DOL Schemes

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SPE-D TRAINING SESSION TITLE: -

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DOL, RDOL, STARDELTA SCHEMES

PSW/SPE-D/TVP Date: -18.06.05

CONCEPTS OF DOL, RDOL & STAR-DELTA

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Different terminology in schemes

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DOL basic concept

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DOL with additional starting/stopping starting/stopping commands

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Different terminology in schemes: Any scheme consists of two types of circuits as mentioned below: 1. Power circuit: It consists of power equipments for switching and protection of the downstream equipment e.g. ACB, MCCB, fuse switch, power contactor, relays, CTs, etc. Power circuit is generally indicated by the use of thick lines in the scheme drawing. Such that, it can be easily differentiated from the control circuit. 2. Control circuit: The control circuit consists of metering , protection and indication devices with necessary switches and interlocks. It includes the coil cir cuit of contactors, ACBs, their auxiliary contacts, shunt trips of MCCBs and all indicating lamp circuits. The supply for control circuit is either tapped from one of the phases and neutral of the feeder directly (or through a control transformer) or from external supplies. Control fuses are used for protection of control circuit circuit.. The rating of control fuses is selected

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viz. power contacts & the auxiliary contacts. The power contacts act in the power circuit, whereas the auxiliary contacts act in the control circuit. •

NO/NC contacts: The auxiliary contacts are further classified into two parts, namely NO (Normally Open) & NC (Normally Closed). This implies that these contacts are opened or closed respectively when the coil is de-energised or is in the OFF condition.

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Stay-put Push Button: This type is a special version of push buttons. As its name suggests these buttons once pressed stay or remain in the same position. Turning the button in the clockwise direction can open these buttons. Another version of this button can be turned to open only by using its own specific key to prevent any unauthorized use of it.

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DOL (DIRECT ON LINE) STARTER SCHEME: Power circuit for DOL: The DOL starter power circuit consists of following devices all connected in series. 1. Short circuit protective protective device device -SCPD(Switch fuse disconnector) or MCCB 2. Power contactor 3. Overload relay To start the motor, the SCPD is closed and the start command is given through start P.B.

Control circuit for DOL: The coil of contactor gets supply through the interlocks in the control circuit. The proper operation of DOL starter mainly depends upon the pick-up and drop-off of the contactor. The indications for the starter are given according to the power contactor status (ON/OFF) and the fault indication through the relay.

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TYPE 1: BASIC DOL STARTER: The basic DOL starter scheme is as explained below •

NO (13,14) contact of the SCPD (Q1) is connected in series with the coil of the power contactor to ensure that unless the fuse switch is closed, the contactor will not pick up.

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NC (95,96) contact of the overload relay (K1) is connected in series of the power contactor coil so that during an overload when the relay operates, the contact opens cutting off the supply to the contactor coil. Therefore, power contactor opens thus disconnecting the supply to the motor.

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The stop push button (S1:11,12) is also in series with the contactor coil so as to cut-off the supply when the stop button is pressed.

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NO contact of the power contactor is connected across the momentary start push button. This is to ensure the continuity of supply even after the start command to the

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TYPE 2: DOL STARTER WITH REMOTE START OPTION: The start P.B. connected in the circuit earlier was connected in the module itself. But many a times a start push button is also required at a remote place like in a control room etc. In order to attain this, another detachable start button is provided across the existing local start push button. The hold on contact for both the push buttons remains the same. Please refer the attached sketch “DOL02”. •

Both start commands are connected in parallel connection since on operation of any of the start commands, the coil should get supply.

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The local and remote stop P.B. contacts are in series with the contactor coil. This ensures that, if any of the stop command is given, the supply to contactor coils cuts off. Also, both local as well as remote stop commands are independent of the start commands.

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TYPE 3: DOL STARTER WITH LOCAL-REMOTE SELECTOR SWITCH: This case is similar to the previous case, except the addition of a LOCAL-REMOTE SELECTOR SWITCH which allows the user to select between local or remote starting only. Please refer the attached sketch “DOL03”. •

This selector switch is used to select from the options of local start and remote start. This ensures that if the user wants the start command to be given only from one location in a particular situation, the other start command will not operate the feeder.

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In such case, two separate hold-on contacts are required for each of the start commands since the two paths are totally separate from each other.

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Both remote as well as local stop commands are independent of the selector switch position. This is to ensure that irrespective of the local/remote position of the switch, operator at any location should be able to stop the feeder.

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TYPE 4: DOL STARTER WITH LOCAL-REMOTE SELECTOR SWITCH AND MAINTAINED REMOTE START OPTION IN ADDITION TO LOCAL START Please refer the attached sketch “DOL04”. •

In this case, the remote start/stop command is maintained type. This may be also a combined start/stop command. Therefore, the remote start circuit does not need any hold on contact across it to maintain the continuity of the circuit.

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Since the start command itself acts as stop command (once start is withdrawn), it is connected in the circuit such that it is independent of the selector switch position. There is no separate start command required in such case.

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TYPE 5: DOL STARTER WITH AUTO-MANUAL AND LOCAL-REMOTE SELECTOR SWITCHES: In this case, there is a selector switch in addition to a local-remote selector switch to make a choice between auto start command and manual start command. Please refer the attached sketch “DOL05”. •

In auto mode, only the auto start command (coming from central control room through PLC/DCS, etc.) will be able to start the motor. This command is generally combined start/stop type and hence, separate auto-stop command is not required.

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In such cases, the contact coming in the circuit should be capable to carry the pick-up current of the power contactor coil. This needs special attention since the contact comes from an electronic circuit. In case the contact is not rated for that much current, we have to use an auxiliary relay to multiply this command. The contact of the auxiliary

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TYPE 6: REVERSIBLE DOL (RDOL) STARTER This kind of starter is required when the motor is required to run in both forward as well as reverse directions according to the application. Please refer the attached sketch “RDOL06”. •

Power circuit: There is one more power contactor connected across the power contactor in the power circuit with two of the phases interchanged to achieve reversal of rotation. The two contactors are mechanically interlocked to ensure that both of them do not operate simultaneously. The shorting of two different phases is avoided through this arrangement.

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Control circuit: The control circuit obviously consists of two start push buttons, one for ‘FORWARD START’ and another for ‘REVERSE START’. Each of them controls the supply to the respective contactor coils responsible for the supply to the motor in

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STAR-DELTA (REDUCED VOLTAGE) STARTER In star-delta starter, the starter starts the motor in star-connection so as to reduce starting current taken by the motor. After the motor picks up sufficient speed, the starter changes the connection of motor from star to delta to give higher torque. Please refer the attached sketch “STDL01”. When the push button S2 is pressed the contacts 23,24 gets closed & the coil of the ETR550 timer gets excited through K3(21,22). ETR550 is a STAR-DELTA timer. It has 2 contacts i.e. STAR contact & DELTA contact. When ETR550 energises, its STAR contact operates & 15, 18 get shorted. As a result the coil of contactor K4 gets excited & the power contacts as well as Auxiliary contacts (NO & NC) change their positions respectively. Now coil of contactor K2 gets excited through K4 (13,14) & the power as well as Auxiliary contacts of K2 change their positions. Now even if the start PB is released the K2 coil continues to get excited through

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A special timer is used along with three power contactors. One contactor connects one end of motor windings to the busbar (known as main contactor). Other two contactors decide the star/delta connection of the motor.

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The timer is used to control the time duration for which the motor runs in star mode and then automatic changeover to delta. The T he timer coil gets supply as soon as the start command is given. There are two types of contacts, which are operated on giving supply to the timer coil. One is immediate changeover contact and another is delayed changeover contact. The immediate changeover contact is connected in series with the star contactor coil, while the delayed changeover contact is connected in series with the delta contactor coil.

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Since instantaneous contact of the timer is connected in series with the star contactor coil, the contactor picks up as soon as start command is given. After this, the main

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Thus, the time delay in the timer consists of two parts: 1. Time for which the motor windings are connected in star. This is the time required by the motor to reach 85%-90% of the full load speed. 2. The time required for transition from star to delta connection.

The first time delay can be set in the timer according to the load characteristics. The range generally available with timer is upto 30s or upto 60s. The second time delay is selectable between 60ms or 120ms. Depending on the circuit condition during transition from star to delta, there are two types of star-delta starters: 1. Open transition: The circuit of the motor remains open in this case during the star to delta transition. The disadvantage of this type of starter is that if the timing between star-delta transition is not selected appropriately then the delta phase becomes equivalent to DOL starter. The motor while changing over from star to delta draws a

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CONCEPT OF DRAWOUT FEEDER: The starter circuitry is generally accommodated in draw out type of feeder in a MCC panel. This type of feeder can be racked in or racked put with the help of racking handle, racking screw & sliding telescopic rails. Power supply to the feeder is given through the STABIN contact mounted on the dropper & power supply to customer is given through the D/O O/G power contact. Control supply from external side as well as interlock/feedeback from/to outside is given through SICs (Secondary Isolating Contacts). There are three types of SICs (i.e. SERVICE, TEST & SERVICE + TEST). SERVICE SICs can be mounted in front as well as rear column. However TEST & SERVICE + TEST SICs are mounted only in front column. Fully draw-out module has 3 distint functional positions additonal 2 positions for maintenance purpose.

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TEST FACILITY IN A STARTER: Sometimes the customer asks for a facility to check whether the feeder is working OK or not. The test facility is provided in the test position of a draw-out module since in test position, the power contacts of the module are disengaged. The Test-SICs however are still in contact and the module can get the control supply through the same. A test push button is given in place of all the commands, which will give supply to contactor coil only in test position. Wherever the customer wants some interlocks to be followed even in test position, Service + Test SICs are to be used. This ensures the interlocks work in both the positions. Please refer to the attached sketch ”DOLTEST” for details of the circuit.

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CRITICAL THINGS TO BE TAKEN CARE WHILE MAKING A STARTER SCHEME: Following things need special attention while making a scheme drawing for any starter feeder: 1. Control cabling: The control cables for commands coming from customer side (remote, DCS, PLC, etc.) are generally multi-core cables. Therefore whenever control terminals are numbered for external commands, attempt should be made to keep the numbers of these terminals serially. This makes the t he cable termination at site easier. 2. Draw-Out / Fixed feeders: The starter feeders can be offered in draw-out as well as fixed type. While making scheme, the type of feeder should be kept in mind. The type of draw-out and fixed feeder affects the type of control terminals and power terminals. The representation of the two types of terminals is different in a scheme diagram. 3. Control Supply: Control supply coming to the control circuit of the feeder should be

DOL Schemes

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