PRACTICE 5. RECEPTORS. THE D.C. ENGINE
1. RECEPTORS IN D.C. CIRCUITS
Characteristic curve of a receptor comes from equation ''IrV
+
ε
=
, being '
ε
the
contraelectromotive force of receptor and r’ its internal resistance. Internal resistance
is a constructive feature of receptor, so a constant, not being '
ε
, depending on running
point of receptor. In this practice we’ll use as a receptor a d.c. engine, where '
ε
is
directly related to the turning speed of engine.
If engine is turning at a constant speed in different running points (different
magnitudes of V and I but always with the same speed), drawing of V versus I of these
measured values will give us the characteristic curve of engine at chosen turning
speed. Computing regression line of such drawing, we’ll be able to compute
contraelectromotive force ('
ε
) and internal resistance (r´) of engine.
Once these parameters computed, we’ll be able to calculate
• Consumed power on engine: P
c
=VI
• Transformed power on engine: P
t
='
ε
I
• Lost power by Joule heating on internal resistor: P
r
= I
2
r’
• Efficiency of engine:
IrP
P
c
t
''
'
+ε
ε
==η
2. GRAPHIC DRAWING AND COMPUTATION OF LINEAR REGRESSION OF A SET OF
MEASUREMENTS USING EXCEL
How correctly make the graphic drawing of a set of measurements can be seen on
attachment “GRAPHIC DRAWING”.
About how calculate the linear regression of a set of measurements using Excel
spreadsheet, can be seen on attachment “LINEAR REGRESSIONS”.
PRACTICE 5. RECEPTORS. THE D.C. ENGINE. CARRYING OUT
1. OBJECTIVE
The objective of this practice is study a receptor, in this case a direct current engine
with permanent magnet. This study is related to obtain its characteristic curve (VI) at
a chosen running speed. From this curve, contraelectromotive force, internal
resistance, consumed power, transformed power, lost power on internal resistance
and efficiency of engine can be computed.
2. MATERIAL
• Gold Source power supply
• One Fluke 45 digital multimeter
to be used as a frequency meter
• One 0100 Ω and 1,8 A variable
resistor (rheostat)
• A enginegenerator set
• One Fluke 45 digital multimeter
to be used as an ammeter
3. CARRYING OUT
We’ll connect the set of engine as can be seen on picture, being careful that:
• D.c. source must have both adjustable outputs connected in parallel, in
order to be able to supply a current twice than each output.
• Input of ammeter must be connected on 10 A input, because currents will
be higher than 100 mA.
• Switch of Marcha/Arrêt (Stop) of speed sensor connected to frequency
meter must be on “Marcha” only when we are measuring; before and later
it must be on “Arrêt/Paro” in order to avoid the battery get empty
(discharged).
The only role of variable resistor is to change the running point of engine.
Frequency meter measures the turning speed of engine, and ammeter measures
intensity of current consumed by the engine (I). The voltage given by the d.c. source
(V) can be directly read on its display.
Measurement process is:
 We adjust voltage given by the d.c. source to 5,5 V (more or less), varying
position of variable resistor until frequency meter measures a speed turning of 30 Hz.
Let’s write values of V and I on table, with their absolute errors. As we don’t have data
about error of power supply voltage, we’ll assume that there is only reading error.
 We increase voltage on d.c. source until 6,5 V (more or less), and we adjust
variable resistor until frequency meter measures a speed turning of 30 Hz. Let’s write
values of V and I on table with their absolute errors.
 We repeat process for voltages on d.c. source of 7,5 and 8,5 V, writing on
table the measured values. On a worksheet on computer on your table, you can build a
table, writing measured values on the four columns to left:
a) From data of table, on the own worksheet, taking in account all the rules for a
graphic drawing:
• Draw measured values of V v.s. I, compute regression line equation,
and obtain values of '
ε
and r’.
• From these values, fill in the other columns on table, computing all
powers and efficiency.
• Verify that addition of transformed and consumed power on internal
resistor P
t
+P
r’
equals consumed power by engine, P
c
(but experimental
errors).
• Draw efficiency v.s. measured intensity.
On drawings, both magnitudes and units on each axis must be correctly written.
4. REPORT
When all measurements and computations have been done, each group should
send the Excel file through email to instructor (
jmas@fis.upv.es
), no longer than
three working days after practice class where measurements were done. After this
time, only in absolutely justified cases will be the report accepted.
5. FOR NEXT PRACTICE (VERY IMPORTANT)
For next practice (Network analysis), is necessary to solve an electrical network
before come to Laboratory. To do it, you must read practice guide of 6
th
practice
(Network analysis) on web site and solve the problem you’ll get there. It’s absolutely
necessary to come to Laboratory to do 6
th
practice after have sent the solution of this
problem to the instructor.
V
(V)
ΔΔΔΔV
(V)
I
(A)
ΔI
(A)
P
c
=VI
(w)
P
t
= I'
ε
(w)
P
r’
=
2
I
r
'
(w)
P
t
+P
r’
(w)
c
t
P
P
=
η
'
ε
(V)= r’ (Ohm) =
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