BRUEL & KJAER 4220 - Motor and Frequency Regulation

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Motor and Frequency Regulation.
The Pistonphone is equipped with a precision DC motor. In order to ensure
the frequency stability, the rotating speed is regulated to within 1 % by a
centrifugal switch and a current regulating transistor. See Fig. 8. This
arrangement completely eliminates any distortion coming from the cam
speed. In addition, such an accurate definition of the frequency is required
for calibration of non-linear sound level meters (e.g. in the case of a sound
level meter having the standardized "weighted" frequency response A, or B,
it is known that the response at 250 Hz is 8.6 dB or 1.4 dB, respectively, lower
than the response at looo Hz).
Check t
Measure r-3~--------~--~~~----~----------~
R1
Off t
<]
Fig. 8. Diagram of the frequency regulator.
Centrifugal
switch
The control switch ("flash-light" type) has three positions: "Off-Measure-
Check". In the "Measure" position the current from the batteries is fed to the
motor through the regulating system, which maintains a rotating speed of
3750 rpm, i.e. a frequency of 250 Hz. In position "Check" the batteries are
directly connected to the motor, and the frequency will be somewhat higher,
e.g. 350-400 Hz for fresh batteries. By pushing the switch back and forth
between "Measure" and "Check" positions a quick check on the batteries is
obtained. If no frequency variation is noted, it is time to replace the batteries.
(The batteries could still give some more hours use, but the frequency of
the produced sound would be lower than 250 Hz).
The frequency of the Pistonphone may be varied up to 800 Hz by using an
external DC power supply instead of the batteries, the control switch being
in
position "Check". The motor has a shunt characteristic and its rotating
speed is approximately proportional to the connected voltage. The upper limit
of the motor rot a ting speed, corresponding to a frequency of 800Hz, is attained
for a voltage of approximately 24 Volts. It should never be exceeded and
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