Lab # 7 – DC Machines Part 3

Separately Excited DC Generator

The students should read, understand and follow the power lab safety instructions before using
this laboratory. In this laboratory, high voltage/current terminals may be exposed for the ease of
measurements of voltage/current in different experiments. Regardless of voltage and current
levels, these should be treated as dangerous to avoid any accidents.

Instructions for students:

OBJECTIVE

• To study the properties of the separately excited DC shunt generator under no load and full-
load conditions.
• To obtain the magnetization curve of the generator.
• To obtain the armature voltage vs armature current load curve of the generator.

DISCUSSION

A DC machine can run either as a motor or as a generator. A motor converts electrical power
into mechanical power while a generator converts mechanical power into electrical power. A
generator must, therefore, be mechanically driven in order that it may produce electricity.
Since the field winding is an electromagnet, current must flow through it to produce a magnetic
field. This current is called the excitation current, and can be supplied to the field winding in one
of two ways; it can come from a separate, external DC source, in which case the generator is
called a separately excited generator; or it can come from the generator’s own output, in which
case the generator is called a self-excited generator.
Assume that the shunt field is excited by a DC current, thereby setting up a magnetic flux in the
generator. If the rotor (or more correctly, the armature) is rotated by applying mechanical effort
to the shaft, the armature coils will cut the magnetic flux, and a voltage will be induced in them.
This voltage is AC and in order to get DC out of the generator, a rectifier must be employed.
This role is carried out by the commutator and the brushes.
The voltage induced in the coils (and, therefore, the DC voltage at the brushes) depends only
upon two things - the speed of rotation and the strength of the magnetic field. If the speed is
doubled, the voltage doubles. If the field strength is increased by 20%, the voltage also increases
by 20%.
Although separate excitation requires a separate DC power source, it is useful in cases where a
generator must respond quickly and precisely to an external control source, or when the output
voltage must be varied over a wide range.
With no electrical load connected to the generator, no current flows and only a voltage appears at
the output. However, if a resistance load is connected across the output, current will flow and the
generator will begin to deliver electric power to the load. The machine which drives the
generator must then furnish additional mechanical power to the generator. This is often
accompanied by increased noise and vibration of the motor and the generator, together with a
drop in speed.

The separately excited DC generator equivalent circuit:

EQUIPMENT REQUIRED

DESCRIPTION MODEL
DC Motor/Generator 8211
DC Voltmeter/Ammeter 8412
Electrodynamometer 8911
Digital Tachometer 8920
Timing Belt 8942
Power Supply 8821
Connection Leads 8941
PROCEDURE

CAUTION!
High voltages are present in this Experiment! Do not make any
connections with the power on! The power should be turned off after
completing each individual measurement!
I. No Load Characteristics

a) Magnetization Curve

1. In this experiment the dynamometer will be used as a prime mover for the DC generator.
Make sure the dynamometer is coupled with the DC generator. Connect the circuit shown
in figure 4.1. DO NOT APPLY POWER AT THIS TIME!

Figure 4.1

2. Set the dynamometer function to “CW prime mover” using the function button.

3. Start the dynamometer and set the speed of the dynamometer using the control knob to
1500 rpm.

4. Turn on the power supply and change the input voltage to get the field currents If listed
in the Table 1-1 below. Measure and record the output voltage of the generator.

Table 1-1
If (mA) EA
0
70
130
170
230
270
300

5. Return the voltage to zero and turn off the power supply. Stop the dynamometer.
 b) Speed versus Voltage

1. You will now examine how the generator voltage changes with speed of rotation while
keep the field excitation current If constant.

2. Turn on the power and adjust If to 0.3A dc.

3. Using the same circuit in figure 4.1, turn on the dynamometer and set its speed according
to the values listed in Table 1-2. Measure and record the generator voltage in Table 1-2.

DO NOT CHANGE If for the remainder of the experiment

Table 1-2
Speed (rpm) EA (V)
0
300
500
700
900
1100
1300
1500

4. Return the voltage to zero and turn off the power supply. Stop the dynamometer.

II. On-Load Characteristics

1. A resistive load will be connected to the generator terminals in order to determine its on-
load characteristics. Connect the circuit shown in figure 4.2.

Figure 4.2
1. Start the dynamometer and set its speed to 1500 rpm.

2. Turn on the power supply and set the field current If until the generator voltage reaches
250V dc. DO NO CHANGE If for the remainder of this experiment.

3. The resistive load to be used in this experiment with different combinations shown in
Table 1-3.

4. Measure and record EA and IA for each of the resistance values listed in the Table 1-3.

Note: Although the nominal output current rating of the generator is 300mA, it may
be 50% overloaded without harm

Table 1-3
R Load (Ohms) IA (A) EA (V) Power (W) Speed (rpm)
Open
2400
1200

5. Do you notice that the driving motor is working harder when the generator is delivering
power to the load?

6. Return the voltage to zero and turn off the power supply. Stop the dynamometer.