Saturday, February 21, 2009

n)General questions

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

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.

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?
The 3TF main contacts are mechanically coded and therefore cannot be interchanged within same contactor sizes.


Need confirmation that 3TF Contactors free of PVC.


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?


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


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.


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


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


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


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


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


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


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


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