero and once the machine have stopped, turn off the supplies and de-energize the bench. Short Circuit Test Fig. 4-16 Synchronous machine panel wiring: Do not adjust the DC panel wiring. Connect open circuit test the synchronous machine panel as shown in Fig. 4-17 and clamp the Fluke 43B current Turn the DC field winding rheostat fully probe to the spade connector lead at the short counter-clockwise. On the synchronous circuited synchronous machine terminals. Turn machine panel, make sure the field winding the probe on to 10mV/A and check the probe variac is set to zero and (if present) make sure settings on the meter are also at 10mV/A and the field winding switch is set to 0-24V. that the probe is properly zeroed. Check that Connect the Fluke 87 to the BNC output on the both the DC machine and synchronous induction machine panel and set the reading to machine field windings are at their minimum “Hz”. Have your circuit and settings checked. settings, the DC drive current limit is at its Energise the bench and turn on the DC maximum and the speed control is set to zero. machine field supply. Press the green start Have your circuit and settings checked. button on the DC machine panel and use the Turn on the DC machine field winding and speed control knob to slowly increase the press the green start button on the DC machine speed to the rated speed of the synchronous panel. Use the speed control to accelerate the machine (1800rpm). Turn on the synchronous synchronous machine to rated synchronous machine field winding. Making sure that the speed. Turn on the synchronous machine field speed is constant, slowly increase the winding supply. Making sure that the speed synchronous machine field variac setting and remains constant, slowly increase the record the field current (Fluke 25) and open synchronous machine field current while circuit line-line voltage (Fluke 43B) at 30V carefully monitoring the short circuit current. increments from 30V to 300V. The speed may change while changing the field current; adjust Record the field current and the short circuit the speed control knob on DC machine panel armature current as it increases in 2.5A steps, to keep the speed constant throughout this test. up to a maximum of 17.5A. Return the synchronous machine variac to zero, reduce the DC drive speed to zero and once the Speed Test machines have stopped, turn off the supplies and de-energize the bench. Adjust the synchronous machine field winding variac until the line-line voltage is approximately 208V. Record the field winding current. Use the DC drive speed control to Revised Winter 2008 4-8 EE 332 Lab Manual Lab 4 3-Phase Synchronous Machine Turn on the DC machine field winding, press the green start button on the DC machine panel and slowly accelerate the machines until they are rotating slightly faster (e.g. 1815rpm) than synchronous speed. Turn on the field winding for the synchronous machine and adjust the field winding current until the line-line voltage reads 208V. Turn on the switch to see the synchronization lamps. At the moment the flashing lamps become dark, press the green start button on the synchronous machine panel to connect the synchronous machine to the supply. Fig. 4-17 Synchronous machine panel wiring: short circuit test Turn the DC drive current control to a minimum (counter clockwise), the speed control to a maximum (clockwise) and increase Generator Load Tests the DC machine field current to its maximum setting. Do not adjust the DC panel wiring. Connect In the follow constant excitation test and the synchronous machine panel as shown in constant power test, you will need to adjust the Fig. 4-18. Clamp the Fluke 43B current probe real power and reactive power frequently. To to measure the line 1 current and insert the get the required real power, you will need to Fluke 43B voltage probes to measure the line- adjust the DC drive current control on the DC machine panel. To get the required reactive power, you will need to adjust the field current of the synchronous machine by turning the variac on the synchronous machine panel. The two adjustments are not independent. You may turn the DC drive current knob and the variac back and forth to reach the required real and reactive power. Constant Excitation Making sure that the Fluke 43B is on the 3 power setting, adjust the DC drive current control to set the synchronous machine terminal power to zero. Increase the Fig. 4-18 Synchronous machine panel wiring: synchronous machine field current by generator load test adjusting the variac on synchronous machine line voltage across the two other lines. Check panel until the armature current is that both the DC machine and synchronous approximately 10A. Record the field current, machine field windings are at their minimum armature current, line-line voltage, power, settings, the DC drive current limit is at its VAR, VA and power factor. Keep that field maximum and the speed control is set to zero. current constant by adjusting the variac while Have your circuit and settings checked. slowly increasing the current control setting on Revised Winter 2008 4-9 EE 332 Lab Manual Lab 4 3-Phase Synchronous Machine the DC drive to repeat the measurements at 1.0kW intervals up to 4.0kW. Constant Power Re-adjust the synchronous machine field winding until the generator is producing 4kW and 4kVAR at a lagging power factor. Record the field current, armature current, line-line voltage, power, VAR and VA. Slowly reduce the synchronous machine field winding current and repeat the measurements at 1kVAR intervals until the machine is producing 4kVAR at a leading power factor. Reduce the DC drive current setting until the synchronous machine power is nearly zero, and then adjust the synchronous machine field current until the armature current is minimized. Press the stop button on the synchronous machine set. Fig. 4-19 DC machine panel wiring: motor load test Reduce the synchronous machine field current to zero. Turn off the 1-phase and 3-phase Try to adjust the DC machine field current to supplies on the DC machine panel and get the synchronous machine input power of synchronous machine panel and de-energize 2.5kW. If the input power cannot reach 2.5kW, the bench. you can change the number of load resistors and adjust the DC machine field current in the Motor Load Tests meanwhile until the input power reads 2.5kW. Increase the synchronous machine field Do not adjust the synchronous machine panel winding current until the synchronous machine wiring. Add a parallel-connected load box to power factor is approximately 0.6 leading. The the terminals of the DC machine armature, as input power may change when you are shown in Fig. 4-19. Make sure that all the adjusting the power factor. You can readjust switches are in the “off” position. Check that the DC machine field current to return to both the DC machine and synchronous 2.5kW. Record the synchronous machine machine field windings are at their minimum armature current, line-line voltage, power, VA, settings, the DC drive current limit is at its VAR and power factor. Slowly reduce the maximum and the speed control is set to zero. synchronous machine field current. Have your circuit and settings checked. Maintaining the power at 2.5kW by adjusting Start the DC machine and synchronize the the DC machine field current, repeat the synchronous machine with the supply, as in the readings at power factors of approximately 0.8 generator load tests. Switch off the 3-phase leading, unity, 0.8 lagging, 0.6 lagging. Reduce supply to the DC drive. the DC machine field to its lowest setting, switch off the load resistors, adjust the Turn the DC machine field current to its synchronous machine field until the armature maximum setting, then, switch on resistors on current is minimized then switch off the supply the load box until the input power to the to the synchronous machine. De-energize the synchronous machine is around 2.5kW. bench. Revised Winter 2008 4-10 EE 332 Lab Manual Lab 4 3-Phase Synchronous Machine Lab Report Questions No formal lab report is required. Hand in 1. From the Open Circuit Voltage vs. Field copies of all spreadsheets, graphs, calculations Current plot explain why a phase voltage is and questions. The report is due one week after present when the field current is zero. the lab date. 2. Why does the Open Circuit Voltage vs. Field Current graph start to level off once it Calculations reaches a certain magnitude? 3. Considering the plot of open circuit voltage Once you have recorded your data, download vs. speed, is the machine excited using the spreadsheet for lab 4 from the website at brushes or a brushless exciter? Explain www.ece.ualberta.ca/~ee332/. Enter your your choice. measurements into the spreadsheet. The spreadsheet will automatically calculate all the 4. Look at the Short Circuit Current vs. Field required information, and plot all the required Current graph. Why is the relationship graphs. Print a copy of the spreadsheet between short circuit current and field Analysis and all the graphs. current linear? 5. Explain the shape of the generator armature Sample Calculations current vs. field current plot at constant power. The following sample calculations must be 6. Explain the shape of the generator E loci handed in with your results: and P-Q plots at constant field current and 1. Calculate Xs (sat) for the data closest to VLLoc constant power, respectively. = 208V. 7. Explain the shapes of the motoring current 2. Calculate the load angle () and excitation and reactive power plot at constant power. (E) for the case where P = 4kW, Q = 8. Explain the shapes of the motoring E and I 4kVAR (in constant power generator test). loci at constant power. You may need linear interpolation to (Hints for question 5 ~ 8: You may need to calculate Xs for this question. use phasor diagrams and relevant equations 3. Draw the phasor diagrams for the above to analyze the trends of those curves.) case. Include Ia, V, E, IaXs, and in your drawings. 4. Draw the phasor diagram for motor operation at 0.8 lagging power factor. Revised Winter 2008 4-11 EE 332 Lab Manual Lab 4 3-Phase Synchronous Machine Name: ID # SECTION # EE332 Lab 4: Pre–Lab Questions 1. What are the goals of this lab? 2. Is an over excited generator operating with leading or lagging power factor? 3. In the open circuit test, what do you use to set the speed of the synchronous machine? 4. In the open and short circuit tests, do you supply electrical power to stator winding of the synchronous machine? 5. What range of current do you expect in the short circuit test? 6. Under what conditions should you close the contact to connect the supply to the synchronous machine? 7. What is used to control the flow of power to the synchronous machine when it is acting as a generator? 8. How is the synchronous motor loaded? 9. How is the reactive power flow to/from the synchronous machine controlled? 10. Which meters are used in this lab? Revised Winter 2008 4-12 EE 332 Lab Manual Lab 4 3-Phase Synchronous Machine Name: ID # SECTION # Name: ID # Name: ID # Measurements Table 4-1 Open Circuit Test Table 4-4 Constant Excitation If (A) VLL (V) If Ia VLL P Q S (A) (A) (V) (kW) (kVAR) (kVA) Table 4-5 Constant Power If Ia VLL P Q S (A) (A) (V) (kW) (kVAR) (kVA) Table 4-2 Open Loop Speed Test nm (rpm) VLL (V) Table 4-6 Motoring Table 4-3 Short Circuit Test If (A) Ia sc (A) If Ia VLL P Q S (A) (A) (V) (kW) (kVAR) (kVA) Revised Winter 2008 4-13