Introduction
The purpose of this experiment was to observe synchronous generator behavior, and perform Open Circuit Test and Short Circuit Test on it.
Generator is an equipment that converts mechanical energy to electrical energy. A synchronous generator is an AC machine where the speed is synchronous, i.e., its speed is proportional to the frequency of the armature current. The rotor, with the magnetic field produced by the field current, they rotate at the same speed or in synchronism with the magnetic field produced by the armature current. This results in a constant torque.
To know the parameters of a synchronous generator we need to analyze two curves:
Open circuit characteristic: represents the relationship between the voltage and the field current. Also known as magnetization curve
Short circuit characteristic: represents the relationship between the armature current and the field current.
Procedure/Equipment
For this experiment we used: 1 Synchronous Machine operating as a generator.
4 multimeters – to measure current and voltage;
1 DC Variable Power Supply – external excitation field supply)
1 Servo drive DM
1 Three Phase Resistive Load (220/680/1500 ohms)
1 Three Phase Inductive Load (0.4/0.8/1.6 henrys)
This experiment was divided in four parts: A. Resistance measurement
B. Open circuit test C. Short circuit test
D. Load test (resistive load and lagging load)
Before beginning the experiment, we collected the data of the synchronous generator, based on its nameplate. These data are shown in table 6.1.
For the first part, we just measured the resistances of the machine, the field resistance and the armature resistance. The data is presented in table 6.2 .
In part B we performed the open circuit test. We connected the machine as a generator, in Y. The generator was connected internally with the servo. Then, we set the speed machine to 1800 rpm, and we recorded the value of the terminal voltage. After this, we turned on the DC source and we adjusted the field current to 0.05 A. We increased the field current until 0.4 A, in
steps of 0.05. Then, we decreased the field current until 0, again in steps of 0.05. The recorded data is shown in table 6.3.
For part C of this experiment, we used the same circuit, but this time we shorted the circuit, by connecting line A, B and C to each other. We turned on the servo system supply and the DC supply, adjusting it until the field current was 0.1 A. Then we recorded the values of the field current and the armature current, which are shown in table 6.4. After this we increased the field current in steps of 0.1 A until 0.4 A, these values are also shown in table 6.4.
For the next part, we performed the load test, first with resistive load, and then with inductive load. The previous connections remained the same. The 3-phase resistive load was connected in Y to the generator. Firstly we used the 220 ohms resistors. The servo was turned on, and the speed was set to 1800 rpm. We turned on the voltage supply, and we adjusted it until the terminal voltage was 220 V. We collected the values for the field and armature currents, shown in table 6.5. After this, we turned off the power supply and we connected for this time the resistance of 680 ohms, then we repeated the same steps for the previous part. We did the same thing with a resistance of 680+220 ohms and 1500 ohms. The recorded values are in table 6.5.
For the last part our load was consisted of inductors and resistors. The circuit remained the same as the previous part. For this part we used just the resistance of 220 ohms connected in series with the inductance. First we use the 0.4 H inductance, and then 0.8 H, 0.8+0.4 H, and 1.6 H. For each connection, we turned the servo on and we set the speed to 1800 rpm. Then we recorded the values of field and armature currents, shown in table 6.6.
Experimental Results
Table 6-1Stator winding’s voltage * (V) 220
Stator’s current (A) 1.2/0.78
P (kW) 0.3 Cos() 1/0.80 Field voltage (V) 140 Speed (rpm) 1800 Frequency (Hz) 60 Table 6-2
Field Resistance (Ohm) 282.5
Armature Resistance (Ohm) 33.3
FIELD CURRENT (A) TERMINAL VOLTAGE (INCREASING FIELD CURRENT) (V) TERMINAL VOLTAGE (DECREASING FIELD CURRENT) (V) 0 4.985 61.5 0.05 119.2 119.1 0.10 216.5 214.4 0.15 290.5 297.5 0.20 354.6 361.7 0.25 396.1 397.7 0.30 426.7 426.3 0.35 445.0 448.6 0.40 461.9 463.0 Table 6-4 FIELD CURRENT (A) SHORT CIRCUIT STATOR CURRENT (A) 0 0.023 0.1 0.197 0.2 0.389 0.3 0.582 0.4 0.784 Table 6-5 Resistance (Ohm)
THREE-PHASE RESISTIVE LOAD FIELD CURRENT (A) ARMATURE CURRENT (A) 220 0.318 0.569 680 0.146 0.184 220+680 0.131 0.140 1500 0.118 0.087 Inf. 0.105 0
Table 6-6 Lagging load with
R= 220 (Ω) and Inductance
(H)
THREE-PHASE INDUCTIVE LOAD FIELD CURRENT (A) ARMATURE CURRENT (A) 0 (from Table 6-5) 0.318 0.569 0.4 0.273 0.356 0.8 0.219 0.237 0.8+.04 0.156 0.106 1.6 0.160 0.115
Analysis of Results
The experimental results are consistent with what has been learned in theory. We could observe that the field current and the armature current changed proportionately. Also, was possible to see that the terminal voltage increased as the field current increased, but from a certain point, this variation became small due the saturation.
Questions
1. Plot the open circuit characteristics in a single plot using the data you recorded in Table 6-3. Based on this curve:
a. Is this curve linear (straight line) or nonlinear? Why?
The curve is nonlinear. It happens because this machine is made of a magnetic material, which saturates at a certain current.
b. When the field current is zero the terminal voltage is not zero. Why? This happens because of the residual magnetism.
2. Plot the short circuit characteristics using the data you recorded in Table 6-4. Is this curve linear or nonlinear? Why?
This curve is linear. Because the field current is proportional to the armature current
3. Using equation 6-1, calculate the synchronous reactance of the AC generator for each value of If in Table 6-4. This will require data from Tables 6-3 and 6-4.
√ ⁄ If = 0 A : √ ⁄ If = 0.1 A : √ ⁄
If = 0.2 A : √ ⁄ If = 0.3 A : √ ⁄ If = 0.4 A : √ ⁄
4. Using the data from the previous item, plot the armature current, terminal voltage, and the synchronous reactance of the generator vs. the field current. Do this on a single plot, with three different vertical scales - one for current, one for voltage, and one for reactance.
5. Using Rs, Xs and the different loads in Tables 6-5 and 6-6, and using the OCC, calculate the armature currents and compare them to the experimental results.
6. Plot Ia versus If for the resistive load using the data you recorded in Table 6-5. How should the field current be changed to keep the terminal voltage constant when the load is increasing? Why?
7. Calculate the power factor for each lagging load. Record your values in a table similar to Table 6-7.
Table 6-7 Lagging load with
R= 220 (Ω) and Inductance (H) Power factor 0.0 0.4 0.8 0.8+.04 1.6
8. Plot Ia versus If for the lagging load using the data you recorded in Table 6-6. In this case the load resistance is constant (R=220 Ohm). Therefore, the active power is fixed, yet for each inductance value there is a different reactive power demand. How should the field current be changed to keep the voltage constant when the reactive power is increasing? Why?
Conclusions
The experiment goals were achieved. It was possible to perform the tests: open circuit test and short circuit test, to find the open circuit characteristic and short circuit characteristic. It was possible to see the behavior of the synchronous generator for different loads too. Any
discrepancy or error in the results may have been caused by error in the readings of the data or in the circuit connections.
