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ELECTRICAL MATTERS variable-speed Inverter drives–
Books: Do
some homework - 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" Switches: 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 The Basics of 1-phase to 3-phase Inverters (not
“converters” – they are covered here) Nearly all large machine tools are equipped with
industrial-type 3-phase motors and to use them without a 3-phase supply
requires either the motor to be changed for a 1-phase type - or a phase
inverter connected. The latter is now the preferred choice: plug an
inverter into the single-phase household supply and it will produce a current
capable of running a three-phase motor with an output able to be varied to
change the motor speed. In addition, various other things can be arranged,
including: · “Soft”
start · Electronically
controlled, safety quick stop · A small
remote-control unit placed where it can be reached with the greatest ease and
safety. In addition to getting the machine to work with the
minimum of effort, using an inverter in conjunction with the machine’s own
belt or gear-driven spindle drive will give an even greater spread of speeds.
For example, an inverter conversion recently seen on a Boxford lathe altered
the bottom speed from 40 to 5 r.p.m. and the top from 1450 to 2000 r.p.m. -
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 “over-speed” function than can allow
spindles to be run at revolutions far in excess of the maker’s original
intentions. In addition chucks are always limited in how fast they can be
spun, especially large ones; being made of cast iron they will eventually
burst and the clamping force of the jaws also weakens as the speed rises. Some practical aspects of using an inverter Normally
the output from an inverter is connected directly to the machine’s motor and
not to the normal power-in box on the side of the machine. Control of stop,
start, reverse (and jog) passes from the machine to a panel on the inverter –
or a remote supplied with it. However, it is important to both follow the
maker’s instructions on how the connection is made - and one should also to
be aware that, in connecting directly to the motor, the action of safety
features will be overridden – for example, when opening a door or moving a
handle that would normally have triggered a micro-switch to cut the current. Once
installed an inverter be used to run other three-phase machine tools within
its power limits - drills, milling machines, saws and shapers, for example. If
you intend having several 3-phase machines in your workshop an 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 an 'inverter' is
the only really practical solution. Important facts about the conversion Most
3-phase motors of recent years are dual-voltage marked with something like
220/240-380/440). An ordinary 1-phase
to 3-phase inverters produces an output voltage equal to the input, i.e.
220/240V and so you must make sure that the motor is set correctly. Following
the simple instructions printed inside the lid of the terminal case, and
simply arranging links between the wiring terminals, will normally allow this
setting to be made. “Advanced” 240V Inverters This
latest development offers all the functions of the basic unit and are known
as “Advanced Vector Inverters” They have the advantage of allowing control of
the motor speed without loss of power. On a “basic” inverter the power of the
motor reduces somewhat as the speed falls, but with the latest vector type
this does not happen - and that's good news for applications like lathes
etc., where torque will be maintained down to very slow speeds. In addition
the Advanced type is able to interface to 3-wire control systems, for example
normal green “on” and red “off” push button switches - and this also makes it
possible to connect the inverter to a machine’s existing controls. Advanced
inverters are also suitable for use with computer control on CNC machines and
can be bought in versions from 0.5 to 5 h.p. and are available with a 240V
1-phase input and a 415V 3-phase output. 415V Inverters for Motors
marked as high voltage only and dual-speed 3-phase motors If
your 3-phase motor is marked only for 400+ volts operation, or is a
multi-speed type, you will need an inverter with an output voltage to suit.
These are now available, and offer the same functions as the basic and
advanced unit making them especially useful for machines such as the
Bridgeport miller where a 2-speed motor is built into the upper frame of the
vertical head. Although
lathes.co.uk used to supply inverters, we
now recommend dealing with a specialist in the field - Drives Direct. This is a company with an excellent
reputation who offer a range of
high-quality, reasonably-priced units – and, even more important than the
price, a full back-up service from 10:00 to 20:00 should you encounter have
any problems with the installation. Click on this link to read a fuller
article about inverters Installing a 1-phase motor in place of a 3-phase 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 continuously near its rated capacity (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. A
machine tool with a 1 h.p. 3-phase motor should, in theory, run just as well
with a 1 h.p. 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 run to the
same top speed without struggling (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 – i.e. where you can misplace it. 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 replace 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 the chain
ripped the headstock assembly from the bed and proceeded to smash it to
pieces as it hurtled, 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” or, in the vernacular, cap-start-cap-run.. The high-starting torque variety,
though a little more expensive, are to be recommended and are generally more
reliable where the motor needs to be switched on and off frequently. There
are a couple of very useful books available to help sort out motor problems:
"Electric Motors" and "Electric Motors in the Home
Workshop". . 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 four 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. 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 causes a cyclic
speeding up and slowing down of the drive and can result in the most alarming
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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Detailed advice is given on how to identify and make good
use of discarded and surplus motors from domestic and industrial sources -
and also how to operate 3-phase motors from a single-phase supply. Lots of
money-saving advice. |
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Principles, characteristics, operation installation, speed
control, braking, etc. Generators, safety considerations, testing and a
useful section on identifying and using 'scrap' motors. An excellent primer
on the electric motor. |
