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What are the alternatives to a phase converter?

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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.

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 !!

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.

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.  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


  • 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.


  • 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.