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DC Generators Separate and Shunt Excitation AELE.3514 Power and Machines
Laboratory 2
1
DC Generators
Separate and Shunt Excitation
Power and Machines Laboratory 2
Session 1, 2004
1
Aim
This laboratory illustrates the rst principles of electric machinery and familiarizes students with the
fundamentals of DC machines. You will learn about the characteristics of a DC generator. These
characteristics are no-load (internal generated voltage) characteristic and the terminal voltage charac-
teristic (load characteristic).
2
Safety
You can work individually or with a partner. Groups of three or more are not allowed.
Please be aware of the following hazards.
(a) Electric motors and generators include parts rotating at high speed (up to 1500 rpm). Care should
be taken when operating such machines.
Note that power will be supplied to a machine only if the variable DC voltage control (a big black
knobe in the center of the power supply panel) is initially set to zero. This prevents the machine
from being accidentally operated at high speed. Every time that you wish to start the machine,
rst set this control to zero.
(b) You will be working with voltages up to DC 220 V and currents up to 10 A DC. These levels
of voltages/currents are dangerous. To avoid electric shock, connecting circuits is permitted only
after power has been disconnected. The circuit should be checked by a demonstrator before it has
been powered on.
(c) Dont connect any equipment on your bench to the power supply from an adjacent bench; this
might connect the equipment to a different phase.
(d) Lateral surfaces of the rheostats may become very hot. You should avoid contacts with metallic
parts of the rheostats. Also, wire should be kept away from the rheostats to avoid damaging the
insulation caused by rheostat overheating.
3
Report
Each student must submit an individual report. Your report which answers the laboratory questions,
is to be handed in at the end of the laboratory class.
Australian Defence Force Academy
School of Electrical Engineering AELE.3514 Power and Machines
Laboratory 2
2
4
Introduction
Advantages of DC machines arise from a wide variety of operating characteristics which can be ob-
tained by selection of the method of excitation of the eld windings, i.e., by selection of the method of
supplying current to the eld windings. The eld windings may be separately excited from an external
dc source, or they may be self-excited, i.e., the machine may supply its own excitation. Various eld
connection diagrams are shown in Fig. 1.
+
-
+
-
+
-
+
-
(b)
(a)
(c)
(d)
Figure 1: Field circuit connections of dc machines. (a) separate excitation, (b) series, (c) shunt, (d)
compound.
The method of excitation profoundly inuences both the steady-state characteristics and the dy-
namic behavior of the machine in control systems.
The connection diagram of a separately excited generator is given in Fig. 1a. The required eld
current is a very small fraction of the rated armature current on the order of 1 to 3 percent in the average
generator. Because of this, a small amount of power in the eld circuit may control a relatively large
amount of power in the armature circuit; i.e., the generator is a power amplier.
The eld windings of self-excited generators may be supplied in three different ways. The eld
may be connected in series with the armature (Fig. 1b), resulting in a series generator. The eld may
be connected in shunt with the armature (Fig. 1c), resulting in a shunt generator, or the eld may be
in two sections (Fig. 1d), one of which is connected in series and the other in shunt with the armature,
resulting in a compound generator. With self-excited generators, residual magnetism must be present
in the machine iron to get the self-excitation process started.
Typical steady-state volt-ampere characteristics of DC generators are shown in Fig. 2, constant-
speed prime movers being assumed. The relation between the steady-state internal generated voltage
E
A
and the terminal voltage V
T
is
V
T
=
E
A
I
A
R
A
Australian Defence Force Academy
School of Electrical Engineering AELE.3514 Power and Machines
Laboratory 2
3
where I
A
is the the steady-state armature current and R
A
is the armature circuit resistance. In a
generator, E
a
V
T
; in a motor the opposite holds, i.e., E
a
V
T
.
0
0
100
75
50
25
100
75
50
25
Load current in percent of rating
Voltage in percent of rated voltage
Series
Compound
Shunt
Separately excited
Figure 2: Volt-ampere characteristics of dc generators
.
The terminal voltage of a separately excited generator decreases slightly with an increase in the
load (armature) current, mainly because of the voltage drop in the armature resistance. The eld
current of a series generator is the same as the load current, so that the air-gap ux and hence the
generated voltage vary widely with load. As a consequence, series generators are not often used.
The voltage of shunt generators drops off somewhat with load. Compound generators are normally
connected so that the mmf of the series winding aids that of the shunt winding (cumulative compound).
The advantage is that through the action of the series winding the ux per pole can increase with load,
resulting in a voltage output which is nearly constant or which even rises somewhat as load increases.
The voltage of both shunt and compound generators can be controlled over reasonable limits by means
of rheostats in the shunt eld.
Australian Defence Force Academy
School of Electrical Engineering AELE.3514 Power and Machines
Laboratory 2
4
5
Equipment
M
=
DC machine MV 1036 (electric torque meter), or DC machine MV 1034, or
DC machine MV 1028 (2.0 kW, 220 V, 1400 rpm) which will operate as a
prime mover
G
=
DC machine MV 1006, (1.2 kW, 220 V, 1400 rpm) which will operate as a
generator
T G
=
Tachometer generator MV 1024, 20.8 V (DVM reading) per 1000 rpm
R
M
, R
G
=
Shunt rheostats MV 1095 or MV 1905
R
L
=
Load resistor MV 1100
I
G
, I
M
, I
L
=
AVOs (ammeters)
V
=
AVO (voltmeter)
S
=
Switch
F
=
Power supply MV 1300.
The two stations with torque meter MV 1036 have the rheostat R
M
and two ammeters available
on the front panel. For the three other stations with the DC machine MV 1034 / MV 1028, an extra
rheostat and two extra AVOs (avometers) are provided. Use them to connect the circuit of Figure 3.
All ve stations should use a DVM to measure the speed. The rpm meters available on the front panel
of torque meters MV 1036 give inaccurate reading, dont use them. A DVM is used only with the
Tachometer generator; for other measurements use AVOs.
AVOs should be switched to the OFF
position when you nish.
Take extreme care while working with AVOs. Negligent handling of the AVO can damage
it; each AVO costs more than $1000, a repair costs another $800.
When measuring DC voltage,
make sure that the AVO is set to measure voltage and not current, and vice versa. Before the system is
powered, make sure that you have chosen appropriate current/voltage range setting. I.e., if you expect
that the current will be of 6 A then the AVO must be set for this current range. Disconnect power if
you need to change any of the settings. Never change AVOs settings while operating the circuit.
Suggested initial settings are shown in circuit diagrams (next to the corresponding circuit element).
6
Ratings
Each electric machine is designed by a manufacturer to operate in a certain range of voltages and
currents. The parameters quoted by the manufacturer are known as rating of the machine. Operating
the machine at a voltage or current larger than the rated voltage and current for an extended period of
time may damage the machine. Before you start connecting circuits write down the ratings of the
machines shown on the rating plates. These ratings must not be exceeded at any time during
the laboratory exercises.
7
Connecting up and starting
This experiment is to check that the machines work as expected.
1. Connect the torque meter MV 1036 or DC machine MV 1034 as motor and the DC machine
MV 1006 as generator in accordance with the circuit diagram in Figure 3. The circuit should
now be checked by the lab demonstrator.
Australian Defence Force Academy
School of Electrical Engineering AELE.3514 Power and Machines
Laboratory 2
5
Figure 3: Circuit diagram for the rst experiment
.
2. Turn on the xed DC voltage switch of the power supply and adjust the shunt rheostat R
M
of
the motor so as to obtain the maximum excitation current (check the rated excitation current of
this machine!)
3. Adjust the shunt rheostat R
G
of the DC generator so as to obtain the minimum excitation cur-
rent.
4. Set the variable DC voltage control of the power supply to zero and turn on the variable DC
voltage switch. Then using the variable DC voltage control, gradually increase the rotor voltage
to bring the machine to 1500 rpm. Make sure you are not exceeding the current ratings. The
machine should rotate in the direction of