--- title: "Rotor rheostat speed control of slip ring motor" subject: "Electrical Machines" chapter: 5 key: "semester_4/electrical_machines:rotor-rheostat-speed-control" source: https://www.bctengineer.com/q/semester_4/electrical_machines/rotor-rheostat-speed-control site: BCT Engineering --- # Rotor rheostat speed control of slip ring motor Electrical Machines · Chapter 5: Three Phase Induction Machines - Asked in: 2065 Chaitra (old course), 8 marks - Video: [IM 23 Rotor Resistance Method of Speed Control](https://www.youtube.com/watch?v=QjrpFiUsiPQ) — Lectures in Electrical Engineering - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_4/electrical_machines/rotor-rheostat-speed-control - Whole chapter: https://www.bctengineer.com/md/semester_4/electrical_machines/chapter-5 ## Question Explain the rotor rheostat method of speed control of slip ring induction motor with neat circuit diagram and T-S characteristic. ## Short answer For wound-rotor motors, external resistance added in the rotor circuit increases slip for the same load torque, reducing speed below synchronous. It provides smooth control and high starting torque, but wastes power as I²R loss, lowering efficiency. ## Answer #### Principle In a slip-ring (wound-rotor) induction motor, extra resistance can be inserted in the rotor circuit through the slip rings and brushes, so the rotor resistance becomes $R_2 + R_{ext}$. This changes the slip at which a given load torque is produced, and therefore the speed. The speed can be lowered below normal, but not raised above it. #### Circuit diagram ```text 3-phase supply R ---+----+----+ Y ---+--+-|--+-|--+ B ---+--|-|--|-|--+ [ STATOR ] [ ROTOR ] winding ------ winding | | | slip rings brushes | | | [Rext] 3-phase rheostat (Y) ``` #### Torque-slip characteristics ```text T | TL ------------------------ load line | _.-*-._ _.-*-._ _.-*-._ | / R2 \ R2+r1 R2+r2 +--+----+-----+---+----+------> s 1 s3 s2 s1 (low speed to high) ``` $T_{max}$ is independent of the rotor resistance, so increasing the rotor resistance only shifts the peak towards the high-slip side. For a constant load torque $T_L$ (in the low-slip region, where $T \propto s/R_2$): $$ \frac{R_2}{s_1} = \frac{R_2 + R_{ext}}{s_2} $$ (the torque is constant, so $R_2/s$ is constant). So $s_2 = s_1\frac{R_2+R_{ext}}{R_2}$, and the speed $N = N_s(1-s_2)$ decreases as $R_{ext}$ increases. #### Operation 1. With $R_{ext} = 0$ the motor runs at normal speed with a small slip $s_1$. 2. When $R_{ext}$ is inserted using the rheostat, the T-S curve flattens and the operating point on the load line moves to a larger slip, so the speed falls. 3. The more resistance inserted, the lower the speed. #### Merits and demerits - Merits: simple, smooth control, good starting torque, and the same arrangement is used for starting. - Demerits: power is lost in the rheostat (part of the rotor copper loss $sP_g$), so efficiency falls as speed is reduced; poor speed regulation (speed varies with load); only speed reduction is possible; the rheostat is bulky and expensive; not suitable for light loads (little change in speed). ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. The rotor rheostat method can: A. Only lower the speed below normal B. Only raise speed above N_s C. Raise and lower speed D. Not change speed Answer: A. Added resistance increases slip, so speed only falls. (Key point: Direction of control) 2. For constant load torque, R_2/s stays constant, so: A. s_2 = s_1 R_2/(R_2 + R_ext) B. s_2 = s_1 (R_2 + R_ext)/R_2 C. s_2 = s_1 D. s_2 = 1/s_1 Answer: B. R_2/s_1 = (R_2 + R_ext)/s_2. (Key point: Constant torque relation) 3. With external resistance, the T-S peak: A. Rises in magnitude B. Shifts toward higher slip with same T_max C. Falls in magnitude D. Shifts toward s = 0 Answer: B. T_max is independent of R_2 while s_m increases. (Key point: Max torque with R_ext) 4. A main demerit of this method is: A. It raises speed B. Loss in rheostat reduces efficiency at low speed C. It cannot start the motor D. It needs no slip rings Answer: B. Power s P_g is lost in the rheostat. 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