COVER FEATURE: MARCH 1997

EMC

Reverse drives smooth harmonics and regenerate loads

Use of multiple, modular inverter drives offers a lower cost way of improving EMC and achieving four-quadrant regenerative operation. Tom Shelley reports

By making use of modular inverter drives designed to work in either direction, it is possible to configure systems which produce virtually no mains harmonics, without constructing special builds.

One inverter in reverse can serve the EMC needs of several inverters aimed forwards, and provide means for regenerative braking.

Applications for harmonic elimination are mainly in large systems, particularly those found in the water industry. But uses for regenerative systems range from scenery shifting in theatres, where it is difficult to dissipate heat, to certain large-scale manufacturing processes, where energy saving costs can be significant.

Dr Colin Hargis of Control Techniques tells Eureka that Unidrive power modules were designed to be used in either direction right from the beginning. In the normal forward direction, they form the basis of inverter drives ranging up to 4, 11, 37, 90 and later this year, 120kW. But by changing a parameter which selects a different software function, they are able to replace the usual diode bridges feeding the DC link.

When used on the front end, the AC supply is connected to the inverter through an inductance. The normal pulse width modulated technique is used to generate a sinusoidal back EMF at mains frequency, which can be controlled in such a way that in conjunction with the inductance, the current drawn is sinusoidal and in phase with the supply voltage (see diagrams). A small change in the phase angle of the back EMF reverses the phase angle of the current and allows energy to be directed back into the main supply, in which case the reversed inverter makes the whole drive regenerative. The key feature in the Unidrive is its ability to lock onto the mains wave form and generate a back EMF whose phase and amplitude are controllable in real time.

 

One large reversed inverter can be used to correct phase angle and eliminate harmonics for several motor driving inverters. It is also possible to devise systems in which the energy from one or more motor driving inverters in braking mode is fed back into the other motor driving inverters.

More than one regenerating drive can be used if the total regenerated power is too high for a single unit. If this is done, the DC bus has to be split and each regenerating drive supplies one set of motor driving inverters. The DC buses of two regenerating units must not be coupled together.

It is further possible to use a regenerating converter as a direct replacement for a brake resistor. If the required braking power is less than the motoring power, a smaller and cheaper reversed drive can be used than for a fully regenerative arrangement where the full motoring power has to pass through the reversed inverter. The regenerating drive is supplied from the mains via an isolating transformer. This is because when the regenerating unit is switching, its DC bus voltage moves with respect to earth and the supply neutral point. On the motor drive, on the other hand, the DC link voltage remains relatively fixed with respect to earth.

Applications for harmonic suppression are particularly seen in the water industry, where pumping stations may be equipped with several large pumps, each driven by its own inverter. Loads can be expected to be large, and all of the same type, usually at the end of a long transmission line shared by few other users. Other applications such as air conditioning are also opening up, with the increased interest in energy efficiency there and in harmonic current limiting,

Most inverter users are quite content to use resistors to dissipate excess energy if loads have to be braked. But in certain applications, such as scenery shifting in theatres, generation of excess heat leads to additional load and cost associated with air conditioning and ventilating systems.

Regeneration is also appropriate in processes with high inertia loads such as paper handling, and where one reel is winding in a web from another reel, using one drive in normal mode and the other in regenerative mode. In such instances, regenerating inverter drives challenge DC drives and motors.

The company is developing integrated four-quadrant regenerating drives to become available some time next year. They will still be based on Unidrives, but will eliminate superfluous components such as diode bridge rectifiers, and duplicated items of control circuitry.

Control Techniques

Design Pointers

* Unidrives can be coupled back to back in order to eliminate mains harmonics and to act as regenerating units to feed braking energy back into the mains

* One mains harmonics suppressing or regenerating drive can work with several motor driving units

* Unidrives can be used to directly replace braking resistors, where heat dissipation poses problems

* Integrated four quadrant regenerating inverter drives, based on Unidrives, but eliminating surplus components are to become available next year.

Debrief Box

Where harmonics matter and where they do not

Harmonics are components of the mains current waveform which are at multiples of the base mains frequency. Low frequencies cause little electrical interference by stray coupling, but may in extreme cases cause problems through excessive distortion of the mains supply wave form, reflected in the power factor. Power factor in this instance means:

Displacement factor (Cosf) X fundamental current/RMS current

Conventional inverters in a bank all draw current from the mains supply whenever it is producing more voltage than that in the DC link. This means that a bank of inverters all draw current in pulses at the same time. This is not a problem with small drives, or in most industrial applications, where effects on the overall load pattern are not considered significant. But with large units, particularly where users are at the end of a long transmission line, they lead to a need to have higher current-rated transformers, cabling and other components.

High frequency emissions, arising from inverter switching frequencies, on the other hand can in some cases cause considerable nuisance. If found to be a problem, they are best eliminated with filters on the mains and/or motor supply side. It may also be necessary to cover inverters and their cables with metal conduit tubes and control panels; to place signal cables well away from power cables; and to use shielded, twisted pair cables for weak electric and signal circuits. Most drive manufacturers give detailed guidelines for installing and wiring drives to avoid problems.

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