Elenco Upgrade Kit SC500 to SC750 - Manual (Page 21)

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The voltage produced by a motor when it is spinning is called its Back
Electro-Motive-Force (Back EMF); this may be thought of as the
motor’s electrical resistance. The motor’s
Front Electro-Motive-Force
is the force it exerts in trying to spin the shaft. This circuit
demonstrates how the Back EMF increases and the current decreases
as the motor speeds up.
Place the fan on the motor (M1) and turn on the slide switch (S1). The
6V bulb (L2) will be bright, indicating that the Back EMF is low and the
current is high.
Turn off the slide switch, remove the fan, and turn the slide switch back
on. The lamp is bright when the motor starts and the lamp dims as the
motor speeds up. Now the Back EMF is high and the current is low.
BE CAREFUL NOT TO TOUCH THE MOTOR WHILE IT SPINS.
Project #537
OBJECTIVE: To demonstrate how the motor works.
Back EMF
Project #538
OBJECTIVE: To demonstrate how the motor draws more current
to exert greater force when spinning slowly.
Back EMF (II)
Place the fan on the motor (M1). Connect the photoresistor (RP) with
the jumper wires as shown, and hold it next to the 6V lamp (L2) so the
light shines on it.
Turn on the slide switch (S1) and watch how the 6V lamp is bright at
first, but gets dim as the motor speeds up. By moving the
photoresistor (RP) next to or away from the 6V lamp, you should be
able to change the motor speed. To slow the motor down even more,
cover the photoresistor.
When the photoresistor is held next to the 6V lamp, transistor Q2 (with
lamp L1) will try to keep the motor at a constant speed.
!
WARNING: Moving parts.
Do not touch the fan or
motor during operation. Do
not lean over the motor.
!
WARNING: Moving parts. Do not touch the fan or
motor during operation. Do not lean over the motor.