3 Phase Converter Frankie's Page


What is a 3 phase converter & why would you want one?

A 10HP Phase Converter

I have a small workshop that I was kitting out with equipment. What I found was that single phase 230v equipment was quite expensive and there was a lot of low quality stuff on the market, most of which was made in China. Through research I found that industrial grade equipment was much cheaper, but there was a catch. This type of equipment was designed to run off 415v 3 phase power, something the small home workshop didn't have. This is a major reason why industrial equipment can be much cheaper than domestic stuff. You can't beat the quality, weight and durability of industrial equipment, so long as you have the space for it (it is normally much larger than domestic stuff).

Well, I had a challenge on the table, how to get industrial equipment running off my domestic supply. Once I could do that, I could just buy whatever equipment I needed.

In order to power industrial equipment, a converter is required. This takes domestic 230v single phase and converts it into 415v 3 phase electricity. I did my research and found quite a few companies selling converters for this purpose. One of the things I needed to decide was how much current I required for my industrial machines. This would govern the size, electrical and physical, of the converter and indeed the price.

One of the first items on my shopping list was an air compressor. As with most stuff, there are different types and sizes. I had previously bought a small 230v 8CFM piston type to run my shot blaster, but it was well under capacity and noisy as heck. I didn't want another piston type. That left me with a choice of rotary vane or screw. The screw type only came in sizes too big and used too much current, so I opted for the rotary vane. The type I chose was a Hydrovane 12CFM model.

I calculated that I would need a converter of at least 10HP, in order to start the compressor under load. The next thing was to decide what type of converter I needed.


Differing types of converter

There are fundamentally three types of converter; digital (known as an inverter), static and rotary. Before I discuss these, I will explain in more detail what the converter does.

The converter has to do two jobs, firstly it needs to convert 230v electricity into 415v. It then needs to create this 415v in 3 phases, each 120 degrees apart. How this is done is different for each of the types of converter.

The digital inverter generates the 3 phase angles electronically and creates the 415v through an inverter circuit (a split mode converter). The static and rotary converters use a regular auto transformer (that's one without a secondary winding), to create the 415v. This also has the advantage of creating two of the required 3 phases, being as they are 120 degrees apart. The third phase is the tricky one to create.

Both the static and the rotary converter use capacitors connected to one of the two phases to alter the phase angle. This becomes the third 'floating' phase. In the static converter all three phases are then connected to the motor of the machine and the third phase proper is created by the motor's third phase. This has the advantage that the converter can be cheap to make, but requires a switch on the converter, which switches in and out differing capacitance values to match the motor's rating. This is a disadvantage and means the operator has to 'tune' the converter to the equipment being used. Also, only one machine motor can be used at any one time and there is a minimum motor size the converter will run.

The rotary converter works in a similar way, but has an additional motor connected to it. This is called a pilot or idler motor. It is this motor that generates the third phase and therefore has to be large enough to supply the required current.. The advantage is that the capacitance is set for the particular idler motor, so it doesn't matter what machine you connect to it, the capacitance doesn't change. Also you can connect many motors simultaneously, up to the rating of the converter, with no minimum motor rating.

I decided to make the converter myself because the cost of buying one was prohibitive and negated the reason for buying industrial equipment in the first place. I can't make a digital unit, these are very expensive and complicated, so I decided to build a rotary converter, because it gave me the greatest flexibility.


Building the converter

The transformer & idler motor

The main expense in a rotary converter is the transformer and the idler motor. I was lucky, in that a friend had a 10HP 3 phase motor lying around, so that cost me nothing. Note that the idler motor is only there to generate the third phase current, so although it spins around, nothing is attached to it. Normally the motor shaft is covered up with a cowl. Despite what you might read, the speed at which the motor rotates has little bearing on the output it generates. If you can get a 950rpm motor, it will be quieter on your ears than one spinning at 2800rpm.

For the true die hard misers, you can use a transformer from a MIG welder that is dual voltage i.e. 230v and 415v. Later on in my converter making life and I have made many converters, I used such a transformer because the welders can be bought cheaply. However, they take a lot of adapting, especially being as they have a mechanically moving core, which adjusts the voltage of them. If you want a good quality converter, buy a transformer. I did just that and it's well worth the money.

3 phase idler motor

The circuit

I have over the years devised a most comprehensive circuit diagram which I will share with you here. One of the problems of building a converter is how to start the 3 phase idler spinning, when you only have 2 phases from the transformer. I have devised a start circuit using capacitors, that will work for small motors and large (10HP+). With large motors, the motor has to be started in STAR, because of the current drawn and then switched to DELTA when running. Other methods I have seen are starting the idler from a single phase motor with a belt & even using string around the pulley to start the motor turning. I prefer a fully automated circuit for ease of use and safety.

The operation of the converter is simple. Turn on the main isolator and the 2 phases are connected to the motor. At the same time the start circuit capacitors are connected to the 3rd phase via a contactor operated by a delay timer which is now active. After a delay of about 1.5 seconds, the delay timer shuts off and disconnects the start capacitors, by switching off the contactor. The motor will be running at full speed by now.

For STAR/DELTA motor starting, the operation is the same, except that another set of contactors connect the idler motor into STAR mode, then the start capacitors are connected. When the start capacitors are disconnected, the idler motor is connected into DELTA mode. Another timer relay is used for this purpose.

Phase converter block diagram

With reference to the diagram, there are three features that I haven't mentioned; 220v 3 phase motor operation, using the start capacitors as run capacitors and the use of an output contactor.

220v 3 phase motor operation

Up to now, I have only discussed 415v 3 phase operation. However, some 3 phase motors are dual speed and as such operate at 220v or 415v. To achieve this the motor is configured in STAR and DELTA mode respectively. What this effectively means is that if you have a dual voltage motored machine, you can make a phase converter for it that does not require the expensive transformer. In order to achieve 220v operation, the motor terminal plate is removed and the 3 links that connecting the motor in DELTA need to be removed and just one link being put back, to make the motor operate in STAR. The switch gear, if voltage conscious, such as contactors on the machine also needs to be replaced with 220v equivalents.

Start capacitors as run capacitors

The larger the idler motor, the higher the capacitance required to start the motor initially and to run the motor. I have a stock of 30uf capacitors and just add these in parallel, in order to get the correct capacitance. Capacitors are not cheap and they also take up space in the converters box, so I always endeavour to keep the amount to a minimum.

The start capacitance for my 10HP idler motor was 180uf, which is six capacitors. It required 120uf (4 capacitors) across L3 to L1 and 60uf (2 capacitors) across L3 to L2, in order to get the 3rd phase angle correct. Clearly a lot of capacitors! I obtain my start current from the L1 side of the transformer, so my start and largest run capacitor bank are fed from L1 (10 capacitors). What I did though is to couple all of capacitors together and put a contactor in between the point where the run capacitors are required. Thus, in the off position and the run position, L1 has 4 capacitors connected to it (run mode). When the converter is first switched on and the delay timer is active, this start contactor is energized and just has to connect 2 extra capacitors (60uf), instead of connecting all 6 (180uf). I have therefore reduced to capacitor count in the converter. This is more clearly seen in the following circuit diagram.

Output contactor

When initially starting the converter, it is important that there is no load attached to it, otherwise your capacitor bank will be trying to start the load as well. Usually this isn't a problem, because any machine connected to the converter is isolated by its own on/off switch or contactor. But to be on the safe side, I have included an output contactor which only connects the load to the converter once it has started.

The main circuit diagram

Main Phase Converter Circuit Diagram


The motor is shown in STAR configuration, this would normally be DELTA, it's just the way I drew it.


If you have a welder that is labelled 3 phase and you want to build a converter to run it, check that it is actually requiring 3 phases and not just one. Many welders, especially arc welders are actually 415v single phase. By checking the power plug, if all 3 phases are connected then it is 3 phase, but if only 2 phases are connected, then it is single phase.

A single phase welder converter doesn't need any capacitors or an idler motor. It can be connected directly to the output of the 415v transformer. A quieter and cheaper solution, but obviously you can't run 3 phase motors off of it.

Inside Converter




After a while, I found that I needed a larger converter, so I built a 20HP unit, which used many more capacitors and a very large 3 phase transformer. I rewired the internal connections of the 3 phase transformer from 415v input and 220v output, to give me a 230v input to 420v step up output. It wasn't just a case of swapping the input and output around! The input coil is lower current than the output coil. I used all 3 coils to give me 420v out of the normal 220v output. Note that 3 phase transformers have 3 electrically separate coils on one shared iron former.


Created and designed by Frankie Miller ©2008