Marvin Horovitz (MMD 10.7.20) expresses a wish to slow down the drive
on his Tangley calliope without changing pulleys. As someone who spent
many years in the electric motor business, I think I can shed some
light on the matter.
First of all, we need to be sure what kind of a motor it is: either a
'universal' (AC/DC) series wound motor which has brushes which contact
an armature, or an induction motor. Good examples of the universal
motor are vacuum cleaners and the suction boxes which are often used
to supplant pedal power on automated players. Such motors will change
speed drastically under different load conditions, like speeding up
when you block off the suction to the vacuum.
An induction motor will have a nameplate R.P.M. which approximates one
of several standard speeds, to wit: 3600 (3550), 1800 (1750), 1200
(1150). The first number in each case represents synchronized R.P.M.
on 60 cycle AC, the second number reflects 'slippage' between
synchronized speed and actual output R.P.M.
Synchronized motors have specially built rotors which cause them to
rotate 'in sync', and there aren't very many of them.
Okay, now that we know which type of motor we are using, we can select
a method of motor control. Up to a certain point, universal motors can
be slowed down with a light dimmer, but only very low power motors will
work without smoking the dimmer. After that, you need a variable
transformer, such as a Variac. If you poke up 'Variac' or 'variable
transformer' on eBay several offerings will pop up and you need to
start matching up specs such as 'voltage' and 'amperage'.
There are probably electronic devices by now which do the same job, but
I've been out of the industry for too long to be familiar with them.
I think some of them are called, actually, 'electronic transformers'.
In the case of an induction motor, you need a variable frequency drive,
or VFD. Until recently these things were so expensive that changing
pulleys was a much more attractive notion, but now the costs have come
down considerably. Again, if you look for 'VFD' or 'variable frequency
drive' on eBay, many examples will show up, and you will have a good
idea of what you are looking for. You might be able to find such
devices locally for a decent price in some industrial supply house
if you live in a big city, and you might not find anything to beat
offerings on the Bay. So There! Hope that helps!