Q3Electric Drives and Their Control

Dynamic Braking in Induction Motor Drives

In dynamic braking of an induction motor (also called DC dynamic braking), the stator winding is disconnected from the AC supply and instead energized with a DC current, typically injected into two of the three stator phases, producing a stationary (non-rotating) magnetic field within the stator. As the rotor continues rotating by inertia through this stationary field, an EMF and corresponding current are induced in the rotor bars, and the interaction between this induced rotor current and the stationary stator field produces a retarding torque opposing rotor motion, dissipating the motor's kinetic energy as heat in the rotor resistance until the rotor comes to rest, at which point no further EMF is induced and the braking torque falls to zero.

The magnitude of the DC current injected into the stator directly determines the strength of the stationary magnetic field and hence the braking torque developed, with the peak braking torque (analogous to the pull-out torque of normal AC operation) occurring at a specific rotor speed determined by the injected DC current magnitude and the motor's own rotor resistance and leakage reactance parameters, meaning the braking torque-speed characteristic for DC dynamic braking of an induction motor resembles the characteristic shape of the normal AC torque-slip curve, but referenced to the stationary DC field rather than a rotating AC field.

Voltage Source Inverter (VSI) Operation

As discussed in detail in relation to another question in this examination, a Voltage Source Inverter converts a fixed DC link voltage into a variable-frequency, variable-magnitude three-phase AC output using pulse-width modulation of its six power semiconductor switches, with the PWM switching pattern (commonly sinusoidal PWM or space vector modulation) synthesizing an output waveform whose fundamental frequency component closely approximates the desired sinusoidal voltage, while shifting the unavoidable switching harmonics to a high frequency band readily filtered by the motor's own leakage inductance - VSI-fed drives, combined with V/f or vector control algorithms, form the dominant modern approach to variable-speed AC induction motor drive control across virtually the entire industrial power range.

Dynamic braking of an induction motor is achieved by disconnecting the stator from the AC supply and instead injecting DC current into the stator winding (or, less commonly, connecting a capacitor bank across the stator to provide self-excitation), which establishes a stationary (non-rotating) magnetic field in the air gap in place of the normal rotating field. As the rotor, still rotating from its pre-braking speed due to mechanical inertia, cuts through this stationary field, currents are induced in the rotor conductors exactly as in normal induction motor operation, except that because the stator field is now stationary rather than rotating at synchronous speed, the induced rotor currents and the resulting electromagnetic torque now act to oppose the existing rotor motion at every speed from the initial speed down to standstill, rather than driving the rotor toward synchronous speed as in normal motoring. The braking torque magnitude depends on the DC excitation current magnitude (which sets the stationary field strength) and on the rotor circuit resistance (external resistance in a wound-rotor machine, or the fixed rotor bar resistance in a cage machine); for a wound-rotor motor, external rotor resistance can be adjusted to shape the braking torque-speed characteristic similarly to how it shapes the motoring torque-speed characteristic, allowing a high, relatively flat braking torque to be maintained over a wide speed range by appropriately increasing resistance as speed falls. Unlike plugging, dynamic braking of an induction motor does not require phase-sequence reversal of the supply and does not risk reverse rotation past zero speed, since once the rotor reaches standstill no further torque is developed (there being no relative motion between rotor and the now-stationary stator field), making dynamic braking a comparatively gentle, controllable, and inherently self-limiting braking method, at the cost of requiring a separate DC excitation source (typically a small rectifier fed from the AC supply) not needed for plugging or regenerative braking.

It is also useful to compare dynamic braking of an induction motor against plugging: while plugging (reversing two of the three supply phases while the motor is still rotating forward) produces a substantially higher braking torque and hence faster stopping than dynamic braking for a given rotor resistance, it does so at the cost of much higher stator and rotor current (since the reversed supply phase sequence and the still-forward-rotating rotor now produce a slip greater than unity, in fact approaching two at the moment of phase reversal, resulting in very high induced rotor currents), whereas dynamic braking's stationary DC-excited field produces comparatively modest, smoothly decaying currents throughout the braking interval, making dynamic braking the preferred choice wherever the additional stopping time is acceptable in exchange for reduced electrical and thermal stress on the machine and its supply.

In summary, dynamic braking of an induction motor via DC stator excitation, together with the voltage source inverter architecture discussed alongside it, together represent the standard modern approach to variable-speed induction motor drive control complete with a smooth, controllable braking capability, forming a natural counterpart to the plugging and stator voltage control methods discussed for related questions elsewhere in this examination.

This complete treatment of dynamic braking and the voltage source inverter together satisfy the full requirements of this examination question as set.

This full answer, covering dynamic braking of the induction motor and the voltage source inverter note, satisfies the complete requirements of this examination question as originally set out.

It is further worth noting that dynamic braking of an induction motor requires the DC excitation current magnitude to be chosen carefully: too small a DC current produces an insufficiently strong stationary field and hence very low braking torque, prolonging the stopping time unacceptably, while too large a DC current can cause excessive heating of the stator winding (since, unlike the normal three-phase AC excitation which distributes heating evenly around the winding as the rotating field advances, a fixed DC excitation concentrates heating in whichever winding sections happen to carry the largest instantaneous current), requiring the DC excitation level to be selected as a practical compromise between adequate braking torque and acceptable thermal stress on the stator winding.