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EXPERIMENT N0.1 DC GENERATORS I. Objectives: To investigate the principles of operation of dc generators by measure some of the characteristics of a separately excited dc generator, dc shunt generator, dc series generator, and dc compound generator such as, no-load characteristics, and terminal characteristics. Also to be familiar with the main advantages and disadvantages of these types of dc generators by comparing the main features among them, specially the voltage regulation and control methods of the output voltage. II. Equipment: 1. 2. 3. 4. 5. 6. 7. 8. D.C Machine (as a Generator) D.C Machine (as a Motor) Torque measuring unit Power supply Shunt regulator Resistor load Load switch Multimeters MV 1006 / TERCO MV 1028 / TERCO MV 1051 / TERCO MV 1300 / TERCO MV 1905 / TERCO MV 1100 / TERCO MV 1500 / TERCO 1 1 1 1 2 1 1 5 III. Theory: Direct current machines are energy transfer devices. These machines can function as either a motor or a generator. DC motors and generators have the same basic construction, differing primarily in the energy conversion. In DC generators, the three conditions that necessary to induce voltage are: magnetic field, conductor, and relative motion between them. So the basic DC generator has four basic parts: magnetic field, single conductor or loop, commutator and brushes, as shown in Fig.(1-1). Figure (1-1): Basic Operation of a dc Generator. 1/16 EXPERIMENT NO.(1) DC GENERATORS A single conductor, shaped in the form of a loop, is positioned between the magnetic poles. As long as the loop is stationary, the magnetic field has no effect (no relative motion). If the loop is rotated, there will be a relative motion between the magnetic field and the conductor, so an EMF (voltage) will induce into it. The magnitude of the induced EMF depends on the field strength and the rate at which the flux lines are cut, as given by the following equation. Eg = K Where: Eg K (1-1) = generated voltage = constant, depending on the construction of the machine = magnetic flux strength = angular speed The direction of the induced current flow can be determined using the "left-hand rule" for generators, as shown in Fig.(1-2). Applying the left-hand rule to both sides of the loop will show that current flows in a counter-clockwise direction in the loop Figure (1-2): Left-Hand Rule for Generators. Commutator converts the AC voltage generated in the rotating loop into a DC voltage. In a simple one-loop generator, the commutator is made up of two semi cylindrical pieces of a smooth conducting material, usually copper, separated by an insulating material. Each half of the commutator segments is permanently attached to one end of the rotating loop. Brushes usually made of carbon, slides along the commutator as it rotates so the brushes make contact with each end of the loop. Each brush slides along one half of the commutator and then along the other half. Every time the ends of the loop reverse polarity, the brushes switch from one commutator segment to the next. This means that one brush is always positive with respect to the other. The voltage between the brushes fluctuates in amplitude between zero and some maximum value, but it is always of the same polarity as shown in Fig.(1-3). 2/16 EXPERIMENT NO.(1) DC GENERATORS Figure (1-3): Commutation in a DC Generator. The magnetic field may be supplied by either a permanent magnet or an electromagnet. The magnetic fields in DC generators are most commonly provided by electromagnets. The current which flow through the electromagnet conductors to produce the magnetic field is known as the field excitation current and can be supplied to the field winding in one of two ways; it can be fed from a separate DC source (Separately excited generator) or it can come directly from the output of the generator, in that it is called a self-excited generator. In a self-excited generator, the field winding is connected directly to the generator output. The field may be connected in parallel with the output (Shunt generator), in series with the output (Series generator), or a combination of the two (Compound generator). The field current produces a flux in the machine core in accordance with its magnetization curve, and since EA is directly proportional to the flux at constant speed, the relation between E A and If (no-load characteristic) will be as shown in Fig.(1-4). EA = o Figure (1-4): The magnetization curve of a generator for a fixed speed. If Dc Generators are compared by their voltages, power ratings, efficiencies, and voltage regulations. Voltage regulation (VR) is defined by the equation VR Vnl V fl V fl 100% 3/16 (1-2) EXPERIMENT NO.(1) DC GENERATORS The Separately Excited DC Generator The equivalent circuit of such a machine is shown in Fig.(1-5). In this figure the armature circuit is represented by an ideal voltage source (EA) and a resistor (RA). The field coils are represented by inductor (Lf) and resistor (Rf). It is clear that the armature current (IA) is equal to the load current (IL) in a separately excited generator. By Kirchhoff's voltage low, the terminal voltage is: VT E A I A R A IF IA (1-3) IL RA + RF + - VF LF VT EA Load Figure (1-5): A separately excited dc generator. - According to the previous equation the terminal characteristic (V T versus IL) of a separately excited generator is a straight line as shown in Fig.(1-6-a). In generators without compensating windings, an increase in IA causes an increase in armature reaction and armature reaction causes flux weakening. This flux weakening causes a decrease in E A= K which further decreases the terminal voltage of the generator as show in Fig.(1-6-b) VT VT EA RAIA EA RAIA drop AR drop IL IL (a) (b) Figure (1-6): The terminal characteristic of a separately excited dc generator with(a) and without(b) compensating windings. There are two possible ways to control the output voltage: 1. Change the speed of rotation. If then EA = K increases, so VT = EA-IARA increases too. 2. Change the field current. If If increases then the flux in the machine increases. Therefore EA = K increases and VT = EA-IARA increases too. 4/16 EXPERIMENT NO.(1) DC GENERATORS The Shunt-Wound DC Generators When the field windings is connected in parallel with the armature of the generator, the generator is called a shunt-wound generator (Fig.(17)). In this circuit the armature current supplies both the field current and the load, so IA IF IL The Kirchhoff's voltage low equation for the armature circuit is: VT E A I A R A IA IL RA + RF + - VT EA LF IF Load Figure (1-7): The equivalent circuit of a shunt dc generator. - The voltage buildup in a dc generator depends on the presence of a residual flux in the poles of the generator as shown in Fig.(1-8) There are several possible causes for the voltage to fail to buildup during starting. Among them are: 1- There may be no residual flux 2- The direction of rotation of the generator or the field current may have been reversed which produces a flux opposing the residual flux instead of adding to it. 3- The field resistance may be adjusted to a high value so the field current will be very small and the voltage never buildup. EA/VT VTnl VT versus If EA versus If Rf =VT/If EResid ual Ifnl Figure (1-8): Voltage buildup on starting in a shunt dc generator. If 5/16 EXPERIMENT NO.(1) DC GENERATORS As the load is increased, IA increases and then armature resistance voltage drop increases, causing VT to decrease. However, when VT decreases, the field current decreases and so the flux also decreases with it. This causes EA to decrease which causes a further decrease in the terminal voltage. By the effect of the armature reaction the terminal voltage will decrease more and more as shown in Fig.(1-9) VT RAIA drop Field weakening effect Figure (1-9): The terminal characteristic of a shunt dc generator . A.R. IL As in the separately exited generator, there are two ways to control the voltage of a shunt generator: 1. Change the speed of rotation. 2. Change the field current. The Series-Wound DC Generators When the field winding of the DC generator is connected in series with the armature, the generator is called a series-wound generator (Fig.(1-10)). IA IF RA + - EA RS IL LS + VT Load Figure (1-10): The equivalent circuit of a series dc generator. - According to this equivalent circuit, it can be noted that IA IF IL and VT E A I A ( RA RS ) (1-4) Since the excitation current in a series-wound generator is the same as the load current, the curve represents the relation between the generated voltage (EA) and the load current, looks like the magnetization 6/16 EXPERIMENT NO.(1) DC GENERATORS curve. The IA(RA+RS) drop goes up by increasing load current which reduces the terminal voltage as shown in Fig. (1-11). EA/VT IA(RA+ RS) drop + Armature reaction Figure (1-11): A series generator terminal characteristic. IL The Compound DC Generators Series and shunt generators have a disadvantage in that changes in load current cause change in generator output voltage. One means of supplying a stable output voltage is by using a compound generator. A compound dc generator is a dc generator with both series and shunt fields, connected so that the magnetomotive forces from the two fields are additive (cumulatively compound) or subtractive (differentially compound). Fig (1-12) shows the equivalent circuit of a compound dc generator. IA IL RA + - EA RS LS IF + RF VT LF Load - Figure (1-12): the equivalent circuit of a compounded dc generator. As load current increases the armature current increases which tends to cause a decrease in the terminal voltage VT = EA- IA(RA+RS). In cumulatively compound generator, increasing IA cause an increase in the net flux which increases EA and VT. These two effects oppose each other, one tending to decrease VT and the other tending to increase VT. In case of few series turns the first effect will be dominant so the terminal voltage decreases. This type of construction is called undercompounded If there are a few more series turns, then at first the flux strengthening effect wins but as the load continues to increase, the resistive drop becomes stronger. If VT at no load is equal to VT at full load the generator is called flatcompounded. 7/16 EXPERIMENT NO.(1) DC GENERATORS If the number of series turns is large, the effect that increases the terminal voltage will be predominant and the generator is called overcompounded. Fig.(1-13) shows the terminal characteristic of the compound dc generator with all previous cases compared with the shunt generator. VT Overcompounded Flatcompounded Undercompounded Shunt Differentially compounded Figure (1-13): Terminal characteristic of a compounded dc generator. IL IFL In differentially compounded generator, as the load is increased, IA increases, causing decreasing in the net flux which in turn decreases the induced voltage and VT. The resistive voltage drop will also reduce the terminal voltage and so it will drops drastically by increasing the load current as shown in Fig.(1-13). 8/16 EXPERIMENT NO.(1) DC GENERATORS IV. Procedures: Part 1: Separately excited dc generator No-Load Characteristic 1.1 Connect the dc machine MV 1006 as a generator, and the dc machine MV1028 as a motor in accordance with circuit diagram (1). 1.2 Make sure that the switch S is off and the two regulators are turned to minimum value. 1.3 Set the field current of the dc motor to 0.7 A, increase the armature voltage whereupon the motor starts rotating. Adjust the speed to 1400 rpm (this speed must be kept constant during the experiment). 1.4 Increase the field current of the generator in steps of 0.1 A up to maximum (see ratings) for each step measure the terminal voltage. 1.5 Repeat step 1.4 after adjusting the speed to 1200 rpm Load characteristic 1.6 Readjust the speed to 1400 rpm (this speed must be kept constant during this part). 1.7 Adjust the shunt regulator of the generator to bring the voltage up to 220 V. Record the excitation current, this value must be kept constant during this part of the experiment. 1.8 Turn the load resistor toward minimum load current. Turn on switch S. By varying the load resistor increase the armature current in steps of 1A up to maximum (see ratings), for each step read the terminal voltage and the shaft torque. 1.9 Turn off the switch S, variable dc voltage, fixed dc voltage, and the main power supply switches (don’t open the field circuit of the dc motor while motor is running) 9/16 EXPERIMENT NO.(1) DC GENERATORS Part 2: Shunt dc generator 2.1 Leave the dc machine MV1028 as it was, and reconnect the dc machine MV1006 as a shunt dc generator as shown in circuit diagram (2). 2.2 Restart the dc motor as in the previous part (step 1.3) 2.3 Adjust the shunt regulator of the dc generator to bring the terminal voltage up to 220 V. 2.4 Turn the load resistor toward minimum load current. Turn on switch S. By varying the load resistor increase the load current in steps of 1A up to 5A, for each step read the terminal voltage, the field current and the shaft torque. 2.5 Turn off the switch S, variable dc voltage, fixed dc voltage, and the main power supply switches (don’t open the field circuit of the dc motor while motor is running) Part 3: Series dc generator 3.1 Leave the dc machine MV1028 as it was, and reconnect the dc machine MV1006 as a series dc generator as shown in circuit diagram (3). 3.2 Restart the dc motor as in the previous part (step 1.3) 3.3 Turn the load resistor toward minimum load current. Turn on switch S. By varying the load resistor increase the load current in steps of 0.5A up to 5A, for each step read the terminal voltage and the shaft torque. 3.4 Turn off the switch S, variable dc voltage, fixed dc voltage, and the main power supply switches (don’t open the field circuit of the dc motor while motor is running) 10/16 EXPERIMENT NO.(1) DC GENERATORS Part 4: Compound dc generator 4.1 Leave the dc machine MV1028 as it was, and reconnect the dc machine MV1006 as a compound dc generator with 100% of the series windings (Overcompounded) as shown in circuit diagram (4). 4.2 Restart the dc motor as in the previous part (step 1.3) 4.3 Adjust the shunt regulator of the dc generator to bring the terminal voltage up to 220 V. 4.4 Turn the load resistor toward minimum load current. Turn on switch S. By varying the load resistor increase the load current in steps of 1A up to 5A, for each step read the terminal voltage and the shaft torque. 4.5 Turn off the switch S, variable dc voltage, fixed dc voltage, and the main power supply switches (don’t open the field circuit of the dc motor while motor is running). 4.6 Modify the circuit by using 70% of the series windings (Flatcompounded) and then repeat step 4.3 to 4.5. 4.7 Again modify the circuit but now with only 30% of the series windings (Undercompounded) and then repeat step 4.3 to 4.5 11/16 EXPERIMENT NO.(1) DC GENERATORS Circuit diagram (1): r.p.m Speed (n) If A F1 M D1 F1 D3 D2 F2 A M TG F2 A V + - + + L1L2L3N 220 Vdc 0-220 Vdc 0-220/127 (VT ) S L1L2L3N 220/127 (IL ) A Resistive load Circuit diagram (2): r.p.m Speed (n) If A F1 M F2 A + - + + L1L2L3N 220 Vdc 0-220 Vdc 0-220/127 D1 F1 D3 D2 F2 A M TG (VT ) V L1L2L3N 220/127 S (IL ) A Resistive load 12/16 EXPERIMENT NO.(1) DC GENERATORS Circuit diagram (3): r.p.m Speed (n) D1 F1 A M F1 G TG D3 D2 F2 A (IL ) + - + + L1L2L3N 220 Vdc 0-220 Vdc 0-220/127 (Vt ) F2 A V S L1L2L3N 220/127 Resistive load Circuit diagram (4): r.p.m Speed (n) If A F1 M F2 A + - + + L1L2L3N 220 Vdc 0-220 Vdc 0-220/127 D1 F1 D3 D2 F2 A M TG (VT ) V S L1L2L3N 220/127 A (IL ) Resistive load 13/16 EXPERIMENT NO.(1) DC GENERATORS V. Results: Part 1: Separately excited dc generator No-Load Characteristic n=1400 rpm IF (A) Eg (V) 0.0 0.1 0.2 0.3 0.4 0.5 0.1 0.2 0.3 0.4 0.5 3 4 5 n=1400 rpm IF (A) Eg (V) 0.0 Load characteristic Calculated Measured N=1400 rpm IF = IL (A) 0 Vt (V) 220 (Nm) Pin(w) Pout(w) (%) Part 2: Shunt dc generator Calculated Measured n=1400 rpm IL (A) Vt (V) IF (A) (Nm) Pin(w) Pout(w) (%) 0 220 1 2 14/16 EXPERIMENT NO.(1) DC GENERATORS Part 3: Series dc generator n=1400rpm Measured IL (A) 0 0.5 1 1.5 2 2.5 3 3.5 4 Vt (V) (Nm) Calculated Pin(w) Pout(w) (%) Part 4: Compound dc generator 100% of the series winding (Overcompounded) Measured N=1400 rpm IL (A) 0 Vt (V) 220 (Nm) Calculated Pin(w) Pout(w) (%) 70% of the series winding (Flatcompounded) Measured n=1400 rpm IL (A) 0 Vt (V) 220 (Nm) Calculated Pin(w) Pout(w) (%) 30% of the series winding (Undercompounded) Measured n=1400 rpm IL (A) 0 Vt (V) 220 (Nm) Calculated Pin(w) Pout(w) (%) 15/16 4.5 5 EXPERIMENT NO.(1) DC GENERATORS VI. Tasks: a. In the same graph, draw the magnetization curve (Eg versus IF) for the two different speeds according to the measured data in part 1. b. Complete the tables in parts 1, 2, 3, and 4 by calculating the input power (Pin = ), the output power (Pout =VtIL), and the efficiency ( Pout ) for each electrical load. Pin c. In the same graph, draw the terminal voltage versus the load current of the separately excited, shunt, and series dc generator. d. In the same graph, draw the terminal voltage versus the load current for the different cases of the cumulatively compounded dc generator and the shunt dc generator. e. In the same graph, draw the efficiency () as a function of the load current for the separately excited, shunt, series, and flatcompounded dc generator. f. According to the terminal characteristics drawn in C, is there any effect of the armature reaction in this machine? How can this problem be remedied? g. Using the terminal characteristic of the separately excited dc generator, determine approximately the internal resistance of the armature of the dc generator (RA). h. Calculate the voltage regulation of this dc generator in case of separately, shunt, and series excitation. Calculate the voltage regulation for the different cases of the cumulatively compounded dc generator. i. What is the critical resistance? j. How can the different cases of the cumulatively compounded dc generator (over, under, and flat) be gotten without changing the number of series windings? k. Explain how to control the output voltage for the different types of the dc generator and how to reverse the voltage polarity? 16/16