n)General questions

1) QUESTION:
How can I select contactors for utilization categories AC-8a and AC-8b? What special features have to be observed?

ANSWER:
This is to confirm that all technical data of contactors as shown for AC-3 can also be used for AC-8a and AC-8b.

2) QUESTION:
Is it possible to use a set of 3TF53 main contacts in a 3TF52 contactor in order to increase the electrical life of the contactor?
ANSWER:
The 3TF main contacts are mechanically coded and therefore cannot be interchanged within same contactor sizes.

3) QUESTION:
Need confirmation that 3TF Contactors free of PVC.

ANSWER:
This is to confirm that the materials of 3TF contactors do not contain any PVC


....

m) Temperature limit of coil

Temperature limit of coil

Q) To which class of insulating material are the solenoids coils of contactors have to subject?
What are their maximum permissible temperatures?

ANSWER:

The maximum permissible temperature of solenoid coils of contactors is 140 °C.
Example: Siemens make 3TF,3TH contactors coil sustain upto 140 degree C.

For these coils the maximum permissible temperature rise limit for coils in air according to class of insulating material is E = 100 K.
Since this is a temperature rise in Kelvin, this value shall be added with the maximum ambient air temperature of +40 °C of the contactors as described in IEC 60947-4-1.
The temperature rise limits applicable only if the ambient air temperature remains within the limits -5 °C to +40 °C.



......

l)Problem in dalher connection

Problem in dalher connection


QUESTION:

Contactors in a Dahlander connection are controlled by a PLC. Very often contactors are welding after a speed changeover commands. No transition times are parameterized into the PLC program. Contactors are electrically interlocked however.

ANSWER:

Enhanced by unfavorable configurations of line frequency and residual magnetic field of rotor, transient reactions inside the motor occur during a changeover of motor speeds. As a result current peaks arise higher than current peaks during ordinary DOL starting.

In the worst case the making capacity of contactors is exceeded with contact welding as a consequence.In order to allow the residual magnetic rotor field in to decay, a transition time of at least 50 ms has to be allowed before the line voltage is re-connected during speed changeovers



.....

k)Series & Parallel combination

Series & Parallel combination

QUESTION:

How do load ratings change when contacts are connected in parallel or in series? Which option should be chosen?

ANSWER:

Series connection of contacts:

A series connection of contacts allows a certain current even at increased voltage values (as stated in technical data of contactors). This is because the switching arc is divided into a certain amount of smaller arcs, depending on the actual amount of contacts in series. A series connection of contacts also strongly reduces contact erosion which means increased contact endurance. Series connection of contacts is used for DC-loads.


Parallel connection of contacts:

Since -due to mechanical tolerances- all contacts within one contactor never close or open simultaneously, a parallel connection of contacts neither increases the making capacity nor the breaking capacity of a contactor. The making- and breaking capacity of each contact is the upper limit also in this instance.
Parallel connection of contacts can be used to increase the continuous (thermal) current of a contactor.

The AC-1 ratings of contactors can be increased as follows:
- parallel connection of 2 contacts: Ie AC-1 x 1,8
- parallel connection of 3 contacts: Ie AC-1 x 2,5


Fig:parallel connected contactors
In order to avoid current unbalance we recommend to connect only neighboring current paths in parallel.

......

j)Mounting position changed

Mounting position changed

QUESTION:

Which technical consequences shall be considered if mounting position other than standard is used?

ANSWER:

Operating the contactor at other than standard mounting position is subject to following restrictions:

- drop out voltage of contactor is reduced.
- shock resistance in OFF position is a little lower
- mechanical lifetime is slightly reduced
- electrical lifetime is not or neglectible influenced
- risk of contact welding is increased by "slow" opening (under incidence of residual currents or under use of free wheeling diode) and by voltage drops.



......

i)Power loss

Power loss

QUESTION:

How to calculate power loss of contactors which is generally required for thermal calculations of a switchboard.

ANSWER:

The power losses of contactors can be calculated as follows:

- with DC-operated contactors: Closed power consumption of magnet coil- with AC-operated contactors: Closed power consumption of magnet coil x cos phi
Note: The above coil power consumptions and power losses per current path


.....

h) Effect of cable capacitance

Effect of cable capacitance

Q)What is Effect of Cable Capacitance on the Operation of AC operated Contactors

When long length of cables is used the stored energy in the capacitance of the cable holds the contactor even after the closing command is removed. Typical value of capacitance for two core cable is 0.3 micro farad/km


Fig:Stray capacitance

Remedy:

1) Use of DC control supply voltage
2) Use of lower supply voltage
3) Short circuiting of contactor coil during off command by additional bridge contacts.
4) Use of large contactors


.......

g)Control Xmer guidelines

Control Xmer guidelines

Q)Is there any guidelines available to determine the correct size of control teansformers?

To determine the size of auto control transformer, the following guidelines may be used.

For switching on contactors, the control rating of the transformer should be:
0.33 times the sum of the pick up VAs of the contactors which are to be switched on simultaneously

For Example: If 3x contactors of 3TF43 & 2x contactors of 3TF 55 are to be switched on simultaneously then the total pickup VA is 3064. Required control

transformer rating: 3064/3 = 1.021 kVA. With 5% regulation.Note: In general, as control transformers have poorer regulation, usage of higher VA of transformer is recommended


Fig:Control transformer


.....

f)Control cable length

Control cable length

Q) How do I calculate the critical length on control cable for contactor?
The permissible length of cable (m) can also be calculated by using the formula below:




U : Rated coil voltage

dU : Voltage drop ( 5%)

Son : Coil VA / W at pick up

Cos φon : p.f of coil at pick up

R: Ohmic resistance/cable values as tabulated




When long control cables are used for actuating the contactor, large voltage drop may occur causing chattering of the contactor or even failure to pick up. If the length of the cable exceeds L, use of higher size of cable or use of contactor relay is suggested as shown in fig.




..

e)High frequency effect

High frequency effect

QUESTION:
What are the derating factors for Contactors at higher frequencies?

ANSWER:
At frequencies equal or higher than 100 Hz the thermal load rating of switchgear is reduced. The reason for this is the so called ''skin effect'' at higher frequencies. This skin effect causes a current flow near the surface of a conductor and not through its full cross section. This effect is increasing with higher frequencies.
As a result the thermal load rating (continuous current) of contactors is reduced according to the following table:
--------------------------------------------------
- at 100 Hz: Ie AC-1 x 0,933
- at 200 Hz: Ie AC-1 x 0,871
- at 300 Hz: Ie AC-1 x 0,836
- at 400 Hz: Ie AC-1 x 0,812
--------------------------------------------------

Note: Above factor applied to Siemens make contactors

Indicated are typical frequencies of practical applications.
Making- and breaking capacities are not reduced within this range.
The contactors are not suitable for frequencies higher than 1000 Hz since internal ferromagnetic parts would be excessively heated by eddy currents



.......

c)Contactors with V.F.D.(load)

Contactors with V.F.D.(load)

Q) Which application criteria shall be considered for use of contactors on load side of variable speed drives?

Ans) For load side of variable speed drives it is appropriate to select contactors for AC-3 utilization category according to rated motor power. Since voltage and frequency of VSD are virtually always directly proportional, load breaking is not critical for the contactor even at low frequencies.

Example:

Referring to a network 400 V 50 Hz, at only 5 Hz a voltage value of 80 V would apply. This value for breaking operation is easily handled by an AC-3 sized contactor.

In theory it may happen that in a failure case DC current flows under full net voltage. In such case the HRC fuses which protect the VSD would blow and disengage the circuit.

ATTENTION!
Vacuum contactors 3RT12 and 3TF6 are not recommended for switching DC and very low frequencies below 16.7 Hz.


.......

b)Contactors with V.F.D.(line)

QUESTION:
Which application criteria shall be considered for use of contactors on line side of variable speed drives?

ANSWER:
Application criteria for use of contactors on line side of variable speed drives:

Variable speed drives incorporate DC voltage link capacitors which cause high making current peaks unless there are suppression measures pre chokes or pre charge resistors provided inside the frequency converter.

If those peaks are higher than the contactor’s making current capacity then even previously closed main contacts will lift off and chatter.
Resulting switching arc melts the contact material and in consequence the contacts will weld together when re-closing.

Since level of occurring current peaks at the converter’s place of installation mainly depends on the prospective short circuit current.


For project planning following options are available:

- under awareness of current peak value:

Select contactor according to its making capacity (10 x Ie AC-3).This making capacity shall not be lower than the current peaks.


- without awareness of current peak value:
Select a capacitor contactor .Capacitor contactors incorporate pre charge resistors. Therewith making current peaks are damped to uncritical values.

Following capacitor contactors Siemens make contactors are recommended for usage on the line side of variable speed drives:

- 3RT1617: up to 5,5 kW / 400 V
- 3RT1627: up to 7,5 kW 11 kW / 400 V
- 3RT1647: up to 22 kW 45 kW / 400 V


.......

a)Indicators of mechanical life

Indicators of mechanical life

Q) What are the indicators of mechanical life of contactors?
Following are the indicators of the end of mechanical life.

1)For E shaped magnets, air gap between the central limbs is reduced to zero. This is indicated when there is rubbing of paint on the central pole face of the upper magnet.



2)Broken shading ring of magnet.




3)Flaring of magnet pole faces, hence difficulty in removing coil.


4)Incurable humming

5)Sluggish operation


6)Contactor does not drop off


....

f)Excessive pitting

A) Excessive pitting and welding of contacts chattering is caused by following reason

1) Reasons: Coil not picking due to low voltage

Remedy: Ensure correct voltage conditions In case of persistent low voltage change coil to lower voltage or change control transformer tapping

2) Reason: Broken Short circuit ring (Shading ring)

Remedy: The mechanical life of contactor is over. Replace the contactor

3)Chattering in switching device
Remedy: Check & correct the condition in the control circuit


4)Reason: Small cross section/long length
Remedy: Select sizes as per guideline in sect on of control cable control wiring

5)Reason: Improper termination of control cable
Remedy: Use proper lugs for termination

6)Inadequate capacity of control transformer
Remedy: Use correct control transformer



7)Reason: Capacitor switching
Remedy: Use correct contactor & follow guidelines

8)Reason: Short circuit during star delta changeover
Remedy: – Use 7PU 60 20 timer

9)Reason: Reversing application (cranes)
Remedy: Use interlocked contactors with electrical interlocking


........

E) Failure to drop out

E) Failure to drop out

1)Coil not disconnected from supply
Remedy: Check wiring of the coil circuit

2)Residual magnetism due to lack of air gap in the central is over limb of the magnet
Remedy: The mechanical life of contactor. Replacement

3)Gummy substance on pole faces causing binding
Remedy: Remove the foreign substance



.....

D) Failure to pick up & seal

D) Failure to pick up & seal

1) Low voltage
Remedy: Correct voltage conditions-In case of persistent low voltage change to lower voltage coil or change transformer tap

2) Coil open or shorted
Remedy: Replace the coil

3) Mechanical obstruction
Remedy: Clean and check free movement of contact assembly

4) Coil excitation without arc chute or arc chute not properly fixed
Remedy: Ensure proper fitting of arc chute


.....

C) Noisy Magnets

C) Noisy Magnets

1) Broken shading ring
Reason: The mechanical life of contactor is over


2) Magnet faces not mating
Remedy: The mechanical life of contactor is over


3) Dirt or rust on magnet faces
Remedy: Wipe/clean lightly with fine emery paper size 00

b)Over heating of main contact path

B) Over heating of main contact path

1) Reason: Foreign matter on the magnet pole faces or jamming of contact carrier preventing contact closing

Remedy: –Remove the foreign matter

2) Reason: Improper termination of cables, improper cross section of cable & busbar

Remedy: Check the millivolt drop across cable/strands & terminals
The mV drop should be < 4.0 mV at rated currents otherwise clean busbar or recrimp cable
.
3) Reason: Presence of harmonics

Remedy: Suitable derating to be employed.

4) Reason: Improper ventilation of panel
Remedy: Provide suitable cooling for the panels.

a)Overheating of coil

Overheating of coil

1)Over voltage

Remedy: Check & correct terminal voltage/replace with higher voltage coil

2)Under voltage failure of magnet to seal in

Remedy: Correct terminal voltage / replace with lower voltage coil or change transformer tap

3)Inter turn short circuit of Coils (Coil getting excessively hot turned brown in colour)


Remedy: Replace coils


4) Dirt or rust on pole faces increasing the air gap
Remedy : Clean pole faces. Don’t use grease solvents or sharp objects while cleaning


....

Star Delta Switching

1 Introduction

Star-delta (wye-delta) starting continues to be used for switching on 3-phase induction motors. When this type of connection method is used, the starting current is reduced to 1/3 of the current required for direct switching.

When switching from a star (wye) circuit to a delta circuit, transients can occur in the motor, which, further intensified by an unfavorable constellation of line frequency and inductor field, lead to the development of larger current peaks than is the case if the stopped motor is directly switched into a delta connection. In an unfavorable case, the following problems could occur:

• Short-circuit devices could trip,
• The delta contactor could become welded or be subject to high contact erosion,
• The motor could be subject to extreme dynamic overloading.

A preferred circuitry configuration for star-delta (wye-delta) starters is described in this Functional Example. When the main circuit is connected in a favorable manner, it is possible to reduce the transient currents and current peaks that develop when the motor is switched over from a star (wye) connection to a delta connection


2 Function

Two different types of motor connections are often listed in star-delta (wye-delta) starter circuit diagrams: One for clockwise and one for counterclockwise motor rotation. When installing the motor, care should be taken to ensure that the transient current peaks are as small as possible. Correct wiring of the motor terminal board is essential


1.1 Description of the Functionality for Clockwise Motor Rotation

2.1.1 The Preferred Circuitry Configuration is implemented

The vector diagram illustrated below shows the voltages that develop in a motor with clockwise rotation when the motor is switched from a star (wye) to a delta connection. In this case, the motor terminals are correctly connected according to the preferred circuitry configuration, meaning that phase L1 is connected to the motor terminals U1 and V2, L2 is connected to V1 and W2, and L3 is connected to W1 and U2:





Fig. 1: Correct connection of the motor phases for clockwise rotation



Fig. 2: Vector diagram for switching from a star (wye) connection to a delta connection for clockwise motor rotation when the motor phases are correctly connected

During the idle changeover delay, the rotor lags behind the phase sequence. The magnetic field induces a damped residual voltage, which is listed here in the voltage vector diagram for phase L1: L N U 1'− . When switching over to a delta connection (Fig. 1 and Fig. 2), the stator winding carrying this residual voltage is supplied with the line voltage L1 L3 U − . Thanks to the favorable vector position of the residual voltage L N U 1'− and the line voltage L1 L3 U − , which have approximately the same values, the differential voltage ΔU is relatively small. Thus, the current peaks resulting from this voltage also remain small.


2.1.2 The Preferred Circuitry Configuration is not Implemented

The motor also rotates clockwise when the motor terminals are connected as follows:

Phase L1 is connected to motor terminals U1 and W2, L2 is connected to V1 and U2, and L3 is connected to W1 and V2.



Fig. 3: Incorrect connection of the motor phases also resulting in clockwise motor rotation

Both the lagging residual voltage and the damped residual voltage are active in the stator. When the motor is switched to a delta connection, the phase winding with the vector L N U 1'− is supplied with the line phase L1 L2 U − . However, each of these voltages has a different vectorial direction. The differential voltage ΔU is high, thus causing auto correspondingly high transient current peak. Switching from a star (wye) connection to a delta connection therefore results in the following vector diagram




2.2 Changing the Direction of Rotation from Clockwise to Counterclockwise
When the motor rotates counterclockwise it is sufficient to exchange two phases in any position. Then the same conditions would prevail as described above for clockwise rotation. In order to also keep the transient current peak as small as possible in this case when switching from a star (wye) connection to a delta connection, the wiring must be carried out as illustrated below



Refer to Section 3 for information regarding the wiring of the main current and the control current. The circuit diagrams for contact assemblies for star-delta

(wye-delta) starting with clockwise and counterclockwise motor rotation are illustrated in the preferred circuitry configuration.

2.3 Device Rating for Standard Start-up

Star (wye) contactor: Ie Motor x 0.33

Line/delta contactor: Ie Motor x 0.58

Overload relay: Ie Motor x 0.58



Comment: When two phases in the circuit are exchanged to change the direction of rotation, the connection is automatically switched from the more favorable connection to the less favorable one and vice versa.
℘ = Transient current factor = transient current peak/starting current peak
In theory, the transient current factor has a maximum value of 2.
For example, measured: Favorable connection: ℘ = 0.8
Unfavorable connection: ℘ = 1.37



3 Assembly and Wiring

3.1 Main Circuit Overview

In the following graphic, the preferred circuitry configuration for the main circuit is illustrated for a star-delta (wye-delta) connection for clockwise and counterclockwise motor rotation



1.1 Control Circuit Overview
The control circuit for the main circuit shown above is illustrated below.


Fig:Siemens contactor assembly for star delta starter

a) R-C element

Circuit with RC Elements



Fig. Basic circuit diagram: RC element


Advantage: 1) Limit the spike to low levels and also limits dv/dt.
2) Can be used for AC & DC contactor

Note: Very careful selection of resistance & capacitance is required to achieve desired switching time of contactor


Fig. 3: contactor with RC Element


........

12)Frequently Asked Questions

List of most frequently asked question list is given in below

: FAQs by customers

Here we are going to discuss following question asked by customers from various fields like cement industries, steel sectors, paper mill etc.

Q1) What are the indicators of mechanical life of contactors?

Q2)Which application criteria shall be considered for use of contactors on line side of variable speed drives?

Q3) Which application criteria shall be considered for use of contactors on load side of variable speed drives?

Q4)What are the derating factors for Contactors at higher frequencies?

Q5) How do I calculate the critical length on control cable for contactor?

Q6)Is there any guidelines available to determine the correct size of control transformers?

Q7)What is Effect of Cable Capacitance on the Operation of AC operated Contactors

8)How to calculate power loss of contactors which is generally required for thermal calculations of a switchboard

9)Which technical consequences shall be considered if mounting position other than standard is used?

10)How do load ratings change when contacts are connected in parallel or in series? Which option should be chosen?

11)How can I select contactors for utilization categories AC-8a and AC-8b? What special features have to be observed?


.......

11)Troubleshooting in contactors

Here we are going to discuss about the Problems in contactors & their solution.

Problems:

1)Over heating of main contact path

2)Overheating of coil

3)Noisy Magnets

4)Failure to pick up & seal

5)Failure to drop out

6)Excessive pitting and welding of contacts



...

10)Communication capable contactors

This post is under consruction

9)Milivolt drop test

• Checking mV drop with an accurate meter which indicates< 0.2mV when shorted

• Twist the connecting leads while measuring to avoid false pick ups

• Measure current with clip on meter

• The values of drop measured should be less than 4.0mV at the rated current of contactor for AC3 duty. mV volt drop can be calculated by given formula:


Milivolt drop test



Corrective action: If milivolt drop is more, then it is indication of bad joints

For case A & B) Clean the surfaces, apply Dowells crimping compound & re-crimp cables



Image A




Image b




For case C) Higher value of mV drop in this case could be due to either case A or B; In case of persistent higher value, replacement of the entire contactor is necessary


Image C


........

8)Problem & soluation

This post is under progress

7)Capcitor duty contactor

Capacitor duty switching contactor

If contactors are used for switching of capacitor ,the capacitor must be completely discharged by means of resistance or discharging chokes.

When capacitor are switch ON depending on the phase angle at the instant of switch ON sever current peaks may occur.

Drawbacks: Welding of contactor
Erosion of contacts

Protective measures: The effective capacitor-load switching capabilities of a.c. contactors & their service life can be increased by following measure.

•Choke coils: The transient condition which is caused when switching in on an existing capacitive load may be damped if an air cored inductance of at least 6 micro Henry connected in parallel connecting leads between the capacitors


Fig. Choke


•Pre-charge resistor: Connect precharge resistance in parallel with capacitors.




fig.capacitor duty contactor

6)Do & Don’ts

5. Do & Don’ts

a) Don’t replace slightly pitted contacts; contacts should be replaced only when 40% of silver tips is remaining.

b) Don’t file the contacts, don’t use emery paper on contacts it will reduce the life of contacts drastically
c) Don’t apply grease on contacts
d) Easily separable contacts can be reused according to IEC947
e) Do inspect the contact condition after short circuit & overload trip.
f) Don’t used broken/cracked & carbonized arc chamber it will cause flashover
g) Don’t replace the contacts after mechanical life of contactor is over.
(You can replace the contacts 7 times after that complete contactor has to be replaced

5)Maintenance tips

Maintenance tips

Siemens NEMA contactors have a robust design and a rugged construction and thus do not require frequent maintenance. Nonetheless the following items can be added to your preventative maintenance program to help further ensure equipment protection and minimum unplanned down time.

1. Routine inspection: Inspection consists of observation for signs of overheating, dirt, loose parts, noise and any other signs of abnormalities. It will help to monitor the state of contactors.


2.Keeping the contactor clean will help eliminate overheating, high voltage leakage and breakdowns. Clean by blowing the dust and dirt away with a low pressure dry air stream.


3.Terminals: Tighten Terminals periodically because

a) Connections have a tendency to loosen with time – particularly those of aluminum which is soft material



b) Vibration may result in loose connections that will eventually cause problems.
Note: Brittle or discolored wire insulation may be an indication of a loose connection.



4.Contact condition: Remove the arc cover and inspect the contacts. Replace all 3 contacts if one or more of the following conditions exists:

a. Silver cadmium tips are unevenly worn,
b. Silver cadmium tips are nearly worn down to the copper material of the contact,
c. Copper material of moveable contact is discolored indicating over-heating.

Note: It is normal for the Silver cadmium tips to be discolored.
It is due to formation of silver oxide which is nothing but good conductor



5.Clean the contacts with CRC2-26. Use care not to drop magnet or scratch the pole faces of either magnet or armature.

6. Magnets In case of rust/dust accumulation on the magnet faces clean them with CRC 2-26 / Chamoi leather. Do not use degreasing agent as petrol. Dust/rust affects contactor performance and cause humming.

Do not clean magnets with share/pointed objects. Filing upsets the mating of the magnet faces. Clean lightly with fine size 00 emery papers if required.

Fig.CRC2-26


7. Corrosive atmosphere: Overheating takes place due to the formation of foreign films at the connections and on the springs. For such applications, contactors have to be suitably derated. Occasional spraying with CRC 2-26 prevents reaction of the silver parts with the atmosphere and minimizes damage.




.......

c)Varistors Circuit

3 Varistors Circuit

When wired parallel to a coil, Varistors, voltage-dependent resistors, limit the maximum over voltage, since they become conductive when a specific threshold voltage is exceeded



Fig. Basic circuit diagram: varistor




Fig. Switching over voltage of a contactor with varistor

Advantages : The increase in drop-out time is minimum and clamps the voltage to acceptable levels.

Disadvantages :Cost is very high




Overview of different protetion circuit





.....

b)Diode Circuit

Diode Circuit


A) Freewheeling Diode Circuit



Basic circuit diagram: freewheeling diode




Fig. Switching over voltage of a contactor with diode


Advantage: Switching over voltage limited to 0.7 volt

Disadvantage: Breaking delay, breaking time increased by a factor 6 to 9.

Note: Above disadvantage can be used to your advantage if, for example, a temporary voltage drop of around several milliseconds has to be bridged



B) Circuit with One Diode / Zener Diode Diode Assembly





Fig. Basic circuit diagram: diode assembly





Fig. : Switching over voltage of a contactor zener & diode


Advantage: Switching over voltage limited to 10 volt

Disadvantage: Breaking delay, breaking time increased by a factor 2 to 6

Note: Above disadvantage can be used to your advantage if, for example, a temporary voltage drop of around several milliseconds has to be bridged


........

e)End of electrical endurance

End of electrical endurance


After the (silver alloy) contact materials are burnt off due to contact wear, the (copper) contact carriers will weld together.

d)Transition times too low

Transition times too low


At star-delta starting and pole changing or reversing of motors transitions between star and delta or low and high speed or between rotation directions it can happen that high peak currents occur. When a motor is switched off, a residual voltage remains in its windings for a period of some milliseconds. If the motor is connected to the network within this period, the summary of residual voltage and line voltage (in unfavorable phase angle) is causing high peak currents which cannot be safely handled by the contactor main contacts anymore. Welding occurs as a consequence. In order to prevent contact welding in these instances we recommend a minimum transition time of 50 ms between star and delta and between pole changing or reversing of motors.


Important Note:


Star-delta-timer relays from Siemens incorporate this transition time in their timing program. If control circuit is operated from a PLC the 50 ms transition time shall be programmed for each rotation direction and as well for each change of rotation speed. This is not only valid above 500 V, even at 400 V it is obligate to obey these rules however one might hear other opinions sometimes

c)Use of diodes

Use of diodes as surge suppressors

Contactors are generally designed in such a way that a slight contact welding during closing is forced open again by the back pressure springs of the contacts during de-energization. Diodes can reduce the drop-out speed of a magnet system to such an extend, that the abovementioned slight contact welding cannot not be opened anymore







In the extreme case the low drop-out speed caused by the diode can lead to a so-called two-step drop-out.

This means that following the first contact opening, another short, weak closing of the main contacts may happen during the breaking process. This second closing (during the arcing time) can lead to welded contacts.

Since two-step drop-out cannot happen with size S00 3RT1 contactors, diodes as surge suppressors are offered for this size only. From size S0 upwards diode assemblies (series connection of diode and zener diode), RC-elements or varistor have to be used as surge suppressors. Some companies on the market offer diodes for Siemens contactors. Their use on Siemens contactors leads to the above problems!

b)Unstable control voltage

Unstable control voltage

Unstable control voltage can be the result of

- an insufficiently sized control transformer




- drop of the line voltage during motor starting




- relay contacts in the control circuit of the contactor which are not sufficiently rated to handle the closing power consumption of the contactor magnet coil. This leads to relay contact bouncing and subsequent welding of contactor main contacts

a)Inrush current

Inrush currents exceeding the making capacity of contactors:


Current peaks often occur on the primary side of frequency converters and are caused by DC-link capacitors during the charging process. The magnetic force of such peak currents is sufficient to open the contacts of an unsuitably sized contactor. During opening an arc occurs that fluidises the contact material. Since the magnet system of the contactor is still energized, the contacts close again and weld together. Inrush current calculated formula given below


Inrush current=2 root 2*Rated current (Ie)





Fig:Inrush current




Solution: Only solution is to use oversized contactor having making capacity of a contactor given by formula.

Making Capacity=10 * Ie (AC-3)* 1.41

Ie- Rated current carrying capacity of contactor



.....
AC 3 is a type of duty of contactor.

3)Reasons for welding

Reasons for welding of contactor

Contact welding occurs only during closing of contacts or during opening and immediate reclosing of contacts because of following reason





1) Inrush currents exceeding making capacity




2) Unstable control voltage




3) Use of diodes as surge suppressors





4) Transition times too low




5) End of electrical endurance





Note: These reasons are explain in well detail in concern heading

2)ABC of contactors

ABC of contactors

Basic Contactor Operation Magnetic contactors operate by utilizing electromagnetic principles. A simple electromagnet can be fashioned by winding a wire around a soft iron core. When a DC voltage is applied to the wire, the iron becomes magnetic. When the DC voltage is removed from the wire, the iron returns to its nonmagnetic state





Fig. Contactor principle




Internal components: There are two circuits involved in the operation of a contactor: the control circuit and the power circuit. The control circuit is connected to the coil of an electromagnet, and the power circuit is connected to the stationary contacts




Fig:Internal components of contactors



Working: When power is supplied to the coil from the control circuit, a magnetic field is produced, magnetizing the electromagnet. The magnetic field attracts the armature to the magnet, which in turn closes the contacts. With the contacts closed, current flows through the power circuit from the line to the load. When current no longer flows through the control circuit, the electromagnet’s coil is de-energized, the magnetic field collapses and the movable contacts open under spring pressure.




Fig:Operation of contactor

1)Introduction

Introduction
Most motor applications require the use of remote control devices to start and stop the motor. Magnetic contactors (similar to the ones shown below) are commonly used to provide this function. Contactors are also used to control distribution of power in lighting and heating circuits.

Types of contactors

• Power contactors

• Auxiliary contactors

• Capacitor duty contactors

• Hoist Duty contactors

• Solid state contactors




Fig:Different types of contactors




In this Article following points are discussed in details


1) ABC of contactors

2)Over voltage damping circuit used in contactors their advantages & disadvantages

3) Reasons for welding of contacts in contactors

4) Maintenance guidelines of contactors

5) Dos & donts

6)Procedure for milivolt drop test

7)Capacitor duty contactors

8)Problen & soluation in capacitor duty contactors

9)Communication capable contactors

10)Trouble shooting in contactors

11)FAQs