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, primarily focusing on the application of variable frequency constant voltage methods through voltage/frequency control. It explains how six-step switching in voltage source inverter fed systems can control pole voltages and subsequently manage the line voltage frequencies. Additionally, the document outlines the relationship between pole voltages and phase voltages in a balanced three-phase operation.

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 - 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 - SIX STEP SWITCHING
HOW FREQUENCY CONTROL IS DONE USING SIX STEP SWITCHING ?
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
“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, then it is possible
to control the frequency of
line voltages
1st leg 2nd leg 3rd leg
V/2
V/2
V 0V
+
-
+
-

3. VA0 +V/2 (S1 turned ON)
-V/2 (S2 turned ON)
+V/2 (S1 turned ON)
+V/2
-V/2-V/2(S4 turned ON)
(S3 turned ON)
(S4 turned ON)
VB0
VC0 +V/2 (S5 turned ON)
- V/2 (S6 turned ON) - V/2 (S6 turned ON)
+V/2 (S5)
VAB
VAB = VA0 – VB0
+V ZERO
-V
+VZERO
+V +V
+V
- V
- V
- V
- V
VBC = VB0 – VC0
VBC
VCA
VCA = VC0 – VA0
ZERO ZERO ZERO
ZERO ZERO ZERO
120 degree 60 degree
HOW CONTROLLED GATE VOLTAGES ARE GENERATED IN SIX STEP SWITCHING?- ARE GENERATED TO GET POLE
VOLTAGES AS BELOW180 degree

4. ONE TIME PERIOD (T) ; f = 1/T
ONE TIME PERIOD (T) ; f = 1/T
HOW TO CONTROL THE FREQUENCY OF LINE VOLTAGE VAB?
“Here Frequency of Line voltage
VAB increases - By increasing
the switching frequency of S1,S2,
S3 & S4 OR in other words by
decreasing the switching time of
S1, S2, S3, S4”
120 DEGREE
120 DEGREE

5. HOW TO FIND THE PHASE VOLTAGE VAN ?
VAN = VA0 – VN0 (1) VBN = VBO – VN0 (2) VCN = VC0 – VNO (3)
For a balanced three phase operation: VAN + VBN + VCN = 0
VN0 = VA0 – VAN (4) VN0 = VB0 – VBN (5) VNO = VCO- VCN (6)
ADDING 4, 5, 6  3VN0 = VA0+VB0+VCO – (VAN+VBN+VCN)
So , 3VN0 = VA0+VB0+VCO (OR) VNO = (VA0+VBO+VCO)/3 (7)
VN0
+V/6
- V/6
VAN VAN = VA0 – VN0
SIX STEPPED WAVE (PHASE VOLTAGES)  THE REASON FOR THE NAME SIX STEP SWITCHING
Substitute (7) in (1)
VAN =
2/3 (VA0) – 1/3 (VB0+VC0)

6. 1) VSI
2) CSI
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