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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. About Three Phase Power

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. About Phase Converters

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. About Boost Energy Systems

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. About Three Phase Power

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.

B. About Phase Converters

Before looking at phase converters in detail we ought to consider the alternatives:

B.1. What are the alternatives to a phase converter ?

  1. Have three-phase power installed.
  2. Move to somewhere with three-phase power.
  3. Change the motors etc. to single-phase motors.
  4. Install a variable speed drive (VSD / VFD).
  5. 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. About Boost Energy Systems

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.

 

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