Drop Off Voltage of Contactors

Just my opinion. May not be the right reason.

DC coils in contactors can be (a) economy resistor type, (b) double-coil type or (c) fully, continuously rated for the control voltage. The last one is the most reliable type (no late-break NC with its own problems), so much so that many industries like steel plants mandate the use of only such DC coils.

Now coming to your question, it is somewhat easier to achieve a 20% or even 40% drop-off in (a) and (b) types. Not so easy in (c). Bearing in mind that the coil must pick up cleanly at ≈80% when hot ! Quite a challenging design problem, since the coil can reach 100°C easily.

i am getting more convinced that this is really the reason.

An extract from the Tesys catalogue of Schneider/Telemecanique shows that the dropoff voltage window starts at 10% for the continuous-rated magnet (image below) of the first two frames, and is 15% for the double coil. (The pickup and sealed consumption values also show the nature of the magnet system)

A nice patent here .. http://www.google.com/patents?id=cGI5AAAAEBAJ&printsec=abstract&zoom=4#v=onepage&q&f=false

Is it anything to do with alternating current crossing through zero at twice the supply frequency and the magnetic field reversing at twice the supply frequency? - Full-wave-rectified, unsmoothed DC hits zero at twice the incoming AC frequency too, though the magnetic field doesn't actually reverse.

But the drop off is same even for DC supply from a station battery bank.

Indeed. Just asking.

I'd say you're right, in the case of AC contactors, at 20% of the voltage, you don't have 100% duty cycle but a couple zero crossings every 16 mSecs. that, for me opens a couple of weakened flux windows, which will lead to the release of the laminated core due to the pulling action of the spring.

Regards

The hysteresis in the magnetic iron core of the electromagnet can prevent drop off a little longer with a DC field because the core may still be slightly magnetic when current is released. Remember, this specification identifies the range that drop off is acceptable for a contactor. This is not a designer's specification but an operations specification.

Redfred has the answer. The core of a contactor is soft iron and should not retain any magnetism , however it does and this will be the reason why dc coils will hold on longer as this is always of the same polarity and not reversed by the ac, as mentioned by others.

If you need a contactor to release at a higher voltage you need to add a non magnetic pole piece between the armature and the core nose. A piece of paper will work as a one off solution. This will also make it release quicker.

regards

capblanc

Be careful with modifying the magnetic circuit on an AC contactor. Doing so significantly lowers the inductive reactance and therefore increases the current consumption which can result in over heating of the coil.

Rcapper,

quite correct. I did not mean to add a pole piece to an ac contactor but that is not clear from my post. thanks for the correction

chas

Due to residual magnetization which minimum on AC