Direct Online Starter - DOL

OL Starter | Direct On Line Motor Starter

A DOL starter is mainly consists of some push switches, metallic contacts, connectors, overload sensing relay coil and a under voltage protection relay coil. Hope you are already aware about the functionality of a relay coil; it generally designed with a predetermined current  value. If somehow the limit is reached then it energies the coil and due to the electromagnetic action, it attracts a mechanical plunger within it. The plunger is fitted with the contacts of the circuit, so pulling the plunger will break the circuit and the current flow will terminate. In this way it helps to clear the faults from the circuit. In DOL starter two relay coils are used for two different purposes, one is used to sense the over current and trip the circuit immediately in order to safeguard the motor from the effect of huge current, and another is used to trip the circuit in under voltage condition. The main functionalities of a DOL starter are depended up on the relay coils.

Relay Coil and the Mechanical Plungers

The animation will show the basic relay operation. Relay coils are designed with proper current value such that it will able to attract the plunger after a certain predetermined value of current. After that the plungers are coupled with the relay coil and the contacts. In this way the entire control that is connecting or interrupting the circuit depends up on the operation of relay coil. Generally three types of relays are used in DOL starter. They are Magnetic Relay, Thermal Relay and Electronic Relay. The operation of magnetic relay solely depends up on the current value, and senses only when the current tends to approach the predetermined limiting value. Thermal relay is able to sense the heat generated due to the high current flow, and also very reliable and fast. The electronic relays are modern engineering marvel, which use the solid state devices for sensing the over current or faults. The operation is quite easy and fast, additionally they are highly reliable.

Modern direct on line (DOL) starters are using this type of relays.

Push Switches and Contacts

DOL starter is generally used in three phase 415 Volt motor starting, but sometime it also used to start 230 Volt single phase induction motors. So operation with 415 Volt requires some safety arrangement for the operator. That is why the entire starter is packed into a casing and the push switches are connected with the circuit for operating. These switches are common button type switches and able to connect the two ports or terminals with lesser risk of sparking. Fuses are installed in series with each phase to increase the safety factor and to protect the starter and motor both. In the wiring diagram the entire connection of the circuit is shown and the positions of push switches are also shown.

Operation of Direct ON Line (DOL) Starter

 The operation of DOL starter is very simple. The full line voltage is applied across the motor, by means of the starter circuit. The coil C is called as the under voltage relay coil as well as it also acts like the starting coil. This coil is controlled via the push switch “S1”. This switch is called as START switch and installed at a convenient place from where the remote operation is easily possible; generally the switch can be pushed from outside of casing. The START switch kept at open position by a spring and if the switch is pressed then the coil C is energized (or charged) from the two line conductors (Here R phase and Y Phase).

A mechanical plunger is connected with the coil C. When it energizes from the line, it attracts the plunger by relay operation. There are one auxiliary contact and three Main contacts which all are synchronized with the movement of the plunger. Three main contacts connect the three phases with the motor terminals and the auxiliary terminal is used to control the starter circuit. When the Coil C is energized, the plunger is attracted by it and automatically all the contacts become short circuited. In this way the motor is energized from the full line voltage. If the START switch S1 is released then also the motor will continue running, because the contact A will hold on the circuit at ON mode. That is why this contact is also termed as hold on contact. The “Off” and “Remote Off” switches are generally closed with the help of spring arrangement. Both of them are in series connection so pressing any of them will break the circuit and terminate the current flow. In this way the coil C will be discharged and automatically it release the plunger, which h will break the main motor contacts and in this way the motor will stop after some time. The coil C should be charged by pressing START switch to start the motor from this condition.

Under Voltage protection of DOL Starter

This protection is done with the help of relay of relay coil C (under voltage protection coil) and the plunger. Two phases are directly connected with the coil and the coil is designed such a way that it is able to attract the plunger for a certain level of current. If the level of current falls due to the low voltage supply then the coil will release the plunger and all the contacts become open circuited. In this way the circuit will open and the current flow breaks. Typically electromagnetic relays are used for this purpose, but in modern days electronic solid state relays are used also.

Overload protection of DOL Starter

The DOL starters are widely used for starting of 3 phase cage motors with 415 Volt rating. Fuses are connected in series with each line conductor, which provides the primary safety from high line current. The 3 phase induction motors are costly so additionally a relay mechanism is installed with the DOL starter with overload coils (OLC) in each phase conductors. If the line current is sufficiently high then the overload release coil are energized with such a value it can push the plunger D. The plunger is generally kept at closed position, so pushing it will break the circuit of relay of C. Which will further breaks the main contacts. The entire operation is very fast, and it depends up on the sensing capability of both of the relays.

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