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Direct On Line and Star Delta Starter

We will come to know about the circuit diagram, main parts,working and necessity, of “Direct On Line (D.O.L.) and Star Delta starters”.As the name indicates “Starter” is used to start and stop the motor. The starter is also used to protect the motor and other connected equipment from sustained overload, under voltage, single phasing, etc., and also provide automatic control whenever required. Now the question arises why we need starter for the safe operation of the motor. As already explained that the motor acts as a transformer with secondary winding (Rotor winding) short-circuited. When the motor is started from standstill position, high starting current will circulate in the rotor winding and simultaneously the stator will draw heavy current from the supply mains. As a result of this heavy current, the system or line voltage will be drastically reduced, which is objectionable from safety point of view. The other sensitive equipments connected with same line may trip on low voltage. Therefore large capacity motors(Above 5 H.P.) should be connected through such a mechanism, which can reduce the starting voltage. If the starting voltage is reduced, the starting current drawn by the motor will be reduced automatically and thus the line voltage will remains almost constant.

Several types of starters are available in the market depending upon the type and rating of the motor. For example, Direct On Line (D.O.L.) starter is used up-to 5 H.P. motors; Star Delta starter is used from 7.5 to 20 H.P. motors and Auto- transformer/Resistance types of starter is used above 20 H.P. rating motors. Except D.O.L. starter, these starters are used to reduce the starting voltage and hence to reduce the starting current. Direct on line (D.O.L.) and Star Delta starters are discussed below.

i. Direct On Line Starter (D.O.L.)

As the name indicates “Direct On Line Starter” a device used to connect a motor directly on line voltage. As already discussed these starters are used up to 5 H.P. capacity motors as full load current of these motors is less and hence lesser starting current. Figure given below shows the internal connections and main parts.

a) No-Volt-coil (N.V.C.) : This coil is connected across two phases in series with interlock switch, overload relay, red button (off switch), green button (on switch) etc. Green button is connected in parallel with interlock switch. When green button is pressed for a few seconds the N.V.C. will magnetized and pulls the plunger down ward against the spring tension. Four copper strips will make the contacts with lower contact points and hence the motor will continue to work even after releasing the green button. When the red button is pressed for a second the N.V.C. will demagnetised and the plunger will go back to its original position. So, the motor will switch off.

b) Over Load Relay: This relay is used to protect the motor from overload which may be due to actual overload, defective bearings and single phasing etc. Thick heating strips are connected in series with the motor. A bimetallic strip is also fixed near these strips. When higher current (more than normal) starts flowing through these heating strips, bimetallic strip will start bending due to heating and opens the over load switch contacts. Hence the electric supply to N.V.C. will interrupt and motor will stop.

c) On Switch (Green button): This switch is used to switch on or start the motor and is normally open (Push to on) type  When this switch is pressed, N.V.C. will magnetized and motor will start running.

d) Off Switch (Red button): This switch is used to stop the motor and is normally close (Push to off) type as shown in the figure above. When this switch is pressed for a second, electric supply to N.V.C. will interrupt and motor will stop running.

ii. Star Delta Starter
These types of starters are used for motors having rated capacities from 7.5 H.P. to 20 H.P. Like direct on line starter it has also off switch, over load relay, no volt coil etc.
 
Star delta starter is used to reduce the starting current drawn by the motor when it is switched on. When the handle is pressed to start position (Star), the motor’s out put terminals A 2 , B 2 and C 2 gets short circuited with a thick copper strip on the handle while the other terminals A 1 , B 1 and C 1 gets connected across the supply. The applied voltage across the motor gets reduced to 58 % of the full voltage and hence the starting current drawn by the motor will also reduce to 58 % of the total starting current drawn by the motor if full voltage is applied. Although the starting torque will also reduced to same proportions. When the motor picks up more than 75 % of the rated speed, the handle is switched over to run position(Delta). Now the motor terminals C 1 A 2 , A 1 B 2 , and B 1 C 2 will get short circuit and the motor will get the full voltage. So, the motor will take full load current and develops full torque. In delta position handle is magnetically picked up and motor will continue to run on full load. An off switch is connected in series with overload relay and no volt coil. This switch is used to switch off the motor. Over load relaymmis used to switch off the motor by disconnecting the supply to the no volt coil.

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