---
title: "Speed control techniques in AC drives"
subject: "Utilization of Electrical Energy"
chapter: 3
key: "semester_7/utilization_of_electrical_energy:ac-drive-speed-control-techniques"
source: https://www.bctengineer.com/q/semester_7/utilization_of_electrical_energy/ac-drive-speed-control-techniques
site: BCT Engineering
---

# Speed control techniques in AC drives

Utilization of Electrical Energy · Chapter 3: Control of Electric Drive

- Asked in: 2081 Bhadra, 8 marks
- Video: [Speed control of 3 phase induction motor |stator voltage control method |stator frequency control](https://www.youtube.com/watch?v=1KZHRK7YQ00) — Mind of Engineer 
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_7/utilization_of_electrical_energy/ac-drive-speed-control-techniques
- Whole chapter: https://www.bctengineer.com/md/semester_7/utilization_of_electrical_energy/chapter-3

## Question

Describe with neat sketch, the various Speed control techniques used in AC drive systems.

## Short answer

Speed is changed by varying slip (stator voltage, rotor resistance, slip-power recovery), supply frequency (V/f control), or number of poles. V/f control with inverter keeps flux constant for wide smooth range; vector control decouples torque and flux for high performance.

## Answer

Speed of an induction motor is $N = N_s(1-s) = \dfrac{120f}{P}(1-s)$. So speed can be changed by changing the **slip** $s$ (at fixed $N_s$), the **supply frequency** $f$, or the **number of poles** $P$. Slip is changed by stator voltage, rotor resistance or slip-power injection.

#### 1. Stator voltage control
- AC voltage controller (anti-parallel thyristors) reduces the rms stator voltage at fixed frequency. Since $T \propto V^2$, the torque curve shrinks and the operating point moves to higher slip.
- Simple and cheap; used for fans and pumps ($T \propto N^2$).
- Narrow range, poor efficiency at low speed (rotor loss $= sP_g$), harmonics.

#### 2. Pole changing
- Stator winding reconnected (e.g. Dahlander) or two separate windings, giving 2–4 fixed speeds. Only for squirrel cage motors; speed changes in steps.

#### 3. Variable frequency (V/f) control
- An inverter supplies variable voltage and frequency, keeping $V/f$ constant so that air-gap flux stays constant.
- Wide, smooth speed range above and below base speed, high efficiency, good starting torque; most common modern method (VSI/PWM or CSI drives).
```text
 3-ph ac -> [Rectifier] -> [DC link L/C] -> [Inverter] -> IM
                                 V/f controller ^
```

#### 4. Rotor resistance control (slip ring motor)
- External resistance in rotor circuit raises slip at a given torque; static version uses a diode bridge, chopper and one resistor.
- Simple, high starting torque, but slip power is wasted as heat.
```text
 rotor -> [diode bridge] -> L -> [chopper || R]
```

#### 5. Slip power recovery (slip ring motor)
- **Static Kramer drive**: rotor power rectified, inverted and fed back to the supply; sub-synchronous speeds only.
- **Static Scherbius drive**: cycloconverter or back-to-back converter in rotor; power flows both ways, giving sub- and super-synchronous speeds.
- Efficient; used for large pumps, fans, compressors.

#### 6. Vector (field oriented) control
- Stator current split into flux and torque components and controlled separately, so the induction motor behaves like a separately excited dc motor; fast response for high-performance drives.

| Method | Motor | Range | Efficiency |
|---|---|---|---|
| Stator voltage | Cage | Narrow, below $N_s$ | Low |
| Pole changing | Cage | Steps | High |
| V/f | Cage | Wide, both sides | High |
| Rotor resistance | Slip ring | Below $N_s$ | Low |
| Kramer / Scherbius | Slip ring | Limited range | High |

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. Speed of an induction motor can be changed by changing:
   A. Only the rotor resistance
   B. Slip, supply frequency or number of poles
   C. Only the supply voltage
   D. Only the number of phases
   Answer: B. N = (120f/P)(1 - s), so s, f or P can be varied. (Key point: Speed relation)

2. Why is V/f kept constant below base frequency?
   A. To keep air-gap flux constant
   B. To keep slip constant
   C. To keep rotor resistance constant
   D. To reduce supply frequency
   Answer: A. Flux is proportional to V/f; keeping it constant avoids saturation and keeps torque capability. (Key point: V/f control)

3. What distinguishes Kramer from Scherbius drive?
   A. Kramer uses stator voltage control
   B. Kramer allows only sub-synchronous speed; Scherbius allows power both ways, so super-synchronous too
   C. Scherbius works only with cage motors
   D. Both give only sub-synchronous speeds
   Answer: B. Kramer uses a diode rectifier (one-way power); Scherbius uses a cycloconverter or back-to-back converter. (Key point: Static Kramer vs Scherbius)

4. Why is stator voltage control suited to fans and pumps?
   A. Torque is proportional to V, so it is efficient
   B. Fan/pump torque rises with speed (T proportional to N^2) so the narrow range suffices
   C. It gives wide range and high efficiency
   D. It gives super-synchronous speed
   Answer: B. Torque varies with V^2 and operating point moves to higher slip; fans and pumps with T proportional to N^2 tolerate the narrow range. (Key point: Stator voltage control)

## Related in Utilization of Electrical Energy

- [Electric braking in AC and DC traction drives](https://www.bctengineer.com/question.md?key=semester_7%2Futilization_of_electrical_energy%3Aelectric-braking-traction-drives) — Utilization of Electrical Energy, chapter 4
- [Closed-loop control of multi-motor drive](https://www.bctengineer.com/question.md?key=semester_7%2Futilization_of_electrical_energy%3Aclosed-loop-multi-motor-drive) — Utilization of Electrical Energy, chapter 2, asked 2×
- [DC vs AC electric traction systems](https://www.bctengineer.com/question.md?key=semester_7%2Futilization_of_electrical_energy%3Adc-vs-ac-traction-systems) — Utilization of Electrical Energy, chapter 4
- [Types of electric drives with merits and demerits](https://www.bctengineer.com/question.md?key=semester_7%2Futilization_of_electrical_energy%3Aelectric-drive-types-merits) — Utilization of Electrical Energy, chapter 2
- [PID control of speed and torque](https://www.bctengineer.com/question.md?key=semester_7%2Futilization_of_electrical_energy%3Apid-control-speed-torque-motor) — Utilization of Electrical Energy, chapter 3, asked 2×
- [Ward-Leonard system of speed control](https://www.bctengineer.com/question.md?key=semester_7%2Futilization_of_electrical_energy%3Award-leonard-speed-control) — Utilization of Electrical Energy, chapter 3

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