Industrial Star Delta Starter for a 3-Phase Induction Motor

Mastering the wiring & Schematics for Electrical, Protection, Automation, Substation Engineers

Contents of Course

•Introduction to Induction Motor

•Construction of Induction Motor

•3D Simulation showing working of induction Motor

•What are issues arises while starting of induction Motor

•Direct Online start DOL of induction Motor

•Advantages and disadvantages of DOL

•What are different types of starting techniques

•Reviewing Induction motor starting curve

•Star Delta Starter

•Purpose of Start Delta Starter

•Advantages & Disadvantages of Star Delta Starter

•Understanding Control Circuit of Star Delta Starter

•Component of Star Delta Starter

•Understanding Power Circuit of Star Delta Starter

•Reading Schematic drawings of Star Delta Starter

•Comparing Different Curves of starting current when use different starters

Introduction Of Induction Motor

An induction motor or asynchronous motor is an AC electric motor in which the electric current in the rotor needed to produce torque is obtained by electromagnetic induction from the magnetic field of the stator winding.

An induction motor can therefore be made without electrical connections to the rotor. An induction motor's rotor can be either wound type or squirrel-cage type..

Three Phase induction motor is self started , because flux is moving at synchronous speed.

Where as single phase induction motor is not self start, capacitor is used to produce rotating field.

•Three-phase squirrel-cage induction motors are widely used as industrial drives because they are self-starting, reliable and economical. Single-phase induction motors are used extensively for smaller loads, such as household appliances like fans. Although traditionally used in fixed-speed service, induction motors are increasingly being used with variable-frequency drives (VFD) in variable-speed service. VFDs offer especially important energy savings opportunities for existing and prospective induction motors in variable-torque centrifugal fan, pump and compressor load applications. Squirrel-cage induction motors are very widely used in both fixed-speed and variable-frequency drive applications.

Cutaway view through stator of induction motor, showing rotor with internal air circulation vanes.

•A 3 Phase Induction motor consists of a stator. Ø It contains 3 phase winding connected to the 3 phase AC supply. Ø The arrangement of the winding is to produce a rotating magnetic field. Ø The rotor of the Induction motor contains cylindrical core with parallel slots that contain conductors.

Industrial Star Delta Starter for a 3-Phase Induction Motor What Problems Can Encounter During Motor Starting?

•During the motor self starting period, torque increases and a large amount of current flows in the rotor.

• Ø To achieve this the stator draws a large amount of current and by the time the motor reaches its full speed

•Ø A large amount of current is drawn and coils get heated up, damaging the motor. Ø Hence there is a need to control the motor starting.

ØOne way is to reduce the applied voltage, which in turn reduces the torque

Starting current of induction motor

•Starting current of induction motor is as high as 5 to 7 times the normal full load current. Therefore different starting of induction motor, methods such as (star delta starter, auto transformer starter and other starting methods) are employed in order to reduce the high starting currents of induction motor.

PURPOSE OF STARTER

If a rated stator voltage is applied to the motor at the time of starting, then the motor
will draw heavy starting current.
This will lead to excess i2R losses in the winding which will overheat the motor.
Secondly due to a heavy current drawn from the AC supply voltage will reduce.
The heavy starting current may damage the motor windings.
In order to avoid these problems, we can use some kind of a starter to start the
induction motor safely.

To avoid this situation, we use star delta starter. At the time of start, motor connections (stator

connections) are made in star mode so impressed voltage is reduced by 1/√3 (Phase voltage = Line

voltage/√3) which reduces the starting current.

Some of the techniques used to reduce the starting current of a squirrel induction motor:

•Soft starting (voltage control)

•Variable Frequency Drives (Voltage and frequency control)

•Star/Delta Starting.

•Stator impedance and/or resistance starting.

•Autotransformer Starting

Direct Online STARTER BASIC POWER CIRCUIT

ADVANTAGES OF DOL STARTER

1.Most Economical and Cheapest Starter .

2. Simple to establish, operate and maintain .

3. Simple Control Circuitry .

4. Easy to understand and trouble‐shoot.

5. It provides 100% torque at the time of starting.

6. Only one set of cable is required from starter to motor.

7. Motor is connected in delta at motor terminals.

DISADVANTAGES OF DOL STARTER

•1. Starting current:- It does not reduce the starting current of the motor.

•2. High Starting Current:- Very High Starting Current (Typically 6 to 8 times the FLC of the motor).

•3. Mechanically Harsh:- Thermal Stress on the motor, thereby reducing its life.

•4. Voltage Dip:- There is a big voltage dip in the electrical installation because of high in-rush current

•affecting other customers connected to the same lines and therefore not suitable for higher size

•squirrel cage motors

•5. High starting Torque:- Unnecessary high starting torque, even when not required by the load, thereby

•increased mechanical stress on the mechanical systems such as rotor shaft, bearings, gearbox,

coupling, chain drive, connected equipment's, etc. leading to premature failure and plant downtimes

Industrial Star Delta Starter for a 3-Phase Induction Motor

•The main advantage of using the star delta starter is reduction of current during the starting of the motor. Starting current is reduced to 3-4 times Of current of Direct online starting. (2). Hence the starting current is reduced , the voltage drops during the starting of motor in systems are reduced.

•Advantages of Star Delta Starter Inexpensive. No heat is produced, or tap changing device needs to be used, hence efficiency increases. Starting current reduced to 1/3 of direct online starting current. Produce high torque per ampere of line current

Star delta starter

In star delta starting, the motor is connected in STAR mode throughout the starting period. When the motor reached the required speed, the motor is connected in DELTA mode

PURPOSE OF STAR DELTA STARTER

At the time of starting the motor, rotor is at standstill and the slip ( between stator's magnetic field and

rotor) is large which causes the large inrush of armature current (which is 6–7 times the rated value).

This large current can damage the stator windings and burn the motor.

2. To avoid this situation, we use star delta starter. At the time of start, motor connections (stator

connections) are made in star mode so impressed voltage is reduced by 1/√3 (Phase voltage = Line

voltage/√3) which reduces the starting current.

3. Once the rotor is at 80–90% speed, centrifugal switch operates changing the connections from star to

delta mode (full line voltage is impressed).

ADVANTAGES AND DISADVANTAGES OF STAR DELTA STARTER

•ADVANTAGES OF STAR DELTA STARTER:

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

DISADVANTAGES OF STAR DELTA STARTER:

•a ) The starter can only be applied to motors where the six leads or terminals can be accessed.
b ) The supply voltage must be the same as the rated motor voltage for Delta connection.
c ) Because the starting current is reduced to approximately one-third of the rated current, the starting torque is also reduced to one-third.
d ) 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. start, thus causing harmful effects to the contacts of the contactors and the connection system brings no advantage to the electrical system.

Understand Schematics Drawings of Star Delta Starter.