Innovative current sensing for brushless DC drives

A method of deriving a motor current signal with a 3-phase inverter is described in which only the bus current and the inverter drive signals are required to determine motor current. The method may be applied without extra hardware to any multi-phase brushless DC drive, provided that no circulating currents are produced in the inverter. Therefore, transistors must be switched on and off in pairs and only two transistors may conduct at any time. This single resistor method eliminates the problem of balancing gains experienced with multi-sensor methods. The size advantages make it an ideal component of a controller designed for robotic or aerospace applications. However, some noise is injected into the current signal and the entire current loop requires careful design to ensure good inverter performance. To simplify description of the method, its operation in an H-bridge inverter is considered initially