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ELECTRICAL
MATTERS
Conversions : 3-phase : 1-phase : Inverters
If you have any doubts about
your ability to undertake work connected with electrical installations, you
should employ an electrician, qualified to work on machine tools, to do the
job for you; you will find it money well spent.
For excellent advice on how to use electric motors in the
small workshop you need a copy of "Electric Motors in the Home Workshop".
This will show you how to identify and use motors from industrial and
domestic sources and how to operate 3-phase industrial motors from a 1-phase
domestic supply.
For more advanced instruction the book "Electric Motors" is
recommended. This was written with the amateur user in mind and covers motor
types, characteristics, operation, installation, speed control, braking and a
host of other matters - as well as advice on using and adapting surplus
motors from industrial sources. To find out how to prepare a complete
workshop for electrical installation try "Workshop Electrics"
Machine-tool switches are often a problem, especially on older examples where
the parts are obsolete or don’t meet current regulations. We can supply a reversing and on/off switch that
will meet the majority of requirements for motors up to 5 h.p. and can be
easily wired to replace built-in switch gear that has become unreliable.
Other switches to control 2-speed motors and various kinds of push-button
starter can also be provided. Just email
your requirements and we’ll try to help
Nearly all larger lathes are equipped with 3-phase motors
and to use them without a 3-phase supply requires either the motor to be
changed, or the use of a phase "converter" or "inverter".
Plug a converter into the single-phase household supply and it will produce
an output capable of running a three-phase motor; an Inverter does the same
job but the output can also be varied to provide a speed-control effect.
Besides getting the machine to work with the minimum of effort, combining an
inverter drive with the machine’s own belt or geared spindle drive will, of course,
have the added advantage of making an even greater range of speeds available.
An inverter conversion recently seen on a Boxford lathe (a South Bend clone)
had the bottom speed reduced from 40 rpm to 5 rpm - and the top speed raised
from 1450 rpm to the 2000 rpm - the same as the now very hard-to-find version
fitted with the maker's original "high-speed" pulley set. A word of
caution: many inverters have an “overspeed” function than can allow machines
to be run at speeds far in excess of the maker’s intentions. Lathe chucks are
always limited in how fast they can be spun; being made of cast iron they
will eventually burst and the clamping force of the jaws weakens as the speed
rises.
Using a "Converter" or "Inverter"
normally allows you to connect directly to the machine’s motor and by-pass
the original electrical system with control of stop, start and reverse being
by the inverter itself. However, it is important to both follow the maker’s
instructions on how the connection should be made and also to be aware that,
in connecting directly to the motor, you may well be stopping the action of
safety features – for example, when opening a door or moving a handle would
normally have stopped the motor. The inverter, once installed in the workshop
can also be used to run other three-phase machine tools - a drill, milling
machine, saw and shaper for example. A 'converter' can, within the limits of
the maker’s instructions, be wired to supply several motors simultaneously;
an 'inverter' can also do this but, as you vary the output to change speed,
the other motors connected to it will change speed as well. If you plan to
use a machine tool equipped with auxiliary motors (to power a coolant supply,
or milling machine table drive, for example) it is worth bearing in mind that
it would be possible to use an inverter for the main drive motor, and a
separate (small and relatively inexpensive) Converter to run the auxiliary
motors.
If you intend having several 3-phase machines in your workshop a
"converter" or "inverter" is almost certainly a more
economical way of running them than changing the motor on each; additionally,
some machine tools have motors so tightly integrated into their construction
that finding a single-phase motor to replace them is impossible - and, unless
you take out a bank loan and have the original motor reconstructed as a
single-phase unit, using a 'converter' or 'inverter' is the only practical
solution.
An important part about the conversion - and often overlooked - is
that 1-phase to 3-phase converters or inverters usually produce 3-phase
output at the input voltage. So, if you want to run a 3-phase 440 volt
machine, you must reconnect the motor in 240 volt "delta"
configuration (as distinct from 440 volt "star") which often
involves just following the simple instructions printed inside the lid of the
terminal case - but can sometimes involve first removing the motor from some
dark, oily and inescapable corner of a cabinet stand and having the local
motor-rewind shop conduct internal surgery on it. However, some of the
recently produced converters and inverters units have a transformer built in
(though at considerable extra expense) to provide a 440 volts (or other)
output, which of course can save a huge amount of time and frustration - you
just configure the inverter, hook up and switch on.
Today inverters have fallen in price dramatically (ranging in price from around £90 to £250 covering motor
from 0.25 to 4 h.p. and larger) and if the 3-phase motor on your machine is
in good order it’s now hardly worth changing it for a single-phase unit. We
stock inverters by SIEMENS - please Email
for details and the latest prices.
Changing Motors
Should you decide to change a 3-phase motor for a 1-phase one some thought is
necessary to get the best out of the conversion. It is as well to bear in
mind that a single-phase motor is not the direct equivalent of a three-phase
type. A 1-phase motor is best run near its rated capacity all the time (i.e.
worked nearly flat out); if the motor is switched on and off frequently
against “no load” the windings will be damaged and, if run through a cycle
where it is started, worked briefly, stopped and started again, the capacitor
will fail prematurely.
A machine tool with a one horse-power 3-phase motor should, in theory, run
just as well with a one horse-power, 1-phase motor - but, life's not like
that. If you need your machine to perform as well as it did with its original
motor (and it is by no means certain that you will) you may find that, for
power for power, for ease of starting and long-term reliability, a successful
and reliable conversion to 1-phase requires a motor which is marked as being
some 30% to 50% more powerful
When changing the motor(s) leave the original 3-phase wiring and switchgear
as intact as possible and store the old motor safely inside the machine’s
stand - where it should be impossible for it to be lost. Do not waste time
trying to modify any of the original electrics; it is much simpler, and a lot
safer, to fit new wiring, a new switch and a "no-volt" safety
cut-out. Should the machine ever be put back on 3-phase,or wired with an
inverter to give a variable-speed drive all you (or the next owner) needs to
do is bolt back the original motor - and hook up the wiring.
If you
have any doubts about your ability to undertake work connected with
electrical installations, you should employ an electrician, qualified to work
on machine tools, to do the job for you; you will find it money well spent.
Motor Power - a Word of Warning
A note of caution might be appropriate here for the owners of smaller machine
tools. If you are going to replace the electric motor on your lathe, miller
or drill, etc. - think carefully about on how powerful it
needs to be. Modern motors are very compact - and it is now all too easy to
fit a massively powerful unit in a tiny space.
If you have a "dig in" when turning, milling or drilling - or other
accident - instead of coming to a dead stop as the load overcomes the motor
power, the machine may continue running and do itself, and you, serious
damage.
I remember a little Grayson lathe that the owner had converted to chain drive
via a motorcycle gearbox. He hooked it up to a 1 h.p. motor from a Bendix
washing machine and revelled, for a time, in the slip-free, powerful drive he
had constructed. When, as was inevitable one day, he allowed the cutting tool
to run into the chuck the lathe failed to stop. Indeed, the irresistible
forces being so efficiently transmitted by chain ripped the entire headstock
assembly from the bed and proceeded to smash it to pieces as it hurled, at
great speed, round and round the countershaft unit.
For lathes up to 3.5" centre height it is surprising
what a 0.25 h.p. (one-quarter horse power) motor can achieve. 0.33 hp
(one-third horse power) is probably a safe compromise (that’s the size Myford
fitted for many years to their ML7). Anything over 0.5 h.p. (half-horse
power) and you need to be aware of the fact that you are driving a powerful
little beast - that can have a savage bite.
Lathes between 3.5" and 5" centre height often
have more complex drive systems - with the motor inside the cabinet stand -
or with variable-speed drive, and require more power to overcome the
frictional losses in the transmission. Unfortunately, modern motors do not
seem nearly (subjectively) as powerful as their rating would suggest,
especially the more affordable ones, and the starting characteristics of
single-phase motors often demand an excess power rating to get the spindle turning
- especially on top speed and when no clutch is fitted. Thus, it becomes
difficult to say exactly what size of motor you should fit to obtain the best
compromise between starting, turning performance and safety. The maker's
original advertising or maintenance literature will
provide a guide of course, or, if that is missing or unobtainable, there is
often a plate on the machine that lists the original electrical
specification. If the only clue is the existing 3-phase motor, replacing it
with a single phase one of the same nominal horse power will almost certainly
leave the machine underpowered; instead, as previously advised, something 30%
to 40% more powerful would be a good starting point. Single-phase motor can
often be obtained in two types: low starting torque and high starting torque
with the latter often referred to as “capacitor start - capacitor run”
(cap-start-cap-run) types. The high-starting torque variety, though a little
more expensive, are to be recommended and are generally more reliable where
the motor might be switched on and off frequently.
There are a couple of very useful books to help sort out motor problems:
"Electric Motors"
and "Electric Motors in the
Home Workshop".
If you are in the UK we can supply a range of good quality electric motors
from 0.25 to 4 h.p. and a selection of (often hard-to-find) reversing,
2-speed control and other switches. Email
for details.
Belts
If a motor is changed, or the drive system otherwise modified, ensure
that the pulleys and belts receive proper attention. Pulleys, preferably in
cast iron (for grip), should be a close fit on their shaft, secured with a
key, locked securely and run without wobbling. If the pulley has to be
changed, and the correct replacement cannot be found, the solution lies in
the industry-standard but more expensive “Taper lock” type. These are made in
two parts: a split centre, tapered on the outside, and an interchangeable
outer rim held on by 4 bolts. The tightening action of the bolts compresses
the inner section to grip the shaft. The locking force to the shaft is so
great that no key is required.
Whether V or flat, belts need to run in line and the easiest way to check
this is to lay a long steel straight edge across the full width of both
pulleys. Assuming the pulleys to be of the same type - and thickness - the
edge should make contact at all four points on the rims. The drive belts also
need to be in good condition. V-belts often wear in patches (or develop hard
and soft spots through standing) and when the different sections run over the
pulleys they either “fall into the
groove” or “climb the hill” and so continuously alter the drive ratio. This
speeding up and slowing down of the drive results in a cyclic motion that can
create interesting vibrations, especially at higher speeds, with the result
that marks are passed to the workpiece and show as vibration bands. New belts
are inexpensive and can make a remarkable difference to the smooth running of
a machine tool.. A supply of belts – V, flat and round, can be found here.
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