Permanent Magnet DC motor with on‑rotor drive system

More than 90 % of all industrial motors are AC induction motors and bruschless DC motors. They are very reliable but not so energy efficient, almost half of all the electricity in the world is consumed in running these motors.

In this project researchers will investigate how a Permanent Magnet DC motor with on-rotor drive system can achieve both the best efficiency and the best performance but with the lowest cost.

Background

The most popular motors nowadays used in various industries are AC induction motor and brushless DC motor, and AC induction motors account for more than 90 % of all industrial motors in use. The AC induction motor was invented more than 120 years ago by Mikhail Dolivo-Dobrovolsky and its design remains to this day almost unchanged. This rotor mechanical structure is a very solid and robust piece, so the AC induction motor proved to be very reliable in different industrial applications, and also because of its high achievable efficiency and low production cost, it has dominated the industry for more than half century.

It is estimated that almost half of all the electricity generated now in the world is consumed in running these induction motors, and electricity bill can accounts for up to 95% of the lifetime cost of owning and running these motors, so obviously the efficiency of a motor becomes a key parameter for industry to optimize.

Objectives

In this project researcher will investigate the possibilities for a new motor and drive combination design called: “permanent magnet DC motor with on-rotor drive system”. This design aims for a motor that achieves both the best efficiency and the best performance but with the lowest cost. This idea originated from the observation that most of the losses in conventional motors are actually on the stator side. The goal of the stator winding and electrical steel design in both induction motor and the synchronous reluctance motor is to create a rotating magnetic field with high flux density in order to deliver strong torque on the rotor shaft.

The problem is that there are lots of big cut-outs around the stator laminate for copper winding, which in turn greatly increases the reluctance of the close-loop path for the generated flux to go through, and that means it become more difficult for the current flowing in the copper winding to generate flux, which then means it take more current and hence become less efficient to generate a rotating magnetic field with high flux density. The synchronous reluctance motor has the same stator design as induction motor, so it also has the same problem.

Permanent magnet brushed DC motor existed a long time ago and it had a lot of troubles with the mechanical commutation brushes and was never efficient compared with modern induction motors. But the trick in the new design is an on-rotor drive system which includes several function modules to directly deliver the utility power from stator and control the voltage and current of individual rotor bar pairs in order to optimally run the motor at different speed and torque. This system completely eliminate the need of conventional brushes and drive the motor with much lower voltage (less than 20V) and higher current (up to dozens of kA) using off-the-shelf multi-phase DC/DC step-down power regulators like the ones used in all the PCs for CPU power supply.

Technology Roadmap

The technologies that are developed and used in this new motor and drive system design can also be easily applied to other application areas. One obvious area is the electrical generator, which in principle can directly reuse the same architecture of the new motor and drive system and just change the direction of the power flow. Here the advantage of high efficiency over wide operating speed range and high controllability would be very attractive factors for the electricity-production industry.

Another area is the electrical vehicle, where the advantage of our high-efficiency and miniaturized motor and drive system directly translates to either longer range between charges or lower-cost energy-storage system, and the low-cost feature of our system would be appreciated.