Energy Management Group
Department of Electrical & Electronic Engineering
Merchant Venturers Building
Woodland Road
Bristol BS8 1UB
United Kingdom
T +44 117 954 5499
F +44 117 954 5206
E energy-management
@bristol.ac.uk
Both the emergence of new materials and efficiency requirements of modern vehicles are driving the development of electric machines and their control systems. Efficiency enhancing direct control techniques are being developed in order to increase the efficiency and reduce the weight of drives.
Permanent magnet synchronous machines are the most promising type of motor for electric vehicles and many other applications, due to their compactness and high efficiency. Most applications using these machines require the shaft position and speed to be controlled accurately; this usually requires the measurement of the rotor position by means of a cumbersome mechanical sensor. The mechanical sensor allows for excellent control performance, however it adds significantly to the size and weight of the machine, thereby degrading system efficiency and reliability.
Sensorless machine control is a technique aimed at taking full advantage of the compactness and high efficiencies of modern machines. The position and speed of a machine are measured without mechanical sensors; the estimation of rotor speed is based exclusively on estimated terminal voltages and measured currents. Sensorless control requires the measurement and feedback of the currents and usually voltages that are applied to the machine, and these two variables can be used to calculate the rotor position and other internal variables such as flux linkage and electrical torque.
The control techniques associated with sensorless operation are fundamentally the same as those used with position or speed feedback. However, it is a requirement within the sensorless control strategy to calculate the rotor position using only measurements of variables such as phase voltages and currents. To calculate the position of the rotor involves time critical and somewhat complicated computation and for this reason it is usual to use a Digital Signal Processor.
Sensorless control is widely used in general purpose induction machine drives where rated or full torque is not required at operating speeds below 3% of rated or base speed and modest dynamic performance is acceptable.
The Energy Management Group is developing sensorless drive systems that operate over the entire speed range. To this end, machine designs are being developed which facilitate sensorless control, and new control methods are being researched.