College of Engineering
Electrical Engineering Department
Electric Machinery Lab
Experiment – 7
DC Generator Characteristics
To experiment the open circuit characteristics of separately excited DC generator with the field variation at different speed.
To study load characteristics of a DC shunt Generator.
To conduct a comparative study for load characteristics of all tested generators versions and to evaluate the performance of the machine.
Universal Motor (DC version)
Variable DC Power Source (40-250 V/ 10A)
Variable DC Field Supply (0-250 V/ 2.5 A)
Regulator for Field Current
1 Sensor Cassy
1 Isolation Amplifier, Four Channel
1 AC Adapter
2 Professional Digital Multimeter
DC generators have three characteristics. First, DC generators have an open circuit characteristic. DC generators also have internal and external characteristic as discussed in detail below.
1. Open circuit characteristic (O.C.C.) (E0/If)
At a given fixed speed, the characteristic of an open circuit shows the relation of field current (If) and generated emf when there is no load (E0). The open circuit characteristic curve is a magnetization curve similar for all generators. If the generator is operated at no load and with constant speed the data for open circuit characteristic curve can be obtained. Field current gradually increases and as a result, corresponding terminal voltage is recorded.
The figure below is an illustration of the connection arrangement to obtain open circuit characteristic curve. For series excited generators, the field winding is connected across an external supply.
According to the emf equation Eg = kɸ illustrates that the generated emf is proportional to field current. It is imperative to note that some emf is generated even at zero field current. This emf is due to residual magnetism that remains within the field poles. Magnetism induces an initial emf in the armature. Induced emf increases residual and field flux. Hence, open circuit characteristic tracks a line. When there is an increase in flux density ɸ becomes constant and the poles are saturated. Eg and ɸ remains constant if there is an increase in If. Open circuit characteristic curve will therefore look like the B-H characteristic.
Figure 2 Is a graph for direct current generators depicting open circuit characteristics
2. Total or Internal Characteristic (E/Ia)
The relation of (Ia) to the generated emf on load is illustrated in the internal characteristic curve. Due to armature reaction emf Eg is less than E0.It is determined by the drop in demagnetized armature reaction..
3. External Characteristic (V/IL)
The relation of load current (IL) and terminal voltage (V) is illustrated by the curves of external characteristic. Terminal voltage V will be less compared to the emf Eg generated due to a drop of voltage in the armature circuit. For that reason, the internal characteristic curve will lie above the external characteristic curve. The suitability of the generator fir a given task is determined by this curve. Therefore, External characteristics can also be referred to as performance characteristics.
For each type of generators, both characteristic curves are shown below.
Characteristics of separately excited DC Generator
Voltage for load current remains constant when there is no armature reaction. In the above graph, the line AB shows the relation load current and on load voltages. AC curve characterizes the internal characteristic for an excited dc generator. Its terminal voltage is lesser. Curve AD represents is the relation between load current and terminal voltage. It is a representation of their external characteristic.
Characteristics of DC Shunt Generator
Load characteristics for shunt direct current generators determined be allowing it build up voltage before an external load is applied. The voltage of the shunt generator, is build up by driving it at the prime mover rated speed. Because of a residual magnetism found at the field poles, initial voltage is induced. Voltage in a shunt generator builds up as described in the curve of open circuit characteristic. After the voltage has built up, a resistive load is loaded and the readings taken at intervals.
A shunt generator has Ia=IL+If. Subtracting the If from Ia. gives us the internal characteristic transmitted to Eg vs. IL.
If the resistance of the load is decreased during a normal running condition, load current increases. However, with continued decrease of the resistance for the terminal voltage decreases. Resistance of the load decreases to a limit, below which terminal voltage falls. This is because of the increased I2R losses and the excessive armature reaction experienced at high armature current. A load resistance limit beyond this leads to a fall of resistance of the load and results at decreased load current. In figure 5, the dotted line demonstrates how the external characteristic curve turns back.
Characteristics of DC Series Generator
The field winding for direct current generators are connected in series with armature and the load. For that reason, the AB curve is identical to curve of the open circuit characteristic. The load current (IL=If) in this scenario resembles the field current. External and internal characteristics are represented in the curves (OD & OC). Terminal voltage increases with load current in a direct current series generator. As the load current increases direct current generator, its field current also increases. Nevertheless, in direct current generators, the demagnetizing effects of the armature reaction make terminal voltage decrease with further increase in load beyond a certain limit.
Characteristics of DC Compound Generator
The DC compound generators has external characteristics as illustrated in Figure 7. A generator is said to be over compounded when series winding amp-turns adjust to increase load current and subsequently, terminal voltage increases. In curve AB, the external characteristic of an over compounded generator are shown. Series winding amp-turns can also be adjusted to maintain a constant terminal voltage subsequently, current of the load increased. Generators such as this are referred to as flat compound. External characteristic as shown in the curve AC curve represents a flat compounded generator.
When a generator’s series winding have lesser turns than required of a flat compounded, it is called compounded. The external characteristics of a compounded generator are illustrated in the curve AD.
A: Preliminary Measurements
First read and enter the rating plate data of the DC Generator in Table 1.
Nominal Voltage (V) 220
Nominal Current (Armature and Series Winding) (A) 4.8
Nominal Current (Shunt Winding) (A) 5.4
Nominal Speed (RPM) 2040
Nominal Power (W) 0.75 kW
Use the Professional Digital Multimeter to measure the Generator resistances to be entered in Table 2.
R_(A1,A2) R_(B1,B2) R_(C1,C2) R_(D1,D2) R_(D1,D3) R_(D2,D3) R_(E1,E2) R_(E1,E3) R_(E2,E3)
45.3 2 3 3.8 1 2.9 0.64k 186.2 0.453k
Read and enter the rating plate data of the Universal Motor in Table 3.
Nominal Voltage (V) 220
Nominal Current (Armature and Series Winding) (A) 5.3
Field Resistance (Ω) 5.3
Nominal Speed (RPM) 2700
Nominal Power (W) 0.8k, 0.75k
Us e the Professional Digital Multimeter to measure the Universal Motor resistances to be entered in Table 4.
B: Generator Open Circuit Characteristics
Connect the circuit as shown in the figure using Cassy measurement Connection for the indicated field current, open circuit voltage, and rotor speed.
Generator Open Circuit Characteristics:
Connect the circuit as shown in Figure 1.
Activate Cassy Lab for terminal voltage (V_t) as UA1, Field current (F_i) as UB1 and rotor speed as UA2. For the quantities UA1, UB1 and UA2, select the “averaged values” and the “zero point” as left.
Adjust the channel A of the isolation amplifier for /100 position and channel B as 1 V/A position.
In the Formula option, define a new formula as terminal voltage, as (UA1*100), to define the field current as (UB1) and to define the rotor speed as (UA2*1000).
Fix the Manual Recording Option in Cassy Lab Measurement parameters settings.
Go to Cassy Lab display option and adjust it to draw the plot between terminal Voltage and field current.
Ask your instructor to check the connections.
Gradually apply the DC motor power supply voltage to start till reaching a speed of 1300 rpm.
Vary the field current using the external DC power supply, make sure that current knob is set to 10 A, in steps of 0.02 A until you reach the 0.24 A, also keep observing the rated voltage and current values from both supplies. At every step you need to measure the voltage between the C1 and A2, which is terminal voltage. You can take the reading for each step from cassy lab through Single Measurement button. This is the no-load voltage and the current displayed on the external power supply is your field current. Keep in mind that whenever you change the current, the speed of the motor must be fixed at 1300 rpm. This can be done by slightly varying the DC power supply going to the universal motor.
Observe and paste both graphs in word file.
Field Current (A) Terminal Voltage (V) (1300 rpm) Terminal Voltage (V) (1500 rpm)
0 1300 1500
0.02 1290 1490
0.04 1285 1485
0.06 1280 1480
0.08 1265 1465
0.1 1235 1435
0.12 1230 1430
0.14 1215 1415
0.16 1190 1400
0.18 1170 1399
0.2 1145 1390
0.22 1130 1385
0.24 1110 1380
Using the graph on the next page, plot the graph of no-load voltage (y-axis) and field current (x-axis).
Question: Explain the characteristics of your graph in your lab report.
The curve which gives the relation between field current (If) and the generated voltage (E0) in the armature on no load is called magnetic or open circuit characteristic of a DC generator.
C: Separately excited DC Generator Load Characteristics
Connect the circuit as shown in Figure 2 using Cassy measurement connection load voltage, for the load current, and for rotor speed. Note that at this stage the load resistance value has to be at 100%.
Disconnect the load from the circuit temporarily then apply and increase the field voltage till reaching the rated field current of 0.24 A, make sure that current knob should be fixed at 10 A.
Now include the load then gradually apply the DC motor power supply voltage (50-60 V) to start the motor till reaching the rated speed of the generator (1500 rpm) and take the measurements for no load voltage. The student in now ready to continue the measurements for online recording.
Carry on the measurement and data recording by varying the resistive load from 100% to 30% in steps as follows 100%, 90%, 80%, 70%, 60%, 50%, 40% and 30%. At every step the rotor speed has to be fixed at the rated generator speed of 1500 rpm by slightly adjusting the DC motor power supply voltage before taking the measurement.
Load Terminal Voltage (V) Load Current
100% 0.11 51.50
90% 0.13 54.00
80% 0.14 54.00
70% 0.16 53.50
60% 0.18 53.00
50% 0.20 52.00
40% 0.26 54.00
30% 0.33 52.00
Reduce the DC motor supply voltage to zero.
Switch all power supplies OFF and bring the resistance rotating control back to 100%, save Cassy Lab File but do not close it.
D: Shunt DC Generator Load Characteristics:
Connect the circuit as shown in figure 3 using Cassy Measurement connection for load voltage, for load current, and for rotor speed. Note that at this stage the load resistance value has to be set at 100%.
Load Terminal Voltage (V) Load Current
100% 0.02 5.00
90% 0.02 4.50
80% 0.02 4.50
70% 0.02 4.50
60% 0.02 4.50
50% 0.02 5.00
40% 0.02 4.50
30% 0.02 4.50
Separately excited Direct Current generators are more advantageous compared to self-excited Direct Current generators. This is because separately excited Direct Current generators operate in a stable condition at all field excitation to give a wider range of output voltage. Nevertheless, separately excited Direct Current generators are very expensive at providing separate excitation source.