boss_notes/T-20_Speed_Control_and_Braking.md

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← T-19: Starting Methods | 🏠 Index | T-21: Induction Generator →

T-20: Speed Control & Braking

Section: B | Priority: 🟡 MEDIUM | Exam Frequency: 4/7 years Sources: Theraja Ch-34-35, Slides L-07

Why This Topic Matters

Speed control and braking questions appeared in 4 out of 7 papers. Plugging was defined in 4 papers (2017, 2018, 2019, 2020). Speed control methods appeared in 2017, 2019, 2023. The rotor resistance speed control numerical appeared in 2021. These are short, definition-heavy questions worth 2-4 marks each.

📝 Key Definitions

Plugging: "A braking method in which the phase sequence of the stator supply is reversed while the motor is running. The stator field now rotates opposite to the rotor, producing a braking torque. The motor decelerates rapidly. The supply must be disconnected before zero speed, otherwise the motor reverses direction." — Theraja, Art. 34.44

Pull-out Torque: "The maximum torque a motor can develop at any operating speed. Also called breakdown torque. If the load torque exceeds pull-out torque, the motor stalls." — Theraja, Art. 34.24

Speed Control Methods

From $N = N_{s} (1 - s)$ and $N_{s} = 120 f / P$ :

$N = \frac{120 f}{P} (1 - s)$

Speed can be changed by varying $f$ , $P$ , or $s$ .

1. Stator Voltage Control

Reduce supply voltage $V \to$ reduced torque $\to$ higher slip $\to$ lower speed.

Simple but inefficient. Torque drops as $V^{2}$ . Only suitable for fan/pump loads (torque $\propto N^{2}$ ).

2. Supply Frequency Control (V/f)

Change $f$ to change $N_{s}$ . Must change $V$ proportionally to maintain constant flux ( $V / f$ = constant).

Wide, smooth speed range. Used in Variable Frequency Drives (VFDs). Best modern method.

3. Pole Changing

Switch winding configurations to change $P$ . Gives discrete speed steps.

Example: $P = 4 \to N_{s} = 1500$ rpm; $P = 8 \to N_{s} = 750$ rpm. Only for squirrel-cage motors.

4. Rotor Resistance Control (Slip-Ring Only)

Add external resistance $R_{e x t}$ in rotor circuit through slip rings. Higher $R_{2} \to$ higher slip at same torque $\to$ lower speed.

Disadvantage: Power $= s P_{g}$ wasted as heat in external resistors. Efficiency drops.

Electric Braking Methods

Regenerative Braking

Motor driven above $N_{s}$ ( $s < 0$ ). Acts as induction generator. Feeds power back to supply. Energy-efficient. Only above synchronous speed.

Dynamic Braking (DC Injection)

Stator disconnected from AC. DC current fed to stator. Creates stationary field. Rotating rotor cuts this field. Braking currents induced. Energy dissipated as heat.

Plugging (Counter-Current)

Reverse two stator supply leads while running. Stator field reverses direction. Braking torque opposes motion. Very rapid deceleration. Must disconnect at zero speed. At standstill, $s \approx 2$ .

Torque-speed curve showing all three operating modes

Worked Example (PYQ 2021)

2021 Q4(c): 4-pole, 50 Hz slip-ring IM. $R_{2} = 0.30 Ω$ /phase, runs at 1440 rpm full load. Find external resistance to lower speed to 1320 rpm at same torque.

$N_{s} = 120 \times 50/4 = 1500$ rpm

Full-load slip: $s_{1} = (1500 - 1440) /1500 = 0.04$

New slip: $s_{2} = (1500 - 1320) /1500 = 0.12$

Constant torque condition (low-slip approximation):

$\frac{s _{1}}{R _{2}} = \frac{s _{2}}{R _{2} + R _{e x t}}$

$\frac{0.04}{0.30} = \frac{0.12}{0.30 + R _{e x t}}$

$0.30 + R_{e x t} = \frac{0.12 \times 0.30}{0.04} = 0.90 Ω$

$R_{e x t} = 0.90 - 0.30 = 0.60 Ω/ phase$

🏆 Golden Questions (Past Exam Archive)

🎯 Q1: Define plugging and pull-out torque.

Appeared: 2017 Q2(a) — (2 marks)

Full Answer:

Plugging: A braking method. The phase sequence of stator supply is reversed while the motor runs. A braking torque opposes rotation. Motor decelerates rapidly. Supply must be cut at zero speed or the motor reverses.

Pull-out torque: The maximum torque a motor can develop. Also called breakdown torque ( $T_{m a x}$ ). If load exceeds this, the motor stalls.

🎯 Q2: Short notes on regenerative braking, dynamic braking, and plugging.

Appeared: 2018 Q5(a) — (3 marks)

Full Answer:

(i) Regenerative braking: Motor speed exceeds $N_{s}$ ( $s < 0$ ). Machine acts as generator. Power fed back to supply. Smooth, efficient. Used in cranes (lowering loads) and electric trains.

(ii) Dynamic braking: Stator disconnected from AC supply. DC fed to stator winding. Stationary magnetic field created. Rotating rotor cuts this field and braking currents are induced. Energy dissipated as heat in rotor.

(iii) Plugging: Two stator leads swapped while running. Stator field reverses. Braking torque opposes motion. Very rapid stop. Must disconnect at zero speed.

🎯 Q3: Mention speed control methods and discuss any one.

Appeared: 2017 Q2(c) — (4 marks)

Full Answer:

Speed control methods: (1) Stator voltage control, (2) Supply frequency control (V/f), (3) Pole changing, (4) Rotor resistance control.

Rotor resistance control: For slip-ring motors only. External resistance added through slip rings. From $T \propto s V^{2} / R_{total}$ (at low slip), for constant torque: $s \propto R_{total}$ . Higher resistance = higher slip = lower speed.

Disadvantage: Power $s P_{g}$ wasted as heat. Efficiency drops. Used where step-speed control and high starting torque are needed (cranes, hoists).

🎯 Q4: Rotor resistance numerical for speed reduction.

Appeared: 2021 Q4(c) — (4 marks)

Full Answer:

See Worked Example above. Result: $R_{e x t} = 0.60 Ω$ /phase.

🎯 Q5: Define braking of IM. How can speed be controlled?

Appeared: 2019 Q7(c) — (4 marks)

Full Answer:

Braking: Applying a decelerating torque to bring the motor to rest or control speed. Three methods: plugging, dynamic braking, regenerative braking.

Speed control methods:

Stator voltage control: Reduce $V$ , torque drops, speed drops. Simple but poor efficiency.
V/f control: Change both $V$ and $f$ proportionally. Wide smooth range. VFDs.
Pole changing: Switch winding configuration. Discrete speed steps. Squirrel-cage only.
Rotor resistance: Add external $R$ (slip-ring). Higher slip = lower speed. Lossy.
Slip energy recovery: Feed slip power back to supply instead of wasting. Efficient but complex.

Exam Variants

Year	Question	Marks	Focus
2017 Q2(a)	Define plugging + pull-out torque	2	Definitions
2017 Q2(c)	Speed control methods + discuss one	4	Theory
2018 Q5(a)	Three types of braking	3	Short notes
2019 Q7(c)	Define braking + speed control	4	Theory
2020 Q8(a)	Define plugging + slip	2	Definitions
2021 Q4(c)	Rotor resistance numerical	4	Numerical

⚡ Exam Tips & Common Mistakes

Plugging appears in almost every paper. Memorize a 3-sentence definition.
Rotor resistance method ONLY works for slip-ring motors. Squirrel-cage has no access to rotor.
In the speed control numerical: The key equation is $s_{1} / R_{2} = s_{2} / (R_{2} + R_{e x t})$ for constant torque.
Don't confuse regenerative braking with plugging. Regenerative = above $N_{s}$ , power flows back. Plugging = phase reversal, no power recovery.

T-15c: Torque-Speed Curves — Braking regions on the curve
T-19: Starting Methods — Related to starting current
T-21: Induction Generator — Regenerative braking = generator mode

← T-19: Starting Methods | 🏠 Index | T-21: Induction Generator →

boss_notes/T-20_Speed_Control_and_Braking.md

7.8 KB · commit d2f3b6e

← T-19: Starting Methods | 🏠 Index | T-21: Induction Generator →

T-20: Speed Control & Braking

Section: B | Priority: 🟡 MEDIUM | Exam Frequency: 4/7 years Sources: Theraja Ch-34-35, Slides L-07

Why This Topic Matters

📝 Key Definitions

Plugging: "A braking method in which the phase sequence of the stator supply is reversed while the motor is running. The stator field now rotates opposite to the rotor, producing a braking torque. The motor decelerates rapidly. The supply must be disconnected before zero speed, otherwise the motor reverses direction." — Theraja, Art. 34.44

Pull-out Torque: "The maximum torque a motor can develop at any operating speed. Also called breakdown torque. If the load torque exceeds pull-out torque, the motor stalls." — Theraja, Art. 34.24

Speed Control Methods

From $N = N_{s} (1 - s)$ and $N_{s} = 120 f / P$ :

$N = \frac{120 f}{P} (1 - s)$

Speed can be changed by varying $f$ , $P$ , or $s$ .

1. Stator Voltage Control

Reduce supply voltage $V \to$ reduced torque $\to$ higher slip $\to$ lower speed.

Simple but inefficient. Torque drops as $V^{2}$ . Only suitable for fan/pump loads (torque $\propto N^{2}$ ).

2. Supply Frequency Control (V/f)

Change $f$ to change $N_{s}$ . Must change $V$ proportionally to maintain constant flux ( $V / f$ = constant).

Wide, smooth speed range. Used in Variable Frequency Drives (VFDs). Best modern method.

3. Pole Changing

Switch winding configurations to change $P$ . Gives discrete speed steps.

Example: $P = 4 \to N_{s} = 1500$ rpm; $P = 8 \to N_{s} = 750$ rpm. Only for squirrel-cage motors.

4. Rotor Resistance Control (Slip-Ring Only)

Add external resistance $R_{e x t}$ in rotor circuit through slip rings. Higher $R_{2} \to$ higher slip at same torque $\to$ lower speed.

Disadvantage: Power $= s P_{g}$ wasted as heat in external resistors. Efficiency drops.

Electric Braking Methods

Regenerative Braking

Motor driven above $N_{s}$ ( $s < 0$ ). Acts as induction generator. Feeds power back to supply. Energy-efficient. Only above synchronous speed.

Dynamic Braking (DC Injection)

Stator disconnected from AC. DC current fed to stator. Creates stationary field. Rotating rotor cuts this field. Braking currents induced. Energy dissipated as heat.

Plugging (Counter-Current)

Reverse two stator supply leads while running. Stator field reverses direction. Braking torque opposes motion. Very rapid deceleration. Must disconnect at zero speed. At standstill, $s \approx 2$ .

Torque-speed curve showing all three operating modes

Worked Example (PYQ 2021)

2021 Q4(c): 4-pole, 50 Hz slip-ring IM. $R_{2} = 0.30 Ω$ /phase, runs at 1440 rpm full load. Find external resistance to lower speed to 1320 rpm at same torque.

$N_{s} = 120 \times 50/4 = 1500$ rpm

Full-load slip: $s_{1} = (1500 - 1440) /1500 = 0.04$

New slip: $s_{2} = (1500 - 1320) /1500 = 0.12$

Constant torque condition (low-slip approximation):

$\frac{s _{1}}{R _{2}} = \frac{s _{2}}{R _{2} + R _{e x t}}$

$\frac{0.04}{0.30} = \frac{0.12}{0.30 + R _{e x t}}$

$0.30 + R_{e x t} = \frac{0.12 \times 0.30}{0.04} = 0.90 Ω$

$R_{e x t} = 0.90 - 0.30 = 0.60 Ω/ phase$

🏆 Golden Questions (Past Exam Archive)

🎯 Q1: Define plugging and pull-out torque.

Appeared: 2017 Q2(a) — (2 marks)

Full Answer:

Pull-out torque: The maximum torque a motor can develop. Also called breakdown torque ( $T_{m a x}$ ). If load exceeds this, the motor stalls.

🎯 Q2: Short notes on regenerative braking, dynamic braking, and plugging.

Appeared: 2018 Q5(a) — (3 marks)

Full Answer:

(i) Regenerative braking: Motor speed exceeds $N_{s}$ ( $s < 0$ ). Machine acts as generator. Power fed back to supply. Smooth, efficient. Used in cranes (lowering loads) and electric trains.

(iii) Plugging: Two stator leads swapped while running. Stator field reverses. Braking torque opposes motion. Very rapid stop. Must disconnect at zero speed.

🎯 Q3: Mention speed control methods and discuss any one.

Appeared: 2017 Q2(c) — (4 marks)

Full Answer:

Speed control methods: (1) Stator voltage control, (2) Supply frequency control (V/f), (3) Pole changing, (4) Rotor resistance control.

Disadvantage: Power $s P_{g}$ wasted as heat. Efficiency drops. Used where step-speed control and high starting torque are needed (cranes, hoists).

🎯 Q4: Rotor resistance numerical for speed reduction.

Appeared: 2021 Q4(c) — (4 marks)

Full Answer:

See Worked Example above. Result: $R_{e x t} = 0.60 Ω$ /phase.

🎯 Q5: Define braking of IM. How can speed be controlled?

Appeared: 2019 Q7(c) — (4 marks)

Full Answer:

Braking: Applying a decelerating torque to bring the motor to rest or control speed. Three methods: plugging, dynamic braking, regenerative braking.

Speed control methods:

Stator voltage control: Reduce $V$ , torque drops, speed drops. Simple but poor efficiency.
V/f control: Change both $V$ and $f$ proportionally. Wide smooth range. VFDs.
Pole changing: Switch winding configuration. Discrete speed steps. Squirrel-cage only.
Rotor resistance: Add external $R$ (slip-ring). Higher slip = lower speed. Lossy.
Slip energy recovery: Feed slip power back to supply instead of wasting. Efficient but complex.

Exam Variants

Year	Question	Marks	Focus
2017 Q2(a)	Define plugging + pull-out torque	2	Definitions
2017 Q2(c)	Speed control methods + discuss one	4	Theory
2018 Q5(a)	Three types of braking	3	Short notes
2019 Q7(c)	Define braking + speed control	4	Theory
2020 Q8(a)	Define plugging + slip	2	Definitions
2021 Q4(c)	Rotor resistance numerical	4	Numerical

⚡ Exam Tips & Common Mistakes

Plugging appears in almost every paper. Memorize a 3-sentence definition.
Rotor resistance method ONLY works for slip-ring motors. Squirrel-cage has no access to rotor.
In the speed control numerical: The key equation is $s_{1} / R_{2} = s_{2} / (R_{2} + R_{e x t})$ for constant torque.
Don't confuse regenerative braking with plugging. Regenerative = above $N_{s}$ , power flows back. Plugging = phase reversal, no power recovery.

T-15c: Torque-Speed Curves — Braking regions on the curve
T-19: Starting Methods — Related to starting current
T-21: Induction Generator — Regenerative braking = generator mode

← T-19: Starting Methods | 🏠 Index | T-21: Induction Generator →

T-20: Speed Control & Braking

Why This Topic Matters

📝 Key Definitions

Speed Control Methods

1. Stator Voltage Control

2. Supply Frequency Control (V/f)

3. Pole Changing

4. Rotor Resistance Control (Slip-Ring Only)

Electric Braking Methods

Regenerative Braking

Dynamic Braking (DC Injection)

Plugging (Counter-Current)

Worked Example (PYQ 2021)

🏆 Golden Questions (Past Exam Archive)

🎯 Q1: Define plugging and pull-out torque.

🎯 Q2: Short notes on regenerative braking, dynamic braking, and plugging.

🎯 Q3: Mention speed control methods and discuss any one.

🎯 Q4: Rotor resistance numerical for speed reduction.

🎯 Q5: Define braking of IM. How can speed be controlled?

Exam Variants

⚡ Exam Tips & Common Mistakes

🔗 Related Topics

T-20: Speed Control & Braking

Why This Topic Matters

📝 Key Definitions

Speed Control Methods

1. Stator Voltage Control

2. Supply Frequency Control (V/f)

3. Pole Changing

4. Rotor Resistance Control (Slip-Ring Only)

Electric Braking Methods

Regenerative Braking

Dynamic Braking (DC Injection)

Plugging (Counter-Current)

Worked Example (PYQ 2021)

🏆 Golden Questions (Past Exam Archive)

🎯 Q1: Define plugging and pull-out torque.

🎯 Q2: Short notes on regenerative braking, dynamic braking, and plugging.

🎯 Q3: Mention speed control methods and discuss any one.

🎯 Q4: Rotor resistance numerical for speed reduction.

🎯 Q5: Define braking of IM. How can speed be controlled?

Exam Variants

⚡ Exam Tips & Common Mistakes

🔗 Related Topics