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EXPERIMENT NO.
AIM: Measurement of speed using magnetic pick up.
APPARATUS: Trainer kit , CRO , connecting wires
THEORY: A magnetic pick up is a coil placed in magnetic field. Variation in magnetic
field induces a voltage pulse in the coil. Amplitude and frequency of induced voltage
pulse depends upon rate of change of magnetic flux.
Magnetic flux produced by magnet can be altered by bringing any external magnetic
material near to the magnetic field of magnet which will induce voltage in coil placed
in it. These voltage pulses can be processed and made available for measurement
purpose . These voltage pulses from magnetic pick up are proportional to the rate of
change of flux .Hence there is a certain minimum speed below which the induction will
be weak to generate a pulse & hence a pulse could be missing here & there .This limit
is below 5V motor voltage (from 3V to 5V), you may observe missed pulses there by
causing erroneous reading of speed.
Every time the tooth of wheel reaches over magnet it cut magnetic lines & hence
induction of a voltage pulse. Hence, divide the frequency obtained at the output of
sensor (tp 13) by 8 to get shaft rotation frequency.
If a motor shaft fitted with ferrous slotted wheel it will cut magnetic field produced
around a coil situated beneath the slotted wheel. Slotted wheel of magnetic material
may alter magnetic field of magnetic pick up periodically on the uniform motion of
motor. In this case voltage pulse from magnetic pick up if measured are found to be
proportional to the motor speed (r. p. m).However, due to 8 slots ,it will produce 8
times pulses.
Magnetic pick up used to measure Motor speed
Panel circuitry consists of an emitter follower to drive motor .For speed variation a
potentiometer is provided. Magnetic pick up gives output voltage in response to a
rotating slotted wheel connected to the shaft of motor. This voltage is converted into
pulses by a comparator. These pulses are converted into a DC voltage by frequency to
voltage converter followed by an amplifier. Calibration requires zero adjustment for
zero speed and span pot ,adjust for full speed
Procedure
Connect+_12v supply to the panel short the terminal as shown in wiring diagram by
patch code for manual speed control of motor through potentiometer on panel .
Select the magnetic pick up sensor by setting rotary 6 position switch at correct
location [1st] as shown.
Connect voltmeter to the output terminals to moniter the speed. Adjust zero pot for
zero output voltage when moniter is stand still and adjust span potentiometer [ 2 v
output voltage] when moniter is running at beyond 2000kpm speed.
Repeat the procedure twice or thrice to confirm zero and full scale adjustment. Thus
reading I n volt *2000 will display RPM measure frequency at tp 13 to confirm
maximum speed of your moniter which will be beyond 2000 RPM. In case it is beyond
4000max then self span such that volt output x 2000 will be equal to RPM vary the
speed of moniter by potentiometer.
Note down the output voltage for each position of potentiometer. Measure voltage
applied to the motor at the tag shown to the left hand lower corner of the wheel
connected CRO to the left test point TP13(F) and observe the wave form for different
motor speed.
Motor speed measurement may be confirmed using external calibrated stroboscope or
on board.
Verify output voltage=[ f/8x16] 2000v so that even higher RPM you can observe the
same on 0-2 range. This is very useful for computer intereferance
Observation table :-