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15 - D.C. MACHINES
Time ~ 6 hours
Object: To study the basic operating principles of D.C. Motors and Generators. To investigate
speed control of D.C. motors and voltage regulation of D.C. Generators.
References:
1. “Electric Machinery”, A. E. Fitzerald, Charles Kingsley Jr. and Stephen D. Umans
2. “Electric Machines”, Mulkutla S. Sarma
Caution: In this experiment, currents in various parts of the circuit may be large; therefore use
higher capacity connecting leads. Ensure the continuity of the field current.
Safety
The voltages used in this experiment are lethal. Assemble or modify a circuit only with the
breakers off. Do not apply power to a circuit until the wiring has been checked by an
instructor. Do not touch any node or component of a live circuit. Be careful when moving
near a circuit so that a wire is not accidentally snagged.
The machines used in this experiment are physically dangerous. Guards must be in place
over any rotating components before applying power. Do not wear loose clothing or
neckties, and keep long hair away from the machines.
A DC motor must be used properly to avoid a dangerous runaway condition. When starting
a DC motor, always energize the field circuit first and then energize the armature circuit.
Never interrupt the field current of a DC motor while it is running
If an emergency occurs, the power for the entire laboratory can be disabled using the red
button on the power distribution panel.
Procedure: (read the appendix before starting)
1. Connect and operate the D.C. machine as a motor with no load. Note the U of S asset number
of your machine. In DC motor/generator units operate the generator as the the motor.
Initially, use one of the power resistances as the variable starting resistance. Subsequently,
investigate and use a suitable starting box to run the machine as a motor. Refer to Figure l for
the connections.
2. With the machine operating as a motor at no load, measure and plot:
(a) speed as a function of applied armature voltage with the field current held constant near
rated value.
(b) speed versus field current with constant armature terminal voltage near rated value.
Revised Jan 06, 2013
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3. Now operate the same machine used in parts 1 and 2 as a generator using the other
mechanically-coupled machine as a motor to drive it. Use separate field excitation for the
generator and determine its open-circuit output characteristic, (i.e. measure and plot the
variation of the induced emf vs. the excitation current with the generator operating at rated
speed and no load).
4. Operate the same machine as a generator and determine its output voltage characteristic
under various loading conditions. Maintain the motor speed constant while increasing the
generator load current from “no-load” up to “full rated load” in 5 or 6 steps. Repeat this
procedure for the four cases of field excitation indicated below. Adjust the field rheostat to
start with the same no-load voltage (rated) in each case and plot all results on a common
graph.
(1) separately excited
(2) self excited
(3) self excited plus cumulative compound excitation
(4) self excited plus differential compound excitation
Notebook: Plot the various characteristics during the experiment and explain your results giving
figures where necessary.
Fig. 1. DC Motor Connections
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NOTES:
I. Terminals AI-C12 and A2-C21 should always be shorted either permanently through the brass
strips or through shorting wires.
2. Include ammeters and voltmeters at appropriate places.
3. While using power resistance box, replace the starting box and use power resistance box as a
rheostat in the armature circuit of the motor.
4. In order to use the set as a motor-generator set, connect the motor as shown in Figure 2.
Connect the field of the generator in the same manner as shown in Figure l. The armature
terminals [C11 and C22] can be used to connect the generator load.
Fig. 2. DC Motor Connection with full utilization of the starter
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Appendix - Background
The main electrical components of a D.C. machine are the shunt field excitation winding and the
armature winding. If the field of a D.C. generator is excited from an external source of power,
the generator is said to be separately excited. However, a D.C. generator can also supply its own
excitation; this feature is called self-excitation. This ability is due to the residual magnetism
which remains in the field poles after the machine has once been separately excited.
A series field winding may be connected in series with the armature to slightly increase the
magnetic field and compensate for voltage drop in the generators armature resistance as the load
current increases. When the shunt field and series field windings are both used this is known as
compound excitation. A series field winding may also be used to increase the starting torque of a
D.C. machine.
As the armature current increases in a D.C. motor or generator, the armature winding MMF
increases and the zero flux density region moves from the q-axis of the machine. This changes
the ideal commutating position and causes brush arcing if no adjustments are made. This
"armature reaction" MMF may be compensated by passing the armature current through an
interpole (or compole) winding. This neutralizes the armature reaction MMF in the coils under
commutation.
The magnitude of the induced EMF in the armature may be calculated as follows:
E =Z"
!
P n
"
"#
a 60
where:
Z is the number of conductors
P is the number of poles
a is the number of parallel paths in the armature winding
n is the speed in rpm
φ is the flux per pole
E is the magnitude of emf induced in the armature
A second equation is derived by applying Kirchhoff’s law to the armature circuit. The armature
drop is added or subtracted depending on whether the machine is operating as a motor or
generator.
V = E ± IR
!
where:
V is the terminal voltage of the machine
I is the armature current
R is the armature and brush contact resistance
END