Manualshelf

Datasheet-1

ManualsBrandsElenco ManualsElectronic ExperimentationSnap Circuits STEM Kit
41424344454647484950
1A
+
Build the circuit, set the switcher (S6) to the middle position (off), set the meter (M5) to the
1A setting, and place the fan on the motor (M1). First, set the switcher to the left position to
start the motor. Then turn on the slide switch (S1) to increase the speed. Then push the
press switch (S2) to increase the speed again. Then hold the magnet next to the reed switch
(S9) to turn it on, further increasing the motor speed. Try different combinations of those
switches. The meter measures the current.
Project 37
4-Speed Motor
!
WARNING: Moving parts. Do not
touch the fan during operation.
!
WARNING:
Do not lean over the motor.
Project 38
Back EMF
Build the circuit as shown, leave the fan off the motor (M1).
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.
+
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.
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.
Why does the fan affect the lamp brightness?
-41-
SC_STEM1_manual_PRINT.qxp_Layout 1 7/13/17 4:43 PM Page 42