Automatic Star

8/13/2019 Automatic Star

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Automatic Star/Delta Starter Using

Relays and Electronic Timer for 3 PhaseInduction Motor

Project Guide:-Mr. Swapnil N. Jani

Submitted By:-Rohan Rustagi (10BEE0029)Rushil Wadhwana(10BEE030)


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Introduction

Most induction motors are started directly on line, but when very large motorsare started that way, they cause a disturbance of voltage on the supply lines dueto large starting current surges. To limit the starting current surge, largeinduction motors are started at reduced voltage and then have full supplyvoltage reconnected when they run up to near rotated speed. Two methods areused for reduction of starting voltage are :

1. Star delta starting2. Auto transformer stating.


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Working Principle of Star/Delta Starter

Voltage reduction during star-delta starting is achieved by physically

reconfiguring the motor windings.

The motor is connected in STAR during the starting and when the motor isaccelerated close to the 75% of the rated speed, the motor is connected inDELTA.


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Starting Voltage Reduction =Vline /3Hence starting current is reduced

Controlling the interchanging star connection and delta connection of

an AC induction motor is achieved by means of a star delta or wyedelta control circuit. The control circuit consists of push buttonswitches, auxiliary contacts and a timer.


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Fig. 1: DOL Starter


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Fig 2: Star-Delta Starter


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Circuit Diagram

1. Power Circuit


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2. Control Circuit (3 wire control)


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The Star and Delta are electrically interlocked and preferably mechanicallyinterlocked as well. In effect, there are four states:

1. OFF State. All Contactors are open

2. Star State. The Main and the Star contactors are closed and the deltacontactor is open. The motor is connected in star and will produce one thirdof DOL torque at one third of DOL current.

3. Open State. The Main contactor is closed and the Delta and Starcontactors are open. There is voltage on one end of the motor windings, butthe other end is open so no current can flow. The motor has a spinning rotorand behaves like a generator.

4. Delta State. The Main and the Delta contactors are closed. The Starcontactor is open. The motor is connected to full line voltage and full powerand torque are available.


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Features of star-delta starting

For low- to high-power three-phase motors. Reduced starting current Six connection cables Reduced starting torque Current peak on changeover from star to delta Mechanical load on changeover from star to delta

Effect of Transients

During transition period of switchover the motor must be free runningwith little deceleration. While this is happening Coasting it may

generate a voltage of its own, and on connection to the supply this voltagecan randomly add to or subtract from the applied line voltage. This isknown as transient current. Only lasting a few milliseconds it causesvoltage surges and spikes. Known as a changeover transient.


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A contactor is an electrically controlled switch used for switching a powercircuit, similar to a relay except with higher current ratings.[1] A contactoris controlled by a circuit which has a much lower power level than theswitched circuit.

Contactors come in many forms with varying capacities and features.Unlike a circuit breaker, a contactor is not intended to interrupt a short

circuit current.

A contactor has three components. The contactsare the current carryingpart of the contactor. This includes power contacts, auxiliary contacts, andcontact springs.

Apart from optional auxiliary low current contacts, contactors are almostexclusively fitted with normally open ("form A") contacts. Unlike relays,contactors are designed with features to control and suppress the arcproduced when interrupting heavy motor currents.

Contactors


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Utilization categories for contactorsaccording to IEC 60947-4-1

The current rating of the contactor depends on utilization category. Forexample IEC categories are described as:

AC-1- Non-inductive or slightly inductive loads, resistance furnaces

AC-2 - Starting of slip-ring motors: starting, switching-off

AC-3- Starting of squirrel-cage motors and switching-off only after themotor is up to speed. (Make Locked Rotor Amps (LRA), Break Full LoadAmps (FLA))

AC-4- Starting of squirrel-cage motors with inching and plugging duty.Rapid Start/Stop. (Make and Break LRA)


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NEMA (National Electrical Manufacturers Association) contactors and

motor starters are rated by sizes. These sizes are grouped by ratedcurrent and horsepower.

NEMA Size


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Size of each part of Star-Delta starter

(1) Size of Over Load Relay:

For a star-delta starter there is a possibility to place the overload protection intwo positions, in the line or in the windings.

Overload Relay in Line:In the line is the same as just putting the overload before the motor as with aDOL starter.

The rating of Overload (In Line) = FLC of Motor.Disadvantage: If the overload is set to FLC, then it is not protecting the motorwhile it is in delta (setting is x1.732 too high).

Overload Relay in Winding:

In the windings means that the overload is placed after the point where thewiring to the contactors are split into main and delta. The overload thenalways measures the current inside the windings.

The setting of Overload Relay (In Winding) =0.58 X FLC (line current).

Disadvantage: We must use separate short circuit and overload protections.


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2) Size of Main and Delta Contractor:

There are two contactors that are close during run, often referred to as the main

contractor and the delta contactor. These are AC3 rated at 58% of the currentrating of the motor.Size of Main Contactor= IFL x 0.58

(3) Size of Star Contractor:

The third contactor is the star contactor and that only carries star current whilethe motor is connected in star. The current in star is 1/ 3= (58%) of the currentin delta, so this contactor can be AC3 rated at one third (33%) of the motorrating.Size of Star Contactor= IFL x 0.33

Automatic Star

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