The BEMF can be reduced by positioning the stator flux such that it opposes the rotor’s magnetic flux. When this takes place, the inverter becomes limited by the back electromotive force (BEMF) generated by the rotor’s magnetic flux as it approaches the DC supply bus voltage and/or component limits, such as capacitor voltages or conductor sizes. Sometimes a user chooses to operate a motor at a higher speed while sacrificing torque generation. To maximize torque in this type of motor, the stator current should be oriented 90 degrees from the direct axis (no direct axis current Id = 0, full stator current = quadrature current). In a SMPM motor, the portion of the current oriented in the positive quadrature direction (Iq) produces torque (DC armature current) and a negative current in the direct axis (Id) reduces torque (DC magnetizing current).

This angle also represents the displacement of the magnetic fields of the rotor and stator. The angle between the stator current with respect to the quadrature or direct axis is referred to as the electrical angle or load angle (θ). The vertical axis represents the quadrature current and the horizontal axis is the direct current. This stator current is often represented in polar coordinates for the purposes of FOC. AC current is applied to the stator windings to produce a rotating magnetic field, causing the rotor to “chase” the stator. The rotor of a permanent magnet motor has its own magnetic field generated by the magnets mounted on the surface or interior of the rotor. In field-oriented control, these modeled winding currents are controlled to regulate torque and speed. The stator winding is responsible for creating magnetizing flux, while the armature winding produces torque. A wound DC motor has a stator winding and an armature winding. This simplifies processing by essentially controlling an AC motor like a DC motor. This allows an inverter’s PI control loop to regulate a handful of non-time-varying (DC) variables. It is important to note that each of these carry their own uncertainty and assumptions.įOC is used by motor controllers (inverters) to simplify the control of multi-phase motors by applying a DC control model at the input of a complex AC commutation output. The techniques illustrated can also be applied to other FOC variables, algorithms, and motor technologies.ġMeasurement, in this context, can also mean computations based upon a combination of actual measurements, equations, and constants. Specifically addressed are direct and quadrature currents of a surface mounted permanent magnet motor (SMPM) with field weakening applied. This application note provides guidance for making measurements 1 related to field-oriented control (FOC) of electric motors and presents an example use case that illustrates how these are accomplished using a power analyzer and/or a ScopeCorder.

Field oriented control