DC relay problem

[JatTiw]

Hi Every body
I Had a circuit where three components, connected in series (2000-ohm resistor then Indication lamp & then relay coil) across the 125V DC source. These components are rated for 120 V DC. Under this situation the relay coil didn’t get energized because of not sufficient voltage to pick-up the relay coil.
Recently I replaced existing relay with different type of relay having same voltage rating. Now I had the problem, as this relay is getting energized when the circuit is powered-up.
Other data I have is original relay coil wattage was 4.5 W & new coil wattage is 7.8 W.
I want to know why the new relay is picking-up.

 

[buzzp]

I would redo this circuit entirely. What is the minimum pull-in voltage of the relay coil for the old and new? I think this is where you are having problems. I have not seen such a large difference in coil wattages for the same coil voltage relays. Look at the spec sheet and I think you will find the old relay had a higher minimum pull-in voltage and the new one has lower minimum pull-in.

Electricpete- I am aware of lot differences in relay coils but the tolerances should not be more than +- 20%. If they are worse than this then the mfg process better be looked at. I am not sure the point you were trying to make.

 

[electricpete]

buzz- these are two different styles of relays. We know they are different styles because they have different published wattage. The only thing we know they have in common is they are both designed for the same voltage.

Certainly both may have different turns and different spring forces. There is absolutely no reason to think they should have the same current required for pull-in. If it's not clear let me know and I'll ramble some more.

 

[electricpete]

By the way, manufacturing tolerances have nothing to do with the discussion unless we are talking about relays which share a common specification such as same style or both supposed to have same pickup current.

 

[buzzp]

You were misunderstanding my point. I was merely pointing out that regardless of the voltage, the current is what will pull the relay in not the voltage. Anyway, we both misunderstood each other and I understand what your saying perfectly.

 

[electricpete]

Hi buzz. I always appreciate your comments. For clarify of communication I would like to highlight my points of disagreement:

"There is some misinformation here. If you had a 4.5W coil and it did not pull in, changed to 7.8W and it does pull in, then changed to coil with 20 ohms resistance and it did not pull in, assuming the voltage is the same, there is something wrong with your statements or your measurements.
The current is what will pull in the coil not the voltage."

I see no contradiction in the data that you say must be wrong, considering these are two completely different relays.

"I am aware of lot differences in relay coils but the tolerances should not be more than +- 20%. If they are worse than this then the mfg process better be looked at"

I don't understand why there are any tolerances that apply to two different coils or what is has to do with the manufacturing process.

It kind of goes to the main subject of the post:
In my mind there is a simple possible explanation... the new style relay has chacteristics which cause it to pick up earlier...lower coil resistance and/or lower pickup current. If I understand your comment correctly you suggest there must be something else going on or something wrong with the data provided to us.

 

[electricpete]

... and if it's just a difference in relay characteristics the solution is to increase the series resistance or change relay back similar to original style.

 

[buzzp]

My post was before the circuit diagram. I was under the impression that the coil was in a different spot in the circuit.
If the voltage across the coil remained the same and the resistances of the coil were the only thing to change, then it would make no sense that the 4.5W would not pull in, the 7.8W would, and the 20 ohm would not, assuming all have the same voltage rating. There is no need to get in a heated debate over what you thought I said and what I thought I said. This problem is likely solved by now with careful attention to data sheets and circuit wiring. In any case, hopefully JatTiw will post what the problem was.

 

[BJC]

Jat Tiw

What kind of switchgear is this?
I have never seen an aux relay wired as you have shown It ( or at least how I think you have shown it.)
I would think the aux relay would be connected through a NC aux contact, not a NO contact as shown. When the breaker is closed the normally closed contact (NC) would be open and the aux relay would be deenergized. Using a normally open contact would cause the aux relay to be energized all the time ( it would have voltage across it wheather or not it closed.)
A 7.8 watt coil on the relay is about 2000 ohms. Tha would drop the voltage across the lamp where it may not function properly (very very dim). That is why I questioned the circuit before.
Is the trip contact a maintained or momentary switch? Is the aux relay a lockout or used as a lockout (87)?

 

[electricpete]

To bjc - the red light has more current with the 120v 7.8w coil than it did with the 120v 4.5w coil, so if it worked before I would think it would work better now. But you bring up a good point that minimum current to make lamp clearly visible should be considered if the series resistance is increased.

To buzz - sorry, as I said I was just searching for clarification.

 

[JatTiw]

Hi BJC & Electricpete,
This whole circiut is big & cannot explain. But I am just describing a portion of it. Under normal conditions (no trip signal is present) the lamp circuit is completed through the bkr. trip coil (20 ohm resistance). It glows fine. Also when bkr. opens the bkr. trip coil is diconnected by bkr. NO contact. Once the bkr. is open I donot care that lamp is glowing or it is very dim. My main question was when the older relay (4.5W) didnot pick-up why new relay (7.8W) is picking-up (Without getting the trip signal). Also the rest of the circuit is untouched.

 

[electricpete]

You are right Jat that I misspoke when I said the new relay would have any effect on the lamp. The relatively large aux coil resistance has very little effect as you say. The resistance of the parallel combination is established by the TC.

Back to main point: increase resistance or change back relay. I see it was suggested very early by Dan.

 

[busbar]

I may be misunderstanding the discussion, but is the “Bkr NO Contact” closed? Could the auxiliary relay coil be connected “below” the 52a, in parallel with the trip coil?

 

[electricpete]

As far as the location of additional resistance.... put it directly next to (in series with) the aux coil, but in parallel with the trip coil. That way there is no significant reduction of current to the red light.

 

[electricpete]

If you don't have any measurements to go on, I would try adding a 2000 ohm resistor at the location I described (in series directly adjacent to the aux relay).

Why 2000 ohms? We know that won't prevent the relay from picking up when the relay trips. (based on the fact that the relay picks up through the other existing 2000 ohm resistor). Still have to test it out to make sure 2000 ohms is enough to keep the relay from picking up but I'm guessing it is.

 

[jbartos]

Suggestion to the original posting: Please, would you elaborate on the design basis of the circuit? Is it originally a manufacturer design product or are you developing this circuit in-house?
There are various concepts available in circuit monitoring (ANSI Dev. No. 30), and annunciation (ANSI Dev. No. 74).
Often, the signaling/annunciation functions to lamps are derived over the relay contacts only.

 

[JatTiw]

Hi Everybody,
I modified this circuit even before I post the problem on August 1st. All I need is some logical answer or possibilities to find why the coil with 4.5W doesn't pick-up & coil with 7.8W pick-up.

 

[DanDel]

JatTiw, it seems like a long time ago, but if you refer to my original post, you will have your answer.

 

[electricpete]

What happened between when the circuit worked and didn't work? You changed the relay. Therefore most obvious (?) likely cause is change in relay characteristics (lower resistance known due to higher watts, also possible lower pickup current).

Do you have any reason to think the relays should have the same characteristics? If so you haven't shared it with us.

 

[jbartos]

Suggestion to JatTiw (Electrical) Aug 8, 2003 marked ///\\Hi Everybody,
I modified this circuit even before I post the problem on August 1st. All I need is some logical answer or possibilities to find why the coil with 4.5W doesn't pick-up & coil with 7.8W pick-up.
///The 4.5W coil has a voltage high enough so that it does not pick up. The 7.8W coil has a lower voltage across it and it picks up. Obviously, this is what has happened and the calculation and measurements should confirm it.
Assume indication light 4W, 120V, 3600Ohms since this information was requested; however, never provided.
4.5W/120V=0.0375A, 120V/.0375A=3200Ohms
125V/(2000Ohms + 3600Ohms + 3200Ohms)=0.014A
.014A/.0375A=0.378
7.8W/120V=0.075A, 120V/.075A=1600Ohms
125V/(2000Ohms + 3600Ohms + 1600Ohms)=0.01736A
.01736A/.075A=0.23
The calculation results suggest that the 7.8W relay can pickup at 23% of its rated current, while the 4.5W relay cannot pick up at 37.8% of its rated current. These are relay characteristics given by the relay design.
Now, measurements are left up to the personnel to confirm it. When are these going to be available since relay data sheets have not been posted?\\

\\\

 

[electricpete]

I'm with you, Dan.