Is this the dawn of the matrix converter era?

For years, drives experts have been talking about a technology known as "matrix conversion" as being the next generation of drives. All of the major drives suppliers, including Rockwell and Siemens, have worked on the technology but the consensus has been that, although matrix converters offer many potential attractions, they would be too expensive to commercialise at present.

Now, however, Yaskawa has revealed that it is almost ready to start selling matrix converter drives. At last month`s Hannover Fair, it demonstrated a pre-production prototype (shown above) and announced that it plans to start selling the drives next year. Several are already being field-tested in Japan.

In essence, a matrix converter uses an array of semiconductor power switches, operated in a precisely controlled sequence, to connect the three phase lines directly to a motor. In Yaskawa`s converter there are nine switches, each consisting of two IGBTs which allow either positive or negative voltages to be applied to the motor. The large number of IGBTs is one of the factors that make matrix converters expensive to produce.

The converter uses all three phases of the input voltage to control the output voltage. This not only absorbs any current disturbances, but also provides a clean output voltage.

This results in one of the matrix converter`s attractions - a level of input current harmonic distortion which, at around 8%, is dramatically lower than the 80% of a conventional inverter. The current of a matrix converter is almost sinusoidal and, when on load, is almost in phase with the voltage. During regeneration, the current is shifted by up to 180 degrees, but is still sinusoidal.

The matrix converter can operate in a regenerative mode without needing a braking resistor or special converter. The regenerative energy is fed back to the supply without needing any extra equipment and the drive operates efficiently in all four quadrants.

Another attraction is that because the matrix converter does away with the need for vulnerable DC bus capacitors, it should operate reliably for a long time.

Initially, Yaskawa expects the new drive to be used mainly in applications which exploit its advantages, such as its regenerative abilities. Examples include lifts, escalators, cranes, centrifuges and eccentric machines such as presses and cutters running in continuous motoring and regenerating cycles. The drives will also be useful in applications that need braking, but where there is no space for a braking resistors, or where the heat generated by resistors could be a problem.

Another set of potential applications are those where the extremely low harmonic distortion would be an attraction. For example, it could allow smaller generators to be used on ships. The matrix converters could also avoid the need to over-size equipment in isolated systems, and remove the need for extra transformers in 12-pulse input systems.

Initially, Yaskawa plans to offer its matrix converters in ratings from 5.5-22kW, 400V and will extend the range later to 75kW. There will also be a 200V range spanning ratings from 5.5-45kW.

Precise pricing has not been revealed, but the matrix converters are likely to cost about twice as much as their conventional counterparts.