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Issued by : EBG - Electrical Systems & Equipment, LARSEN & TOUBRO LIMITED, Powai Campus, Mumbai 400 072

Prospect / Retrospect

Designed to Withstand

In the Electrical Systems, the field operatingengineers has started experiencing peculiarphenomenon. Sometimes without giving thecommand a drive starts. On some other occasions,even after giving a stop command the drive doesnot stop. In some cases, the PLC DI contact isobserved to burn frequently even on control signalinput. Such cases are always reported to theequipment suppliers for failure of the control systemsuch as PLCs or Numerical relays.

However, system study and analysis has proventhat it is not the apparent component which is atfault but the physical parameters such as controlvoltage level, type of cable, length of cable, etc. areresponsible for this behaviour.

In early days, except for the material handling plants,other plants had smaller spread out. Also control

cable where run from one device to the other devicedirectly without involving multi-core cables andjunction boxes. For higher rated motors, controlsupply used to be DC.

Now the plant sizes have expanded in capacity andalso geographically. Extensive instrumentation tosupport automation has resulted in long cablesbetween fields sensors to PLCs or MCCs. Use ofmulti-core cables and junction boxes has simplifiedcabling and troubleshooting. But all such measureshave resulted in a new phenomenon - increasedcable capacitance experienced in the control circuit.This cable capacitance is the root cause of theunexplained behaviour referred above.

In this issue of L&T Current Trends, we have triedto analyse the causes and recommend solutions toovercome this effect.

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FEATURE

Effect of Cable Capacitance on Contactor Operation- S.S. Khare

Manager, Switchboard Design.

Introduction

Contactor Controlled Motor Starters areused in Motor Control Centres inprocess plants. Switching of thestarters is routed through DCS or PLCor numeric relays. Many times,sensors are used for sequencecontrolling and interlocking of variousprocesses. The sensors are distributedin the field, away from the MotorControl Centres, resulting in longcontrol cables between theswitchboards and the field devices.Sometimes, multicore cable is usedto connect different field devices to theMCC using field junction boxes.

In normal operation the remotecontrolled contactor should pick upcleanly when the “Close / Start”command is given, hold on till the“Open / Stop” command is given.However, some times the capacitanceeffect experienced by the control circuitdue to the long cable causesmalfunction. The malfunction occurmostly executing “Stop” command. Insuch cases the starter does not dropoff. It is also experienced thatcontactor picks up even when there isno “Start” command. This also is theeffect of capacitance experienced bythe field cables. This article explainsthe reasons for this effects andprobable solutions to overcome.

CAUSES :

� Large distance between thefield controls and the MCCCapacitance of cable increases inproportion to the control cablelength. Due to this, even if the“Stop” command is given, voltagestill appears across the contactorcoil. If this voltage is more thanthe drop off voltage, contactor doesnot drop off. Similarly due to cablecapacitance, start command is

experienced if the accumulatedcharged voltage is higher than thepickup value. If the substationlayout and control logic is designedso as to restrict the control cablelength to 200-250 meters, then thecapacitance effect even thoughpersists in small amount, doesn’tinterfere with the starter operation.

� Higher control voltages like 240VAC.As shown in Fig. 2, thecapacitance between the pair ofcables shunts the remote signalcontact. This provides an alternatepath to the coil current and if thecapacitance is high enough, willprevent the contactor from droppingoff even when the switch is opened.As the cable capacitanceincreases with length, a limit isplaced on the distance at whichthe switching device can be locatedto avoid latching of the contactoreven after Stop / Open command.The limiting distance is impactedby control voltage. Lower thevoltage, higher the permissiblelength of the control cable.

The limiting value of cable length canbe calculated as follows, which in turnindicates the maximum limit on thelength of control cable.

(Effect of the capacitance of the cableis neglected, as the length of the cableis assumed to be small)

1) Let P = UdU

2) Z1 = R+jXL

3) ThenZ1 = U2

M

4) I = UZ1

5) Id = PI

When the single pole switch closes,the contactor picks up. However, whenthe switch opens, there will be a cur-rent maintained through the capaci-tance reactance given by :

6) I1 = UZ2

Where Z2 = R - j ( X

C - X

L )

Necessary condition for the contactorto drop off is :

7) I1 < I d

i.e. U < P U ; Z2 > Z1

Z2 Z1 P

R2 + ( XC – X

L)2 > U4

P2 M2

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FIG 1

MOTOR CONTROL CENTRE

FIG2

Fig-1 is a schematic diagram of acontactor coil connected in serieswith a single pole switch (locatedat a distance place from thecontactor) to the supply and fig-2shows its equivalent circuit.

L < BM P 109 metres2 frU2 P+1

As can be seen from the formula, thelimit on length of cable can be increa-sed by using lower control voltage.

So, using 110V AC instead of 240VAC will allow more length betweenMCC & control room. However, theselection of lower voltage level shouldbe selected such that it doesn’t affectthe pick up operation of contactor.

� Type of control voltage (AC orDC)

The AC control supply systemexperiences effect of capacitanceeven with comparativelylesser length of cables.The DC control supply system

does notexperience thecapacitanceeffect.

� Unused conductors in amulticore cableIn a multi-core cable unused con-ductors increase the capacitanceeffect. To overcome this effect,earthing of the unused conductorsin a multi-core cable will reducethe capacitance effect.

Recommendations to overcomethe Capacitance Effect:

� Use of lower control voltage:Using 110V AC instead of 240VAC in the control circuit allows

higher length of control cables.� DC control voltage:

In DC control system the effect ofcapacitance is eliminated.

� Earthing of unused cores:Earthing the unused conductorsin a multicore cable minimize thecapacitance effect to a largeextent.

� Use of resistance in parallelwith the contactor coil:The effective load resistance isreduced by this method. Thevoltage drop across the contactorcoil reduces, which helps inde-energising the contactor.

� Use of cables with lowdielectric constantThe cables with low dielectricconstant can reduce thecapacitance effect to a significantlylower value as compared to anormal PVC cable.

The dielectric constant of air orvacuum is given a reference valueof 1.0. Poor quality PVCinsulation may have a dielectricconstant of 5.0 to 6.0.Polyethylene has a much betterdielectric constant ofapproximately 2.0. Foamedpolypropylene or polyethyleneinsulations have constants as lowas 1.6. Use of insulation of lowerdielectric constant reduces thecapacitor. This helps in reducingthe problem of drop off.

� Use of potential divider circuitThis solution, in particular, appliesto the cases in which the motorsstart in absence of any starting

10) 1 L < 1 XL + U4 – R2 x 103 metres.

2 f P2 (bM)2 r x 10-6

Xc > XL + U4 – R2

P2 M2

8) 1 C < 1 XL + U4 – R2

2 f P2 M2

It may be necessary to modify theequation (8) as shown below:9) 1

C < 1 XL + U4 – R2

2 f P2 (bM)2

Where ‘b’ is a factor, which is lessthan 1. Introduction of this factor bcan be explained as follows:

a) Normally, manufacturers ofcontactors publish the maximumpower consumption of the coil andthere could always be a variationtowards the lower side.

b) Secondly, it is known that a part ofthe ampere-turns is required to over-come the high reluctance offeredby the air gap in the magnet. As thenumber of operations increases inthe field, there will be a tendency forreduction in this air gap, which alsomight result in a reduction in powerconsumption. These two factorscombined together will reduce thepower consumption from M to bM.

If capacitance of the control cable is

r mF / km.

Equation (10) can be further simplifiedby making the approximation R < < X

L

XL » Z1

and Z1 > > R

P

Z1 2 - R2 ~ Z1

2

P P

Then equation (1) can be approximatedto :

List of symbols Units

U Rated Control Circuit Voltage. VOLTS

Ud Drop Off Voltage. VOLTS

M Hold On VA, At Rated Voltage. U VA

I Hold On Current At Rated Voltage U A

Id Hold On Current At Drop Off Voltage Ud A

R Equivalent Resistance OHMS

C Equivalent Capacitance FARADS

F Frequency Hz

XL Equivalent Inductive Reactance OHMS

Xc Equivalent Capacitance Reactance OHMS

r Capacitance Of The Cable/ Km. MICRO FARADS/KM

L Length Of The Control Cable METERS

For further details on this subject, please contact:EBG - Electrical Systems & Equipment, Larsen & Toubro Limited, Saki-Vihar Road, P.O. Box 8901, Powai, Mumbai 400 072

Fax: 022-6705 1553 * E-mail: kotnisnd@Intebg.com

Printed by Printania Offset Pvt. Ltd.,D 20/21, Shalimar Industrial Estate, Matunga (East), Mumbai 400 019. Tel.: 2407 7996/8866/4540 Fax : 2402 4703 Email: pds6@rediffmail.com The views expressed in this magazine are not necessarily thoseof the management of Larsen & Toubro Limited. The contents of this magazine should not be reproduced without thewritten permission of the Editor. Not for sale-only for circulation among the customers. Editorial Coordinators : R. S.Mahajan, Neelam D. Kotnis, EBG-ESE, Powai.Member

ABCI

For example - Using the equation Tc=C x R

Where Time Taken to dissipate 70%of charge = TcValue of Capacitor = C = 470µFValue of Resistor = R = 470k

Putting these values into the equationwe get a Tc value of 250ms.

Therefore the capacitor will discharge70% of its value in 250ms and 99% ofit value in under a second. (Fig. 5)

� Separate route for power andcontrol cablesIf power and control cables arerouted together, the surges inpower cables induce voltage in thecontrol cables. To minimize thiseffect, the power cables andcontrol cables should be routed inseparate trays, atleast for theinstrumentation system.

� Use of shielded twisted wireSometimes magnetic interferenceinduces voltage in the control wire.The effect can be nullified by usingshielded twisted wire (for bringingthe commands from remote) ascompared to normal PVC wire.

Conclusion:

All the solutions discussed above haveto be evaluated on case-to-case basis.Looking at the pros and cons of allthese solutions, the following solutionsturn out to be most effective:

� Layout to restrict length of controlcable to 250-300m.

� Reduce control supply voltage to110 V AC.

� Use of lower dielectric constantcables for bringing the signals

� Use of twisted wire for bringing theremote signals.

command.

A capacitor is placed on the inputto coil and it taken to ground. Thiscreates a “Potential Divider” circuitand creates a path to ground.

The Capacitor C must be rated at 400VDC and 250 V AC, eitherpolycarbonate or polyester. (Fig. 3)

The value of the capacitance iscalculated as follows:

C = 10x TCCABLE

Where C= Value of Added CapacitanceTC= The total capacitance of the cable.

A typical value for the capacitor wouldbe 470µF

We also recommend a resistor beplaced in parallel to the capacitor.

Resistor R acts as a “bleed”, safelydissipating any energy that may bestored in the capacitor. We wouldrecommend a 470k resistor. (Fig. 4)

This value is chosen to ensure thatthe capacitor discharges safely inunder a second.

Fig. 5

STARTSWITCH

START COMMAND

CSTARTSWITCH

START COMMAND

C

Fig. 3

S TARTS W ITCH

S TART C OM M A ND

CRS TARTS W ITCH

S TART C OM M A ND

CR

Fig. 4