Peter Hammond is the father of the Perfect Harmony topology—a medium-voltage (MV), variable-frequency-drive (VFD) design that changed the industry. With the U.S. patent now expired, the Perfect Harmony cell-based topology is the most imitated MV VFD topology globally. More than 30 companies provide similar cell-based solutions. In fact, a large fraction of all MV VFDs produced globally are similar to the original Perfect Harmony topology.
Hammond’s career coincides with the emergence of the power electronics market. This market was miniscule until several years after the thyristor or silicon-controlled rectifier (SCR) was invented in 1957 by GE, because the initial voltage and current ratings were too small. The thyristor followed a path similar to the transistor, which was invented in 1947, but did not mature enough for wide application for several years.
Hammond received his MSEE from Cleveland’s Case Institute of Technology in 1966, about the time the thyristor matured enough to build a drive that could handle 100 hp. After graduating, he worked for several small companies designing ac drives.
father of the Perfect Harmony topology
“In 1977, I joined a company called Robicon, now part of Siemens," says Hammond. "Back then, we made ‘me-too,’ low-voltage drives, at 600 Vac or less, using thyristors. Our competitors were making similar drives. Everyone knew there was a big market if we could get beyond the low-voltage range into the medium-voltage range—4,160 Vac, for example—with power in the thousands, not the hundreds, of horsepower. We were looking for ways to scale up our existing low-voltage circuits, but there were lots of drawbacks."
One drawback: “If the semiconductors in a typical low-voltage circuit have the highest voltage ratings available, it’s still not possible to achieve 4,160 Vac,” says Hammond. "A second drawback is that, as you go up in horsepower, the power quality becomes more critical," he says. "Just scaling up the existing circuits doesn't give you any improvements in power quality."
Hammond was looking at ways to connect semiconductor devices in series for higher voltages, but that was very difficult, because the series devices needed to turn on and off at exactly the same moment. If one device turned off too soon, the other device would need to support the entire voltage and would fail.
Birth of the Perfect Harmony drive
Over a weekend in 1993 Hammond had an idea for a new approach for medium-voltage drives. "Mulling over the difficulty of connecting switching devices in series, it suddenly dawned on me that it would be easy to put complete converters in series, and that there would be many collateral benefits." says Hammond.
His disclosure memo to managers outlined the benefits, including low harmonics, absence of torque pulsations, quiet operation, near unity power factor, and reduced stress on the motor (Figure 1). “These benefits would allow a medium-voltage drive to be installed without a special motor, without worrying about torsional resonance and without worrying about harmonic distortion. Management approved the project, and we had a prototype drive running in about a year."
Hammond started working on the Perfect Harmony project with himself and one technician and later added a draftsman. "We were designing as we went along,” says Hammond. "The converters—we call them ‘cells’—were actually easy because once you decide to put a number of low-voltage converters in series to get the medium-voltage you need, the power required from each converter comes down. For example, if the number of converters, or cells, is 12, each cell only needs to supply one-twelfth of the power." With 12 cells, 1,000 hp can be produced with individual cells that are only rated at 84 hp (Figure 2).