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MMD > Archives > April 2004 > 2004.04.28 > 07Prev  Next

Vacuum Transducer for Motor Speed Control
By John A. Tuttle

Hi All,  Roger Waring asks about vacuum sensors that can monitor
a specific vacuum level and then send a signal to a device which will
control the speed of a 240 volt A/C motor that produces the vacuum
needed to operate a player piano.

All fine and good, but it seems to me that he needs to establish what
constitutes the desired vacuum level first.  That's not very difficult,
but "the desired" level depends more on the condition of the player
than it does any specific vacuum level (measured in inches of water
vacuum).  Further, determining the desired level (or desired volume
or loudness of the music) is something only he knows.  (Average vacuum
levels for a 'medium' volume of music in a player piano generally range
from 10 to 18 inches of water vacuum in a "tight" system where
everything is well regulated: both the piano and player mechanism.)

As for an adjustable vacuum sensing device that provides electrical
'on-off' contacts to operate a relay or power switch for turning an
AC motor 'on and off", they are relatively inexpensive and readily
available.  Story & Clark uses a couple of them in their modern player
pianos, and I have a few in stock.

However, what I'm getting from Roger's posting is that he's looking for
a device that is constantly varying (or regulating) the speed of the AC
motor as the vacuum level changes due to the load or vacuum needs of
the player system.  Naturally, there's a big difference between turning
an AC motor 'on' and 'off' at a high rate of speed and raising or
lowering the voltage to that motor with a variable electronic voltage
regulator which is controlled by a vacuum sensing device that produces
a variable output.  So, what he needs is an "adjustable vacuum-operated
AC voltage regulator" capable of regulating 240 VAC motor.  That device
could be expensive and hard to find...

At the link below, I found a 500W 230 VAC mains dimmer control for
6.95 UKP.  Based on the information, it sounds like it uses the same
electronics that's used in the dual-stage electronic motor speed
controller that comes with a standard electrification kit for player
pianos.  A simple diac/triac circuit with a variable gate, which is
manually adjusted to the desired levels (Play and Rewind).  However,
unlike the dual-stage controller, it is a single-stage controller.

(This brings to mind another consideration.  What about the different
vacuum requirements during Play and Rewind?)

Locating a vacuum sensing device that provides a variable output
(in either VAC or VDC) which could be used to control an AC voltage
regulator was not easy.  I finally found a few at the link below.
However, it seems that integrating the analogue output of the device
with the voltage regulator would require some expertise in

The above devices monitor vacuum from -1 to +5 bars.  One bar of vacuum
equals 401 inches of water vacuum.  I'm not sure what "0" or "-1" bars
would equal.  See the conversion chart at

Furthermore, the problem of needing two vacuum levels (for Play and
Rewind) is still not addressed.

All-in-all, it seems to me that it would be much less expensive, much
easier, and just as effective to use a 110 VAC electrification kit
(with dual-stage control) in conjunction with a simple 240-to-120 VAC
step-down transformer.

Then again, I might be wrong!!


John A. Tuttle
Brick, NJ, USA

(Message sent Wed 28 Apr 2004, 16:58:42 GMT, from time zone GMT-0400.)

Key Words in Subject:  Control, Motor, Speed, Transducer, Vacuum

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