The document discusses various speed control methods for induction motors, including line voltage control, line frequency control, rotor resistance control, and slip power recovery schemes. It emphasizes techniques like voltage/frequency (v/f) control and pulse width modulation (PWM) for controlling the motor's operation. The document also outlines the mathematical framework and implementation strategies for both open-loop and closed-loop v/f control systems.

1. SPEED CONTROL METHODS OF INDUCTION MOTOR
1) Line voltage control
2) Line frequency control
 Variable frequency constant voltage
 Voltage/Frequency control (V/F)
Voltage Source Inverter fed induction motor drive
Current Source Inverter fed induction motor drive
3) Rotor resistance control
4) Slip power recovery scheme
Six Step operation
Pulse Width
Modulation
(PWM) operation
Used only in slip ring induction motor

2. 2.1) LINE FREQUENCY CONTROL- (variable frequency constant voltage)
Ns  N
(synchronous (rotor speed)
speed)
𝟏𝟐𝟎 𝐟
𝐏
 N=
Load
Torque
Ns1 Ns2
N1 N2
Ns3 Ns4 Ns5 Ns6
N3 N4 N5 N6
TORQUE
SPEED
E = 4.44 f Ø N Volts
E ∝ f Ø
Ø ∝
𝐄
f
𝐓 ∝ Ø ∝
𝐕
f
Why Torque decreasing?
(Voltage drop in R1 & X1
are small then, E ≈ V)
VERY IMPORTANT POINT
𝐈𝐟, Ø(𝐂𝐎𝐍𝐒𝐓) ∝ T (CON𝐒𝐓) ∝
𝐕
f
(CONSTANT)
High
Saturation!!
SLIP ∝ (Ns- N) = LOW; %η = HIGH & VERY WIDER SPEED VARIATION
f2
f1
f3
f4
f5
f6
To
Overcome

3. 2.2) V/F CONTROL
Ø(𝐂𝐎𝐍𝐒𝐓) ∝ T (CON𝐒𝐓) ∝
𝐕
f
(CONSTANT)
1) HOW TO MAINTAIN V/F AS CONSTANT?
𝐕
f
 CONSTANT
3) HOW TO CHANGE THE VOLTAGE WITH RESPECT TO FREQUENCY?
2) HOW FREQUENCY CHANGES? Based on the desired rotor speed N*, we need to change the
frequency by which the synchronous speed Ns and hence the
actual rotor speed N
Frequency
Voltage
Small
voltage
boosting
V= Constant
Rated
voltage
Base frequency
EQUATION (1)
T = DECREASES
We need
to sacrifice
this to
avoid
Insulation
Breakdown
in stator
windings!
Why voltage boosting @ low frequencies ?

4. TORQUE – SPEED
CHARACTERISTICS
- V/F CONTROL
T = CONSTANT
Ns1
N1
Ns3
N3
Ns4
N4
Ns5
N5
VERY WIDE SPEED
VARIATION & THE
SLIP ∝ (Ns- N) = LOW; SO,
EFFICIENCY = HIGH
LOAD
TORQUE
Ns2
N2
Ns6
N6 N7
Ns7 Ns8
N8
NO
STIFFNESS;
SLIP ∝
(Ns-N) =
HIGH; SO,
EFFICIENCY
= LESS
Variable Voltage & Frequency Variable Frequency (Const voltage)
Voltage
T
Rated
Voltage
Small
voltage boosting
Base frequency
V= constant (has to be)
frequency
CONSTANT TORQUE REGION

5. IMPLEMENTATION OF OPEN LOOP V/F CONTROL
Rectifier Inverter
A.C TO D.C D.C TO A.C
IM3 Ø
SUPPLY
CONTROL CIRCUIT (PWM)
Variable “V” & “f”
P/120
f*
f
v
ω*
V*
∫
Vα = V cos ωt
Vβ = V sin ωt
ωt*
2Ø to 3Ø
Vα* Vβ*
Inverse Clarke
Transform (I.C.T)
2*Π
Sin 1
(0 deg)
Sin 2
(120deg)
Sin 3
(240 deg)
Triangular
signal
“Amp”
Carrier
Signal
Modulating
Signal
I know only the
desired rotor speed
(N*)
of the motor
N*
Va* Vb* Vc*
𝐕𝐚 = 𝐕𝛂
𝐕𝐛 = −
1
2
𝐕𝛂 +
3
2
𝐕𝛃
𝐕𝐜 = −
1
2
𝐕𝛂 −
3
2
𝐕𝛃
Look-up table
Math Function
(PWM OPERATION)
(V sin ωt)
“f”

6. IMPLEMENTATION OF
CLOSED LOOP
V/F CONTROL (PWM)
Tacho
Rectifier
A.C TO D.C
Inverter
D.C TO A.C
CONTROL CIRCUIT (PWM)
Sin 1
(0 deg)
Sin 2
(120deg)
Sin 3
(240 deg)
Triangular
signal
N* PI
Regulator
∫
Vα = V cos ωt
Vβ = V sin ωt
IM
2Ø to 3Ø (I.C.T)
Vα* Vβ*
2*Π
Look-up table
3 Ø
SUPPLY
Σ Σ
P/120
Error!
(N*-N)
Slip
frequency*
= f- fN
Slip
frequency*
below B.D.F
f*
f N
ω* ωt*
V*
(f-f N)
N
+
-
+
+
v
f
Controller
Va* Vb* Vc*B. D. F Break Down Frequency
(freq @ maximum torque developed)
I. C. T  Inverse Clarke Transform

7. } PWM OPERATION
(V & f CONTROL)
INVERTER CIRCUIT