DIRECT CURRENT GENERATORS SEPARATELY EXITED, SHUNT AND COMPOUND CONNECTION INTRODUCTION

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1 Islamic University of Gaza Faculty of Engineering Electrical Engineering department Electric Machine Lab Eng. Omar A. Qarmout Eng. Amani S. Abu Reyala Experiment 6 DIRECT CURRENT GENERATORS SEPARATELY EXITED, SHUNT AND COMPOUND CONNECTION INTRODUCTION The dc generator is the machine that converts the mechanical energy into electric energy, in other meaning, it is considered a speed to voltage converter figure 6.1 (a, b) and a torque to current converter figure 6.1 (c, d). (a) DC Generator as a Speed-to-Voltage Converter (c) DC Generator as a Torque-to-Current Converter (b) Output Voltage versus Speed Figure 6.1: DC Generator (d) Output Current versus Torque The magnet of the generator is electrical magnet which depends on its field current. The output voltage of the generator is proportional with the flux so the output voltage is directly proportional with the field current at fixed speed figure 6.2. Figure 6.2: Versus for a Separately-Excited DC Generator Operating at a Fixed Speed.

2 When the output current increases the output voltage decreases under fixed speed since there is an internal resistance n the armature which consume part from the resultant voltage Figure 6.3. (b) voltage Versus Current Characteristic of a (a) Simplified Equivalent Circuit of a DC Generator. Separately-Excited DC Generator (Fixed Speed) Figure 6.3: the effect of increasing the output current on the output voltage in the DC Generator There four connecting methods for the DC Generator, one needs external voltage for the magnet coil and the other three depend on the output of the generator. The relation between the output voltage and the output current is different from one to another Figure 6.4. Figure 6.4: Voltage Versus Current Characteristics of Various DC Generators. PART1: SEPARATELY EXITED DC GENERATOR OBJECTIVE: After completing this part, you will be familiar with: The relation between the input speed and the output voltage of the DC Generator. The relation between the input torque and the output current of the DC Generator. The relation between the magnetic current and the output voltage of the DC Generator. The effect of the changing the load on the DC Generator's outputs. INTRODUCTION: At first a separated source will be used to obtain the current in the magnetic circuit of the stator that is the magnetic circuit is not connected electrically with output of the DC Generator so it's not affected with the changing of the outputs and has approximately constant current. (a) Separately-Excited DC Generator Coupled to a Prime Motor (b) Separated Magnetic Circuit Figure 6.5: Separately-Excited DC Generator without load.

3 PRACTICAL 6.1.A: OUTPUT VOLTAGE VERSUS INPUT SPEED CHARACTERISTIC PROCEDURE: 1- Ensure that the power supply (unit ) is switched off. Then connect the circuit shown in Figure 6.5 (Connect the Prime Mover input between point 7 and N from the power supply). 2- Make the necessary connecting between the Prime Mover and the Data Acquisition Interface in order to receive the result on the computer. 3- On the DC Generator set the Field Rheostat so that the field current is On power supply, adjust the voltage control knob to increase the generator speed n by 150 r/min increments up to 1500 r/min. Record the output voltage while adjustment (table 6-1). Speed (r/min) Voltage (V) Table Turn of the power supply after finishing recording. 6- Draw the graph of the output voltage as function of input speed (what do you observe from the resulting graph? Why?) then determined the value of from the slope. PRACTICAL 6.1.B: OUTPUT CURRENT VERSUS TORQUE CHARACTERISTIC (a) Separately-Excited DC Generator Coupled to a Prime Mover. (b) Separated Magnetic Circuit. Figure 6.6: Separately-Excited DC Generator with load. PROCEDURE: 1- Modify the previous circuit by connect a resistive load (unit ) to the output of the DC Generator. 2- Turn on the power supply then adjust the input voltage to reach the speed to 1500 r/min. 3- Modify the setting of the resistive load to according to table 6.2 then record the demand quantities. Resistive Load Value Ω Output Voltage Output Current Field Current Input Torque. Input Speed /

4 4- Draw the graph of the output current as function of input torque (what do you observe from the resulting graph? Why?) then determined the value of from the slope. PRACTICAL 6.1.C: OUTPUT VOLTAGE VERSUS FIELD CURRENT CHARACTERISTIC 1- Turn on the power supply then set the input load on 629Ω. 2- Turn the Rheostat counterclockwise to set the smallest value of field current. 3- Adjust the input voltage of the prime mover to set the input speed at 1500 r/min then measure the output voltage. 4- Set the field current at 190 and 300 ; adjust the input speed at 1500 r/min and measure the input voltage in every time (Table 6-3). Field Current Output Voltage Table Draw the graph of the output voltage as function of field Current (what do you observe from the resulting graph? Why?) PRACTICAL 6.1.D: OUTPUT VOLTAGE VERSUS OUTPUT CURRENT CHARACTERISTIC 1- From the result of Practical 6.1.C Draw the graph of the output voltage as function of Output Current (what do you observe from the resulting graph? Why?) PART2: SHUNT DC GENERATOR ELEMENTS OBJECTIVE: After completing this part, you will be familiar with the characteristic of the shunt DC generator. INTRODUCTION: Separately exited connection is not a feasible method on running DC generator since it needs an external DC source for the field current, so other connection methods will be applied to avoid the need of the external DC source. One of these methods is connecting the coil of electric magnet in parallel with output voltage. This method is called Shunt DC Generator (figure 6.7), which has some problems as the output current increase. Figure 6.7: Shunt Generator Coupled to a Prime Mover (with an Electrical Load).

5 PRACTICAL 6.2: SHUNT DC GENERATOR 1- Ensure that the power supply (unit ) is switched off. 2- Make the needed change on the previous connection to get the connection in figure Turn on the power supply whole the value of the load is infinity and change the input voltage of the prime mover until the input speed reach 1500 r/min. 4- Adjust the field current around 190 and keep the input speed 1500r/min. 5- Record the values of the input torque, input speed, field current, output current and output voltage. 6- Change the value of the load according to table 6-2 and repeat Draw the graph of the output voltage as a function of the output current (what do you observe from the resulting graph? Why?). PART3: COMPOUND DC GENERATOR ELEMENTS OBJECTIVE: After completing this part, you will be familiar with the characteristic of: The cumulative compound DC generator. The differential compound DC generator. INTRODUCTION: In the shunt DC generator the field current affected with changing of the output voltage according to the changing of the output current so the cumulative compound method is used to solve this problem. In this method, an electric magnet is connected in series with the load so that its flux is cumulative with the flux of the shunt coil, that is, the magnetic field remains approximately constant as the output current increases figure 6.4. When the connection of the series coil is reflected the direction of the current changed and the resultant flux is the difference between the flux of the shunt coil and the flux of the series coil, that is, the magnetic field turns weaker as the output current increases figure 6.8. Figure 6.8: Cumulative Compound Generator Coupled to a Prime Mover (with an Electrical Load).

6 PRACTICAL 6.3.A: CUMULATIVE COMPOUND DC GENERATOR 1- Ensure that the power supply (unit ) is switched off. Then connect the circuit shown in Figure Adjust the input voltage of the prime mover in order to get input speed 1500 r/min while the field current is 190 when the load is. 3- Record the values of the input torque, input speed, field current, output current and output voltage. 4- Change the value of the load according to table 6-2 and repeat Draw the graph of the output voltage as a function of the output current (what do you observe from the resulting graph? Why?). PRACTICAL 6.3.A: CUMULATIVE COMPOUND DC GENERATOR 1- Ensure that the power supply (unit ) is switched off. Then connect the circuit shown in Figure Adjust the input voltage of the prime mover in order to get input speed 1500 r/min while the field current is 190 when the load is. 3- Record the values of the input torque, input speed, field current, output current and output voltage. 4- Change the value of the load according to table 6-2 and repeat Draw the graph of the output voltage as a function of the output current (what do you observe from the resulting graph? Why?). Figure 6.9: Differential Compound Generator Coupled to a Prime Mover (with an Electrical Load).

Experiment 3. The Direct Current Motor Part II OBJECTIVE. To locate the neutral brush position. To learn the basic motor wiring connections.

Experiment 3. The Direct Current Motor Part II OBJECTIVE. To locate the neutral brush position. To learn the basic motor wiring connections. Experiment 3 The Direct Current Motor Part II OBJECTIVE To locate the neutral brush position. To learn the basic motor wiring connections. To observe the operating characteristics of series and shunt connected