ORNL’s inverse segmented motor drive cancels common-mode voltage in simulation

Researchers at the US Department of Energy’s Oak Ridge National Laboratory (ORNL) have developed a motor drive design that in simulation cancels two sources of electrical stress in multi-phase drives: neutral-point current and common-mode voltage. The work comes out of ORNL’s National Transportation Research Center and is intended for aircraft, marine vessels and heavy-duty trucks.

ORNL calls the topology an inverse segmented motor drive. It uses two inversely synchronized active neutral point clamped (ANPC) inverters. Configured this way, the neutral-point current and common-mode voltage cancel topologically, giving zero total neutral-point current and zero common-mode voltage under ideal conditions, according to the researchers. Zero total neutral-point current also minimizes the neutral-point voltage imbalance that ANPC inverters are prone to.

Simulations show about a 90% reduction in neutral-point voltage fluctuation and about a 43% drop in RMS current stress in the DC-link capacitors compared to a conventional segmented motor drive. The researchers also present a space vector modulation (SVM) scheme tailored to the topology, along with a carrier-based implementation.

Implementing the drive requires modifications to the motor windings, and the researchers say a standard motor can be readily adapted for the configuration. The design has not been demonstrated in hardware.

Neutral-point current creates excess heat inside a motor drive, and common-mode voltage is stray voltage that causes electrical interference and equipment damage. Common-mode voltage is also what drives shaft and bearing currents in inverter-fed motors, so suppressing it at the source reduces what the filtering has to handle. Conventional fixes rely on complex control schemes or additional hardware, which add cost, size and inefficiency, ORNL says.

ORNL lists electric and hybrid vehicles, aerospace propulsion and industrial multi-phase drive systems as applications, and has listed the design for licensing.

“This design change requires no additional hardware,” said Gui-Jia Su, Distinguished R&D Staff in ORNL’s Electric Drives Research Group.

Source: Oak Ridge National Laboratory