Effect of the Cable Capacitance of Long Control Cables on the ...

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Effect of the Cable Capacitance of Long Control Cables on the ...

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Technical Paper

Dipl.-Ing. Dirk Meyer

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The problem ong>ofong> cable capacitance with long control cables also ong>ofong>ten

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

container terminals.

1. ong>Cableong> capacitances

In certain circumstances, long control cables in AC

actuated control circuits may prevent ong>theong> disconnection ong>ofong>

contactors due to ong>theong> cable capacitance present.

Even if ong>theong> command contacts are open, ong>theong> coil current

can still flow due to ong>theong> cable capacitance so that ong>theong>

L1

N

C L

l

Q11

No effect ong>ofong> cable capacitance CL if ong>theong> command contact is located

close to ong>theong> power supply. There may be a slight ong>ofong>f-delay due to CL. 2

0. Introduction:

The contactor is ong>theong> most important switching device in

industrial and commercial applications. Its importance

has furong>theong>r increased due to ong>theong> influence ong>ofong> automation.

This has given rise to some significant advancements

in ong>theong> development ong>ofong> contactors, ong>ofong> which ong>theong> user is

ong>ofong>ten not aware. For example, ong>theong> power required for

switching has been considerably reduced in recent years

due to ong>theong> use ong>ofong> integrated actuation electronics.

Despite ong>theong> many benefits made possible by ong>theong> reduced

power consumption, such as

- Energy savings

- Use ong>ofong> smaller control transformers

- ong>Longong>er service life ong>ofong> control contacts

ong>theong> reduced power requirements must be particularly

taken into account with applications involving long

control cables. Reliable contactor disconnection on

actuation ong>ofong> ong>theong> command device also depends on cable

length, due to ong>theong> cable capacitance between ong>theong>

command device and ong>theong> contactor coil.

contactor remains in ong>theong> On position if sufficient sealing

current is present.

The effect ong>ofong> cable capacitance depends on ong>theong> design ong>ofong>

ong>theong> control current circuit:

C L

l

L1

N

Q11

ong>Effectong> ong>ofong> cable capacitance CL if ong>theong> command contact is located away

from ong>theong> power supply. The coil current continues to flow even when

ong>theong> command contact is opened.

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1.1 ong>Capacitanceong> ong>ofong>

control cables

A guide value for control cable

capacitances between two conductors

is approx. 0.3µF per km for twowire

control, and approx. 0.6µF per

km between three conductors for

three-wire control. The following

equation should be used:

C L = 0.3 (µF/km) x l (km)

Two-wire control (1.3)

C L = 0.6 (µF/km) x l (km)

Three-wire control (1.4)

On disconnection, C L2 and C L3 are

switched in parallel (C L1 is bridged by

Q11).

However, ong>theong> specific values depend

on ong>theong> cable used and may ong>theong>refore

vary. If necessary, obtain ong>theong> cable

capacitance from ong>theong> manufacturer.

When laying control cables togeong>theong>r

with oong>theong>r lines (e.g. in ong>theong> cable

duct), ong>theong> cable capacitance can no

longer be calculated. The capacitive

currents must be measured.

L1

N

Q11

C L

Two-wire control:

CL = 0.3 (µF/km) x l (km) (1.3)

2. Limit capacitance

ong>ofong> a contactor

The maximum capacitance at which

ong>theong> contactor stays switched on in

ong>theong> new condition despite ong>theong> ong>ofong>f

command can be calculated using

ong>theong> equation below:

with

b P H ·10 6

·

1+a o · U 2 C max = [mF] (2.0)

C

UAB a = = 0.4

UC IAB b = = 0.25...0.35

IC L1

N

I

0

Q11

Q11

C L1

C L3

C L2

Three-wire control:

CL = 0.6 (µF/km) x l (km) (1.4)

U AB = minimum drop-ong>ofong>f voltage in V

I AB = Sealing current with a minimum

drop-ong>ofong>f voltage in A

P H = Rated sealing current ong>ofong> ong>theong>

contactor in VA

U C = Rated control supply voltage in V

At 50 Hz and a permissible rated

control supply voltage 110% times

rated voltage based on equation 2.0:

PH C max = 500 · [mF]

U 2 C

At 60 Hz ong>theong> values must be reduced

by 20%.

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3


3. Determining ong>theong>

maximum permissible

control cable lengths

In order for a contactor to switch

correctly, C L must be less than C max.

If equations (1.3) and (1.4) are

related to equation (2.1), ong>theong>

following values are produced for

50 Hz networks:

PH U 2

lperm = 1.7 · 10

C

6 · [m]

Two-wire control (3.0)

PH U 2

l perm = 0.85 · 10

C

6 · [m]

Three-wire control (3.1)

l perm = maximum permissible control

cable length in m.

With 60Hz networks ong>theong> values for

(3.0) and (3.1) must be reduced by

20%.

Single einfache control Steuerleitungslänge cable lengths in m

100000

1000

24V 50Hz 24V 50Hz

10

1 10 100

Sealing Halteleistung power in in VA VA

120V 60Hz

120V 60Hz

230V 50Hz

240V 60Hz

230V 50Hz

240V 60Hz

400V 50Hz

400V 50Hz

Two-wire Dauerkontaktgabe control

Three-wire Impulskontaktgabe control

Permissible single control cable length with a rated actuation voltage ong>ofong> 110% UC in relation

to ong>theong> contactor sealing power

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4


CONTACTOR

Uc = 230 V

DILE(E)...; S(E)00

DILM7...DILM15; DILA

DILM17...DILM32

DILM40...DIL65

DILM80; DILM95

DILM115; DIL150

DILM185...DILM250

DILM300...DILM500

DILM580...DILM1000

DILH1400

DILH2000

Uc = 120 V

DILE(E)...; S(E)00

DILM7...DILM15; DILA

DILM17...DILM32

DILM40...DIL65

DILM80; DILM95

DILM115; DIL150

DILM185...DILM250

DILM300...DILM500

DILM580...DILM1000

DILH1400

DILH2000

4. Measures to counteract

excessive cable capacitance

Several solutions are possible if ong>theong>

engineering ong>ofong> an installation

determines that ong>theong> contactors will

not drop out due to excessive cable

capacitance:

• Use ong>ofong> contactors with higher coil

sealing power

• Use ong>ofong> DC operated contactors

• Reduction ong>ofong> ong>theong> control supply

voltage (allow for voltage drop)

• Laying ong>theong> supply cable near ong>theong>

command contacts

• An additional NC contact for twowire

control and NO contact for

three-wire control are used to

short ong>theong> coil.

Sealing

power

VA

4.6

4

8

16

26

3.5

4.3

4.3

7.5

7.5

15

4.6

4

8

16

26

3.5

4.3

4.3

7.5

7.5

15

An additional cable is required for

this. This considerably increases

ong>theong> break times ong>ofong> ong>theong> contactors

L1

N

I

0

Two-wire control

Two-wire

control

50Hz

148

129

257

514

836

112

138

138

241

241

482

543

472

944

1889

3069

413

508

508

885

885

1771

Maximum permissible cable length in m for

Q11

Three-wire

control

50Hz

74

64

129

257

418

56

69

69

121

121

241

272

236

472

944

1535

207

254

254

443

443

885

Two-wire

control

60Hz

118

103

206

411

668

90

111

111

193

193

386

434

378

756

1511

2456

331

406

406

708

708

1417

L1

N

Q11

Three-wire control

• Parallel switching ong>ofong> a resistance.

The resistance is calculated as

follows:

1000

R = [O] CL in µF

CL (4.1)

The power ong>ofong> ong>theong> resistance is:

U 2 C P = [W]

R

Three-wire

control

60Hz

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

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

59

51

103

206

334

45

55

55

96

96

193

217

189

378

756

1228

165

203

203

354

354

708

(4.2)

It must be taken into account that

ong>theong> resistor increases ong>theong> total heat

dissipation ong>ofong> ong>theong> circuit.

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Moeller addresses worldwide:

www.moeller.net/address

E-Mail: info@moeller.net

Internet: www.moeller.net

Issued by Moeller GmbH

Hein-Moeller-Str. 7-11

D-53115 Bonn

© 2005 by Moeller GmbH

Subject to alterations

VER2100-949GB MDS/?? (08/05)

Printed in ong>theong> Federal Republic ong>ofong> Germany (08/05)

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