3 Phase contactor 24Vac coil

Thread Starter

fjb2069

Joined Nov 16, 2016
5
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?
 

LowQCab

Joined Nov 6, 2012
3,457
Only if the manufacturer specifically states that it does, on that particular part-number.

With an AC-rated-Coil, it would probably be a "back-to-back" TVS-Suppressor, not a Diode.

As far as I know, few, if any, Contactors come with a Suppressor from the manufacturer.

In most installations a Suppressor is not required.
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MaxHeadRoom

Joined Jul 18, 2013
27,658
In the CNC installations I was involved in , we always used a snubber (AC coil) or rectifier (DC coil).
Most DIN style contactor manuf. supply a special custom snubber to fit their contactor coils.
If you use a diode on an AC contactor coil, you are asking for trouble!
 

GetDeviceInfo

Joined Jun 7, 2009
2,175
Depends on the need of 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. The armature is attracted by either pole. Any conditioning is typically dictated by the driver.
 

Ian0

Joined Aug 7, 2020
8,407
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)
The contacts are pulled in by an AC. When the contacts are open there is a gap in the magnetic field so the inductance is lower and the current is higher. When the contacts close, the gap in the magnetic field also closes increasing the inductance and lowering the current. That gives AC coil contactors a useful advantage in efficiency.
A snubber across the coil is a good idea, but they generally don't cause too much trouble without.
 

MaxHeadRoom

Joined Jul 18, 2013
27,658
A snubber across the coil is a good idea, but they generally don't cause too much trouble without.
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. ! :cool:

On systems I was involved with, I only ever fitted contactors with DC coils and diode snubber.
 

Ian0

Joined Aug 7, 2020
8,407
One advantage of the AC coil (especially 230V AC coil) is that it can be switched with an opto-triac/triac combination which creates an extra isolation barrier between the interference and the circuit.
Another advantage is that there is generally a live and a neutral nearby (going to the contactor terminals)

The coil definitely cause more interference than the circuit the contactor is switching, even when the contactor is switching 63A of three-phase, but I have never had it cause problems in my control circuit; although it frequently upsets the microprocessor debugging probe which loses connection and has to be reset.
 

MisterBill2

Joined Jan 23, 2018
15,698
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. ! :cool:

On systems I was involved with, I only ever fitted contactors with DC coils and diode snubber.
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.
 

Thread Starter

fjb2069

Joined Nov 16, 2016
5
Depends on the need of the driver.


No. The armature is attracted by either pole. Any conditioning is typically dictated by the driver.
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?
 

MaxHeadRoom

Joined Jul 18, 2013
27,658
It is not attracting a magnet, but an armature, the polarity alternates every cycle, The AC version is inefficient and requires a shading ring in order for the armature to stay retained as it transitions through zero.
This is the reason the AC versions run hotter also.
 

Ian0

Joined Aug 7, 2020
8,407
I guess I like the extra efficiency of the DC coil, runs quieter, cooler and less problems in general.
I was curious to test whether that was true.
I found Two CHINT 4-pole 25A contractors, one with a 24V DC coil and one with a 230V AC coil.
The DC contactor takes 0.34A, the AC contactor takes 34mA. That seems like they both take 8W, but that’s 8VA for the mains contactor as the current is actually 2.4+33j mA as its impedance is 479Ω in series with 21.4H. The dissipated coil power is only 0.5W. There will be power dissipated associated with iron losses but I’ll have to think about how I would measure that.
However the drop out voltage of the DC contactor is 6V, so the power could be reduced by circuitry in which case it will also take 0.5W.
Interestingly, the AC contactor is two thirds of the size of the DC.
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!
 

MaxHeadRoom

Joined Jul 18, 2013
27,658
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!
Or when a shading ring wears open, (there is two of them).!
Normally the only time an AC contactor has the edge is at pull in time due to the high inrush.
I always spec'd in DC solenoids, the AC variety were found very prone to failure,
I don't recall ever changing the coil on a DC version.
 

MisterBill2

Joined Jan 23, 2018
15,698
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.
 

MaxHeadRoom

Joined Jul 18, 2013
27,658
My comment was aimed at Solenoids, as in valves etc.
The big problems I had with AC contactor coils was noise, EMI, heat.
Now with so many controls gone to the practice of DC control, it is much easier to apply than the old days of fully AC control cabinets.
 

GetDeviceInfo

Joined Jun 7, 2009
2,175
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?
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.
 

Ian0

Joined Aug 7, 2020
8,407
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.
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.
 

MisterBill2

Joined Jan 23, 2018
15,698
If the TS is an electrical engineer then the understanding of the magnetics should be adequately understood.. That is covered in the first year physics class, "electricity and magnetism" section, and later in the "AC circuits" segment, second year.
 
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