Previous postings have referred to variable voltage ac controllers.  If
you are to achieve full closed-loop feedback control of the motor speed
according to the suction level or flow rate achieved, you will need to
control the motor speed.  This much is obvious, however, ac motor speed
cannot be controlled by varying the supply voltage because the speed is
a function of the ac frequency.

To do the job properly you will need a variable frequency controller.
The standard cheap 'dimmer' circuit will also not work as this varies
the mark-space ratio of the given frequency of the supply thus varying
the effective voltage.  Driving an ac synchronous motor through one of
these circuits will keep the speed the same but just reduce the power.
Variable speed do-it-yourself power drills work because they are not
synchronous motors.

Variable frequency ac drives have been developed over the last 10 years
or so and are used widely in industry and rail vehicles (my own field)
but they are not cheap.

So, in conclusion, you can either use a variable ac drive and your
existing synchronous motor, or get a series wound motor and use a
thyristor drive.

If Roger is using a Motora type blower he may be able to use a thyristor
controller as I think the Motora has a series wound motor rather than a
synchronous motor.  Perhaps someone will confirm this?

Low pressure transducers are readily available, even down to player
piano pressures.  These are used in air conditioning and other air
management systems.

The price of such a system may make it prohibitive; the system
controller, transducer and motor controller could cost more than a
second-hand piano.

Cheaper, and certainly worth trying, is what is commonly called the
'bang-bang' control system.  This is where the motor is switched on and
off by a control signal from a pressure switch.  A suitable pressure
switch can be obtained from a domestic washing machine where they are
used to detect water level to stop the fill cycle.  Surprisingly, these
are not float switches but diaphragm switches operated by the head of
water in a tube.  We must be talking about similar heads here.  One
could be tweaked to operate at the desired pressure.  Some
experimentation will be required to avoid unsteady control or hunting.

I can't help wondering if all this controllery is really worth the
effort.  A centrifugal pump can be allowed to blow off through the
spill valve or even have its flow rate reduced at maximum reservoir
lift as in pipe organ installations, by the use of a supply gate valve.
A feeder pump however, must be allowed to blow off for obvious reasons.
I even once saw a fair organ with a centrifugal blower feeding the pipe
chest directly without any reservoir or spill valve, and the pipe
voicing was steady as a rock!

If the reason is as an academic exercise, then I applaud it.  If it
is to save electricity, then it will take more than a dozen lifetimes
to recoup your expenditure.  If it is to quiet the vacuum pump, then
a lot can be done very cheaply with baffle boxes.  I have a piano with
a large box pump, unmodified by me, which you can hardly hear when
standing next to it even when no notes are being played, so it can be
done.

I hope Roger will keep us informed of his progress.

Nicholas Simons, GB