Field-Weakening Control

Voltage is proportional to RPM * field flux. So RPM ~ voltage / field flux. When the voltage is maxed, you can increase RPM by decreasing or weakening the field flux.

But! Torque is proportional to field flux * current. So field weakening will cause an increase in current to maintain torque.

The load curve for the vehicle is such that an increase in speed needs an increase in motor torque. So even more current is required when field weakening is used to increase speed of the vehicle.

Battery voltage droops more with increased current tending to decrease motor RPM. So field weakening has limitations. When correctly applied it can be a nice feature.

Some argue against FW saying if the motor is capable with a weakened field, it was oversized to start with. For a chosen material, the field flux determines the motor size.

What is field weakening in DC Motor? Parameter description and relationship between DC Motor Field current, Armature Voltage and Speed, field weakening

Field-weakening or flux-weakening is a technique for increasing the speed of an electric motor above its rating at the expense of reduced torque. Field-weakening is used for motor control in automation applications and traction motor control for electric vehicles and locomotives to achieve higher motor speed when lower torque is acceptable.

The permanent magnet synchronous motor (PMSM) is popular in these applications because of its high power density, high speed, and fast dynamic response.  However, PMSM speed is limited when the stator terminal voltage reaches the inverter output limit.  Therefore, a PMSM requires field-weakening to increase its shaft speed above its design rating.  One approach to achieve higher motor speed is to regulate the inverter power electronics to manipulate the stator d- and q-axis currents to counter the magnetic airgap flux generated by the rotor magnets.

Field-weakening control involves reducing the resulting d-axis flux, λd , by lowering the effect of the resulting air-gap flux linkage associated with the permanent magnets, λpm . This is done by driving the component of the magnetizing d-axis stator current negative in the PMSM as shown in Figure 1 below.

Fig. 1 Vector representation of resulting d-axis flux, λd

The torque speed characteristic curve in Figure 2 shows that the motor’s back EMF (stator voltage) rises in proportion to the motor speed. This behavior occurs in the constant torque region of the PMSM, where field-oriented control (FOC) is an accepted way to regulate the motor. However, when the speed continues to rise, the applied voltage reaches maximum and the back EMF voltage exceeds the applied voltage, preventing the motor speed from increasing. To increase the motor speed above its base speed, field-weakening mode is used while maintaining a constant output power, which is the product of torque and motor speed. During field-weakening, the motor can rotate faster at the maximum available voltage, at the expense of reduced maximum torque.

Fig. 2 Torque and speed characteristic of PMSMs

Figure 3 illustrates the field-weakening control operation as the intersection of the voltage limit ellipse and the current limit circle on the left-hand side of the stator currents (id, iq) plane.

Fig. 3 Voltage and current limit of a PMSM

To understand field-weakening, the current vector trajectories can be evaluated using trajectories that bound the field-weakening region OABC. Trajectory I along OA is the maximum torque per ampere (MTPA) curve, where MTPA can be achieved by manipulating the current vector trajectory to match the OA curve. Trajectory II follows the current limit circle from A to B. The current limit is defined by the constraints of the DC bus and power electronics. Trajectory III represents deep field-weakening along BC, the maximum torque per volt (MTPV) curve.  During MTPV operation, the motor generates the maximum speed and torque allowed within the voltage constraint ellipse, which is bounded by the DC bus.  Regardless of the torque transient response, the optimized field-weakening trajectories or operating points are always located within the grey area.

Figure 4 shows the system-level block diagram for field-weakening control of a PMSM in Simulink®. The outer speed control loop generates a torque command as an input for the MTPA field-weakening control block. The inner current loop is composed of Clarke and Park transformations and a space vector generator.

Fig. 4 Overview of PMSM field-weakening control

KINETIC POWER SYSTEM provides reference examples showing field-weakening control, and code generation deployment to help you implement field-weakening control using Simulink.

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