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Topic Title: Contactor Rectifier
Topic Summary: Never come across before - please explain
Created On: 14 January 2013 09:04 AM
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 14 January 2013 09:04 AM
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scrumpy

Posts: 55
Joined: 26 October 2005

Just been handed a rectifier module that was taken out of an old MEM lighting contactor. The module had failed so the lights didn't work. Removed the module, wired direct to the contactor coil and all is ok.
Could anyone explain what these rectifier modules are for as I've never seen them before? A quick Google search didn't reveal much apart from something about silent running...
 14 January 2013 09:23 AM
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burn

Posts: 125
Joined: 06 June 2003

Google is correct. They are usually fitted to stop humming or in extreme cases buzzing noises. The contactor will work ok without it.

burn
 14 January 2013 09:33 AM
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daveparry1

Posts: 6200
Joined: 04 July 2007

Probably to do with suppresing back-emf at the contacts, helps prevent arcing, most small relays have a diode across the coil for the same reason,

Dave.
 14 January 2013 10:11 AM
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broadgage

Posts: 1292
Joined: 07 August 2007

Yes, as others post a rectifier is sometimes included to reduce noise.
It also means that same contactor coil will work on AC or DC thereby reducing spares stockholding in organisations that use both.

I would expect that a contactor coil designed for DC (whether rectified AC or from a battery) would have a higher impedance than an AC coil, and I am therefore a bit suprised that the same coil works ok without the rectifier.
 14 January 2013 10:32 AM
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potential

Posts: 1264
Joined: 01 February 2007

Originally posted by: broadgage

Yes, as others post a rectifier is sometimes included to reduce noise.

It also means that same contactor coil will work on AC or DC thereby reducing spares stockholding in organisations that use both.



I would expect that a contactor coil designed for DC (whether rectified AC or from a battery) would have a higher impedance than an AC coil, and I am therefore a bit suprised that the same coil works ok without the rectifier.

I agree.
Early coils and inductor cores will almost certainly be designed for DC use.
Using AC will create eddy currents and may cause overheating unless the core has been designed for AC working.
 14 January 2013 02:49 PM
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leckie

Posts: 1863
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A DC rectified contractor can be used to reduce the noise that you get with an AC coil due to it being 50Hz. Think of a fluorescent light flickering at 50Hz, similar thing happens with a contactor but it buzz's. So if you want a contactor but don't want it to buzz because the noise would unacceptable for the environment in which it is required, you can fit a DC rectified coil contactor. No Hz so no buzz.
 14 January 2013 02:56 PM
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Ricicle

Posts: 840
Joined: 23 October 2006

In my experience the coils were always marked as RAC (rectified ac)

EDIT: Hang on though - that might have been solenoids.

-------------------------
Empty barrels make the most noise.
 14 January 2013 03:45 PM
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jcm256

Posts: 1866
Joined: 01 April 2006

Fit a small bridge rectifier in place of the old selenium rectifier. Size does not matter they are a lot smaller and carry a lot more current than the old one, attach it to a small heat sink don't let it dangle on the cables in mid air.
Enbray use to be the whole go used for night storage heaters, you just heard a click and that's that.

http://www.maplin.co.uk/bridge-rectifiers-19088
 15 January 2013 07:25 AM
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ebee

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Surely a rectifier will not stop hum unless a smoothing capacitor is added too?

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Regards,
Ebee (M I S P N)

Knotted cables cause Lumpy Lektrik
 15 January 2013 09:30 AM
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potential

Posts: 1264
Joined: 01 February 2007

Originally posted by: ebee

Surely a rectifier will not stop hum unless a smoothing capacitor is added too?


No.
The residual magnetic flux in the core between each rectified half phase does the same thing as a capacitor would do.
Also remember that AC produces a reversing magnetic field whereas DC, even if not smoothed, will produce a steady field.

Mind you, it is possible for DC to produce contactor/solenoid noise if for any reason the magnetic core is not a completely closed loop.
For instance a wayward nut, bolt or any magnetic scrap caught in the contactor/actuator core's gap may cause the contactor not to close properly.
That can produce noise.
Stray magnetic particles becoming lodged between the actuator core gap can be a problem with DC particularly where the core is deliberately made to hold some magnetic field to delay opening.
Whereas AC contactors do not normally collect debris at all.
(not completely unknown though)

Edited: 15 January 2013 at 09:55 AM by potential
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