Effect of Control Cable Capacitance on Contactor Actuation

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Effect of the Cable Capacitance of Long Control Cables onthe Actuation of Contactors

Technical PaperDipl.-Ing. Dirk Meyer

Contactors DIL

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Q11CL

l

L1

N

L1

N

CL

l

Q11

0. Introduction:

The contactor is the most important switching device inindustrial and commercial applications. Its importancehas further increased due to the influence of automation.This has given rise to some significant advancementsin the development of contactors, of which the user isoften not aware. For example, the power required forswitching has been considerably reduced in recent yearsdue to the use of integrated actuation electronics.

Despite the many benefits made possible by the reducedpower consumption, such as

- Energy savings

- Use of smaller control transformers

- Longer service life of control contacts

the reduced power requirements must be particularlytaken into account with applications involving longcontrol cables. Reliable contactor disconnection onactuation of the command device also depends on cablelength, due to the cable capacitance between thecommand device and the contactor coil.

No effect of cable capacitance CL if the command contact is located

close to the power supply. There may be a slight off-delay due to CL.

The problem of cable capacitance with long control cables also often

occurs in large-scale installations such as with crane systems in

container terminals.

Effect of cable capacitance CL if the command contact is located away

from the power supply. The coil current continues to flow even when

the command contact is opened.

1. Cable capacitances

In certain circumstances, long control cables in ACactuated control circuits may prevent the disconnection ofcontactors due to the cable capacitance present.Even if the command contacts are open, the coil currentcan still flow due to the cable capacitance so that the

contactor remains in the On position if sufficient sealingcurrent is present.The effect of cable capacitance depends on the design ofthe control current circuit:

2


NQ11

CL

L1

CL 2

Q11

CL 3

CL 1I

0

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L1

N

3

2. Limit capacitance of a contactor

The maximum capacitance at whichthe contactor stays switched on inthe new condition despite the offcommand can be calculated usingthe equation below:

with

UAB = minimum drop-off voltage in VIAB = Sealing current with a mini-

mum drop-off voltage in APH = Rated sealing current of the

contactor in VAUC = Rated control supply voltage in V

At 50 Hz and a permissible ratedcontrol supply voltage 110% timesrated voltage based on equation 2.0:

At 60 Hz the values must be reducedby 20%.

b PH 106

1+a o U 2C

Two-wire control:

CL = 0.3 (F/km) x l (km) (1.3)

Three-wire control:

CL = 0.6 (F/km) x l (km) (1.4)

1.1 Capacitance of control cables

A guide value for control cablecapacitances between two conduc-tors is approx. 0.3F per km for two-wire control, and approx. 0.6F perkm between three conductors forthree-wire control. The followingequation should be used:

CL = 0.3 (F/km) x l (km) Two-wire control (1.3)

CL = 0.6 (F/km) x l (km) Three-wire control (1.4)

On disconnection, CL2 and CL3 areswitched in parallel (CL1 is bridged byQ11).

However, the specific values dependon the cable used and may thereforevary. If necessary, obtain the cablecapacitance from the manufacturer.When laying control cables togetherwith other lines (e.g. in the cableduct), the cable capacitance can nolonger be calculated. The capacitivecurrents must be measured.

Cmax = [mF] (2.0)PH Cmax = 500 [mF]

UABUC

a = = 0.4

IABIC

b = = 0.25...0.35

U 2C


424V 50Hz 24V 50Hz100000

1000

101 10 100

Halteleistung in VA

ein

fach

e St

euer

leit

un

gsl

ng

e in

m

Dauerkontaktgabe

Impulskontaktgabe

120V 60Hz

120V 60Hz

230V 50Hz

240V 60Hz

230V 50Hz

240V 60Hz

400V 50Hz

400V 50Hz

Permissible single control cable length with a rated actuation voltage of 110% UC in relation

to the contactor sealing power

3. Determining themaximum permissiblecontrol cable lengths

In order for a contactor to switchcorrectly, CL must be less than Cmax.

If equations (1.3) and (1.4) arerelated to equation (2.1), thefollowing values are produced for 50 Hz networks:

Two-wire control (3.0)

Three-wire control (3.1)

lperm = maximum permissible controlcable length in m.

With 60Hz networks the values for(3.0) and (3.1) must be reduced by20%.

PH U 2C

lperm = 1.7 106 [m]

PH U 2C

l perm = 0.85 106 [m]

Sin

gle

co

ntr

ol c

able

len

gth

s in

m

Two-wire control

Three-wire control

Sealing power in VA


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4. Measures to counteractexcessive cable capacitance

Several solutions are possible if theengineering of an installationdetermines that the contactors willnot drop out due to excessive cablecapacitance:

Use of contactors with higher coilsealing power

Use of DC operated contactors

Reduction of the control supplyvoltage (allow for voltage drop)

Laying the supply cable near thecommand contacts

An additional NC contact for two-wire control and NO contact forthree-wire control are used toshort the coil.

Three-wire control

Parallel switching of a resistance.The resistance is calculated asfollows:

(4.1)

The power of the resistance is:

(4.2)

It must be taken into account thatthe resistor increases the total heatdissipation of the circuit.

1000CL

R = [O] CL in F

U 2CR

P = [W]

An additional cable is required forthis. This considerably increasesthe break times of the contactors

Two-wire control

CONTACTOR

Uc = 230 VDILE(E)...; S(E)00DILM7...DILM15; DILADILM17...DILM32DILM40...DIL65DILM80; DILM95DILM115; DIL150DILM185...DILM250DILM300...DILM500DILM580...DILM1000DILH1400DILH2000

Uc = 120 VDILE(E)...; S(E)00DILM7...DILM15; DILADILM17...DILM32DILM40...DIL65DILM80; DILM95DILM115; DIL150DILM185...DILM250DILM300...DILM500DILM580...DILM1000DILH1400DILH2000

Sealing power

VA

4.648

1626

3.54.34.37.57.5

15

4.648

1626

3.54.34.37.57.5

15

Two-wire control 50Hz

148129257514836112138138241241482

543472944

18893069

413508508885885

1771

Three-wire control50Hz

7464

129257418

566969

121121241

272236472944

1535207254254443443885

Two-wire control60Hz

118103206411668

90111111193193386

434378756

15112456

331406406708708

1417

Three-wire control60Hz

5951

103206334

4555559696

193

217189378756

1228165203203354354708

Maximum permissible cable length in m for

Maximum permissible single control cable length for a rated control supply voltage of 230V and 120V and a maximum control supply voltage of

1.1 x UC for 50 Hz and 60 Hz mains frequency.


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Issued by Moeller GmbHHein-Moeller-Str. 7-11D-53115 Bonn

2005 by Moeller GmbHSubject to alterationsVER2100-949GB MDS/?? (08/05) Printed in the Federal Republic of Germany (08/05)

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Effect of Control Cable Capacitance on Contactor Actuation

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