Dropping of Contactors

Low or interrupted voltage to their coils.

Check wiring.
Adjust voltage, if within your control.
Consider mechanically latched relays/contactors.
There may be other coil designs more accommodating of voltage dips.

Use a DC control circuit with battery backup....

http://www.geindustrial.com/Newsletter/ind_nov01_left1.pdf

You have not given details of Service Condition, when dropping of contactors is being experienced.

If it is occurring during normal operation of plant or motors, during low voltage of the day, on a particular starter, which is very remote as coils drop at 65% of rated voltage unless the shaded ring is damaged (explained below)

On Yoke of AC Contactors, there is a shaded ring (Cu ring circling half of core).Purpose is to convert flux due to single phase control supply in to 2 phase flux, flux in shaded portion lagging approx. 90 Deg of un-shaded portion so that we get smoother average flux. If this ring is broken (Cu becomes hard with core vibration - cold working -> brittle -> develop crack.

In contactors with damaged ring, it is common to hear humming noise, which on lower coil voltage turns to chattering and further drop in voltage, dropping of contactor.

If dropping is occurring on a particular contactor check its ring and solder it if cracked.

Otherreason of dropping of a particular contactor of a Starter can be if it has self reset thermal overload or any other protection without target. (Bi-metallic overload relays can be set for self or hand reset). In this case if the overload is not set correctly or momentary overload occurs, the contactor will drop of its own and you would not know the cause.

Otherreason can be if a number of Contactors drop during starting of a large motor.In this case if the dropping occurs when power is supplied by DG Set, then either 3 phase synchronous alternator is undersized or both engine and alternator are undersized.

If there is heavy dip in both, frequency and voltage, then complete DG Set isundersized. In case only voltage dips, the 3 phase alternator is undersized for the duty.

if a number of Contactors drop off during starting of a large motor. In this case if the dropping occurs when power is supplied by Utility fed Distribution Transformer, the size of transformer is smaller. In this case you can select one or two taps to get higher voltage.

Another solution in both of above cases (undersized DG or transformer) is; if it DOLstarter for that motor change to Y/D or go for VFD to start large motor so that you can limit the starting current and resultant voltage dip.

In case of DG Sets, besides dropping of contactor other production problems alsoexist, there are other complex solutions, such as volts per Hz AVR, type of speed governing etc. For this you will have to engage Application Engineer from DG Set manufacturer to give solution.

I have tried to suggest all possible causes and solutions I could think off. If any more cause occurs later I shall come back.

A common cause of relays dropping out, or an entire machine stopping, is an intermittent "Stop circuit". Since the stop contacts are at the front of the line,so to speak, in most control circuits,and intermittent in this circuit can cause all `controllers downstream to stop.

Refer to the schematics of the machine in question.All stop buttons and E-Stop buttons will be in series.A thorough testing of these switches and all wiring is required to isolate the problem.A very simple method can reveal dirty contacts in a particular switch:

A GENTLE tap on the side of the switch with a very light hammer or screwdriver will reveal the bad switch, if indeed that is the problem.All crimped-on connectors and wire splices should be tested by pulling and bending also.

Terminal strip screws should be closely examined for proper insertion depth of wires, and screws should be tightened using a properly sized screwdriver.Forcing an over sized screwdriver into a strip can cause problems later.

Sometimes repairs are made in a wire trough,and are undocumented,so if all else fails, do a physical-hands on- inch by inch trace of the stop circuit(s) wiring,feeling and tugging on the wires.I have found wires that were broken inside of the insulation and were not visible,but when pulled on, the insulation stretched at the point of breakage.

Good luck. Intermittents are notoriously hard to pinpoint.