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 details the operation of PWM systems using voltage source inverters and how to control voltage magnitude and frequency through modulation techniques. Additionally, it explains the significance of gate voltages in PWM and the relationship between sinusoidal and triangular signals in controlling motor speeds.

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

2. VOLTAGE SOURCE INVERTER FED 3 PHASE IM (SINUSOIDAL PWM)
HOW VOLTAGE (MAGNITUDE) & FREQUENCY CONTROL IS DONE USING PWM OPERATION ?
S1 S3 S5
S2 S4 S6
VA0
VB0
VC0
A
B
C
VA0, VB0 , VC0 = POLE VOLTAGES
(You can directly control these pole voltage
by controlling the gate voltages of IGBT switches!)
For example:
• When S1 (upper) turned ON then, VA0  +V/2
• When S2 (lower) turned ON then, VA0  - V/2
• When S3 (upper) turned ON then, VBO  +V/2
• When S4 (lower) turned ON then, VBO  -V/2
• When S5 (upper) turned ON then, VC0  +V/2
• When S5 (upper) turned ON then, VC0  -V/2
But in V/F control, what we need to control is
“magnitude” & “frequency” of line voltages
(VAB, VBC, VCA)
VAB (0 DEG Phase shift) = VA0 – VB0
VBC (120 DEG Phase shift)= VB0 – VC0
VCA (240 DEG Phase shift)= VC0 – VA0
{
If we control the pole voltages (by
controlling the gate voltages of
IGBTs switches), then it is
possible to control the magnitude
& frequency of line voltages
1st leg 2nd leg 3rd leg
V/2
V/2
V 0V
+
-
+
-

3. HOW CONTROLLED GATE VOLTAGES ARE GENERATED IN PWM?
Sinusoidal /
Modulating
signal
Triangular/
Carrier
Signal
Comparators
G1 (S1)
[Upper]
G2 (S2)
[Lower]
G3 (S3)
[Upper]
G5 (S5)
[Upper]
G4 (S4)
[Lower]
G6 (S6)
[Lower]
So that both upper & lower transistor
will not turn ON @ the same time – So
Short circuit will not happen!
0 degree
120 degree
240 degree
1st leg
2nd leg
3rd leg

4. Sin 1 Triang
Sin 2
(120 degree)
Sin 1
(o degree)
Sin 3
(240 degree)
Triang
Comparators
G1 (S1)
G2 (S2)
G3 (S3)
G4 (S4)
G5 (S5)
G6 (S6)
POWER CIRCUIT
PWM CONTROL CIRCUIT
GATE
VOLTAGESG2
(S2)
G1 (S1) G1(S1)
VAO
-V/2 -V/2 -V/2
+ V/2 + V/2
Lets analyze how gate voltages are generated for
single sinusoidal signal and triangular signal
-- What we need to control is magnitude &
frequency of line voltages !
T>S T>S T>SS>T S>T
VAB = VA0 – VB0
G2
(S2)
G2
(S2)
So lets analyze for single line voltage VAB
G1
G2
G3
G4
G5
G6

5. Sin 1 ( 0 degree) Sin 2 ( 120 degree) Triang
VA0
VB0
VAB
VAB = VA0 – VB0
HOW TO CONTROL MAGNITUDE OF
LINE VOLTAGE VAB?
HOW TO CONTROL THE FREQUENCY OF
LINE VOLTAGE VAB?
“f VAB = f Sin”
For example,
TO INCREASE THE FREQUNECY OF
VAB YOU NEED TO INCREASE THE
“FREQUENCY OF THE SINUSOIDAL
SIGNALS” & VICE-VERSA
TERMINNOLOGY 1 :
CARRIER RATIO = ft/ fs
fc = frequency of triangular signal
fs = frequency of sinusoidal signals
For example,
TO INCREASE THE VOLTAGE VAB
YOU NEED TO INCREASE THE
“AMPLITUDE OF THE SINUSOIDAL
SIGNAL” & VICE- VERSA
TERMINNOLOGY 2 :
MODULATION INDEX = As/ At
Am = Amplitude of sinusoidal signal
Ac= Amplitude of triangular signal
NOTE : Pulses in each half cycle have different widths (central pulse
is wider). Less harmonics (Fourier analysis)
+ V/2 + V/2 + V/2 + V/2
- V/2 - V/2 - V/2 - V/2
+ V/2+ V/2+ V/2
- V/2 - V/2 - V/2 - V/2
+ V + V
- V

6. VSI
(SIX STEP SWITCHING)
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