Whether an application requires precision motion control for medical apparatus or for powering a megawatt motor, the omnipresent transistor invariably sits at the crux of it all. Boris Sedacca reports on the latest developments in power electronics for motors.
In July 2013, the UK power electronics industry received an £18 million boost from the Engineering and Physical Sciences Research Council (EPSRC) with the opening of the first EPSRC National Centre of Excellence for Power Electronics.
David Willetts, Minister for Universities and Science, said: “This National Centre will bring together universities and businesses to ensure industry has access to the latest science and technology, as well as helping to maintain a supply of skilled people.”
A central co-ordinating hub will be led by Professor Mark Johnson at the University of Nottingham, and will involve the universities of Manchester, Newcastle, Greenwich, Bristol, Warwick, Nottingham and Imperial College London. A series of four technical programmes will cover devices, components, convertors and drives.
The opening of the new centre comes two months after the launch of the PowerelectronicsUK Forum, a network backed by industry, academia and the government that aims to boost the number of people within the power electronics industry.
Steve Burgin, Chairman of PowerelectronicsUK and UK President of Alstom said: “The new EPSRC Centre for Power Electronics will be key to the future success of UK Power Electronics. It will help to keep UK industry and academia at the forefront of next generation Power Electronics technologies.”
The new forum was launched as a direct result of the influential strategy document ‘Power Electronics: A Strategy for Success’, which was developed with input from the Technology Strategy Board, the Department for Business, Innovation and Skills and the EPSRC.
“Gambica supports the PowerelectronicsUK forum and we have significant expertise in power electronics, particularly with variable speed drive manufacturers like Siemens, Alstom (now GE), Parker SSD, Invertek and Control Techniques,” says Steve Brambley, deputy director of GAMBICA.
“Soft starts also feature in power electronics for motor control, like those manufactured by Fairford Electronics, which exports 90 percent of its products around the world.”
Signal or power amplification
A transistor can either amplify a signal level or an electrical power level. Signals are specified by voltage whereas electrical power is specified by current, and this is where the transistor family splits in two. Broadly speaking, a field effect transistor is effectively a signal voltage 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 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 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. Drury points that although silicon carbide and gallium nitride hold out the opportunity for significant energy efficiency improvements, they remain expensive and the bulk of devices will still be silicon-based five years from now.
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.
“With the bulk of industrial drives being three-phase, you end up with substantial fifth, seventh, eleventh and thirteenth harmonics and although the power decreases at higher the higher harmonics, the fifth and the seventh can be almost as high as the fundamental.”
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 which can be made from much smaller components to provide a smooth supply current waveform. The higher the frequency, the easier it is to filter, whereas filtering a harmonic current at say 250Hz requires a sizeable inductor or transformer.
Drury concludes: “Control Techniques drives are modular for higher powered motors and offer an IGBT input bridge option. For lower powers you can just take two standard drives and connect their DC links together. The downside of that obviously is that you pay for two drives.
“Another alternative if you have several drives is to feed them from a common DC bus. Although PWM switching does not cause harmonic problems, it can cause interference on electronic equipment and radio communications, so drives have to be installed properly and comply with appropriate electromagnetic compatibility (EMC) standards."