Robert H. Park

1902-1994

The American engineer whose 1929 Park transform freezes a spinning motor's three-phase currents into a steady two-axis frame, the change of viewpoint that makes modern field-oriented motor control possible.

Robert H. Park was born in 1902 in Strasbourg, where his father, the sociologist Robert E. Park, was studying. Raised in Massachusetts, he earned his electrical engineering degree at MIT in 1923, then went to the Royal Institute of Technology in Stockholm to deepen his command of operational calculus, the mathematical machinery he would soon put to spectacular use. He then joined General Electric, the same company where Edith Clarke worked on related problems.

At GE in 1929 he published his landmark paper, the two-reaction theory of synchronous machines. In a survey of the most influential power engineering papers of the entire twentieth century, it ranked second. The reason is the Park transform. An AC motor's currents and voltages are constantly rotating in time, which makes the equations of the machine hard to work with. Park's transformation mathematically rides along with the rotor, projecting the three phase quantities onto a set of axes (called direct and quadrature, or d-q) that spin with the machine. In that rotating frame, quantities that were oscillating sinusoids become steady constants.

That single change of viewpoint turns an intimidating, time-varying problem into something far more tractable. It built on the alpha-beta idea from Clarke's work and extended it into the rotating reference frame, and the combined Clarke-Park sequence is now the standard front end of motor analysis. Park himself was a prolific inventor well beyond machines: he developed naval mines at the Naval Ordnance Laboratory during World War II, worked across mechanical, chemical, and physics problems, and generated 64 US patents over his career.

Park's transform is the heart of how a microcontroller actually drives a brushless motor. In field-oriented control, the firmware applies the Clarke and then the Park transform to the measured phase currents, lands in the d-q frame where torque-producing and flux-producing currents are cleanly separated, runs simple control loops there, then transforms back to drive the phases. Without Park's rotating-frame trick, the smooth, efficient, precise control inside every modern drone ESC, EV inverter, and robot joint would be far harder to achieve.

Fun facts

In a ranking of every power engineering paper published in the twentieth century, Park's 1929 two-reaction theory paper came in second. Few engineers ever write a single paper that shapes a field a century later.
He was a transformer of more than equations. Across his career Park accumulated 64 US patents, ranging from synchronous-machine theory to naval mine development during World War II at the Naval Ordnance Laboratory.
His transform pairs naturally with Edith Clarke's, and the two worked at the same company, General Electric. The Clarke transform lands you in a fixed two-axis frame; Park's then spins that frame to follow the rotor, which is exactly the sequence motor-control firmware runs today.