Phase change - converting to three-phase

Three-phase motors are more efficient, cost less, last longer and are more dependable than their single-phase counterparts; yet, often a three-phase supply is not available.

The industrial world runs on three-phase power.

Three-phase motors are more efficient, cost less, last longer and are more dependable than their single-phase counterparts.

Yet, often a three-phase supply is not available, particularly in remote areas where it may not be economical to install the necessary overhead lines cabling and distribution transformers.

Phase conversion is often needed on remote sites such as farms - because the electricity supplier often finds it impractical to provide a three-phase supply to the site, it provides a split-phase arrangement instead.

A standard UK three-phase supply will typically consist of a star connected three-phase arrangement, with 120 degrees between phases, with neutral connected to the star point.

This commonly gives 415V between phases and 240V between phase and neutral.

By contrast, a typical split phase arrangement has only two phases available, 180 degrees apart.

This gives 480V between phases and 240V between phase and neutral.

If a single-phase or two-phase supply is all that is available to a motor, then some form of phase conversion is needed.

There are three main methods - static, rotary and variable speed drives - with drives being the most modern and preferred option.

Static convertors use capacitors to produce a three-phase supply - the two input phase wires are connected to two of the inputs on a three-phase motor and a capacitor is then connected to one of the single-phase inputs and the third leg of the motor.

This method is used in so-called capacitor-start, capacitor-run single-phase motors.

A phase shift through the capacitor shifts the voltage in time from its parent voltage, producing a voltage distinct from the input phase lines.

Some devices will start a three-phase motor on capacitors but will then allow the motor to run on single-phase.

This will seriously limit the motors overload rating.

The static method is useful when operating moderately loaded motors.

Most motor-driven tools are generally suitable for static conversion and most mills, drills, saws, grinders etc will work well.

Even mechanical shears, brakes, and punch presses work well if they have a flywheel.

The system is vulnerable to power quality disturbances, and the harmonic content of a network can cause premature failure of the capacitors.

For two-phase to three-phase, the main static method is the Scott transformer.

This uses taps at 50 and 86.6% in the primary winding, to which the two incoming phases are connected, with three phases on the secondary.

In this case, the currents in the secondary windings are 15.5% greater than in transformers delivering the same kVA single-phase at the same voltage, causes the kVA output of the secondary to be 86.6% of regular transformers of the same physical size.

The modern version of this connection is in a duplex (two units in one tank) transformer, called a Y connection.

This simulates the various Y or delta connections, with the exception that no neutral is available on the primary side.

A more versatile phase convertor is a rotary convertor, with a capacitor-start, capacitor-run motor and three-phase generator.

Rotary convertors are inherently inefficient - with losses in both the motor and generator; they also have a major drawback in that rotary convertor will not deliver the starting inrush current of three-phase motor, and the rotary unit must often be greatly oversized relative to the largest motor operated to produce high starting torque.

Variable speed drives (VSDs) are the method most commonly used today to convert to three-phase power.

A VSD can convert 240V single phase to 240V three phase, or two phases to three phase.

It operates by rectifying the input phase AC into DC and then converting this into three-phase AC - the output is approximately equal to the supply voltage therefore users need to check on the motor nameplate that the motor can run at this voltage.

Their big advantage compared with rotary convertors is that they are vastly more economical, with a 95% efficiency compared with 50%.

Other advantages of using a VSD is that it is smaller, cheaper, and can control the speed and acceleration of the motor - a very useful feature on a machine such as a lathe or drill.

A limitation is that the maximum power that can be drawn from the network using this system is usually 1.1-1.5 kW (the input current is normally limited to 16A).

Typical drives used in this application include ABB's low voltage AC drives, which are finding applications among many small businesses operating a variety of machines from lathes and milling machines to irrigation pumps and multiple-motor sewing machines.

The output of these drives is not dependent on the power supply, which can be single phase, three phase or even DC.

The convertor controls the speed of a standard squirrel cage motor and operates over a wide power range at high efficiencies.

This is achieved by converting the mains supply to DC and then inverting it to AC at a variable frequency and amplitude.

The speed of the motor varies with frequency.

The frequency convertor incorporates all necessary motor protection and control and should be connected directly to the motor terminals.

Although machines with mechanical gearboxes can still use the facility of speed variation with the convertor, for the purpose of motor cooling it is recommended that the gearbox be used for major speed variations and the convertor for fine-tuning.

This feature is particularly useful for precision work at low speed.

A typical application is the Winston village pumping station, operated by Northumbrian Water.

The pumping station has two duty standby submersible pumps on a sewage pumping application.

The pumps are 3kW units designed for three-phase operation, yet the supply to the site is 480V split phase.

Upgrading the supply to three-phase would have cost up to GBP 50,000.

Using an ABB low voltage variable speed drive, the existing two-phase supply was used to create a synthetic three-phase supply.

With all these systems, the economics of buying and running the equipment dictate when to move to a more conventional solution to run three-phase motors - if the total motor power of the installation is around 3-4kW it will probably be worth upgrading the supply to a three-phase system.

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