Elenco Snaptricity® - Manual (Page 65)

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Back EMF Project #55
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Educational Corner:
The voltage/current 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 overall current decreases as the motor
speeds up.
Electric Paths
Build the circuit as shown, leave the fan off the motor (M1).
Assembly
!
WARNING: Moving parts. Do not touch
the fan or motor during operation.
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The voltage from the batteries (B3) pushes an electric
current through a coil in the motor, which spins the shaft
using magnetism. But the spinning shaft also uses
magnetism to produce a current in the coil, which opposes
the current from the batteries.
The result is that the motor has low resistance when the
shaft isn’t spinning fast, allowing a higher current to make
the lamps bright. When the shaft is spinning really fast
without the fan, the motor has high resistance, limiting the
current and keeping the lamps dim.
Description
Place your finger on the top of the motor shaft to prevent it
from spinning, then push the press switch (S2) - the lamps
(L4) are bright. Now release the motor shaft and press the
switch again - the lamps get dim or go out as the motor
speeds up. DO NOT TOUCH THE MOTOR WHILE IT SPINS.
Next, place the fan on the motor and push the press switch
again - the lamps stay bright as the fan speeds up.
Operation
!
WARNING: Do not
lean over the motor.