It's true that Motora pumps, as found in many small, late UK Duo-Arts,
were not very well made, and they were certainly deficient in that they
would not play more than a few rolls in succession before overheating
-- often apparently terminally!

None seems to have been fitted originally with over-temp protection,
the only warning seemingly being a smoky smell of hot insulation.
They'd probably play even fewer rolls today before catching fire, owing
to their ancient cotton winding insulation, were they not run at our
50 Hz instead of the intended USA 60 Hz.

But in fact they're a competent design and if rebuilt carefully and run
at optimum speed can give about 30" suction and sufficient flow to get
proper reproduction from a well-regulated Duo-Art with a late small UK
stack.  UK Motoras came with toasty variable resistors(!) or transformers
whose ratio was sometimes adjustable, to compensate for the frequency
difference.  Nowadays it's much better to run them on a triac or
thyristor power controller.

But they will still overheat unless rewound -- not a cheap option.
One friend successfully had one Motora rewound to 200 volts (rather
than our then 240 volts) to achieve the right speed straight from the
mains.  We fitted suitable thermal cut-outs to the several Motoras we
played about with.  Like most modern vacuum cleaners, the one rewound
with modern wire never tripped despite playing rolls continuously.

The fact that late UK Duo-Arts with both types of pump are usually
fitted with an inbleed regulator to give a desirable constant reference
suction for the expression system helps, but it's also important to
ensure perfect sealing between the sections of the Motora to prevent
hot air being recycled internally and the suction head reduced.  This
usually means discarding the perished rubber seals and fabricating new
ones from neoprene cord or sponge.

Even perfectly rebuilt Motoras aren't that quiet, despite some
clever design features.  I spent some months discovering how to make
off-the-shelf vacuum cleaner motors effectively silent (which requires
quite a lot of space).  Nowadays it's possible to buy powerful 1400
watt cleaners here for about £25 which are so compact and quiet that
they can be boxed up complete and secreted in the bottom of an upright
Duo-Art, and run just about silently on their built-in speed control or
an external power controller.

Turbines (especially those with forward curvature blades) have somewhat
self-regulating head/flow characteristics and are actually ideal for
reproducing pianos.  Dr Hickman was attracted by the concept, if not
the achieved results.  On Thursday 29 October 1925 he recorded in his
diary: "Pres. of Motor Player Corp called me and said he had made test
that I suggested and all was as I predicted.  At 5000 rpm ten stages
give 40" with 3/8 spill.  He was much elated and they are going ahead
with 10-stage pump."  Interesting that Motor Player's own designers were
apparently less clued up than Hickman.

Nowadays multi-stage turbine pumps are commonly used in industry,
often belt driven by an external AC motor.  The suction head is roughly
proportional to the speed of rotation, the diameter of the fans and the
number of stages in series.  Flow is proportional to the width of the
fans and the number of stages in parallel.  A 10-stage pump as suggested
by Hickman would have been about 7" diameter and 12" long without the
motor, and would have had to be very well engineered and insulated to
be as quiet as a rotary pump.

To achieve high suction with reasonable flow in a small volume as in
domestic vacuum cleaners usually means two stages in series of fans no
bigger than about 5" diameter run at about 20.000 rpm, and the series
wound "universal" brushed motors needed to turn them will always be
extremely difficult to make quiet -- as in the Motora...

Patrick Handscombe
Wivenhoe, Essex, UK