Q5Electric Drives and Their Control

Regenerative Braking of Synchronous Motor with VSI

Regenerative braking of a synchronous motor driven by a Voltage Source Inverter is achieved by reversing the direction of real power flow through the VSI: during normal motoring operation, the VSI draws power from the DC link and delivers it to the synchronous motor's stator windings, but during braking, the motor (now driven mechanically by the decelerating or overhauling load, acting as a generator) delivers power back through the VSI to the DC link, and provided the front-end converter (connecting the DC link to the AC supply) is itself capable of bidirectional power flow (such as a back-to-back PWM converter rather than a simple diode rectifier), this recovered energy can be returned to the AC supply rather than being wasted as heat.

During this regenerative braking process, the VSI's control system must appropriately adjust the phase and magnitude of the stator voltage/current relative to the synchronous motor's rotor field position to produce a negative (retarding) torque while maintaining synchronism with the rotor's continuously changing (decelerating) speed, typically requiring rotor position feedback (from an encoder or a sensorless position-estimation algorithm) to correctly orient the stator excitation throughout the braking maneuver, exactly as required for normal self-controlled synchronous motor operation but with the power flow and torque direction reversed.

Control Characteristic of Open-Loop V/f Controlled Synchronous Motor

In an open-loop V/f controlled synchronous motor drive, the inverter's output voltage magnitude is varied in direct proportion to output frequency, maintaining a constant V/f ratio (and hence approximately constant air-gap flux) below base frequency, exactly analogous to induction motor V/f control. Since synchronous motor speed is rigidly locked to applied stator frequency (unlike an induction motor, which always operates at some finite slip), the V/f-controlled synchronous motor's speed is set directly and precisely by the commanded frequency.

A critical distinguishing characteristic of open-loop V/f control for a synchronous motor, compared to an induction motor, is the risk of loss of synchronism (pole-slipping): if the commanded frequency changes too rapidly, or if load torque momentarily exceeds the motor's maximum developed (pull-out) torque at the present flux and load angle, the rotor can fall out of synchronism with the rotating stator field entirely, causing the motor to stall or oscillate uncontrollably rather than simply settling to a new slip as an induction motor would. This is why open-loop V/f control of synchronous motors requires more conservative frequency ramp-rate limiting than induction motor V/f control, and why many practical synchronous motor drives instead employ closed-loop, self-controlled operation (using rotor position feedback) to directly track and maintain the correct rotor-to-stator-field angular relationship, avoiding this open-loop synchronism-loss risk entirely.

Regenerative braking capability and open-loop V/f control together represent two important but distinct aspects of synchronous motor drive design: regenerative braking addresses how energy is recovered during deceleration or an overhauling load condition, while the V/f control characteristic addresses how the drive maintains stable, synchronized operation across its normal motoring speed range, with the synchronism-loss risk inherent to open-loop V/f control being a fundamental limitation that closed-loop, self-controlled synchronous motor drives (discussed in relation to another question in this examination) are specifically designed to eliminate.

It is worth emphasizing that regenerative braking capability in a VSI-fed synchronous motor drive fundamentally depends on the front-end converter (rectifying the AC mains supply to the DC link that feeds the VSI) also being capable of bidirectional power flow, since during regenerative braking the mechanical energy of the decelerating synchronous motor is converted to electrical energy by the machine acting as a generator, delivered to the DC link by the VSI operating in its inverting-from-motor-side, rectifying-from-supply-side mode, and this recovered energy must then be returned to the AC supply through the front-end converter - if the front-end converter is a simple uncontrolled diode bridge rectifier (as in many cost-sensitive drive applications), this reverse power flow to the supply is not possible, and regenerative braking energy must instead be dissipated in a braking resistor connected across the DC link, exactly as in dynamic braking, unless a fully bidirectional front-end converter is used.

The open loop V/f controlled synchronous motor drive, discussed alongside regenerative braking here, differs importantly from the self-controlled synchronous drive in that the stator supply frequency is set directly by an independent oscillator or reference signal following a fixed V/f profile, rather than being derived from a rotor position sensor; this means the rotor must accelerate and pull into synchronism with the applied rotating stator field entirely on its own, typically requiring either a damper (amortisseur) winding to provide induction-motor-like asynchronous starting torque, or a carefully ramped frequency profile that increases slowly enough for the rotor to track the slowly accelerating synchronous speed without slipping. Once synchronized, the open loop V/f drive maintains an approximately constant ratio of stator voltage to frequency (identical in principle to the induction motor V/f scheme) so as to maintain approximately constant air-gap flux and hence approximately constant available torque capability across the controlled speed range, but because the control is entirely open loop with respect to rotor position and torque angle, a sudden increase in load torque demand can cause the rotor to fall out of step (lose synchronism) if the resulting torque angle exceeds the machine's pull-out torque limit, at which point the motor stalls or continues to rotate asynchronously rather than in synchronism with the applied stator frequency - this vulnerability to pull-out under load disturbances is the fundamental practical limitation that self-controlled (rotor-position-feedback) synchronous drives are specifically designed to eliminate.

It is further worth noting that the choice between armature voltage control (analogous to voltage-controlled operation below base speed) and field-weakening-like flux reduction (analogous to operation above base speed) in a V/f-controlled synchronous drive follows broadly the same constant-torque, constant-power logic described for DC drives: below base speed, the V/f ratio is held constant so that flux (and hence available torque per ampere) remains at its rated value, giving a constant-torque operating region, while above base speed, the applied voltage is held at its maximum (rated) value while frequency continues to increase, causing the effective flux to fall in inverse proportion to speed and giving a constant-power operating region similar in principle to field weakening in a DC or wound-field synchronous machine, though the practical realization in a synchronous machine additionally depends on maintaining an adequate margin between the operating torque angle and the pull-out torque angle at all times to avoid loss of synchronism, a consideration that has no direct counterpart in DC motor field weakening.