No. The armature is attracted by either pole. Any conditioning is typically dictated by the driver.Does a 3 phase contactor (240 VAC) that is controlled by a 24 VAC signal have a "diode" in the coil to allow for 24VAC to only flow current in direction to pull in contactor?
No - it wouldn't be AC if it did (but there are some contactors which include a bridge rectifier and have a DC coil - Iskra, amongst others, make them)Does a 3 phase contactor (240 VAC) that is controlled by a 24 VAC signal have a "diode" in the coil to allow for 24VAC to only flow current in direction to pull in contactor?
I recall the first time I came across a problem, the machine was an in-floor lathe that was used for (re)-truing locomotive wheels without removal, (the lathe came up out of a floor pit)!A snubber across the coil is a good idea, but they generally don't cause too much trouble without.
My experience is that the snubber solved the problem that was created by an improper arrangement of the contactor control wiring. Not wrong connections, but a wrong arrangement. Similar to a "ground loop" but not quite the same. Usually it is a problem caused by saving wire or taking short cuts. Thus the application of snubbers becomes a requirement , as Max noted.I recall the first time I came across a problem, the machine was an in-floor lathe that was used for (re)-truing locomotive wheels without removal, (the lathe came up out of a floor pit)!
The system was built by a large N.A. RR equipment manuf and was PC based for control.
Every so often the PC would reset right in the middle of a cut.
Watching the M/C over several hrs, it was noted it was happening intermittently when an AC contactor coil picked up
A coil snubber cured the problem. !
On systems I was involved with, I only ever fitted contactors with DC coils and diode snubber.
How is the armature "attracted" and pulled closed by both poles? Wouldn't it only be "pulled in" by one pole and pushed out by the other?Depends on the need of the driver.
No. The armature is attracted by either pole. Any conditioning is typically dictated by the driver.
I was curious to test whether that was true.I guess I like the extra efficiency of the DC coil, runs quieter, cooler and less problems in general.
Or when a shading ring wears open, (there is two of them).!I can’t disagree about the DC contactor being quieter especially when an AC contactor gets a particle of dust stuck in the magnetic circuit!
The flux from the coil passes through the iron armature due to the irons conductance. There are a few ideas on what happens as a consequence. One is that poles form on the iron piece opposite to coil polarity which creates attraction. Another is that the fields strength pulls the iron conducted flux into the shortest path between poles, drawing the conducting material along with it, to overcome the highly reluctant air gap. Either way, regardless of the flux polarity, attraction occurs.How is the armature "attracted" and pulled closed by both poles? Wouldn't it only be "pulled in" by one pole and pushed out by the other?
Failures amount to about 5 in the last 5 years, all 5 happened around the same time. The contactor failed to close its magnetic circuit (I don't know why: contaminants, or moulding failure for the bobbin perhaps). As a result it took too much current and overheated. The bobbin distorted and completely prevented it from closing.I have never had a contactor coil fail for me in a machine. Of course, in a cycling application like temperature control I did use SSRs for control and the contactor for enable/disable. And never a coil failure in a motor starter contactor, either.
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