3 Phase Induction Motor Direct On Line Starter

3 Phase Induction Motor Direct On Line Starter 

Introduction:- 3 phase induction motors comes in the range of  0.5 hp to 200 hp and above. Depends on load motors power requirement is decided. Motor starters are used to safely start/stop motor either locally from near motor or remotely from control room. Many operational interlockings are also used in industries where complex process in involved. Now a days programable logic controller (PLC) are also used for control interlockings. In industries where large motors are used very big high current capacity MCC are used to control motors operation. When MCC current capacity is very high the motors can be started directly with dol starting principle where direct 3 phase supply is given to motor with the help of contactor. But in cities where supply comes from pole through cable high starting current of dol is not acceptable so we use different starting methods.

1 motors up to 5 hp direct on line starter is used
2 motors above 10 hp
        a. star delta starter   b auto transformer starter

In this post i am taking direct online starter

Circuit Diagram:-

As told above the starting current of small motors is less so dol stater is used safely.
normally a main switch or mcb is used as main power on off switch. A electromagnetic contactor is used to on/off three phase power to motor with start / stop switch. an over load relay is used to protect motor from operational over load .

Electromagnetic contactor:- It has a coil and set of no and nc contacts. coil is generally rated for 230 volt ac or 400 volt ac. There are three normally open power contacts used in power circuit of motor. Two sets of no and nc contact used for holding circuit, interlocking and indication circuit.

Start push nutton :- It has two contacts one no and one nc. The no contact is used in circuit. when start button is pressed supply goes to coil and contactor picks up. As soon as contactor picks up three phase supply goes to motor through three power contacts and motor starts. Also one no contact connected in parallel of start button is closed which maintains the supply to coil even after releasing the start button and motor runs continously. this contact is known as holding contact as it keeps contactor holdon.

Stop button:- It has two contacts one no one nc. Its nc contact is used in control circuit. As stop button is preesed the supply to contactor coil brakes and contactor drops out and motor stops.

Over Load Relay:- Over Load relay has three input power terminals and three output power terminals and one change over contact for control circuit. Overload relay input power terminal are connected to output of contactor. The output power terminals are connected to  motor power terminal with the help of cable. And the nc contact is used in control circuit in series with stop button. While running of motor if motors load is increased beyond overload set current value the overload operate and its nc contact becomes open so contactor drop out and hence motor stops (tripped)
so that overload relay protect motor from over load conditions.

Power fuse:- Power fuses are used to protect circuit from any short circuit in cable or motor. In case of any short circuit power fuse blown out and safe the other items from damage.

Control fuse or control mcb:- Generally used in mcc control panels where it needs to test circuit with out power. In case of some short circuit in control line control fuse blows out. Thanks



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