Motor Control Edition Beyond Bits Motor Control Edition

Permanent Magnet Synchronous Motor Control High-performance and power-efficient motor control

Introduction Figure 1: Field-Oriented Control Vector Explanation Permanent magnet synchronous

motors (PMSM) are typically used for The “d” axis refers Axis of phase B high-performance and high-efficiency to the “direct” axis of the rotor flux motor drives. High-performance motor The “q” axis is the axis of Rotor position control is characterized by smooth motor torque along against phase A rotation over the entire speed range which the stator field (measured by must be developed position sensor) of the motor, full torque control at N

zero speed, and fast acceleration and Rotation Axis of phase A deceleration. To achieve such control, S vector control techniques are used for PM synchronous motors. The vector Stator windings control techniques are usually also referred to as field-oriented control (FOC). The basic idea of the vector maximal, when they are perpendicular How to Simplify Control control algorithm is to decompose to each other. It means that we want of Phase Currents to a stator current into a magnetic to control stator current in such a Achieve Maximum Torque field-generating part and a torque- way that creates a stator vector DC motor control is simple because generating part. Both components perpendicular to rotor magnets. As all controlled quantities are DC values can be controlled separately after the rotor spins we must update the in a steady state and current phase/ decomposition. Then, the structure stator currents to keep the stator flux angle is controlled by a mechanical of the motor controller (vector control vector at 90 degrees to rotor magnets commutator. How can we achieve that controller) is almost the same as a at all times. The reactance torque of in PMSM control? separately excited DC motor, which an interior PM type PMSM (IPMSM) simplifies the control of a permanent is as follows, when stator and rotor DC Values/Angle Control magnet synchronous motor. magnetic fields are perpendicular. First, we need to know the rotor Let’s start with some basic Torque= position. The position is typically 32pplPMIqs FOC principles. related to phase A. We can use – Number of pole pairs pp an absolute position sensor (e.g., Torque Generation – Magnetic flux of the permanent lPM resolver) or a relative position sensor A reactance torque of PMSM is magnets (e.g., encoder) and process called generated by an interaction of two – Amplitude of the current in alignment. During the alignment, the Iqs magnetic fields (one on the stator and quadrature axis rotor is aligned with phase A and we one on the rotor). The stator magnetic know that phase A is aligned with As shown in the previous equation, field is represented by the magnetic the direct (flux producing) axis. In reactance torque is proportional to the flux/stator current. The magnetic field this state, the rotor position is set to amplitude of the q-axis current, when of the rotor is represented by the zero (required voltage in d-axis and magnetic fields are perpendicular. magnetic flux of permanent magnets rotor position is set to zero, static that is constant, except for the field MCUs must regulate the phase stator voltage vector, which causes that rotor weakening operation. We can imagine current magnitude and at the same attracted by stator magnetic field and those two magnetic fields as two bar time in phase/angle, which is not such to align with them [with direct axis]). magnets, as we know a force, which an easy task as DC motor control. tries to attract/repel those magnets, is Beyond Bits Motor Control Edition

1. Three-phase quantities can Figure 2: Basic Principle of Field-Oriented Control transform into equivalent two-phase quantities (stationary reference Measured Current Generated Voltage frame) by Clarke transformation. 2. Then, we transform two-phase

quantities into DC quantities by rotor Phase A α d d α Phase A Three-Phase Stationary Rotating electrical position into DC values Phase B Phase B to β to q q to β SVM Phase C Two-Phase Rotating Stationary Phase C

(rotating reference frame) by Control Process Park transformation. The electrical rotor position is a Electrical rotor Electrical rotor position position mechanical rotor position divided by numbers of magnetic pole pairs pp. 7. Using the space vector modulation, voltage. The calculation of the BEMF After a control process we should the output three-phase voltage observer requires math computation generate three-phase AC voltages is generated as multiply accumulation, division, on motor terminals, so DC values sin/cos, sqrt which is suited for DSCs, of the required/generated voltage A complete FOC speed PMSM control Kinetis ARM core-based MCUs or the should be transformed by inverse structure with Freescale motor control Power Architecture family. Park/Clarke transformations. library functions is shown in the Beyond Bits: Motor Control Edition article titled, Amplitude Control “Industrial/Appliance PMSM Drive.” Field/Flux Weakening All quantities are now DC values, Control which are easy to control, but how do Sensorless Control The operation beyond the machine we control them in magnitude? The rotor position information is base speed requires the PWM inverter For magnitude control we use PI needed to efficiently perform the to provide output voltages higher than controllers in the cascade structure. control of the PMS motor, but a rotor its output capability limited by its DC We can control many state variables position sensor on the shaft decreases link voltage. To overcome the base as phase current (torque loop), speed the robustness and reliability of the speed limitation, a field-weakening or position as with DC motors. overall system in some applications. algorithm can be implemented. A Therefore, the aim is not to use this negative d-axis required current will FOC in Steps mechanical sensor to measure the increase the speed range, but the To perform vector control: position directly but instead to employ applicable torque is reduced because some indirect techniques to estimate of a stator current limit. Manipulating 1. Measure the motor phase currents the rotor position. These estimation the d-axis current into the machine 2. Transform them into the two- techniques differ greatly in approach has the desired effect of weakening phase system (a, b) using Clarke for estimating the position or the type the rotor field, which decreases the transformation of motor to which they can be applied. BEMF voltage, allowing the higher 3. Calculate the rotor position angle stator current to flow into the motor At low speed, special techniques like with the same voltage limit given by 4. Transform stator currents into the high frequency injection or open-loop the DC link voltage. d,q-coordinate system using start-up (not very efficient) are needed Park transformation to spin the motor over the speed Freescale Enablement 5. The stator current torque (isq) and where BEMF is sufficiently high for the Reference designs, application notes flux (isd) producing components BEMF observer. Usually, 5 percent of and software solutions for PMSM are controlled separately by the base speed is enough for proper control applications are available at the controllers operation in sensorless mode. freescale.com/motorcontrol. 6. The output stator voltage space- At medium/high speed, a BEMF vector is transformed back from the observer in d/q reference frame is d,q-coordinate system into the two- used. The PWM frequency and phase system fixed with the stator control loop must be sufficiently by inverse Park transformation high to get a reasonable number of samples of phase current and DC bus Beyond Bits Motor Control Edition

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