Inserting Resistances in series with Contactor Coils

[appunni]

Sir,
I wish to include resistance in the coil circuits of some contactors, which uses for running motors in order to limit holding current as minimum. For that we have planned to connect the resistances across the NC contacts of the contactors so that the resistance will come in series with coil only after the contactor has been energized.
How is the value of the resistor is determined? I have planned to find out the drop out values for current and voltage in the coil, which are the minimum values required for just holding the contactor after once it has been energized. Is this method enough? What additional checks are to be carried out? How the chattering of contactors can be avoided even with lowest permissible supply voltage? I request for your valuable comments. Thanks in advance.
With regards,
Appunni

 

[rbulsara]

The coil will draw what it needs to draw..it does not draw more current just because voltage is applied to it. Current do drop down after initial inrush.

Reducing the coil voltage will only exacerbate chattering if any, I would think. Chattering occurs becaus of low coil voltage. And the coils are rated for certain voltage, why would you reduce it?

Why do you want to reduce coil current anyway? What problem is it causing? Certainly losses are not of any concern, I would think.

 

[jbartos]

Suggestion: The resistor approach to the scheme to be accomplished has weaknesses:
1. Chattering
2. The relay may drop out on fast transients or short voltage dips
3. The relay may not pick up during voltage dips or transients
The resistor value may be obtained by:
1. Calculations. This will require to have the relay data sheet with detailed relay parameters, e.g. resistance or coil, coil inductance on pick up, coil inductance on hold, tolerances, etc.
2. Empirically. By using a variable resistor, e.g. rheostat, the resistor value may be found. This is probably the fastest way and more accurate for given conditions than the calculations.

 

[DanDel]

appunni, I have installed different methods of doing this, usually with a timer, but only for DC contactor coils. It is not necessary for AC coils, since they are self-limiting(will draw less current when closed). Damage to DC coils from heat is common when the closing voltage is kept on a large coil after it is closed.
Typically, the manufacturer will give you a value for closing and holding voltages or currents for particular DC coils. Then, using the manufacturer's values for the coil resistance, the added resistance can be calculated from Ohm's Law.
Don't count on the NC contact to work for you, since it might open up as the contactor is closing and reduce the voltage before the contactor is fully closed, and then probably give you some serious chattering.

 

[appunni]

Sir,
I was thinking about inserting resistor in series with coil of the contactor but in parallel with NC contact of the contactor on the assumption that the current in the coil increase after the contactor is closed. And I thought that the increase in current will increase the heat loss.
I request you to explain how the current will not be increasing or self limiting for AC coils. Thanks in advance.
With regards,
appunni

 

[DanDel]

Right, appunni, and as soon as the current passes through the NC contact and the coil, the contactor starts to close, opening up the NC contact. This may happen before the contactor is closed, reducing the voltage to the coil and therefore the current and magnetic field, possibly limiting its closing force and causing chattering of the contactor. It's just a possibility for which I think you should be prepared.
As far as the AC coil question, once the magnetic circuit is complete(with low reluctance, when the magnet surfaces are touching) in an AC contactor, the current is limited by the counter-emf generated.
For a very good detailed explanation of this, check out 60hzhmmm's posts in this thread: thread237-64355