Inverter or Converter?

I have both in use in my workshop, they have quite different properties. My experience with buying, installing and using them prompted me to write about these differences primarily to help those who have limited funds or space and / or those who only wish to buy one unit to cover all their needs. I have kept things simple technically in order to get the general principles across.

Note that there is no description of the bulky, inefficient and outdated Rotary Converters available to date, which were felt to be outside the scope of this article. I've not doubt that these will continue to evolve and improve as time passes, just as Inverters and Static Converters will.

Static Converters

The Benefits

Converters normally take 240V single phase as an input and give 440V 3 phase as an output. A Converter will therefore run a 440V 3 phase motor from a single phase 240V supply or a 240 / 440V 3 phase motor wired in 'Star' mode (440V) close to its full rated power. (Inverters will only run a 440V 3 phase motor using a 'Delta' connection to around 60% of its rated power (although RPM would be the same at 50Hz) from a 240V supply).

When considering the above, buying a Converter can help you avoid changing the motor on a machine fitted with a 440V only unit. This could influence your choice if the motor you want to run is specially mounted and / or awkward to get at.

Converters are usually built into a cabinet and can be simply plugged into a 240V Outlet. You probably won't need the services of an Electrician to install one as long as the 240V supply you intend to use is up to the job.

Converters are inherently quite robust and easily portable. They often have a 'Lewden' plug fitted to allow easy connection of different machines. An Ammeter is often provided to show the current demand being made by the motor connected.

Converters are often capable of running two-speed 440V motors, and all of the machine's orginal 3 phase control gear (switching etc.) can be used.

The Drawbacks

Converters can be noisy in operation and only provide 2 1/2 phases in reality, not full 440V 3 phase. The third 'phase' produced by a converter is synthesised and not the same as a full third phase that you would get from a regular 440V 3 phase mains supply. For this reason it may be that a connected motor refuses to start, requiring 2 of the connections to be swapped around.

Converters do not (to my knowledge) give you inherent speed control, nor do they give you programmable facilities such as 'Jog', reverse running, soft start (Ramp up) and soft stop (Ramp down) etc.. They are also quite bulky and have to be set to run the motor connected to give the most suitable (normally the quietest) results. Converter controls are placed on the front of the cabinet and do not give the opportunity for a remote control box to be placed by, or fitted to, a particular machine.

The cabinet of a Converter will perhaps include air vent slots for cooling and maybe an internal fan. In both cases these vents should not be blocked. Adequate space should be available for the access and egress of air which often means no 'stacking' of the cabinet amongst other things on a shelf when space saving (a definite Model Engineer trait!)

Converters are relatively expensive - Even when bought second-hand, my 3 HP converter (a very hopeful upper power limit) cost 250 whereas I bought a good second-hand 3 HP inverter for 165. The Lewden plugs and cabling I bought to install my inverters didn't amount to more than 10 per machine.

The Static Converter is quite bulky, although it often has an integral Lewden plug socket outlet (bottom left of the front panel, coloured in red) and is easy to move or install.


The Benefits

Most Inverters give the benefit of 'Jog' and reverse running. Benefits like soft start (Ramp up) and soft stop (Ramp down) can also be gained. These facilities are usually programmable. It helps if you are used to setting modern menu-based things like TV's or Video recorders and have a Computer to run the CD instruction manuals that are commonplace these days. It must be noted that Inverters will only run 240V 3 phase motors (connected Delta) at close to their rated power as opposed to 440V 3 ph motors which would run at reduced power due to the reduced voltage (240V). However, most 3 Phase motors made after the 1970's can be rewired (at the winding connections) and run in 240V mode (Delta). All you need to check is that the motor you want to run is dual voltage (240 / 440V) which is normally shown on the motor's label. Note that this dual voltage motor connection facility is not the same thing as a Dual Speed motor which should be avoided for use with an inverter.

Using an Inverter can greatly reduce the amount of spindle speed changes that have to be made by moving belt / pulley arrangments or changing gears. If a speed / power setting is unattainable by the speed control facility you still have the ability to change spindle speeds to your advantage by using those methods. Spindle speed changes by these 'old' methods still get made now and then but in practice this happens nowhere near as often as it did before conversion.

This 1 HP Inverter runs my Drilling machine, but it can be used to run other machines fitted with 3 phase motors up to
1 HP rated outout via the added Lewden socket outlet.

The Drawbacks

Inverters do not run a motor at the same power level as a Converter would, unless set to supply at exactly 50Hz. This becomes especially clear when running at lower speeds (below 50 Hz) where torque can be reduced. Also note that the benefit of being able to reduce the running speed of a motor can bring new problems - Less RPM can mean less cooling from the motor's integral fan, resulting in potential overheating. It is wise to ensure short duty cycles, periodic rest periods and good ventilation when running any motor at low RPM.

Two-speed 3 phase motors are not suitable for inverter operation.

Inverters are much harder to wire up and set up initially.

I have not seen an Inverter with an Ammeter included in its casing, although they may exist. If required, it may be necessary to add one, introducing further complexity and expense to installation.

Inverters are NOT robust and do not like vibration. Wall mounting the Inverter reduces the risk of vibration damage to the delicate electronics within. It would be very unwise to mount an inverter directly on a machine. Inverters also need their own airflow for cooling, the larger sizes often have their own fan built in. They do not come with a Lewden plug socket outlet because they are designed primarily to be dedicated to run one motor used in industrial applications.

Getting the best from an inverter

The good news is that you can get around most of the Inverter's shortcomings. All of my Inverters are wall mounted and connected for output to a Lewden plug socket designed for 3 phase output. All of my machines have a suitable Lewden plug fitted on the end of their supply cable so I can switch them around or connect other machines at a later date. As long as the Inverter's output is greater than the maximum requirement of the motor then you can run any motor or any amount of motors (one at a time of course) from one single Inverter. It is not necessary to change the settings on an inverter, even if the last motor it ran was 1/2 HP and the next to be run is 3 HP. The existing settings will give similar results for any motor up to the maximum rated power of the Inverter.

My 3 HP Inverter runs a Colchester Student (3 HP motor). A simple plug swap allows it to run my 3/4 HP Drill grinder which stands close by.

Note that my 3 HP Converter did not run the Lathe's 3 HP motor anywhere near as well as this Inverter does, although both units are rated at 3 HP.

The Inverter's remote control box can be a huge bonus. It can be connected by cable long enough for the box to drop into a suitable place and control whatever machine you are working on at the time (say a bracket of some kind) and move it on to another machine with ease. In this way you only need one control box to run any number of machines from one inverter (one at a time). The wiring connecting the control box to the Inverter is very low voltage and presents no significant safety risk, save the ordinary care required where cables and rotating machine spindles are concerned.

The remote control box of the Inverter shows some of the facilities gained.
The box is easily moved around for convenience or can be moved to control another machine which has been plugged in to the corresponding inverter output socket.

Finally, if replacing a motor to run a machine from an Inverter (Machine Mart have very good supplies quite cheap) then get a dual voltage ( 240 / 440V ) model at least 1/2 HP more than the original fitment. This will give much you better slow running torque. It requires more forward planning on your parts, as if you intend to convert any machine to inverter control (say currently having a 1 HP motor) then you'll need to buy a 1.5 HP motor AND make sure to buy an inverter that is rated for at least 1.5 HP. Note that 2 pole motors run at 2880 RPM and 4 pole at 1440 RPM which is something I learned by getting it wrong first off....... Motor spindle diameter must also be considered when replacing one motor with another although pulleys can often be bored out to suit a larger diameter. Lack of spindle length can be a problem sometimes and in that case you may have to make up an adapter.


All things considered, If I could only have one kind of device to run a 3 phase motor it would be the Inverter. It brings with it a lot of extra features as described above. Its shortcomings can be overcome by using the Lewden plugs and socket and fitting a motor of greater HP. That said, only a Converter will run a 440V only motor at close to its full rated power and if you want the benefits of an inverter you may have to change motors for dual voltage models ( 240V / 440V ) However, once this has been done you get all the benefit of speed control and extras such as 'Jog', ramp up and down and reverse running. I actually changed some of the motors on my machines from single phase 240V to 3 phase 240V to get the benefit of speed control (simply marvellous for a Lathe or Drilling machine!). Given Lewden plug connections and the ease of moving the control box around from machine to machine it is entirely feasible to have only one inverter mounted in a suitable place to run all of your workshop machines (one by one).

All of my inverters were bought used in good condition and at very reasonable cost from Gavin Oseman of Malvern. Gavin advertises regularly on the Homeworkshop site found by clicking on the link or on this website's 'Links' page. I can recommend this source with confidence.

Things to be aware of

Inverters can be supplied as 440V input voltage and 440V / 3 phase output or 240V input voltage 440V / 3 phase output. If you buy a 440V input Inverter in error it will not be much use when connected to a 240V supply!

Also note that when buying 3 phase dual voltage (240/440V) motors they can be either 2 pole (2880 RPM) or 4 pole (1440 RPM).

Some contactors inside switch boxes fitted to a 3 phase machines work on 440V only. Even if the machine's motor can be wired to 240V (wired in 'Delta' mode) it does not follow that the contactor coils will work with 240V. I found this out when I wired up my Colchester Student's motor in Delta to run it from the inverter, only to find that it would not run from the lever operated contactor unit. The same goes for 'No Volt' switching units. In a case such as this, the contactor coils must be changed for 240V units or the motor wired direct, although the lever/switch facility will then be disabled.

Following on from that, also note that a machine with a main motor of 440V / 240V might have secondary motors (power feed or coolant pump for example) or a worklight. None of these will work as before (if at all) if given the 240V 3 phase supplied by an Inverter, unless they can be similarly connected / adapted for 240V operation.

One Horsepower (1 HP) in 'old money' is the same as the modern equivalent 0.75 Kw. A motor rated 1.5 Kw would therefore have an output of 2 HP.

Remember that RPM of a motor are directly proportional to the amount of 'Hertz' (Hz) it is supplied with. 50 Hz is standard for motors intended for use in UK which will give roughly 1440 RPM from a 4 pole motor. The Inverter provides speed control by varying the amount of Hertz supplied to a motor whereas a Converter will supply the power at a fixed amount of Hertz (normally 50 Hz). The power developed by a motor (its 'Torque' or twisting force) is quite a separate thing. It is governed by the amount of Voltage and Current supplied to the motor and its maximum power rating. It is therefore possible to run, for instance, a 440V motor from an inverter at 50 Hz giving the same RPM (say 1440) but at reduced voltage ( 240V ) it would only have roughly 60% of its rated power expressed as Torque.

Note that a 3 HP converter may not run a 3 HP 440V motor (either single or dual voltage) in a satisfactory manner. It may not even have enough 'grunt' to start a 3 HP motor, especially if the machine (say a Lathe) is in a high speed gear. If you are in this position you may have to buy the next model of Converter up, which is often 5 HP and a great deal more expensive.

Also note that a dual voltage motor connected in 'Delta' for 240V operation should not be connected to a 440V output Converter until it has been rewired for 'Star' (440V) operation. If this is forgotten, the workshop may be full of smoke pretty quickly!

Remember that whatever choice you make, if the speed / power combination you need is not attainable it is still quite legitimate to try and acheive it by moving belt / pulley arrangements or making gear changes. Often in the case of an inverter, for instance, if the required power cannot partner a low speed, an increase in motor speed (using more 'Hertz') and a decrease in spindle speed by mechanical belt moves or gearchanging will produce more power.

If you intend to get a dual voltage 3 Phase motor from Machine Mart or similar it is likely that you'll need to order it in advance as they do not stock all the models they list in their catalogue or on their website.

Just coming onto the market are Inverters that will produce 440V 3 Phase output from a 240V input. At the moment they are relatively expensive (400 for a 2 HP model). Although the market for them is relatively small (UK etc.) they may decrease in price in the future. If these units become more popular it is likely that the familiar single purpose Converter will have had its day. Faced with the extra virtues of a 440V output Inverter, Static Converters will probably drop in price when new or used. This might make a decision for you if purchase cost is paramount. In any event, if you do decide to go for a 240 - 440V inverter then make sure it is CE marked. If it isn't and interference is passed along the grid, you could end up in a whole heap of trouble.

The usual disclaimers apply. Health and Safety rules should be adhered to by anyone installing either item. If you are not fully conversant with electrical installations of this type, read and understand the recent Part 'P' legislation and get a qualified electrician to install the equipment for you.

Finally, whether you choose one device or the other, and if buying new or second-hand, make sure you get a FULL set of installation and programming instructions (crucial for a programmable Inverter) along with the unit.

I hope this item helps you to make the right choice for your 3 Phase needs.

Steve Sparrow BSc (Hons)

March 2005