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 (V-I) 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 0-100 Ω and 1,8 A variable

resistor (rheostat)

• A engine-generator 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 e-mail 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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