Hi Bob,


It's really far simpler than all that, at least for Unimats and similar
sized machines.


Yes, series wound universal motors have poor speed characteristics, but
are adequate for these little machines. Way back in the forum archives
there are examples of people using sewing machine motors quite happily,
whether standard ones with mounting difficulties, or overlocker motors
which are more or less a drop in replacement. Rewired to have a separate
field supply, and using something like a Variac to control the armature
voltage might give a good characteristic, but who has that sort of stuff
kicking abut in their garage? Well I do, but I've never bothered to try
it out!


Constant speed is not a requirement. A bit of give in the motor makes it
easy to judge how hard the lathe is working. This applies to both small
machines and larger lathes using standard induction motors. If you give
the operator a precise constant speed he will lose a useful bit of
feedback from the machine. Then he would need to watch the ammeter, or
some equivalent gauge, and that means diverting attention away from the
actual cut. Auto production machines used ammeters to give an indication
of tool wear, and similar problems, but for a hobby machine, of whatever
size, it is not needed. Bear in mind I run Unimats at home, a large
lathe (1 ton weight, 3 HP motor) in my brother's workshop and have
access to bigger machines in a friend's industrial machine shop. The
small variations in speed when working are not a problem - quite the
opposite.


The permanent magnet DC motors currently available are fine for the
Unimats, and I would look at them for other small machines too. They are
rated at much lower speeds under no load, but because of their very
different characteristics actually deliver a similar range of speeds
when working. Running on a simple unregulated power supply I get a
reasonably constant speed and lots of torque - a much better
characteristic than the original Unimat motors. Using a PWM speed
controller, with no feedback, the speed remains constant enough for our
purposes. The motors run cool enough that I can stop worrying about
overheating, and they are nice and quiet. I've used my SL for many hours
with one of these, with no problems, but that lathe is now in Australia.
I shall be fitting a similar motor, maybe a 150 W this time, to my U3.
Re-motoring the PC/Basic is a different exercise, since that will
include fitting modern high torque steppers for the NC movements.
Including programmed speed control and provision for NC screw cutting
may take it into a different realm of complication.


The whole question of speed control on RC motors is muddled by the Kv
factor. The Kv only tell you the voltage needed to achieve a given
speed. The motors themselves (all the ones I've dealt with) are three
phase motors with a synchronous permanent magnet rotor. The speed is
defined by the drive frequency, so constant speed working is simple. The
current available determines the maximum torque and the Kv, along with
the supply voltage gives you the maximum speed, at which the torque
falls to zero. If you have plenty of voltage in hand the motor will
maintain a constant speed under varying loads. The problem comes when
motors are specified by the size of prop they suit, but it is getting
easier to find fuller specifications giving the more usual definitions.


So for the little Unimats, and for some bigger machines, I find the DC
permanent magnet motors are fine. They only need simple controls, they
are cheap to buy, and fit in the available space. Mounting them can be a
problem, but with careful planning can be dealt with using the Unimat to
make the parts before changing the motor. If you have access to a
slightly larger lathe it is simple.


There is no point in doing anything more complicated.


Regards,



Keith


On 2021-09-09 00:10, Bob T wrote:

Hi Keith,
? ? ? ? ? ? ? I have a unimat 3 and also being an electrical engineer
and having two other bigger lathes (a MD65 and a Boxford) I have tried
most things to drive my lathes. In the end what you want is a drive
that doesn’t change speed when you are cutting. In other words a
“constant speed” drive. Most small motor drive solutions are not that.
universal ac motors (that the unimat uses) are series connected motors
and are quite sensitive to loading. Adding a pedal will tend to make
things worse. Using a “light dimmer” circuit to reduce voltage ?and
thus speed is ok….. but doesn’t improve things. Using a DC motor is
better since (unless connected as a series motor) it will try to keep
a constant speed as load builds up on the lathe. A tread mill motor
and its drive uses an optical sensor/encoder to digitally sense the
speed and control the excitation to the motor so in theory it’s a very
good speed control. It’s not that good being a very slow response and
relies on a large flywheel to smooth out load disturbances. Recently
PWM controllers for DC motors and tread mill motors have been
available. Some of the good ones (the $50 dollar types) have a speed
reference pot and digital control systems that measure the back emf of
the motor and control that. Those work well and I have one on my MD65.
For my unimat I wanted better and built my own drive with a DC motor
and drive controller that used a tachometer (small dc motor) mounted
on the main motor shaft to measure speed and control the drive to keep
constant speed. My larger lathe uses a VFD on to a small (1kW) AC
motor. The VFD has an speed reference pot and puts out a frequency to
match the pot setting. The motor has-to try and follow that frequency
(it can’t do anything else) and so keeps a good constant speed.
The modern RC drives are very good for what they do but, they are
?torque drives whose speed will change as the load changes……fine if
you have propeller as a load but not so good for a lathe. To make one
to suit a lathe you would need to add a speed feedback circuit like I
have done on my unimat, to convert the torque drive to a speed
control. Do-able but not for the faint hearted !
I hope that, this clears up a lot of the ideas that have been floating
around in the group.

Regards Bob