Motor StarterTypes and Technology of Motor Starter And Its Applications

Types And
Technology of Motor
Starter And
Applications

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Types And Technology of Motor
Starter And Applications
Introduction:
 An induction motor is a 3 phase motor consisting of the 3 phase
winding as the stator with a permanent magnet and the rotor as
another 3 phase windings. It works on the principle of rotating
magnetic field, i.e. the formation of the magnetic flux from 3 phase
winding fluxes, which rotates about its axis, causing the rotor to rotate.

Block Diagram

Need for Motor Starting
 In an induction motor, when supply is given to the stator windings.
 The rotating magnetic field flux and the produced flux in the rotor
windings due to the back emf.
 It causes the motor torque to increase, causing a high rotor current.
 During the time between the application of electric supply to the motor
and the actual acceleration of the motor to its full speed.
 A large amount of current is drawn by the stator from the supply.

Need for Motor Starting
 This starting current is about 5 to 6 times more than the full load
current.
 This time duration can be for few seconds or longer.
 This causes the electrical equipments to damage because of the
increasing voltage drop in electrical systems.
 Due to flow of larger currents across the cable.
 For this reason, a definite method of starting the motor is needed.

Definition of Motor Starter
 It is the device connected in series with the motor to decrease its
starting current and then increase it as the motor starts rotating
gradually.
 It consists of a connector which acts as a switch to control flow of
current to the motor and a overload unit which measures the flow of
current through the motor and controls the halting of motor in case of
large current being drawn.

Principle of Motor
Starter
 The current drawn by the motor can be controlled by reducing the
back emf (possible by reducing the supply voltage) or by increasing the
rotor resistance during the motor starting.

Types of Motor
Starters
 Direct On line
 Star Delta
 Auto Transformer starter
 Stator Resistance Starter
 Rotor Resistance Starter

Direct On line
 It consists of simple push button as the controller.
 When the start button is pressed.
 The switch connecting the motor and the main supply is closed and
the motor gets the supply current.
 In case of over current, the stop button is pressed and the bypass
auxiliary contact is opened.

Direct On line

Star Delta
 The 3 windings are first connected in star connection.
 After some time (decided by the timer or other controller circuit) the
windings are connected in delta connection.
 In star connection, the current drawn is 0.58% of the normal current and
also the phase voltage is reduced to 0.58%.
 Thus the torque is reduced.

Star Delta

Auto Transformer starter
 It consists of an auto transformer in star connection.
 It reduces the voltage applied to the motor terminals.
 It consists of 3 tapped secondary coils connected to the three phases.
 In the starting period the transformer allows application of lower
voltages to the three windings.

Auto Transformer starter

Stator Resistance Starter
 It consists of three resistors in series with each phase of the stator windings.
 It causes a voltage drop across each resistor and as a result a low voltage is
applied to each phase.

Rotor Resistance Starter
 It consists of 3 resistances connected in series with the rotor windings,
thus reducing the rotor current, but increasing the torque.

Application of Star Delta Starter to Control Starting
of Induction Motor

of Induction Motor
 The star delta starter is the cheapest among all the starters and is suitable
for applications like machine tools, pumps, motor generators etc.
 A star delta starter can be used in starting an induction motor by using 2
relays as the connector and the timer as the controller.
 1 connector is used to provide the mains supply while the other connector
controls the connection of motor in star or delta connection.
 Transformers are used, primaries of which are connected to the 3 phase
supply and the secondaries are connected to the relays and the timer.

of Induction Motor
 The two relays are used to trigger the timer, which develops a high logic
output at pin 3.
 Thus switching on the relay 4, causing the supply in star connection
 It offers low power intensity to the load by isolating the load from the
normal 3 phase supply through relay 3.
 Some more regarding this starting of induction discussed below.

Soft Start of Induction Motor by Stepped Delay of Reducing
Firing Angle
Soft Start and Soft Stop
 In normal start up of the induction motor, more torque is developed.
 It causes the stress to be transferred to the mechanical transmission
system.
 It results in excessive wear and failure of the mechanical parts.
 As acceleration increases, high current is drawn which is about 600%
of the normal run current.
 This can be seldom solved using a star delta starter

 Soft start provides a reliable and economical solution to
these problems by delivering a controlled release of power
to the motor, thereby providing smooth, steeples
acceleration and deceleration. The damage to windings and
bearings are reduced, resulting in an extended motor life.

 With this technique, a controlled starting and stopping is achieved with
proper selection of ramp times and setting of the current limit.
 Less mechanical stress.
 Improved power factor.
 Lower maximum demand.
 Less mechanical maintenance.
 This technique is suitable for applications where torque transients are
frequent, such as in pumping fluids, which can ultimately lead to rupture
pipes and couplings.

Technology followed in Soft Starter:
 A soft starter is a type of reduced voltage starter for AC induction
motors.
 The soft starter is similar to a primary resistance or primary reactant
starter in that it is in series with the supply to the motor.
 The input current to the started equals its output current.
 It consists of solid state devices to control the current flow and the
voltage applied to the motors.
 The soft starters can be connected in series with the line voltage or
connected inside the delta loop.

Voltage Control
 Solid state AC switches are arranged in series with one or more phases
to achieve voltage control.
Use of Solid state switches
 1 x Triac per phase

Use of Solid state switches
 1 x SCR and 1 x Diode reverse parallel connected per phase.
 2 x SCRs reverse parallel connected per phase.

Advantages
 Star-Delta starters are widely used due to their relatively low
price.
 There are no limits to the number of times they can be
operated.
 The components require very little space.
 The starting current is reduced to approximately one-third.

Disadvantages
 The starter can only be applied to motors where the six leads or
terminals can be accessed.
 The supply voltage must be the same as the rated motor voltage for
Delta connection.
 Because the starting current is reduced to approximately one-third of
the rated current, the starting torque is also reduced to one-third.
 If the motor does not reach at least 90% of its rated speed at the time
of switching from Star to Delta the current peak will be as high as in a
D.O.L.

Conclusion
 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 current surges. 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 speed.

Motor StarterTypes and Technology of Motor Starter And Its Applications

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