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Choosing Your Digi-Phase Converter

This page aims to help your selection of a Digi-Phase digital rotary phase converter. In particular, which model might work best for your machines and some things to look for as you compare models. We will try to make you as well informed as possible in a short time, however please feel free to contact us with any technical questions before you make your final selection.


The magnetic field that causes a motor to rotate is produced from the electricity supplied by the power company, called the utility in many countries. Electricity is measured in terms of voltage and current. Using the analogy of water supply, voltage is analogous to pressure, and current is analogous to flow. Voltage is measured in Volts (V). Current is measured in Amperes (A). Power is voltage multiplied by current, measured in Watts (W) or thousands of watts, kilowatts (kW). One kW is equivalent to 1.34 horsepower.

Electricity is distributed as Alternating Current (AC). Whereas a battery has two terminals, one that is always positive (+), and one that is always negative (-), AC changes, or alternates, from positive (+) to negative (-) at a set frequency, usually 50 times a second (50 cycles per second or Hertz or Hz) in Europe and Australasia, and 60 times a second in the US and Canada.

At the power company's local distribution transformer, voltage is reduced from sometimes 11,000V to either 230V or to 3 phases, each 400V (Europe) or 230V between phases (US). There are still some variations found in different countries, e.g. 220V and 240V phase to Neutral in Europe or phase to phase in the US. US and Canadian industrial three-phase voltages are 3x 460 to 480V between phases.

In an electric motor, as the electrical polarity on the AC line changes (from + to -), the magnetic poles in the motor change from north to south in relation to the rotor poles, causing the motor to rotate. With each change in polarity the voltage rises and falls as a wave, passing through zero voltage, called a zero crossing point. Each time the voltage rises, either above or below zero crossing, the motor receives power, much as a motor vehicle receives power each time the engine fires.

Why Three Phase is Better

Using the motor vehicle analogy, the pulsing power from an internal combustion engine needs to be smoothed by a heavy flywheel. Single phase AC has somewhat similar characteristics, although less marked. The zero crossings produce a subtle but persistent power interruption. Single phase motors above 5 hp are not common.

Two power peaks (positive and negative) every 1/50th second seems smooth enough. A motor running at 3000 rpm, i.e. 50 revs/s receives only 2 power strokes per revolution. This is analogous to the crank in a 4 cylinder, 4 stroke internal combustion engine. With three phase electrical power a motor running at 3000 rpm receives 6 power strokes per revolution, analogous to a 12 cylinder internal combustion engine. This is only an approximate analogy because the electrical power peaks are more gently rounded than the sharp pulses from exploding fuel but it serves to illustrate the point.

Three Phase Power Supply

Users in rural or remote locations (in the US or Canada or anywhere in the UK and Ireland) will find that a three phase supply is not easily obtained from their local power company. Unless you are in an urban area within a country where a three phase service is shared among many customers, installation costs can be prohibitive.

Users have also been surprised to learn that even if the power company has already installed three phases, many extra costs, in the form of daily or monthly line charges and "demand" billing based on peak use and higher kilowatt hour rates, serve to drive up the price of three phase supplies far higher than the investment required for a 1ph to 3 ph converter.

As such converters work from the secondary side of the power grid (after the utility-supplier's street or pole transformer), installation is simple. Only three phase loads are applied to the converter. All existing lighting and other wiring to single-phase loads remains unchanged.

A Digi-Phase can be connected to your three phase loads in about 30 minutes. The T and F models can operate any combination of motors, heaters, welders, or three phase rectifiers (AC to DC), CNC machines or VSDs. The E Model, (which has Boost start for fast starting of motors but no phase balance control, similar to other machines on the UK market), is for standard machines with three phase motors that you would have no concern about them running hotter than normal.

The Digi-Phase converter you buy once, and may expect it to last many years with virtually no maintenance and repairs. With an operating cost of only about 4% of the operated load it is easy to see that phase conversion is one of the best industrial bargains available. The next step is to determine the model that is best suited to your needs.

Three Phase Conversion Methodology

SMP Limited is not the only company producing single to three phase converters. To the best of our knowledge, most manufacturers still use timers and mechanical contactors together with electrolytic capacitors in their rotary converters. As this method is not free from maintenance and cannot provide hard-start capabilities without oversizing the converter and/or use of a manual boost button, we have developed the solid state converter based on modern capacitor-switching technologies. These products are being used in the UK, Australia, New Zealand, Saudi-Arabia, Singapore, Mexico, Argentina, Bhutan, Canada and USA.


A capacitor can be viewed as temporary electrical storage. Alternating voltage is delayed as the capacitor is charged. Capacitors have been used to operate three phase motors on single phase power for decades. In this method, the two single phase wires are connected to two of the inputs of a three phase motor. A capacitor is then connected between one of the single phase inputs and the third terminal of the motor.

A motor requires about five to six times as much current to start as it does to run, so a capacitor-type single to three phase converter must have some means of switching a large group of capacitors in and out during motor starting. This solid state switch is a high-voltage high-current semiconductor, tested at 2200V and, in the smallest converter, withstanding short circuit currents of 2300 A. Switching is precisely triggered at zero voltage and zero current transitions in order to minimise stress on components, to the motor and to the supply line.

The trigger and switch circuitry, using state of the art CMOS logic, is protected against any kind of moisture, dust, electric noise, magnetic and electrostatic fields, overvoltage and undervoltage.

Digi-Phase Operation

The Digi-Phase T or F Models generate all three phases internally. Such a device distributes three phase power to multiple motors and to many machines. Electric power is injected into a running motor-generator, resulting in three useful sine wave phases separated by 120 degrees as with utility power. The equipment may then be started and stopped in any combination up to the Digi-Phase's total load capacity. Any type of three phase load may be operated with a Digi-Phase.

Motors starting under heavy load should only be connected to a Digi-Phase. This unique single to three phase converter will produce up to 600% of maximum continuous power. For how many minutes? Don't worry, if the start-up time of a motor is too long, the motor rated fuse in your fuse box will blow or the overload protection in your machine will trip. A Digi-Phase is a very tough device, not easily overloaded.

The output of a Digi-Phase is nearly as high as the supply power, as long as the single phase supply is stable enough to provide short high power bursts required by the Digi-Phase, i.e. when starting motors or when motors are under excessive load. When the output currents rise during motor the start up period, the input current will also momentarily rise.

If your supply cable is not rated sufficiently, severe voltage drop may occur at the input side. The same relative voltage drop will be found at the converter output. Motors will then not accelerate as fast as they should and will not cope with excessive loads as well as they would with stable voltages. When connecting a Digi-Phase, always use a heavier single phase supply cable than actually needed.

Some Digi-Phase Construction Details

The internal motor-generator shares many characteristics with the three phase motors it operates. There is a set of stationary field coils, or stator, which determines the magnetic poles in the electrical steel of the rotary. These coils and their poles have 120° spacing to produce a uniform three phase wave form. A squirrel-cage type rotor produces the poles of the rotating magnetic field, very much like a rotating transformer. The rotor has a good bearing support in aluminium end-bells. The rotor-to-stator air gap is smaller than in many motors, since a magnetic "flux" that produces three phase voltages must pass this air gap.

The phase-shifted current from the capacitors is absorbed by the electrical steel of the rotary motor-generator and then distributed in a three phase waveform that is usable by any type of equipment. The type of motor-generator used in a Digi-Phase has the highest efficiency found on the market.

Capacitors in the internal capacitor bank are automatically switched when needed. Switching is performed at zero crossing transitions of each sine wave. Using this method, there is no stress to any part. Polypropylene capacitors are the guarantee for long life expectation, (80000 hours and more when switched at zero crossings as in a Digi-Phase). Due to the zero crossing switching, EMC, or electromagnetic radiation, is kept low and always within limits in all countries. The Digi-Phase complies with EMC regulations in all countries.

The compact and smart switch-controller is the key to the Digi-Phase performance. Inputs and outputs are filtered against incoming spikes, noises and other disturbances. The controller measures output conditions and senses the need for high currents to accelerate external motors. It also contains the high voltage and high current power switches activating capacitors in the Digi-Phase's capacitor bank. They are designed to withstand at least 2300 A and 2200 Volts in 3x 230V and 3x400V versions and 3000V in 3x480V versions, well above the highest peaks found in urban or rural areas. The CMOS logic is completely embedded in Epoxy resin giving lifetime protection against dust and moisture.

Life expectancy is practically unlimited.

What to Expect From a Digi-Phase

Compared with three phase supply from your local power company, a Digi-Phase is a viable compromise, even coping with motor hard-starts.

This is stated to prevent unwarranted expectations of the equipment. Converters have weaknesses and strengths which should be considered before a purchase is made. As mentioned earlier, Digi-Phase also have a low purchase cost, far lower than the cost to bring three-phase to your premises via power lines. Operating cost is very low, about four percent of the electrical cost of the operated load.

A Digi-Phase E in a standard, multi-motor installation will not quite balance each line's power as well as a utility-supplied, three-wire, three-phase system. A Digi-Phase T or F will achieve a voltage balance similar to that of utility supplied power. The quality of the Digi-Phase's three phase output depends a lot on the quality and stability of the single phase input line. Since output currents sometimes increase to 600% of the maximum continuous currents, the input lines are also loaded with these higher currents.

A Digi-Phase T4 or E4 draws about 18A continuous input current operating at rated power. During motor starting it can increase to about 110A peak current for a fraction of a second. This could be a longer period of time if the starting motor has to accelerate a heavy mass. These currents can be scaled pro-rata for other models. Motor rated fuses accept high currents during the start-up time of a motor.

High input currents may result in input voltage drops. When motors are starting under load, we have seen voltage drops from 230V down to 170V. Because the nature of a Digi-Phase is a transformer, this will result in a voltage drop at the three phase output from 3 x 400V to about 3 x 280V. Under these conditions, motors will not accelerate as fast as they should. If the time period of excessive input currents is extended, either the input fuse will blow, or the protective overload switch in your equipment will trip. To overcome these problems, it pays to install oversized cables between your fuse box and the Digi-Phase's input. If the voltage drop of the supply side is reduced, motors on the output side will accelerate faster. Low input or output voltages will not do any harm to the converter. They only reduce the overall efficiency.

Compared with the power company's grid, a Digi-Phase E might not maintain close voltage balance over a wide range of operation. Line to line three phase voltages may vary somewhat with changing loads. If you have voltage-sensitive equipment such as some computerised machine tools (CNC), the best results will be obtained by using a Digi-Phase T or even better a Digi-Phase F.

By analysing the strengths and weaknesses of each option, utility three phase or a converter, you minimise your disappointment with either. Utility supplied three phase power brings higher electrical costs than single to three phase converter power because someone has to pay the purchase and maintenance cost of the extra lines and extra street transformers, out in all weathers. A Digi-Phase is owned by you: when you don't need three phase, you turn it off, and you're not paying any line charges. It is 100 percent tax deductible for your business, and you can take it to a new location.

Balanced Voltages; Interpreting Current Balance

Balancing a three phase load is important to avoid one phase being overloaded while the other two are not contributing their share. Excessive unbalanced power can harm a motor if the imbalance is significant it will cause the windings to overheat. CNC machines, variable speed drives and other pure electronic loads always require a stable and balanced three phase supply.

A Digi-Phase operated on a multi-motor system is very efficient, although not quite 100%. The Digi-Phase E is 95% efficient, and the models T and F offer 96% efficiency.

In case of hard-start conditions, some manufacturers suggest using a single to three phase converter about 2-3 times the size of the operated motor. This is not necessary with a Digi-Phase, e.g. a 7.5 Kw/10 Hp Digi-phase will start and run a 7.5Kw/10Hp load. All of our converters are 100% rated output.

Three Phase Voltages and Neutral

The voltages between the three output connectors will be about 1.73 times (square root of 3) higher than the 230 V input, i.e. about 400 Volts between phases. If voltages are measured between one of the phases and ground, different values are found. This is because of the step-up input transformer configuration.

All large motors and machines use the voltage between the three phases. Therefore you should not connect a load between a phase and Neutral. The Neutral line is fitted to the 5 Pin output socket for light loads only, such as an auxiliary transformer in a machine, in this case use Neutral and L3 only. L3 is directly connected to the single phase input line and may be used for any kind of external control circuitry or other general purpose.

Digi-Phase and Welders

Welding current is always the secondary current from an internal transformer, possibly rectified from alternating current (AC) to direct current (DC). Welding voltage is low for resistance welding but (momentarily) high for arc welding. The three phase welder nameplate will need to be interpreted to determine the power consumption in kW.

Irrigation Pumps and Digi-Phase

Some rural power companies do not supply three phase electricity to farms. The cost-efficient and dependable way to operate the irrigation system is with a large Digi-Phase. 400V two-phase or 460V dual- or split-phase is normally available from your local power company, and is easily processed into 400V three-phase power through the Digi-Phase.

To size your Digi-Phase, add the total horsepower of pivot, pump, and end gun motors you wish to operate. Divide the sum by 1.34 to find the kW value. Please note: When windshield-wiper type (reversing) pivots are operated, install a Digi-Phase with a heavier supply cable because such a Digi-Phase will often enter the Boost mode.

Pivots and Other Technical Information

If your pivot operation is a "windshield wiper" configuration, that is, a pivot that runs a partial circle and then reverses direction, then you should choose a Digi-Phase. Under normal pivot operation, only one-half the motors will start at once. However, when a pivot is reversed, all the motors that were "off" when stopped will now start. Thus, a 10 tower pivot may reverse, restarting 7 or 8 motors, and exceed a normal converter's capacity.

A Digi-Phase should always be mounted as close to the single phase service as possible to minimise the heavier single phase wiring required. 24, 32, 48 and 64 kW Digi-Phase can be equipped with a soft-starting feature that reduces the starting inrush of the Digi-Phase to approximately half of normal on start-up to prevent line disturbance.

All Digi-Phase are high-efficiency models. Power consumed by a Digi-Phase itself will amount to approximately 4 to 5% of the operated load.

Harmonics and Power Factor

Due to the zero crossing switching technology, no harmonics are produced. No filters are needed.

A Digi-Phase T and F corrects the power factor caused by any equipment.

For further details on the Digi-Phase models available, please click on the model links below.

Type E
Type F
Type T
Self Build
Specialist Machine Parts Limited, Baldwin Road, Stourport on Severn, Worcestershire, DY13 9AX