Motors and Drives


amplifier while a bipolar junction transistor shunts an amplified current to a load. The latter technology has evolved into the insulated gate bipolar transistor (IGBT) device widely used in today’s motor variable speed drives (VSDs), which has changed out of all recognition and drive manufacturers have been able top jump on the back of semiconductor device developments. There are particular design challenges for motor


drives, the first of which is to turn a machine smoothly, effectively and efficiently according to Professor Bill Drury, one of Control Techniques’ research alumni. Then there are interfaces to the outside world and control functions. A motor drive today


Fig. 2. Steve Brambley: “Gambica supports the PowerelectronicsUK forum.”


typically sports a built- in motion controller and a programmable logic controller (PLC) for sequencing machine operations, but no amount of intelligence can remove the energy losses associated with power electronics unless additional power electronics countermeasures are employed.


“We have a lot of heat to get rid of,” rationalises


Drury. “A heat sink is not the first thing we may think about but this was less of a consideration about five years ago, other than making the drive run as efficiently as we could to reduce the size of the heat sink. “Processing power is always getting cheaper so the


control of the motor is becoming less of a problem and more things are possible, although sensors continue to be an issue - particularly current sensors where high performance sensors add significant cost. “There has been a lot


Fig. 3. Bill Drury: “Wide bandgap semiconductors are still years away.”


of talk about new power semiconductor devices which go under the generic term ‘wide bandgap semiconductor’ devices made primarily of silicon carbide and gallium nitride. They are in the same material family as diamond, which is almost the perfect


device material but still years away.” Wide bandgap semiconductors are semiconductor materials with electronic band gaps significantly


larger than one electron volt (eV) for commonly- used semiconductors like Silicon at 1.1eV or gallium arsenide at 1.4eV, and are used where high-temperature operation is important. Diamond has ideal semiconductor characteristics


with outstanding voltage insulation and regular crystalline structure.


Diode bridge harmonic problems


Silicon devices are considered mature and better known in terms of reliability than ever before, but the diode bridge at the electrical supply side of a drive is always problematic, emitting undesired harmonic current on top of the fundamental 50/60Hz alternating current (AC) waveform. “This is a simple rectifier which takes the fixed


frequency supply and converts it to direct current (DC) through a capacitor bank that smoothes the ripple current,” Drury continues. The DC current is then squirted through the IGBTs


as a pulse width modulated (PWM) voltage into an AC motor’s windings, which has the same effect as applying an AC voltage. PWM is a high frequency switching method and harmonics emission has nothing to do with the high frequency pulsing in kilohertz from IGBTs to the motor. Thyristor controlled DC drives have the same problem with harmonics - not only AC drives. One proven method drive manufacturers use to


combat harmonics is to place a series inductor before the parallel capacitor in the DC link. Another is to use a rectifier pulse number that is higher than the minimum of six required for a three-phase drive to eliminate the crucial fifth and seventh harmonics altogether, producing a source waveform nearer to the sinusoidal ideal.


Drury adds: “There are also different types of


passive and active harmonic filters, transformers and topologies, some of which can be expensive and bulky but they would not come with a standard inverter drive. “An active parallel filter can be used to generate


harmonic anti-phase current to cancel out harmonic current. This may be a piece of equipment for the entire site supply, rather than for individual drives, so an entire plant may use just one piece of installed equipment, unless a particularly large motor is used which may need its own harmonic filter. “The diode bridge at the root of the problem has


an endearing quality of being very cheap, but if it were to be replaced with an IGBT bridge instead, then you could start switching PWM on the drive’s supply side as well as its motor side.” Typical PWM frequency is above 3KHz, which is


too high above the normal 50/60Hz current to cause harmonics problems. It can also be filtered easily with a high frequency filter. ●


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