Starters

Starters

By Allwyn A

Why do we need starters

The three-phase induction motors are self-starting due to rotating magnetic field. But the motors show tendency to draw very high current at the time of starting. ... Hence there should be a device which can limit such high starting current​

Starters are those devices that are used to limit the high current.​

Direct on line for low HP motors

DOL

The star-delta starter is equivalent to an autotransformer with a 57.7% tapping.​

Star-delta

A three-phase star-connected auto-transformer is used to reduce the voltage applied to the stator.

Autotransformer

Direct-On Line Starter

For small motors

In a DOL Starter, an induction motor is connected directly across its 3-phase supply, and the DOL starter applies the full line voltage to the motor terminals.

If voltage drop is needed, then Star - delta must be used.​

Since the DOL starter connects the motor directly to the main supply line, the motor draws a very high inrush current compared to the full load current of the motor

DOL

1. Green and red switch​

2. 3 normally open switch​

3. Hold on contact (magnetising coil)

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DOL

1. ​Overcurrent above the load (overload)

2. Short circuit protection (fuse)​

   (short current for short time will not trip the fuse(as in starting of the motor))​

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DOL

DOL

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Disadvantages

1. Simple and most economical starter.

2. More comfortable to design, operate and control.

3. Provides nearly full starting torque at starting.

4. Easy to understand and troubleshoot.

5. DOL starter connects the supply to the delta winding of the motor.

Advantages

1. High starting current (5-8 times of full load current).

2. DOL Starter causes a significant dip in voltage, hence suitable only for small motors.

3. DOL Starter reduces the lifespan of the machine.

4. Mechanically tough.

5. Unnecessary high starting torque

Multiple Choice

Starter is need in DC motor?

• Yes

• No

Multiple Choice

The starter is need in AC motor?

• Yes

• No

Multiple Choice

There are _____________ types of starters in AC motors

• 4

• 3

• 2

• 1

Multiple Choice

DOL can be used if voltage is to be limted

• True

• False

Multiple Choice

Starters are used to limit the

• high voltage

• high in load resistance

• high in rush current

• None of the above

Poll

The short circuit can be avoided by

• Resistors

• Thermal overload protector

• Fuse

• Capacitors

Multiple Choice

Magnetizing coil is used to hold the switches in on position

• True

• False

Multiple Choice

Magnetizing coil is used to hold the contacts in on position

• True

• False

Multiple Choice

There are _________ normally open switches

• 4

• 3

• 2

• 1

Star Delta starter

Here, we control the input voltage to the stator

Starters

Star Delta starters

A star-delta starter is the most commonly used method for the starting of a 3 phase induction motor.

In star-delta starting an induction motor is connected in through a star connection throughout the starting period.

Then once the motor reaches the required speed, the motor is connected in through a delta connection.

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Current is reduced in the starting

Three resistance reduces the current

hence, 1/3 of the current only reaches the motor​

Star connection

at starting​

IL, supply current = Ips, current per phase

As star connected, the Voltage is VL across all the resistors (Z),

Hence total current , IPS = VL / Sqrt(3)

Star connection

at starting​

VL = Supply Line Voltage,

ILS = Supply Line Current and, IPS = Winding Current per Phase and

Z = Impedance per phase winding at stand still condition.​

ILS = 1/3 (ILD)

From this we can say that the Star connection current is 3 times lower than the delta current.​

STAR to DELTA​

A TPST switch when turned on makes the star connection into a delta connection.

ILD = Sqrt(3) IPD ​; Voltage is VL; IPD = VL/Z

​Therefore, ILD = (Sqrt(3) VL ) / Z

Star connection

to DELTA​

Star - Delta

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Disadvantages

1. Inexpensive

2. No heat is produced, or tap changing device needs to be used, hence efficiency increases.

3. Starting current reduced to 1/3 of direct online starting current.

4. Produce high torque per ampere of line current.

Advantages

1. Starting torque is reduced to 1/3 of full load torque.

2. A particular set of motors required.

Autotransformer

An Autotransformer starter is used to decrease the applied voltage to a motor throughout its beginning.

Voltage is reduced using transformer technique

Voltage drop is not done through the usual step-down, but a spl transformer with the different primary and secondary coil

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A single winding is used for both the input and output​

​​Transformer tapping is used (50 - 80 %)

​The direct connection to the winding of the transformer

Autotransformer

​

Switch 1 and 2 are connected, small voltage flows to motor.

Then switch 1 is removed, making further less voltage flow to motor, meanwhile switch 3 is connected

Full voltage flows with respect to the load torque.

​

Tapping is the switch 2

To turn off the motor, switch 3 is turned off.​

​

Soft starting occurs

Autotransformer

​

Multiple Choice

Types of starters are

• 4

• 3

• 2

• 1

Multiple Choice

Starter that limits high inrush current is

• DOL

• Star delta

• Autotransformer

• All the above

Multiple Choice

Starter that limits high starting voltage is

• DOL

• Star delta

• Autotransformer

• All the above

Multiple Choice

Starter that uses only a single coil is

• DOL

• Star delta

• Autotransformer

• All the above

Multiple Choice

What percentage of the starting is star connected in star delta connection

• 100

• 80

• 60

• 40

Multiple Choice

Star current is more than the delta current in Star delta connection

• True

• False

Multiple Choice

There are 3 switches in autotransformer

• True

• False

Poll

To turn off the motor in autotransformer, which switch should be removed

• 1

• 2

• 3

• 4

Poll

Percentage of tapping in star delta is

• 50

• 57

• 60

• 50-60

Multiple Choice

Tapping is direct__________

• connection at the end of the winding

• connection at the middle of the winding

• connecting anywhere in the winding

• connection at the start of the winding

Multiple Choice

Z is

• Zener

• Impedance

• Resistance

• Load

Multiple Choice

All these are the starters of

• Squirrel cage

• slip cage

• Both

• None

Starters forslip ring induction motors

Rotor resistance type

Rotor is connected to rheostat through slip rings

Stator is given full supply​

Rheostat is adjusted as per the needed supply​

​Some text here about the topic of discussion.

Slip Ring