Contactor Coils Failure in GRR Panel

1.What is the contactor coil voltage,frequency and resistance ? You need to provide me with true information in order for you to get the right information from my experiences

Dear sir,The contactor coil Supply is 230 volts AC.Frequency:50Hz, Contactor coil resistance is 4.2 ohms.All contactor coils resistance is in the range of 4 to 5 ohms.Two nos.contactor coils are 71 and 74 ohms.The failed contactors coils resistance is in the range of 4 to 5 ohms.These values are measured by me.

Thanks and Regards,

NVRSrinivas

Web search for "siemens 3tf57" got :-

https://cache.industry.siemens.com/dl/files/436/6006436/att_45953/v1/53402261.pdf

This document for 3TF57 contactor indicates that there is a main coil K1 and an auxiliary relay K2. This K2 is a separate spare part from main coil and it operates the ballast resistor, see Siemens diagram below.

When contactor is first energised current flows through ballast resistor Rv and both coils of K1 in series.

Then K2 energises through K1 auxiliary contact (which is again a distinct spare part), shorting out the ballast resistor.

K1 energises at full power, opening its auxiliary contact K1. This de-energises K2, putting the ballast resistor in circuit.

I suggest you have replaced the failed K1 coil, but not K2 or the K1 auxiliary contact, which may be the cause of the trouble, not the coil. There is also a note about pressing in K1 contact block after replacement.

67model

You ask, "Kindly suggest us on how to eliminate contactor coils failure."

Hire a knowledgeable electrician to analyze the system and the have him talk to the contactor manufacturer, if he determines that the contactor is the problem.

Is there a reason why you are not on the phone to the contactor manufacturer?

You left off the most probable causes, especially when there are closely spaced multiple failures:

6. Coils that are under-rated or incorrectly specified for the duty imposed on them,

7. Inferior or cheap contactors substituted as a cost cutting measure.

7 - so THAT'S why our friend hasn't rung the manufacturer!

Hi, RAMConsult

Often true. A reference to a major survey of U.S. military equipment failures [in 1950's] in a textbook on reliability impressed me long ago. The survey concluded that 60% of failures were due to design failures. It also indicated that a common design failure was using components outside their specification, like assuming a short-term overload is OK because the prototype survived it. My experience is that little has changed except reliability is generally much higher and user's experience of fault finding has declined as a consequence.

I left out "wrong for duty" and "inferior" because I thought this was a well tried "second user" equipment and Siemens make listed in original post is not inferior or cheap or obviously under-rated for motor size given.

I guess from your CR4 name that you usually get engaged because there is a design fault that no-one thought about or wishes to admit. Consequently your experience has your items 6 & 7 high on the suspect list.

Failure to read the instruction manual or read the failure reports carefully or talk to those on site who saw what happened often delays solutions too!

Regards,

67model

Hi '67, Yes, I really enjoy Forensic Engineering, I did Root Cause Analysis (RCA) before it was called RCA! In this case we have continuing similar failures of a component (the coil) that's part of an assembly (the contactor); i.e., it's not the assembly that's repeatedly failing.

A little more digging into the Siemens literature indicates that the 3T57 contactor uses physically interchangeable coils to accommodate differing control voltages, both AC and DC, the only observable difference is the labeling, and therein may be the problem. There are two coils in the voltage of interest, M4 and M7. The first character is the voltage level, the second character denotes AC or DC; "7" for AC, "4" for DC. I would be checking to see that the right coil is being substituted for the failed coil.

One other thing, the M7 coil is rated (by Siemens) 220-240Vac, the M4 is rated 220Vdc, while OP indicates a control voltage range of 210-240V without specifying AC/DC. If it's AC then the coil may overheat when operated below its rating, if it's DC then it will draw 9% more current at 240Vdc and dissipate 20% more heat than it's designed for.

It would be interesting if OP would report back what his inspection turns up.

Back again RAMConsult.....

nvrsrinivas, who posted the problem, did answer about coil voltage/Hz, to reply #1, in his post #10. nvrsrinivas also reported about the resistance of failed coils - nothing definite - but a lot of failed coils!

I also looked at Siemens' 3TF57 data and found that it has an auxiliary relay K2 associated with the ballast resistor (apparently replaceable) and a replaceable cut-off contact. I posted this in reply to nvrsrinivas and suggested the problem was with these - not the coil itself.

Thanks, RAMconsult, for the additional info. , which will help nvrsrinivas to check if it is a "wrong coil type" problem.

Which reminds me about the occasion when I realised that site folks had been putting aside failed PLC output modules (probably as a "last ditch" repair-component mine). Given the reluctance of customers to have a stock of spares (even for parts they know have failed and put them out of service), the next "fireman" on site was using these - not knowing they were u/s! Always tie a paper luggage label "Failed, but could be a parts source" on such.

So please nvrsrinivas , let us know what happens - at least two folks are keen to know!

Siemens stopped selling the 3TF contactor line in the previous millennium! If he called them, they likely told them what I am going to tell him.

It's time to replace your contactors with products that are not in a state of constant decay.

Even when you buy replacement coils, you are buying coils that were wound in no later than 1995; you are buying "NOS" (New Old Stock) products. The insulation on them is constantly deteriorating and after 20+ years, it's not surprising that when subjected to the heat of battle, they fail quickly.

I'm also very curious as to the selection in the first place. Your motor spec says the rotor voltage is 1185V, yet those contactors are apparently rated for 500V? Even if you split the voltage because the contactors are between the grid resistors, it still seems to me they are not correct. I'd be curious what the schematic looks like.

They aren’t inferior contactors so something else is the culprit. From the specification sheet the holding power isn’t great, closing is considerably higher. Dirt on the coil armature faces is a possibility but purely a supposition.

So you are trying to prove that the contactor coils that are failing, are not failing because they failed?

Must be black magic then. Someone obviously is putting a voodoo curse on your perfectly good 20 year old contactor coils. That's why they are failing.

I don't know, but it might help experts here to pinpoint the cause - is it a new installation or has it worked properly up to now?

Dear sir,

This is not a new installation.The GRR is installed in 2011.The failure of contacts is observed since last year.Before that,I have no record.

Thanks and Regards,

NVRSrinivas