This presentation describes the Line Voltage Control - Torque Speed Characteristics - Methods -Advantages - Disadvantages - Applications of Voltage control

1. NATIONAL ELECTRICAL MANUFACTURERS ASSOCIATION (NEMA)
TYPES OF SQUIRREL CAGE IM
• CLASS A- LOW STARTING TORUE
(Tst – A)
• CLASS B – CONSTANT SPEED DRIVES
• CLASS C & D– HIGH STARTING TORQUE
(Tst – C & Tst – D)
• CLASS E – HIGH EFFICIENCY
(Low Slip Region)
Tst - A
Tst - B
Tst - C
Tst - D
SPEED (N)
TORQUE
SPEED CONTROL METHODS OF INDUCTION MOTOR
SLIP (S)
0
1
Ns
0
VERY IMPORTANT NOTE
High rotor resistance  High Starting Torque Tst
Low Slip (for wide range Torque)  High efficiency

2. 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
Why Induction motor?
Six Step
Pulse Width
Modulation
(PWM)
– Rugged construction (no commutators) &
availability of solid state controllers
Used only in slip ring induction motor

3. 1) LINE VOLTAGE CONTROL
INDUCTION
MOTOR
FAN LOAD
TORQUE
SPEED
Variable
Three
Phase
Stator
Voltage (Vs)
• EASIEST METHOD
• ECONOMICAL
TORQUE
SPEED
25% Vs
50% Vs
75% Vs
100% Vs
N1 N2 N3 N4
NEMA CLASS D CAN BE SUITABLE
TO GET WIDE SPEED RANGE
SPEED RANGE
Ns0
SPEED
TORQUE

4. TORQUE- SPEED CHARACTERISTICS
OF VOLTAGE CONTROL
How to change the “voltage”? (Implementation)
Method 1
Method 2
- Using “Auto transformer”
- Variation is smooth
- O/P -> Sinusoidal voltage
- Using “Solid State Controllers”
- O/P –> Non Sinusoidal !!
- Input Filter required – Large Applns
By changing the
“Firing angle - alpha”

5. 2. Actual Rotor
Speed N
4. Error = N*- N
5.Firing angle
6. Voltage
Method 3 – Using Solid State Controllers -- Closed loop
1. Desired
Rotor speed N*
-+3. Error Generator
“Used when
Precise speed
Control is required”
Manual
Speed setting
Potentiometer!
INPUT VOLTAGE
OUTPUT VOLTAGE
T1
T2
• Turn Off Time (T.Off) thinner
• So AVG V (1/2 cycle)= greater!
• Turn off time (T.Off) larger
• So AVG V (1/2 cycle) = smaller!
Controller
Speed
sensor
g1
g2
A K
AK
+
-
+ -
- +
T1
α
30
T.Off
T2
α
180 +30
T1
T2
T.Off
α
180 +60

6. TORQUE- SPEED CHARACTERISTICS
OF VOLTAGE CONTROL
• EASIEST
• ECONOMICAL
• INCREASE THE SPEED RANGE WITH
CLASS D MOTORS
• SLIP ∝ (Ns-N) IS HIGH @ LOW
SPEEDS – SO, LOW EFFICIENCY
NOTE
“SYNCHRONOUS SPEED “Ns” IS
NOT CHANGING”
• FANS
• PUMPS
• CENTRIFUGAL DRIVES
APPLICATIONS
SLIP
SPEED RANGE

7. LINE FREQUENCY CONTROL