Phase Converter Frequently Asked Questions (FAQ)
Many readers of this website will be familiar with the issue: they have either acquired or are considering acquiring a piece of three-phase machinery yet they do not have three-phase electrical power in their premises. The purpose of this FAQ is to outline the options available to them and the factors they ought to take into account. It is not intended as a technical electrical engineering FAQ but some basic electrical terms are introduced so that it makes sense. The author is an engineer at the phase converter manufacturer Boost Energy Systems but the FAQ attempts to present all the options objectively.
Please remember that this FAQ and website is copyright © Boost Energy Systems, 2004 and you should contact us for permission if you intend to quote more than brief extracts. We would not ordinarily refuse such enquiries and are happy to write articles for magazines etc.
Before reading any further take a look at this table of typical applications for single to three-phase converters. If your project looks anything like one of these then maybe you should talk with us. We are also quite happy to do specials and our phase converters have ended up in some pretty strange uses from Sweden to Spain, the Hebrides to Hull. Follow the menu on the left to phase converter prices and specifications. Remember that at Boost we guarantee we will not be beaten on price.
PROVEN APPLICATIONS |
AGRICULTURE
Pumps / Compressors Corn dryers / Submersible pumps Fans / conveyors Hoists / Wine press Grain mill / Olive oil processor Slurry systems Refrigeration units |
METAL WORKING
Power hacksaws Grinders / Drills Lathes / Milling machines Guillotine MIG and TIG welders Polishers Presses |
WOOD WORKING
Saws / Sanders Spindle moulder Edge benders / Dust extractor Planner thicknesser Copy lathe / Fans Milling machines Mortisser / Router |
GARAGES
Car lifts Wheel balancers Brake testers Hoists / Cranes Spray booths Compressors |
CATERING
Mincers / Saws Ice cream machine Potato peelers / Dough mixers Conveyers / Ovens Packaging and shredding Vacuum packers Air conditioning & refrigeration |
PRINTING
Printing machine Folding machine Stitching machine Paper drills Guillotines |
A.1. What is the difference between DC and AC ?
A.2. What are the different voltages in AC and what does rms mean ?
A.3. What is three phase AC ?
A.4. What voltages are used in different parts of the world ?
A.5. What is the difference between single phase and split phase ?
A.6. Why is three phase power used and why should I buy three phase machinery
?
A.7. What is power factor ?
A.8. What is zero point crossing (noiseless) switching ?
A.9. What are the different sorts of connectors (plugs and sockets) ?
A.10. Can I use my UK phase converter in continental Europe ?
B.1. What are the alternatives to a phase converter ?
B.1.1.
Install
three-phase
B.1.2.
Move premises
B.1.3.
Change the motors
B.1.4.
Install a VSD
Summary of VSD/VFD advantages and disadvantages
B.2. What types of phase converter are there?
B.3. So tell me about static phase converters .. and phase diagrams
B.4. . and now tell me about the problems with statics
Summary of advantages and disadvantages of statics
B.5. So why are rotary phase converters better ?
B.6. How powerful are phase converters ?
B.7. How efficient is a phase converter ?
B.8. How large a power supply will I need?
B.9. Does welding equipment require a larger power supply ?
B.10. What breaker or fuse should I fit ?
B.11. What about line drop, voltage variations, electrical noise and wire
sizes?
B.12. What are bleed resistors ?
B.13. Do phase converters make a noise and where should I put mine?
B.14. Are then any other features to look for in a phase converter ?
B.15. Tell me about customer support and installation
B.16. How long will a phase converter last ?
B.17. Do you have a selection guide for phase converters vs. VSDs etc.?
Table of selection guidelines
C.1. How is Boost different to other phase converter manufacturers ?
C.2. How does Boost keep its prices so low ?
C.3. Where will Boost ship phase converters to ?
C.4. What other products does Boost make ?
A.1. What is the difference between DC and AC ?
Electricity comes in either alternating current (AC) or
direct current (DC). With direct current two wires are maintained with a steady
potential difference between them, measured in volts (V), and when a load is
connected between the two wires current flows from one to the other doing work.
The symbol for DC is straight line above a straight dashed line, which
represents the fact that the voltage remains fairly constant in a DC system. The
system of wires and loads is colloquially known as a network.
However for the vast majority of systems consuming
significant amounts of power electrical networks make use of alternating current
for generation, transmission, and distribution because with AC we can make use
of transformers to obtain different voltages. In a single-phase AC network the
potential difference between two wires oscillates in a sinusoidal manner at a
constant frequency. It so happens that one wire (known as the neutral, N)
normally remains at the same potential difference as ground (literally the
earth, variously designated E or PE) whilst the other wire (known as the live,
L) alternates from being positive with respect to neutral through to being
negative.
A.2. What
are the different voltages in AC and what does rms mean ?
With AC the voltage is constantly changing and so as to get
a single number to use for discussion and calculation purposes, engineers use
normally the root of the mean of the square (RMS) voltage, or occasionally
either the peak, or the peak to peak voltage. With a pure sinusoidal waveform
the voltage that is generally discussed is the RMS voltage because this is
equivalent to the DC voltage that produces the same heating effect for a given
current. So 240V RMS is equivalent to 339 V peak, or 679 V peak to peak and can
be written as 240 Vrms. (the formula is Vrms = Vmax
/ √2). This is illustrated in Figure 1 below which shows a sinusoid
varying about a neutral, and which can also be drawn as a vector with a single
arrow pointing away from neutral. When a load is connected in an AC network the
electrons oscillate backwards and forwards rather than flowing through it as
they do in a DC network. Broadly speaking current is related to voltage and
resistance by the formula V=IR and so in a given system as voltage rises and
falls so will current although there can be a time lag between the two as a
result of capacitance and inductance in the circuit.
A.3. What is three phase AC ?
In a three-phase network there are three live wires called
L1, L2, and L3 and a potential difference exists between each of these and
neutral. If a potential difference of 240 Vrms exists between each
phase and neutral, and if the phases oscillate at the same frequency, and if the
frequency of each phase is offset by a constant and equal amount from each of
the other two phases then the waveform looks like the Figure below. This diagram
can also be drawn as a vector except that now each of the three phases occurs at
0, 120, and 240 degrees around the neutral.
The magic thing is that there is also a potential
difference between L1 and L2, and L1 and L3, and L2 and L3, and simple maths
reveals that the voltage between phases is 415 Vrms and that this
voltage also varies as a sinusoid (the formula is 240 V = 415 V / √3).
This is the system in the UK - see below for other countries.
A.4. What
voltages are used in different parts of the world ?
In the UK the voltage between phases is 415 Vrms
and it happens to operate at 50 Hz whereas most North American systems operate
at 60 Hz. This is how electrical distribution systems are set up so that
three-phase 415 V phase to phase is provided to industrial customers whereas
domestic customers are provided with single-phase (N-L) 240 V from the same
network. In North America and some other countries they also have 110 V
distribution systems, which is simply the phase to neutral voltage of a 220/240
V three phase distribution network.
Continental Europe formerly operated at 380V phase to
phase, 220 V phase to neutral but all of Europe including the UK is now
harmonising around a common voltage specification of 400 V phase to phase, 230 V
phase to neutral. This has been done by changing the tolerances in the
generating and transmission systems, and so there is now more latitude for the
UK voltage to drop and the continental voltage to rise (fortunately we were
already all on the same 50 Hz frequency). The subject of voltage harmonisation
is considerably more complicated than this brief mention, and has aroused some
controversy over various technical details, but rest assured that it is being
done with the best of intentions. It is because of this harmonisation that much
machinery is dual labelled as either 220/240V or 380/415V and in practice even
machinery that is not labelled in this manner is extremely unlikely to be
adversely affected by the change.
A.5. What
is the difference between single phase and split phase ?
The electrical distribution companies tend to erect their
distribution systems on a hub and spoke basis with three phases out to a hub,
and single phases to various users. In order to balance consumption across the
phases they tend to put one phase per street or per apartment block etc. In some
locations they may install two of the three phases - sometimes in order to
balance load across phases and sometimes because they are reusing wiring from
old DC networks. This situation is known as split phase. When they put two
phases into premises it is possible to connect the phase converter across the
two lives in which case the phase converter 'sees' 460/480 V. This has the
advantage that the current consumption of the phase converter will be lower than
for a 220/240 V single-phase connection.
A.6. Why
is three phase power used and why should I buy three phase machinery ?
Three-phase machinery is typically cheaper than
single-phase machinery of the same power rating as it can be more easily located
on the used machinery market through adverts, at auctions or from machinery
refurbishment specialists. The reason for this is that historically single-phase
machinery has only been of low power and so there is, as yet, little stock of
ex-industrial high power single-phase machinery in circulation.
This situation is unlikely to change much in the short or
medium term as most industrial machinery continues to be manufactured using
three-phase motors. Unless exotic designs and/or materials are used it is simply
not cost-effective to manufacture compact high power single-phase machinery.
Single-phase motors of less than 1hp or so are readily
available whereas motors of greater than 3hp (2.2 kW) are seldom single phase.
Moreover, the poor starting torque characteristics of single-phase motors and
their increased cost are such that if a machine has a primary three-phase motor
in it - and therefore can be assured of having three phase power available -
then it is quite likely that the designers of the machine will have installed
three-phase motors in even the smallest drives.
A.7. What is power factor ?
Not all the current in the electrical distribution system
is used for doing useful work at customers' premises. Simplifying dramatically
some of it is simply current that washes back and forth. Power factor is a
measure of how much of the current is doing useful work as a fraction of the
total current - the electricity company would like the power factor to be as
close to one as possible. The power factor is never greater than one and often
is between 0.6 and 0.9 and is sometimes written as cosθ because it can be
depicted as the angle in a triangle that electrical engineers draw. When the
power factor is low the electrical distribution companies have to put in extra
large cables, generating stations, and special power factor correction equipment
to make sure that the consumers at the end of the line still get the power they
need. In some countries consumers who have poor power factors are directly
charged extra for this, and in other countries the extra cost is spread across
everyone irrespective of whether they have good or bad power factors. The extra
generating capacity also ends up contributing to increased pollution, as some
spare capacity needs to be kept in service even if it is not on line.
At Boost we can build phase converters that will improve
your power factor. This makes it cheaper for you as a client, means that your
electricity company will be happy, and reduces pollution.
A.8. What is zero
point crossing (noiseless) switching ?
In an AC circuit ideally one should switch off the banks of
capacitors as the voltage (or current) crosses zero. In this way there is not
only no chance of creating any unwanted electromagnetic emissions, but also the
phase converter puts least stress on the electrical network, and finally the
phase converter itself is subject to least stress. However all phase converters
except for Boosters use electromechanical contactors (relays) to switch the
different banks of capacitors on or off. It is impossible to set up these
electromechanical devices such that they will reliably and consistently achieve
a zero point crossing switching. Even those few manufacturers who try to do this
can only guarantee it when it leaves the factory - and most don't even try
that. The result is that the electromechanical components fail in service which
is expensive (not just the parts but also the service calls and lost production)
and the neighbours get annoyed with the interference. The solution is simple:
buy a Booster !!
A.9. What
are the different sorts of connectors (plugs and sockets) ?
Please look at our accessories page for a full listing of
the different types and an explanation of how to choose between them.
A.10. Can I
use my UK phase converter in continental Europe ?
Yes you can almost always use a Boost phase converter in
continental Europe and we regularly ship units to our clients overseas. It is
best if you contact us first so that we can discuss your circumstances,
especially how best to deliver it to you cheaply.
Before looking at phase converters in detail we ought to
consider the alternatives:
B.1. What are the
alternatives to a phase converter ?
- Have
three-phase power installed.
- Move
to somewhere with three-phase power.
- Change
the motors etc. to single-phase motors.
- Install
a variable speed drive (VSD / VFD).
- Install
a single to three-phase converter.
B.1.1.
Install
three-phase from the line company (utility company)
In many parts of the world three-phase power is installed
everywhere and costs no more than single-phase to have connected. However for
a mixture of economic reasons and because of industrial history this is not
the case in the UK and some other countries such as the Republic of Ireland,
New Zealand, Australia, and the United States. Nevertheless it is always worth
contacting your local electricity company ("line company") to ask whether
three-phase can be installed to your premises and at what price. In the UK the
utility companies will often charge connection fees of £10,000 or more, but
it might be as low as £500 if you are lucky and are prepared to do all your
own site work. Even getting an answer out of a utility company can take a few
months and some perseverance but it is always worth asking. If you do get
three-phase installed you might be charged an industrial tariff and, as a low
consumption user, you might not get a particularly good rate.
B.1.2.
Move
premises
Quite frequently small businesses that are not aware of
phase converters are driven to move premises in order to get three-phase
power. This is probably not an option for people in 'domestic' premises.
Ironically some commercial tenants who pay to get three-phase power installed
find themselves subject to a rent increase because they've improved the
premises !!
B.1.3.
Change
the motors
In fairly simple machinery it is possible to change a
three-phase 415 V motor for a single-phase 240 V motor. The single-phase motor
will be bulkier and this can cause problems if the motor is embedded in a
constrained space. Also a single-phase motor might have insufficient starting
torque - this might not be a problem for most machine tools but is a real
issue for loads such as car hoists and lifts in which the motor must develop
maximum torque from rest. It is also necessary to check the coil voltages
inside machinery - anything that uses motor starters or relay logic will
have electromagnetic coils in them and these might need changing to suit 240 V
supplies. Lastly there could be three-phase heating elements or other
components in some machines that would need rewiring or replacing. All in all
it is seldom cost-effective to change components in complex machinery but it
might be a sensible option in simple, easily accessed machinery.
B.1.4.
Install
a VSD
Variable speed motor drives (also known as inverter
drives or variable frequency drives - VSD or VFD) work by taking the
constant frequency sinusoidal AC input, rectifying it to DC, and then chopping
it into a variable frequency AC output with a fairly blocky waveform. By
altering the frequency of the waveform they are able to control the speed of
the motor, and by altering the nominal amplitude (i.e. the voltage) they are
able to control the power available. Inverter drives almost always have a
microprocessor embedded in them and a simple user interface. In their raw form
they typically appear as per the Figure below. A positive aspect of motor
drives is that they provide a lot of control over the operation of a motor -
one can vary the frequency (which determines motor speed) over a wide and
continuous range, reverse rotation, accelerate motors gradually from or to
rest, and jog motors backwards and forwards.
However all is not perfection and motor drives do have
limitations. Firstly they should only be used to control motors. This is
because the waveform is not sinusoidal and in fact contains quite a lot of
high frequency components that can easily damage any control circuits. Because
of these high frequency components motor drives can also cause radio frequency
interference (which either propagates along the supply cables or is broadcast
to air) and so all motor drives installed in the European Union must have
filters fitted (the North American regulatory environment is more lenient).
Secondly they should only control one motor at a time as otherwise all the
motors will change speed in lockstep with the main motor (as changing speed
means changing frequency and voltage this is another reason why they can
damage any control circuitry). Thirdly although a motor drive might be able to
turn a motor at low speed this can cause the motor to burn out, as the cooling
fan in the motor is shaft mounted and becomes ineffective at low speeds. This
is why motor manufacturers now sell bolt on fans with integral motors and an
independent power supply which mounts behind or instead of the normal fan -
these tend to be quite expensive. From a technical perspective there are
issues regarding the energy efficiency of motor drives - this is why they
have such large heat sinks - but these are unlikely to trouble most readers.
Because of these drawbacks it is typically necessary to
rewire the machine tool to take a motor drive. Firstly the drive itself needs
to be packaged with a filter and a circuit breaker or other protective device,
and often this needs stuffing into a small cabinet with sufficient passive
cooling allowance. Then the drive needs to be wired directly through to the
main motor (bypassing the existing motor stop / start control and any existing
speed controls) as the motor drive can be damaged if the driven circuit (i.e.
the motor) is abruptly disconnected, and so the drive itself must be used to
stop or start the motor (or the start/stop be connected upstream of the
drive). Depending on the exact motor drive used it might also be necessary to
rewire the motor itself or even change it (many three-phase 415 V 'Y'
connected motors should be changed to 'Δ' connected 240 V motors for
use with a 240 V motor drive). Lastly an alternative power supply needs to be
arranged for any other items in the machine tool and these in turn may need
modifying.
If one enjoys rewiring machine tools this can be quite
good fun but one does need to keep an eye on the cost of it all. An
alternative is to buy in a commercial VFD modification kit to suit your
machine tool. A British company called Newton Tesla has standard packages
built around off-the-shelf motor drives to suit the most common machine tools
- as one can appreciate it is not commercially economic to configure a
bespoke package for anything other than the most common as although the drive
itself may be standard for many tools much of the remainder needs careful
preparation. Newton Tesla's packages are highly regarded by some of our
customers.
Summary of
VSD/VFD advantages and disadvantages
Advantages:
- Single
to three phases, a true 3-phase output but at 220V 3-phase. So
motor must be connected into Δ.
- Full
and smooth variable speed control across entire speed range.
- Quiet
operation.
- High
torque down to lowest speed, with automatic compensation with load - to
maintain shaft speed.
- Complete
electronic and thermal overload protection.
- Forward
/ Reverse direction control functionality.
- Generally
very competitively priced vs. a phase converter.
- Very
compact frame size by comparison.
- Excellent
starting torque characteristics.
Disadvantages:
- Motor
must be wired into Δ otherwise will have reduced torque. If an
existing motor cannot be wired into Δ this is a problem, and the
easiest solution (providing you don't want speed control) is a phase
converter.
- Capacity
range 0.2 to 2.2-kW. Commercial single to 3-phase inverters only tend to
go up 2.2-kW / 3-hp. Therefore for particularly large machine tools (such
as bigger Colchester lathes etc) the phase converter is the solution.
- Inverters
are best suited to run one dedicated motor only. Therefore for
applications where one or more motors need to be run, of mixed size,
and/or at the same time a phase-converter is required.
B.2. What types of phase
converter are there?
The final option is to use a single to three-phase
converter. The easiest of these to visualise is a single-phase electric motor
driving a three-phase electric generator and such motor generators do indeed
exist. However this approach is uneconomic for almost all applications and tends
to be restricted to exotic frequency changing problems - for example
converting from a 50 Hz three-phase input to 400 Hz three-phase output. Having
discarded the conceptually simplest we are now left with three other types of
common phase converter: static phase converters, rotary phase converters, and
sine wave inverters (there are some uncommon ones but let's keep it simple).
Sine wave inverters are exactly the same as motor drive
inverters in that they first rectify to give DC, and then invert it back to AC
but on three different phases. The difference is that they add extra circuitry
and so are able to give out a cleaner sinusoidal waveform than motor drives, and
they maintain a constant frequency. They are not manufactured in the UK and have
proven uneconomic to import from the USA where they take a share of the phase
converter market. These could reasonably be termed 'electronic converters'
which unfortunately is a rather ambiguous term that also gets misused to market
variable speed motor drives and static phase converters as well as many other
things. A disadvantage with these is their poor efficiencies.
Static converters and rotary converters are variants on the
same basic design, which has been utilised for several decades.
B.3. So
tell me about static phase converters .. and phase diagrams
In a static converter the 240 Vrms input (phase
to neutral, L-N in the diagram below) is first passed through a transformer to
raise it to 415 V (in these diagrams the transformer outputs are L1 and L3, but
other manufacturers may label their connections differently). Then one 415 V
connection is exposed to a capacitor bank which yields an L2 where L2-L3 is also
of 415 V but which is at almost exactly the same phase angle as the first, i.e.
L1-L2 is almost zero when at rest. When this is connected to a three-phase motor
the inductance of the motor is added in to the circuit and once the motor is up
to speed the 'back emf' generates the voltage L2-L3.
Figure XXX shows the
situation once a static phase converter has the driven motor running at speed
(or that of a rotary phase converter). You will see that whilst the 415 V
equilateral triangle is the same size and shape as that for the electrical
supply from a utility company, it has been rotated a bit and lifted up. This is
because most manufacturers use what are called autotransformers in order to
reduce the cost (at Boost we also do versions with more expensive isolating
transformers which create the same phase diagram as the electricity company -
we think we are the only manufacturer in the world to offer these as a
standard). Therefore, as the diagram shows, only L3-N will be 240 V whilst L1-N
will be 175V and L2-N will be 360 V. This is important because some three-phase
machinery uses 'phase to neutral' as a 240 V supply to drive lighting and/or
control circuits. If so, all these have to be supplied from L3 rather than
randomly choosing a phase. Fortunately in most such machinery the manufacturers
place all the single-phase loads on one phase as typically there is a step down
transformer in the design and this means the manufacturer only needed to buy one
transformer - you can imagine the problems of rewiring a machine where a
manufacturer has carefully balanced all the single-phase loads across each of
the three phases. Similarly - and especially with static phase converters -
it is best to make sure that all the control circuits that use 415 V are fed
from L1-L3 which is the maintained phase and does not vary at all.
It is worth explaining all of the problems that can occur
with static converters and then discussing rotary converters and how far they
overcome the problems.
B.4. .
and now tell me about the problems with statics
Firstly there has to be a motor in the circuit. A static
converter simply cannot produce three phases on its own and so it will only
power two of the three phases used in equipment such as ovens (unless they have
a huge three-phase fan in them). However this point probably will not trouble
most readers unless they are contemplating using welders, wire erosion machines,
or plasma cutters.
Secondly the capacitance in the circuit needs to be matched
to the inductance in the circuit. Unfortunately the amount of capacitance
required will change as the load varies on a given machine, or as different
machines are operated. This characteristic is most pronounced when the primary
motor starts as it will draw up to six times the normal current for a very brief
period and the capacitance must vary accordingly. Because of this the
'boost' circuit was introduced which switches in a much larger capacitance
at motor start (long ago our company was named after this circuit). This boost
can either be manual or automatic. The automatic circuits sense either voltage
or current changes and, in the case of most manufacturers, use electromechanical
contactors to switch in the additional capacitance for a short period. However
in addition to the boost circuit there needs to be more ability to tune the
amount of capacitance so as to match the needs of the motor. Therefore
manufacturers provide a capacitor bank with multiple levels controlled by
another switch so that the operator may manually select the appropriate amount
of capacitance. Manufacturers also provide an ammeter or voltmeter so that an
operator has a guide in choosing the correct amount of capacitance (when the
ammeter reads lowest the capacitance level is probably correctly tuned - in
practice the human ear is a better guide as it can fairly reliably tell when a
motor is running most sweetly). By now readers can probably see a few issues
with statics: they might constantly need to adjust the level setting, and
unreliable automatic boost circuits can lead to chattering contactors which burn
out rather quickly.
There are a number of other problems with statics which
arise from all this. Thirdly, because of the way the main motor is effectively
generating the third phase it will only generate about two thirds of its maximum
torque. Thus it might not reach its maximum speed (especially if it has had very
little margin in the machine tool's design). Fourthly the largest motor must
always be started first. This is probably OK in simple lathes and mills where
operators can choose to switch the suds pump on after the main drive, but
certainly cannot be guaranteed in CNC machinery or indeed in many common
machines with semi-automatic features (e.g. industrial washing machines). Fifth,
if the motor stalls then it will start to 'two-phase' and burn out its
windings more than if it had been on a regular three-phase supply.
This stalling issue is why phase converter manufacturers
dislike selling statics to supply woodworking equipment.
Because of the requirement for the operator to manually
tune the level of capacitance, accurate phase balancing is always doubtful and
so there exists the potential for sensitive machinery to be damaged -
similarly because there is a lowest level of capacitance there is a smallest
size of motor. The bad reputation for statics burning out motors partly arises
from this, and partly because if motors are frequently stopping & starting
only two phases have to bear the load - just at the time when the motor most
needs three phases it has only two available. Thus motors operated with a static
phase converter can easily overheat. This is most pronounced with modern motors
as, unlike older motors, they are designed with very little margin in either
their electrical or thermal properties.
Summary of
advantages and disadvantages of statics
We think static phase converters are imperfect devices -
here is why:
Advantages
- Cheap,
noiseless, compact.
Disadvantages
- Only
really useful for driving motors.
- Require
frequent boosting (chattering contactors syndrome).
- Require
constant manual level setting.
- Only
generate two-thirds torque.
- Largest
motor must be started first.
- Stalled
motors will two-phase and burn out.
- Motors
can overheat especially if frequently starting or of light construction.
- There
is a lower limit to the size of motor that can be used.
If after reading this you insist on buying a static then
please don't say you haven't been warned. In general if you are considering
a static it should only be in an engineered situation such as agricultural
irrigation where all the components of the system have been selected to work
with each other, and where a consistent operating pattern can be predicted.
B.5. So why are rotary
phase converters better ?
After all this you are probably wondering why a phase
converter manufacturer can remain in business for any length of time and still
sleep at night. Well a rotary converter improves things dramatically and so
reputable manufacturers encourage clients to purchase rotaries rather than
statics wherever reasonably possible. A basic rotary converter simply adds a
correctly sized motor to a static converter. The motor has no mechanical
function whatsoever and is simply there as an electrical component to create the
third phase irrespective of what is happening to the driven loads. From a
packaging perspective a motor is basically iron and copper, and a transformer is
basically iron and copper, and there are designs on the market that wrap up the
transformer inside specially wound motors, in which case they are called rotary
transformers (this design is more widespread in the USA). The alternative
approach is to use standard components, in which case the motor is known as a
pilot, donkey, slave, idler, or auxiliary motor and it can either be housed
within the phase converter cabinet or placed separately. For a basic converter
from a technical perspective there is little to choose between using standard
components versus manufacturing rotary transformers (each design has slightly
different characteristics, but other technical factors are more important) and
so commercial factors sway manufacturers to one design or the other. However for
an advanced rotary phase converter there are technical reasons to use standard
components (with some special tweaks) and so at Boost we don't use rotary
transformers.
Irrespective of whether it uses a rotary transformer or a
motor generator a rotary greatly reduces the need to adjust the capacitance and
so phase converter manufacturers can then guarantee a certain amount of phase
balancing across a given operating range - indeed there might be no level
switches at all in a rotary phase converter. Figure XX shows the degree of phase
balancing from a 4-kW rotary converter with no level switches. Nevertheless some
of Boost's more advanced designs do incorporate automatic electronic
'noiseless' switching to finely tune the capacitance and thereby increase
the quality of the phase balancing.
A rotary completely eliminates the problem of having to
start the largest load motor first (or the need to have a load motor at all),
and similarly eliminates the possibility of load motors ever two-phasing.
Provided a sufficiently good quality rotary phase converter is purchased it will
give an output that is equal in quality to the utility company's three-phase
supply.
There is an extra purchase cost associated with a rotary
converter, as the motor and its starter etc need to be provided and transported.
Indeed in general transportation costs are significant for such a heavy and low
volume item as a phase converter, which is why there is little competition from
non-UK producers.
B.6. How powerful are phase
converters ?
An issue with phase converters is their power rating. Up
until a few years ago manufacturers used to place an fairly arbitrary label on
their converters in horsepower and then advise clients to buy a converter "two
to three times the size of the largest motor or larger". Well such a vague
piece of advice is not a great deal of use, and even though some ambiguity will
always arise from the necessary translation of a client's mechanical need into
a manufacturer's electrical rating, most (not all) phase converter
manufacturers have now moved to a system where they label their converters
clearly as to maximum and minimum useful power ratings (in both horsepower and
kilowatts, and taking into account internal parasitic power losses) for typical
single motor and multi-motor cyclical duty. Provided this has been done
correctly by the manufacturer there is generally not much point in buying a much
larger phase converter than they advise, unless you are considering purchasing
larger machine tools in the future. Equally it is unwise to buy a smaller
converter on the basis that you are just a 'hobby' user as the phase
converter is typically most stressed at start-up, which is an unavoidable
element of the duty cycle and because an overstressed phase converter will in
turn stress your machinery and can cause its motor(s) to burn out.
B.7. How efficient is a phase
converter ?
A commonly asked question is what is the efficiency of a
phase converter. Efficiency is a function of the useful power output so when
expressed as a % it can be highly misleading if the useful load is very small.
Instead it is more straightforward to discuss the parasitic power consumption of
a phase converter. A typical 4-kW rotary has a fairly constant 500-W of
parasitic losses (windage, bearing losses, iron losses, and copper losses). So
at full power - supplying a 4-kW load, it is 89% efficient. Clearly as load
falls off this efficiency figure worsens, e.g. to 80% for a 2-kW load, and 67%
for a 1-kW load. This is one reason why it is better not to buy too large a
phase converter as ideally one would like to keep parasitic losses to a minimum.
In the UK the electricity companies charge for Watts
consumed rather than current. This is important because the same 4-kW rotary
phase converter that consumes 500-W when idling will actually draw about 8-Amps
of current rather than the 2-Amps that the parasitic losses would lead one to
expect (500W/240V=2A). The other 6-Amps is simply current flowing in and then
out of the phase converter as the capacitors charge and discharge, and you do
not have to pay the electricity company for it. This is something that causes
many clients confusion as they try to understand why their electricity meter is
not spinning wildly even though their ammeter is reading a large current.
Outside the UK things are often different and electricity
companies may base a proportion of their bill on current. In this case the power
factor of the phase converter is important and in these instances our Boosters
become even more attractive as we can improve the situation for clients.
B.8. How large a power supply
will I need?
Irrespective of how you get your power there needs to be
enough of it. In this section it is assumed that you are using a phase converter
from a domestic supply to power a small detached workshop, but the same points
would apply equally in other locations, or if you had selected a motor drive or
fitted single-phase motors etc.
Any piece of machinery has a normal operating full load
current (FLC) at a given voltage and/or a full load power. So if say it is a
3-hp (2.2-kW) lathe it will draw 9.2 Amps at 240 Volts single phase when fully
loaded and in steady state operation:
2200 W / 240 Vrms = I = 9.17 A single phase
2200 W / 415 Vrms = I = 5.3 A total at unity
power factor and 100% efficiency
5.3 A/√3 = 3.06 Arms per phase which is
what you should find on the motor tally plate
However it will draw 50 Amps or so from the single phase
when it starts up. Exactly how much current it will draw and for how long
depends on the inertia of the load - clearly a lathe with a clutch is an
easier start than a clutchless lathe with a heavy large diameter workpiece (you
are probably now beginning to appreciate some of the challenges the electrical
engineer faces in designing a cost-effective phase converter) and for interest
here is a plot of a 4-kW phase converter starting a 4-kW compressor at the same
time as itself - poor operating practice but this was a worst case test (note
how the peak voltage is ten times the steady state).
So the house's supply will need to be at least 9.2 Amps.
However even if the utility company has provided a 60 Amp supply this might not
be enough if, at the same time as you start your heavily loaded lathe, the
cooker, the washing machine, the tumble drier, and several electric heaters are
all switched on. It is possible to get quite technical about fuseboard
('consumer unit') design and various sorts of load factors so as to achieve
a given probability of not tripping out, but in practice you simply need to
think about the likely domestic situation at the times you are going to use your
machinery and make due allowance.
B.9. Does
welding equipment require a larger power supply ?
In most small workshops only one piece of equipment is in
use at a time, and we have observed that with one exception all the items of
equipment tend to be similar in power consumption, which eases this sort of
calculation. The exception is welders and similar items (x-ray machines, lasers,
wire erosion machines, plasma cutters, etc.) which tend to have about four or
five times the power consumption of anything else in a given workshop. With
welders etc. it is very important to obtain the actual power (kW) consumption of
the item as simply knowing the weld current can be highly misleading unless one
is absolutely certain of weld voltage and duty factor.
B.10. What breaker or fuse should
I fit ?
The power supply to your workshop will have fuses or
circuit breakers (often called miniature circuit breakers, MCBs), which are
rated in Amperes for a given voltage. A circuit breaker is a resettable
electro-mechanical switch that automatically trips if the current through it
exceeds a preset threshold. Because its core component is a thermal or magnetic
operated device it can accept an overcurrent situation for a pre-determined
length of time. Most modern domestic premises will have type 'A' or 'B'
rated circuit breakers installed, which will trip quite rapidly in an
overcurrent situation. However machine tools are industrial items that will draw
an overcurrent at start up for somewhat longer than the average hair dryer and
for this reason a motor rated circuit breaker should be installed throughout the
circuit that feeds the workshop: ideally a 'D' rated breaker but a 'C'
rated breaker might be acceptable. A similar situation exists with fuses. Your
phase converter supplier will advise you as to which rating breaker to install
as they know the time for which a given breaker will hold in, e.g. 63 Amp
'D' - they should not be much more expensive than a domestic breaker.
Returning to the average hair drier for a moment; it has
three components which each have an electrical role to play. The switch is used
to start or stop it in normal operation or if it is failing in service; the plug
is used to disconnect it from electrical power so that one can safely take it
apart; and the fuse in the plug is there to prevent an excessive current flowing
and thereby protect both the user and the hairdryer. A phase converter is just
the same. It needs a switch that can be operated to start and stop it in normal
operation and should be immediately adjacent to it - in converters of up to
6-kW or so it is reasonable for this to be an MCB, and from 8-kW and above this
should be a motor starter. In the smaller converters of 6-kW or less the MCB
also acts as the overcurrent device and in the larger converters the overload
relay in the motor starter fulfils this function.
Converters of 8-kW or above should also have an isolator
fitted to positively isolate the internals when any covers are open, but this
isolator is not a suitable device for using as a starter - it is essentially a
switch that should only be operated with no current flowing as it is simply
there to securely de-energise a section of a circuit. All these components
should either be integrated into the converter by the manufacturer or added into
the total cost of purchase when comparing the prices of the different
manufacturers. If the manufacturer integrates them into the unit then the
purchaser has less wiring to do and the final installation will be neater and
less failure prone. The equivalent of the plug is the MCB at the supply end of
the cable, which has already been discussed in the paragraph above.
B.11. What
about line drop, voltage variations, electrical noise and wire sizes?
The current in a wire causes heating in proportion to the
square of the current times the voltage (known as I2R heating). So
the voltage available at the start of the wire is not quite the same as the
voltage available at the end of the wire. In rural location where a single-phase
line supplies a couple of farms the voltage can drop as low as 200 V at times at
the end of the line (such a large line drop is excessive but instances of this
do exist where the utility companies are being parsimonious to the extreme).
Also the utility company is permitted to vary the line voltage over a given
range - the range was recently widened as part of the European harmonisation
process and will reduce over the course of the decade as the nominal supply
voltage is reduced from 240 V to 230 V. So what you get at your house's fuse
board might not be the steady 240 V you expected - at Boost's premises we tend
to get about 242-243 V but with a slightly clipped waveform. The easiest thing
is simply to measure what you have a few times a day over the course of a week
and make a note of the likely supply voltage. Good quality phase converters are
provided with a range of tappings on their transformers and you can then shift
the tapping in use as appropriate when you install your converter. The reason
that the phase converter manufacturer cannot do this in advance is that there is
a legal obligation to despatch newly constructed electrical equipment with the
230 V tapping selected.
The same line drop phenomenon can occur on a smaller scale
at your premises. Typically the consumer unit is towards the front of the
premises whereas most workshops tend to be tucked away at the back of the garage
or down the garden. So there can easily be a hundred feet or so of cable that
might not be of sufficient size (i.e. cross sectional area) to transport the
required current. However you must not shift the transformer tapping to
compensate for any steady state line drop within your premises, instead you
should install a larger cable !! Remember that an overloaded cable is a long
electrical heater and at some point it is doing the heating in your house and
can act as an ignition source.
In any case it is in your machinery's best interests to
install an adequately sized cable (a cable is the term for something with more
than one wire in it). This is because whereas the line drops discussed above are
steady state affairs, a similar thing occurs over a much shorter timescale when
starting a motor. If there is a constraint in the electrical system, when
current increases voltage will fall - and your motor will not accelerate to
full speed as rapidly as it ought to. We have observed the voltage fall to 180 V
or so when conducting tests at a client's premises where the cable to the
workshop was simply inadequate.
Sizing wires to ensure that you don't have too much
voltage drop is most important in the longer cable runs. A good on-line
calculator for this has been written by Jeff Lucius and is at www.stealth316.com/2-wire-resistance.asp
together with explanations.
To a certain extent a rotary phase converter is better than
a static in locations where the electrical supply is weak. This is because the
motor or rotary transformer acts as an energy storage device (both because of
the mechanical flywheel effect and the equivalent magnetic field storage), which
is available for release during start-up of the driven load. Automatic line drop
compensation is possible but to date has not been economic to provide.
Low frequency voltage / current variations can be
transmitted via the phase converter. These occur because the machine tool is a
pulsating load and, if the electrical supply is constrained, then either the
voltage or current will vary in sympathy. The worst case of this that I observed
was a 15-hp hydraulic blacksmiths hammer with a reciprocating motion at about
two cycles per second. Even though the highly geared motor turned at about 1200
rpm the torque variations were sufficient to cause a noticeable flicker in the
lights at a rather annoying 2 Hz or so which affected the nearby houses. This
was at the end of a very long electrical supply line in a rural location. It is
possible to insert filters to overcome these effects but they tend to be
expensive and it might be cheaper to buy your neighbours a bottle of wine -
and you can be pretty sure that everyone's lights flicker when a washing
machine starts. Converting your machine tool to single phase motors would
actually exacerbate such a situation as it is the machine tool that is the
source of the flicker rather than the phase converter, and a single phase motor
has a less constant torque delivery characteristic than a three phase motor.
The only high frequency voltage components a static or
rotary phase converter is associated with originate from switching the capacitor
banks (for boosting or to adjust power levels). This is an infrequent exercise
unless a fully automatic control system is installed and the load is varying a
lot (or the automatic circuit is failing), or unless a boost circuit is
chattering. If this is the case then the phase converter will probably cause
radio frequency interference, as the electromechanical contactors that are used
are terrible in this respect, but this switching is not a normal operation. In
fact in normal operation a phase converter cannot cause radio frequency
interference (again, unless something is failing) and in this respect they are
superior to inverter drives.
B.12. What are bleed resistors ?
A safety issue is that any motors should either have their
shafts parted or have shaft guards fitted. Other safety issues are that any
ventilation holes should be small enough to prevent childrens' fingers from
reaching through to electrical harnesses. It is also possible for the capacitors
in a phase converter to hold their charge for a considerable time (days even)
and give a very nasty electrical shock. For this reason converter manufacturers
fit warning labels and/or discharge resistors (colloquially referred to as bleed
resistors as they short circuit the capacitors). These bleed resistors are a
potential cause of converter failures and so they might only be fitted on the
larger capacitors.
B.13. Do
phase converters make a noise and where should I put mine?
Most phase converters should be installed in dry locations
where there is no possibility of water spraying or dripping into them, or should
be designed with an enclosure suitable for outside installation. Ideally they
should not be located where metal shavings or metal dust collect for obvious
reasons. Because of static, dust and wood shavings do tend to accumulate
internally, especially in bakeries and carpentry shops, but we have yet to
observe a unit that has actually failed due to this.
Static phase converters make very little noise (just a
transformer hum) whereas rotary phase converters will make the same noise as an
unloaded motor, i.e. the rush of air through the motor fan and a limited amount
of mechanical vibration. The three ways to overcome this noise are to enclose
the motor in a cabinet, to mount the motor on anti-vibration mountings, and to
install the entire converter in a separate enclosure with sufficient ventilation
- under stairs, in the roof space, or in a lean-to are the most common. We
have not seen any scientific noise measurements given by any manufacturer.
B.14. Are
then any other features to look for in a phase converter ?
There are a medley of other physical packaging
considerations including switch location, socket location, cable entry/exit
location, weight, manual handling, split or integrated units, handles, and
delivery. Switches and other controls should be towards the front of the unit,
and should still be accessible if the unit is mounted sideways on due to
workshop space limitations. Any sockets should also be towards the front of the
unit, but cable entry / exit location can be more tricky - in general the rear
or rear/side is preferred but some clients ask for the front. Although phase
converters are quite compact they are fairly dense and so weigh a lot (in fact
checking the weight of a unit is a fairly good way to check the quality of the
key components) and this can create manual handling difficulties. This is one
reason why some customers prefer the slave motor to be supplied separately
although at Boost we are coming round to the view that this is best fitted
internally in units of up to 4-kW / 5-hp and externally on a common frame for
8-kW / 10-hp and above. In any case the units of up to 6-kW really ought to be
supplied with robust handles for general positioning purposes. The weight
affects delivery and all the good manufacturers will ensure that their larger
units arrive at customers using tail-lift lorries.
Finally almost all modern phase converters are blue!
Working independently most have standardised on pretty much the same rather
boring RAL 5017 shade. The other colours tend to be produced for large
distributors who rebrand the products.
B.15. Tell me about
customer support and installation
Crudely there are three stages of customer supports - pre
sale, at sale, and post sale. Few phase converter manufacturers accept on-line
ordering and so you will need to talk to them by telephone or write/email/fax
them in order to make a purchase. In Boost's case we deliberately operate this
policy because we want to ensure that clients purchase a product that will suit
their needs as we have seen too many instances of clients innocently
pre-selecting inappropriate products over the years. Once you have talked
through your situation with a manufacturer they will be able to advise you on
your options and then you will purchase. Following this you will need to be in
contact with them to arrange receipt (these are not packages that should be left
on a doorstep) and often clients make use of the phase converter manufacturer
for telephone support during installation. Lastly you might need support
in-service if a failure occurs, or if you are considering purchasing new machine
tools or even selling your workshop and want pricing advice. It follows from all
of this that the quality of service matters a lot and so you should be
evaluating who is easy to contact, the calibre of their marketing literature
(which may indicate the calibre of their phase converter manual), the duration
and terms of any guarantees, and in general the extent to which they genuinely
support their products. A related point I would like to mention is that many
phase converters are bought at the same time as clients purchase second-hand
machinery. My personal experience is that I spend a lot of time on the phone
talking customers through fault-finding where the outcome is that the machine
tool was faulty in some regard having been bought 'as is' at auction. Whilst
a certain amount of this is reasonable please would clients not look upon us as
a free electrical engineering service, bear with us when we are not familiar
with the intricacies of their particular machine tool, and excuse us if we do
not give this quite such a high priority as our core phase converter business
(it is very interesting for us but makes no money). In this regard the feedback
I am often given is that manufacturers - including inverter drive
manufacturers such as Newton Tesla - all support their phase converters and
clients to a much greater degree than the various distributors or the
second-hand market.
The very small converters (say of 1.8-kW / 2.5-hp) are
simple devices that just plug in to a 13 Amp socket and, unless clients are
completely allergic to electricity, they will be able to install themselves. The
medium sized converters up to say 6-kW / 8-hp will need to be connected to fuse
boards (via an MCB) and have cabling run to the machinery. Many clients choose
to run the cabling and position the equipment themselves, but then use the
services of a professional electrician or knowledgeable friend to make the final
connections. Above 8-kW / 10-hp it is increasingly common for professional
electricians to be more involved.
B.16. How long will a phase
converter last ?
It is worth mentioning that all products have a natural
lifetime. In the case of inverter drives and phase converters these are likely
to be of the order of 5-15 years before the first components start to fail,
depending on usage of course. In VFDs the semiconductor layers eventually start
to delaminate due to thermal effects. Similarly the capacitors and any
electromechanical contactors in phase converters tend to fail due to thermal
cycling.
B.17. Do
you have a selection guide for phase converters vs. VSDs etc.?
The simple table below can be used a guide to select the
most appropriate electrical power solution for your circumstances. Just answer
questions in numerical order until you locate a suggested solution. As with all
such guides it is not perfect but you are welcome to contact us if you need more
help.
The general message is that remotoring and/or installing an
inverter drive might suit the simpler and/or single machine workshops, but that
a phase converter will suit a wider range of machinery. If buying a phase
converter it is almost always worth stretching your budget to a rotary phase
converter rather than a static.
Table
of phase converter selection guidelines
|
About your machinery
|
Only one
three phase machine
|
Multiple
three phase machines
|
No motors
|
One motor
|
One motor
& suds pump
|
Multiple
motors
|
Questions
about your circumstances
|
1. Three phase cheap to install
or easy to move premises?
|
Yes = Install a three phase
electrical supply at your premises or move to premises with three phase.
|
2. Are the motors easy to
replace?
|
|
Yes = install single-phase motor
unless high starting torque required.
|
|
|
|
3. Accurate speed control
necessary?
|
|
Yes = consider a variable speed
inverter drive if a package is available for your machine tool.
|
|
|
4. High starting torque or high
speed required?
|
|
No = consider a static phase
converter (but you will get better performance from a rotary converter).
|
|
|
5. All other cases.
|
It is probably most
cost-effective to install a rotary phase converter.
|
C.1. How is Boost different to other phase converter manufacturers ?
We have a healthy respect for some of our colleagues in other firms. All we
would like to say is that Boost has been in this business for fifty years and is
working very hard indeed to be the most professional phase converter
manufacturer in the world. We are very proud that we have made the first
significant advance in the last decade or so by introducing our range of small
Boosters with a symmetric neutral and many other advanced features as yet
unmatched by any other manufacturer in the world.
C.2. How does Boost keep its prices so low ?
We are frugal people who live simply, work hard, and believe in providing
good products at affordable prices. We also actively invest in research and
development into our products, our manufacturing facilities, and of course our
people.
C.3. Where will Boost ship phase converters to ?
We will ship worldwide. If we cannot serve you cost-effectively ourselves
we will put you in touch with a distributor or colleague who can.
C.4. What other products does Boost make ?
We also make specialist power supplies and supply a small range of renewable
and distributed energy equipment.