Inserting Resistances in series with Contactor Coils
Inserting Resistances in series with Contactor Coils
(OP)
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
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






RE: Inserting Resistances in series with Contactor Coils
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.
RE: Inserting Resistances in series with Contactor Coils
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.
RE: Inserting Resistances in series with Contactor Coils
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.
RE: Inserting Resistances in series with Contactor Coils
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
RE: Inserting Resistances in series with Contactor Coils
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