Achieve accurate current sensing in 3-phase inverters

Article By : Stello Matteo Billè

Accurate motor current measurement is essential for satisfactory drive performance in home appliances. Here tips on designing current sensing circuits using 2 or 3 shunt resistors.

Market demand for energy efficient systems is pushing towards the adoption of more efficient electric motors (e.g. 3-phase synchronous motors) and drives. The field oriented control (FOC) scheme meets this demand while allowing, at the same time, the achievement of a better regulation of electric motor torque and speed together with a higher efficiency compared with many other solutions available on the market today.

This leads to energy savings but at the same time to better performing systems: more silent dishwashers and washing machines, better temperature regulation in air conditioned environments or in refrigerators, higher autonomy in electric vehicles and much more. The FOC scheme requires knowledge of the controlled 3-phase motor current; very often (for sensorless implementations) this is the only direct feedback between the control unit and the electric motor. A precise and accurate motor current measurement is therefore essential for achieving satisfactory drive performance and, on the contrary, an untailored sensing circuit may prevent the systems from even running. Thus, design for motor current sensing circuit was provided where two (or three) shunt resistors, placed on the bottom of two (or three) inverter legs, are used.

Also, in order to maximise the signal by noise ratio it is very important to accurately design the layout of the PCB by following some basic principles.

Learn more: Download the full application note