Chopper circuits for contactor coils
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Posted by CS on July 26, 2008, 8:00 am
Please log in for more thread options 12kw contactor coils and think that coil chopper circuits will do the job. Now I need to find them, or make them, for 240v and 24v coils. Any ideas?? | |||||||||||||||||||
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Posted by Ian Macmillan on July 26, 2008, 9:35 am
Please log in for more thread options In general, contactors are best used as the designer intended. AC contactor coils are designed to hold the armature closed with the minimum current that will ensure that they do not buzz or release at the lowest design line voltage. The coils draw a great deal more current when the armature is open because the magnetic circuit is not closed, and this higher current enables them to close positively. The high current is only momentary, and becomes quite low when the armature closes. This is a valuable characteristic that only works with AC contactors. You could use a variac to see at what minimum voltage you get satisfactory closing and holding, and then use an autotransformer to give you a reduced voltage, allowing a margin of error. Think about using a small transformer, with the secondary winding in opposing series with the supply. Contactors with DC coils often have a normally closed contact across a resistor in series with the coil, so that once the contactor closes, the coil current is reduced for holding. The same scheme could be used to reduce only the holding current on an AC contactor, perhaps using an inductor instead of the resistor. A chopper could be used in principle, but I think it would be an over complex solution with some problems. You have not said why you need this solution. All the best Ian Macmillan | |||||||||||||||||||
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Posted by legg on July 26, 2008, 11:54 am
Please log in for more thread options wrote:
>I am looking to reduce the power used by standard industrial 5 and -
>12kw contactor coils and think that coil chopper circuits will do the >job. Now I need to find them, or make them, for 240v and 24v coils. >Any ideas?? The power consumed/required by the solenoid is not reduced by the means of power delivery, it is determined by the coil and armature. Continuous power consumption can be reduced by mechanically latching the hardware, but this requires added contacts, timing considerations or state-sensitive drive circuitry. Semiconductor switch alternatives trade off control power and versatility for conduction loss. RL | |||||||||||||||||||
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Posted by Paul E. Schoen on July 26, 2008, 4:45 pm
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>I am looking to reduce the power used by standard industrial 5 and -
> 12kw contactor coils and think that coil chopper circuits will do the > job. Now I need to find them, or make them, for 240v and 24v coils. > Any ideas?? If these are AC coils, the increased inductance when the relay closes will automatically reduce the coil current to a lower value, and typically the contactor will buzz and chatter if the applied voltage drops below about 70% of design value. It is possible to drive an AC coil using DC, but there is a large inrush, and the current must be limited once the contactor has closed. This is usually done by an "economy resistor" that is put in series when the armature has almost closed. This can also be accomplished by driving the coil with a capacitor large enough to provide the surge current long enough for the relay to operate (about 30 mSec), and then a resistor across the capacitor will supply the holding current, which is roughly 5-10 times less than the pull-in current. But, when the contactor is deenergized, you must wait long enough (about 500 mSec) for the capacitor to discharge before trying to reenergize. The same effect could be achieved by using a PWM circuit which initially applies full voltage, and then reduces it to the holding current. A spare contact could be used to detect closure, or a timing circuit could be used. A DC coil will draw less holding current than an AC coil, but the hold-in power will be about the same for a given size contactor. The inductance causes a phase shift and a low power factor. But the current requirement is very real, so you can see some advantages of smaller wiring and smaller control transformers. Paul | |||||||||||||||||||
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Posted by LVMarc on August 10, 2008, 6:29 pm
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CS wrote: > I am looking to reduce the power used by standard industrial 5 and -
If you need to reduce energy for a commmmerial endeavor, please contact
> 12kw contactor coils and think that coil chopper circuits will do the > job. Now I need to find them, or make them, for 240v and 24v coils. > Any ideas?? me; I have significant experience in lower power in all circuits and relay actuation in [particular. Marc | |||||||||||||||||||
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> 12kw contactor coils and think that coil chopper circuits will do the
> job. Now I need to find them, or make them, for 240v and 24v coils.
> Any ideas??