DOL Schemes

SPE-D TRAINING SESSION TITLE: - PRESENTED BY:

PSW/SPE-D/TVP Date: -18.06.05

ELECTRICAL SYSTEMS & EQUIPMENT DESIGN STANDARDISATION & DOCUMENTATION GROUP

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DOL, RDOL, STARDELTA SCHEMES

CONCEPTS OF DOL, RDOL & STAR-DELTA

STARTER SCHEMES




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Different terminology in schemes

DOL basic concept

DOL with additional starting/stopping commands

DOL with L/R switch

DOL with A/M switch

RDOL

STAR-DELTA




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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 circuit 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. The rating of control fuses is selected according to the equipments connected in the control circuit.

The following nomenclatures are used while referring to a scheme drawing.

Cross-reference: For reading convenience, the drawing page on which scheme is drawn is divided into equal parts. The scheme drawing consists of various different

equipments like contactors, relays, ACBs, MCCBs, etc. These devices consist of

power contacts & auxiliary contacts. A specific number denotes each device & its

terminals used in the scheme. The family of a device alongwith its contacts & terminals

are drawn in one of the sections of the drawing page & the section where each

terminal is used is also written alongside. Now the section in which a specified terminal

is used also bears a notation alongside, which indicates the section in which the family

of that specified terminal exists & the number of the device to which the terminal

belongs. This is used as a cross-reference for the referring device and vice-versa.

(Please refer the drawings attached )

Contacts / Push Buttons: The push buttons are the switches, which can close or open the circuit whenever it is pressed. The contact is a part which is actually

responsible for opening or closing of the circuit. The contacts are basically of two types




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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.

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 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.

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.

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.

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

P.B. is relieved. This contact is called as a hold-on contact.

INDICATION CIRCUIT: Red colour LED is used for 'ON' indication. NO contact of

power contactor is connected in series with the red LED. Therefore, the red LED

illuminates on energizing of the contactor coil. Green colour LED is used for 'OFF' indication. Hence, a NC contact of power contactor is connected in series with the

green LED. Therefore, the LED illuminates on de-energizing of the contactor coil.

Amber colour LED is used for 'AUTO TRIP' indication. NO contact of thermal overload

relay is connected in series with the amber LED. Therefore, the LED illuminates on

operation of the relay.




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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.

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.

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.

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.

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.

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

relay is then used in the circuit of the power contactor coil. The auxiliary relay in such

cases is known as Interposing relay.

Also, the surge coming at the time of switching the coil may harm the circuitry

connected to the DCS/PLC contact. To avoid any such situation, a surge suppressor is

used across the coil of the power/auxiliary contactor. This is an R-C circuit through

which the voltage of surge is circulated at the time of switching. Thus, the damage to

the electronic circuit is avoided. (Same practice is followed in case of

intelligent/electronic relays used.)

In manual mode, the remote and local start commands can start the motor. The local-

remote selector switch is used further to differentiate between the two starts (local and

remote).

The local and remote stop commands will be independent of the type of start

command. The auto-stop command since combined with start command should not

depend on position of any selector switch.




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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.

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

forward and reverse modes. The stop command is common for both modes since it is

independent of type of operation. The NC contact of forward contactor is connected in

series with reverse contactor coil and that of reverse contactor is connected in series

with forward contactor coil. This is the electrical interlock, which ensures that only one

contactor will pick up at a time avoiding any mal-operation and consequent damage to

the feeder and problems to the dependant processes.

Indication circuit: One more Red LED is required to indicate the Reverse ON mode.

One NO contact of the reverse contactor is connected in series with it.




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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

K2 (43,44) & K2 (53,54).

After the set time of STAR operation, STAR contact of timer ETR550 deenergises.

So contactor K4 drops off opening the star point of the motor. It is recommended that after

the STAR contactor drops off there should be some time delay before the DELTA

contactor picks up. This is to ensure that contactor has fully opened. This period is called

as transition period & generally 60 to 80mS is sufficient time. DELTA contact of ETR550

picks up after this time delay. Now the coil of K3 gets excited through K2 (43,44), K2

(53,54), K4 (21,22) & D1 (25,28) & its contacts changes its position. As a result the coil of

the ETR550 relay loses its excitation & gets isolated from the circuit. Meanwhile the coil of

K3 continues to be excited through its hold on contacts K3 (13,14) & the coil of K2 through

the above mentioned path & the motor continues to run in the delta connection.

Now when the stop PB is pressed all the coils get de-energised and the motor stops

running.

Following are some features of a star-delta starter:




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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.

The timer is used to control the time duration for which the motor runs in star mode

and then automatic changeover to delta. The 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.

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

contactor also picks up immediately. The star contactor is closed before main

contactor to avoid arcing at the contacts while shorting the other end of motor

windings.

After set time delay in the timer according to load characteristics, the instantaneous

contact of the timer drops down. Therefore star contactor also drops down. The

delayed contact closes after the instantaneous contact has opened. The delta

contactor therefore closes after the star contactor drops down. This is again ensured

by putting NC contact of the star contactor in series with the delta contactor coil.




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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

large current. The advantage of using this starter is its simplicity and lesser space

requirement. The sketch STDL01 shows open transition type of starter.

2. Closed transition: The circuit of the motor remains closed in this case during the star to delta transition. The advantage of using this starter is that the motor keeps on

getting supply even during the transition time. A resistance of suitable rating is inserted

at the other end of the windings. Thus, the windings of motor are actually connected in

star during the transition time, only the resistance is added in series with the windings.

The disadvantage of this type of starter is larger space requirement since it requires an

additional contactor for connection of resistance and the space required for the

resistors itself. This type of starter is very rarely used since most of the motors/loads

are suitable for open transition type of starter.




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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.

1.SERVICE: The module has stab-in type of contacts and power contacts on its rear side (behind the base plate). The stab-in contacts get connected to the droppers and the

power contacts get connected to the outgoing power terminals in this position. In this

position SERVICE SICs & SERVICE +TEST SICs remain engaged. However TEST SICs

remain disengaged.

2.TEST: After opening the door module is racked out to bring it to TEST position. In this position, STAB-IN as well as O/G contact gets disengaged. Hence no power supply is

available to module. In this position SERVICE SICs gets disengaged. However TEST &

SERVICE +TEST SICs remain engaged. In this position, Control circuit can be tested

selectively.

3.ISOLATED: In this position, both power and control SICs get disengaged.

4.MAINTENANCE: In this position, one can get access to all components.

5.HINGED OUT: In this position, one can get access to components on rear and incoming contacts without removing the module.




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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 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

shown appropriately. It may be coming from inside the feeder or from the switchboard

or any external supply. The auxiliary bus, from which the supply is tapped, should be

named correctly in accordance with the general arrangement drawing. 4. Control circuit protection: The fuses or MCBs given for the control circuit protection

should be appropriately selected. The rating of the fuses/MCBs is to be selected

according to the equipments connected in the circuit. According to the application, the type of SIC is to be selected.

5. SIC arrangement: The SICs have specific position in a draw-out module. Therefore the arrangement of the SICs should be checked in a scheme. The number of SICs that

can be accommodated also should be checked with respect to the feeder size. Also it

should be kept in mind that Test SICs and Service + Test SICs can only come in the

front column of the module. 6. Star-Delta scheme logic: If any interlocks are there in a star-delta starter feeder, the

scheme should be re-checked for logic. The operation of the feeder in star and delta

modes should be analysed.

DOL Schemes

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