Q5Electric Drives and Their Control

VSI Fed Self-Controlled Synchronous Motor Drive

A self-controlled (also called load-commutated or rotor-position-controlled) synchronous motor drive uses a rotor position sensor (or a sensorless position-estimation technique) to directly determine the exact instantaneous rotor position, and uses this position information to control the precise instants at which the VSI's semiconductor switches are gated, ensuring the stator's rotating magnetic field is always maintained at the correct angular relationship relative to the rotor's own field, regardless of the motor's actual instantaneous speed.

This self-controlled (rotor-position-synchronized) operating principle fundamentally distinguishes such a drive from an open-loop V/f controlled synchronous motor drive (discussed in relation to another question in this examination), since the inverter's output frequency in a self-controlled drive is not an independently commanded setpoint but is instead directly slaved to the rotor's own actual mechanical speed via the position feedback loop, completely eliminating the loss-of-synchronism (pole-slipping) risk inherent to open-loop V/f control, since the stator field can never become desynchronized from the rotor by definition of the control scheme itself - this is precisely why self-controlled synchronous motor drives (often described as functioning similarly to a 'brushless DC motor' at a conceptual level, since the position-feedback-based commutation principle is directly analogous) are preferred over open-loop V/f control for demanding, high-performance synchronous motor drive applications requiring reliable operation across a wide speed and load range without any synchronism-loss risk.

Dynamic Braking of Synchronous Motor with VSI

Dynamic braking of a VSI-driven synchronous motor is achieved by disconnecting the DC link from the AC supply (or otherwise preventing power from flowing back to the supply) and instead dissipating the motor's regenerated braking energy as heat in a dedicated braking resistor connected across the DC link, switched in via a chopper circuit whenever the DC link voltage rises above its normal operating level (as it would during regenerative braking when the motor delivers power back into the DC link faster than the load side can absorb it) - this provides a simpler, lower-cost braking solution than full regenerative braking (which requires a bidirectional front-end converter capable of returning power to the AC supply), at the cost of wasting the braking energy as heat rather than recovering it, making dynamic braking an appropriate choice for applications where braking occurs only occasionally or where the additional cost and complexity of a fully regenerative front-end converter is not economically justified by the relatively modest energy-recovery benefit it would provide.

Self-controlled synchronous motor drive operation and dynamic braking with VSI together illustrate two complementary aspects of practical synchronous motor drive implementation: the self-controlled operating principle ensures reliable, synchronism-loss-free operation throughout the drive's normal speed range, while the dynamic braking capability provides a straightforward, cost-effective means of dissipating the motor's kinetic energy during deceleration whenever the additional cost and complexity of a fully regenerative front-end converter is not economically justified for the specific application at hand.

It is useful to further note that the self-controlled synchronous motor drive discussed here is the fundamental operating principle underlying the widely used brushless DC motor and the load-commutated-inverter synchronous motor drives used in very large power ratings (tens of megawatts) such as large compressor and pump drives, precisely because self-control (deriving the inverter firing signals from a rotor position sensor so that the stator MMF is always maintained at the correct angle relative to the rotor) eliminates the risk of the motor pulling out of synchronism that would otherwise occur under an open-loop, fixed-frequency supply if the mechanical load torque were to change suddenly, making self-controlled synchronous drives inherently as robust to load disturbances as a DC motor drive, unlike open-loop V/f-controlled synchronous motor drives which remain vulnerable to pull-out under sudden load changes.

The dynamic braking of a synchronous motor fed from a VSI proceeds by disconnecting the machine from the inverter output and connecting a braking resistor bank across the stator terminals instead, while the rotor field excitation is maintained (or, in a permanent magnet synchronous machine, is inherently always present); as the rotor continues to rotate under its own mechanical inertia, the rotating rotor field induces a three-phase EMF in the stator windings exactly as in normal generator action, and this induced EMF drives current through the connected braking resistors, dissipating the kinetic energy of the rotating system as heat and producing a retarding electromagnetic torque that opposes the rotor's motion. The braking torque magnitude for a given speed depends on the value of the braking resistance selected - a lower resistance draws higher current for a given induced EMF, giving higher braking torque but also higher instantaneous power dissipation in the resistor, while a higher resistance gives gentler, lower braking torque; because the induced EMF is directly proportional to rotor speed for constant excitation, the braking torque (and hence deceleration rate) is largest at high speed and progressively diminishes as the machine slows down, giving dynamic braking of a synchronous machine a naturally decaying, non-uniform deceleration profile unless the braking resistance or field excitation is actively varied during the braking interval to compensate and maintain more uniform braking torque throughout the speed range.

It is also worth comparing dynamic braking against the regenerative braking method discussed for the preceding question in this same examination: while regenerative braking is more energy-efficient since it returns recovered energy to the supply rather than dissipating it as heat, dynamic braking has the practical advantage of not requiring the front-end converter or supply-side infrastructure to support reverse power flow at all, making it usable even with the simplest, lowest-cost front-end rectifier arrangements, and dynamic braking can typically bring the machine to a complete stop reliably using only a passive resistor bank and a simple switching contactor, without requiring the more sophisticated, actively controlled bidirectional converter control needed to achieve stable regenerative braking, particularly at low speeds where the induced EMF (and hence available regenerative braking torque) becomes small.

In summary, dynamic braking of a synchronous motor via a VSI-connected resistor bank provides a simple, reliable means of bringing the drive to rest using only passive components once disconnected from the inverter, complementing the self-controlled VSI drive and open-loop V/f drive architectures discussed elsewhere in this examination as the standard set of operating and braking modes expected for synchronous motor drives in this unit.