Necessity of starter
At starting ,the speed of motor is zero so
that the back e.m.f. In the armature is
Armature resistance is so low, if it is
connected to power supply directly ; huge
current will pass thru armature.
The huge current may damage the
machine, major heat, very high speed in
case of DC series motor.
Ia = V/Ra
Function of starter
Start and stop the motor.
Limit inrush current where necessary.
Permit automatic control when required
Protect motor and other connected
equipments from over voltage, no voltage,
under voltage, single phasing etc.
Motor Starter Features.
Rated by current (amperes) or power
Remote ON/OFF control
Motor overload protection
Starting and stopping (electrical life)
Plugging and jogging (rapid making and
Type of starter
Two point starter for DC series motor
Three point starter for shunt motor
Four point starter for compound motor
Variable Frequency drive
MotorStarting Characteristics on DOL
Available starting current: 100%.
Peak starting current: 6 to 8 Full
Peak starting torque: 100%
Advantages of DOL Starter:
Most Economical and Cheapest Starter
Simple to establish, operate and maintain
Simple Control Circuitry
Easy to understand and trouble‐shoot.
It provides 100% torque at the time of starting.
Only one set of cable is required from starter
Motor is connected in delta at motor terminals.
Disadvantages of DOL Starter:
It does not reduce the starting current of the
High Starting Current: Very High Starting
Current (Typically 6 to 8 times the FLC of the
Mechanically Harsh: Thermal Stress on the
motor, thereby reducing its life.
Voltage Dip: There is a big voltage dip in the
High starting Torque: Unnecessary high
starting torque, even when not required by the
DOL is Suitable for:
Small water pumps, compressors, fans and
Motor rating up to 5.5KW
DOL is not suitable for:
The peak starting current would result in a
serious voltage drop on the supply system
Motor rating above 5.5KW
Star delta starter
Most induction motors are started directly on
line, but when very large motors are started
that way, they cause a disturbance of voltage
on the supply lines due to large starting
To limit the starting current surge, large
induction motors are started at reduced
voltage and then have full supply voltage
reconnected when they run up to near rotated
MotorStarting Characteristics of Star-
Available starting current: 33% Full
Peak starting current: 1.3 to 2.6 Full
Peak starting torque: 33% Full Load
Advantages of Star-Delta
The operation of the star-delta method is
simple and rugged
It is relatively cheap compared to other
reduced voltage methods.
Good Torque/Current Performance.
It draws 2 times starting current of the full
load ampere of the motor connected
Disadvantages of Star-Delta starter:
Low Starting Torque, only 33% starting torque
Break In Supply – Possible Transients
Six Terminal Motor Required (Delta Connected).
It requires 2 set of cables from starter to motor.
The delta of motor is formed in starter and not on motor
Applications with a load torque higher than 50 % of the motor
rated torque will not be able to start using the start-delta
Low Starting Torque: reduction of the line voltage by a factor
of 1/√3 (57.7%) to the motor and the current is reduced to 1/3
of the current at full voltage, but the starting torque is also
reduced 1/3 to 1/5 of the DOL starting torque .
A variable -fre q ue ncy drive (VFD), also termed
adjustable-frequency drive, variable speed
drive, AC drive, micro drive or inverter drive is
a type of adjustable-speed drive used in
electro-mechanical drive systems.
It co ntro ls AC m o to r spe e d and to rq ue by
varying m o to r input fre q ue ncy and vo ltag e .
Principle of VFD
Variable frequency Drive, principle of the
same is by regulating the frequency, we can
regulate the speed of drive
Example - n=120f/p
n - speed
f - frequency
p – nos. of poles
Circuit and function
An auto-transformer starter makes it possible
to start squirrel-cage induction motors with
reduced starting current, as the voltage across
the motor is reduced during starting.
In contrast to the star-delta connection, only
three motor leads and terminals are required.
On starting, the motor is connected to the
tappings of the auto-transformer.
Motor terminal voltage less than line voltage
(by transformer ratio).
Motor current exceeds line current (by inverse
of transformer ratio).
Starting torque is reduced by the square of the
Where complete acceleration at reduced
amperes is needed.
Where line ampere reduction requirements are
severe, and load torque is not minimal.
Difference between DOL/Star delta
DOL Starter Stardelta starter Auto transformer
1 Used up to 5 HP Used 5 HP to 20HP Used above 20 HP
2 Does not
Decreases the starting
current by 1/3 times
starting current as
3 It is cheap It is costly It is more costly
4 It connects
directly the motor
with supply for
starting as well as
It connects the motor first in
star at the time of starting in
delta for running
It connects the motor
according to the
taping taken out from
the auto transformer