ELECTRIC BRACKING OF DC MOTOR
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1. Overview
2. 30+ Important One-liners (Exam focused)
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Overview
This short guide lists exam-oriented one-liners and memory tricks for DC motor braking methods (plugging, rheostatic/dynamic, regenerative), practical resistor selection tips, common formulas and safety points — curated for AE/JE style questions.
30+ Important One-liners (Exam-focused)
Plugging: Reverse supply polarity to the armature quickly - produces large braking torque but torque and current spike; use only for short durations. (Exam: "fast stop" method)
Rheostatic (dynamic) braking: Connect armature to resistor — kinetic energy converts to heat in resistor; ideal when supply can’t accept regenerated power.
Regenerative braking: When armature acts as generator and returns energy to supply or DC link — requires suitable converter/field conditions for energy flow back to source.
Braking torque relation: Braking torque in DC motors is approximately proportional to armature current — remember "Torque ↔ Current".
Back EMF & braking: During dynamic braking the back-EMF becomes the source that drives current through the braking resistor — resistor sizing controls braking current.
Speed dependence: In rheostatic braking, current (thus torque) roughly falls with speed because back-EMF reduces loop voltage — high speed gives stronger braking current initially.
Torque spike in plugging: Plugging produces torque opposite to rotation but doubles current — use time-limited contactors or timers to avoid armature damage.
Field weakening impact: Stronger field increases generated voltage in regenerative braking; weak field can prevent regeneration — exam tip: check field excitation condition.
Braking resistor selection: Start with resistor giving safe maximum current at rated speed; power rating must handle energy for expected stopping interval.
Energy calculation trick: Kinetic energy ≈ 0.5·J·Ï‰² — estimate resistor duty by converting motor speed to rad/s and using rotor inertia J (easy exam trick).
DC injection (for AC motors vs DC motors): DC injection is used to stop AC motors; avoid confusing with DC motor braking methods in exam answers.
Eddy-current brakes: Contactless braking using magnetic field — used where mechanical wear must be avoided; often mentioned in comparisons with rheostatic methods.
Thermal duty: Braking resistor must be sized for short bursts—look for 'duty cycle' or 'energy in kJ' questions in AE/JE papers.
Braking time formula tip: Use average braking torque and initial kinetic energy to estimate stopping time — common short exam problem approach.
Regeneration limits: Regenerative braking requires supply/system able to absorb returned power (battery or DC link); otherwise use rheostatic method.
Safety interlocks: Always put mechanical contactors with time delay in plugging circuits to prevent continuous reverse connection — exam loves safety-first answers.
Contactor wear trick: Frequent plugging shortens contactor life — choose soft stops (rheostatic/regenerative) where frequent stopping is needed.
Losses during braking: In rheostatic braking energy is dissipated as heat — mention resistor ventilation in practical/maintenance questions.
Control methods: Modern braking uses choppers or converters to control braking current — note difference between simple resistor and controlled chopper-based rheostatic braking.
Generator mode check: For regenerative braking ensure generated voltage > supply voltage so current flows back — quick exam check step to pick correct method.
Braking in series motors: Series DC motors show strong braking but may behave unpredictably at low speeds—state-specific exam cautions.
Armature reaction: During abrupt braking armature flux changes may affect commutation—ensure brushes and commutator condition questions addressed in exams.
Dynamic braking controller: Always mention a resistor switch or chopper in answers asking 'how to control braking torque' for modern exam marks.
Mechanical brakes: For safety-critical stops combine electrical braking with mechanical brake — exam will usually favor 'electrical + mechanical' for emergency stops.
Indicator lamps: Use indicator to show 'braking engaged' for maintenance and fault isolation questions — small detail that saves marks in viva or descriptive answers.
Regenerative efficiency: Regenerative is energy-efficient and reduces wear — mention in comparison questions (regenerative > rheostatic economically).
Voltage and resistor match: Quick trick — lower resistor ⇒ higher current ⇒ higher torque but more heat; exam answers often ask 'choose resistor' using this logic.
Testing after braking retrofit: Perform insulation and thermal rise tests for new braking resistor installations — typical AE/JE practical test question.
Braking in DC drive systems: When using power electronics, regeneration may be handled by converters — mention converter/rectifier compatibility in answers.
Short trick for remembering methods: "Plug, Resist, Return" → Plugging, Rheostatic (resist), Regenerative (return energy) — quick recall for MCQs.
Exam tip: If asked 'fastest stop' choose Plugging (but mention high stress); if asked 'safest frequent braking' choose Rheostatic or Regenerative.
Maintenance tip (exam-friendly): Always ensure resistor cooling, contactor health and field supply before approving a braking system for field use.
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