--- title: "Shunt generator build-up conditions, critical speed and resistance" subject: "Electrical Machines I" chapter: 3 key: "semester_4/electrical_machines_i:shunt-buildup-conditions-critical" source: https://www.bctengineer.com/q/semester_4/electrical_machines_i/shunt-buildup-conditions-critical site: BCT Engineering --- # Shunt generator build-up conditions, critical speed and resistance Electrical Machines I · Chapter 3: DC Generator - Asked in: 2079 Chaitra, 6+2 marks - Video: [voltage build up in self excited DC generator | occ | critical field resistance and critical speed](https://www.youtube.com/watch?v=36JNLiG45gk) — Reddaiah Polisetty - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_4/electrical_machines_i/shunt-buildup-conditions-critical - Whole chapter: https://www.bctengineer.com/md/semester_4/electrical_machines_i/chapter-3 ## Question What is the process of voltage build up in dc shunt generator and what are the conditions to be satisfied for the voltage build up? Also find the critical speed and critical field resistance of the generator. ## Short answer Voltage builds from residual flux: a small EMF drives field current that aids flux, raising EMF cumulatively until the OCC intersects the field resistance line. Conditions: residual magnetism, correct field connection, Rf < Rc, speed > Nc. Critical resistance is slope of tangent to OCC; critical speed Nc = N * Rf / Rc. ## Answer #### Process of voltage build-up A self-excited shunt generator uses its own output to supply its field. 1. The field poles retain a small **residual flux** $\phi_r$. When the armature is driven, a small EMF $E_r = \frac{P\phi_rZN}{60A}$ (a few volts) appears. 2. $E_r$ drives a small current through the shunt field, which is connected across the armature. 3. If this current's MMF **aids** the residual flux, flux rises, so EMF rises. 4. Higher EMF gives more field current, which gives more flux and more EMF: the voltage builds up cumulatively. 5. Build-up stops at the point where the **OCC** cuts the **field resistance line** ($V = I_fR_f$). Beyond that point, the EMF generated is less than that needed to drive more field current, so the voltage settles there. ```text E ^ Rf line | / | / ___---- OCC | / .-' | /.' <-- operating voltage | .'/ | .' / | .' / Er +'---/------------------> If ``` #### Conditions for voltage build-up 1. **Residual magnetism** must be present in the poles. (If lost, "flash" the field from a battery.) 2. **Correct field connection:** field MMF must aid the residual flux; otherwise the residual flux is wiped out. If wrong, reverse field terminals or the direction of rotation. 3. **Field resistance less than critical resistance** $R_c$. 4. **Speed above critical speed** $N_c$ for the given field resistance. 5. **No load or high-resistance load** at starting (load resistance above critical load resistance), so that the terminal voltage is not pulled down. 6. Brushes on the correct neutral axis with good contact. #### Finding critical field resistance and critical speed No numerical OCC data is given, so the standard graphical method is described. **Critical field resistance:** plot the OCC at rated speed $N$. Draw a **tangent from the origin** to the initial straight part of the OCC. Its slope is the critical resistance: $$ R_c = \frac{\Delta E}{\Delta I_f}\Big|_{\text{tangent}} $$ **Critical speed:** for the actual field resistance $R_f$, draw the field line of slope $R_f$. Take any current $I_f$ on the linear part; read the tangent voltage $E_c$ (= $R_cI_f$) and the field-line voltage $E_f$ (= $R_fI_f$). Since EMF ∝ speed: $$ N_c = N\,\frac{E_f}{E_c} = N\,\frac{R_f}{R_c} $$ **Example:** OCC at 1000 rpm gives 120 V at $I_f = 0.6$ A on the linear part, so $R_c = 120/0.6 = 200\ \Omega$. With $R_f = 160\ \Omega$, $N_c = 1000 \times 160/200 = 800$ rpm. The generator builds up only above 800 rpm, or at 1000 rpm only if $R_f < 200\ \Omega$. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. If the field MMF opposes residual flux, what happens? A. Residual flux is wiped out and voltage does not build up B. Voltage builds up faster C. Speed rises D. Nothing changes Answer: A. Correct connection is needed; reverse field terminals or direction of rotation to fix it. (Key point: Wrong field connection) 2. For build-up, field circuit resistance must be: A. Less than the critical resistance B. Greater than the critical resistance C. Equal to armature resistance D. Zero Answer: A. Rf > Rc puts the field line above the OCC, so only residual voltage results. (Key point: Condition on resistance) 3. Critical speed is found from Nc = N x: A. Rf/Rc B. Rc/Rf C. Rf x Rc D. Ef/Rc Answer: A. EMF is proportional to speed, so Nc = N Ef/Ec = N Rf/Rc. (Key point: Critical speed method) 4. OCC at 1000 rpm gives 120 V at If = 0.6 A and Rf = 160 ohm. Nc is: A. 800 rpm B. 1250 rpm C. 625 rpm D. 960 rpm Answer: A. Rc = 120/0.6 = 200 ohm; Nc = 1000 x 160/200 = 800 rpm. (Key point: Numerical example) ## Related in Electrical Machines I - [Shunt generator voltage build-up, critical resistance and speed](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines_i%3Ashunt-buildup-critical-define) — Electrical Machines I, chapter 3, asked 9× - [Starting shunt generator on no load and voltage build-up](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines_i%3Ashunt-generator-no-load-buildup) — Electrical Machines I, chapter 3 - [Critical resistance, critical speed and shunt generator characteristics](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines_i%3Acritical-resistance-speed-shunt-characteristics) — Electrical Machines I, chapter 3 - [Voltage build-up in isolated induction generator](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines_i%3Ainduction-generator-isolated-buildup) — Electrical Machines I, chapter 5 - [Shunt motor speed with 30 Ω field resistance added](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines_i%3Ashunt-motor-220v-30ohm-field-torque) — Electrical Machines I, chapter 4, asked 5× - [Shunt motor speed with 50 Ω field resistance (1200 rpm)](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines_i%3Ashunt-motor-250v-25a-half-load) — Electrical Machines I, chapter 4, asked 5× ## Related in other subjects - [Voltage build-up, critical resistance and speed](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines%3Avoltage-buildup-critical-resistance-speed) — Electrical Machines, chapter 3, asked 3× - [Voltage build-up; series generator at no load](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines%3Avoltage-buildup-series-generator-no-load) — Electrical Machines, chapter 3 - [DC shunt motor speed at reduced voltage and starting resistance](https://www.bctengineer.com/question.md?key=semester_7%2Futilization_of_electrical_energy%3Adc-shunt-reduced-voltage-starting-resistance) — Utilization of Electrical Energy, chapter 3 - [Voltage build-up in induction generator](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines%3Ainduction-generator-voltage-buildup) — Electrical Machines, chapter 5