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DC relay problem

DC relay problem

DC relay problem

(OP)
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.

RE: DC relay problem

Hi JatTiw:

Check the relay specs in terms of coil resistance and pull in current, I suspect they are different. Since the 2K ohm and lamp are in series with the coil, there may not be enough current to pull in the contacts. 125V/2000 ohms will limit current through the coil to 63mA (this assumes lamp and relay coil resistance is zero. If the coil turns cannot produce enough force with this current, the contacts will not make. Pull in current/voltage is typically higher than steady state conditions. Hope this helps

RE: DC relay problem

Suggestion: A variable resistor, e.g. rheostat, might be used to find out the resistance in series that is appropriate for your circuit specification.

RE: DC relay problem

If both relay coils are rated 120VDC, and the old coil wattage is 4.5 and the new is 7.8, then the new coil has less resistance than the old. (1846 ohms compared to 3200 ohms). The difference is causing higher current flows through the circuit and may be causing enough of a magnetic field to close in the relay.
To reproduce the original circuit, you may have to increase the series resistance to compensate.

RE: DC relay problem

(OP)
HiEvery body
Thanks for all suggestions. I think when ever a current of 63mA passes through this circuit, whole voltage shall drop across 2000 ohm resistor & lamp. At input of coil, there should be only 3-4 volts. Is it possible that this much volts(3-4 Volts) & 63mA shall be able to pick-up this relay?
Other experiment carried out was, I replaced new relay with another relay coil (of resistance 20 ohm) & circuit was not able to pick-up this relay

RE: DC relay problem

I don't think you can get 63mA dc through the circuit.  You could if the 2000 ohm resistor was the only thing in the circuit

As pointed out above the 4 and 8 w coil (round numbers) have resistance
R = (120v)^2/(watts) ~ 1800 or 3600 ohms

The highest current you can get is using the higher wattage lower resistance coil which is 1800 ohms

I = 120/(1800+2000) = ~ 30 milliamps.

The voltage across coil you can compute by voltage divider:
V = 120*(1800/[1800+2000]) ~ 56v

All above has assumed the lamp has no significant resistance above the 2000ohms.

RE: DC relay problem

(OP)
Hi Electripete,
You are right. Actually I was trying to say that with 56V & 3mA, is it possible to pick-up the relay rated for 120V & 7.8W

RE: DC relay problem

Jattiw,
    I would also pay close attention to the polarity, alot of older dc relays operated the coil with out regard to  polarity. That is not the case for the newer generation of dc relays I have replaced lately, The polarity has be correct!

Regards,
Afterhrs  

RE: DC relay problem

  I would also trace out the positive and negative lines that is energizing this relay and all other componets in this circiut and ensure the polarity is correct all the way back to the power supply.

Regards,
Afterhrs  

RE: DC relay problem

(OP)
Dear Afterhrs
I checked the polarity & any ground fault. Everything is fine.

RE: DC relay problem

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.
Please double check your statements and/or measurements. Also, verify the voltages of the relay coils you used were all the same.

RE: DC relay problem

(OP)
Hi Buzzp,
The infromation I put is correct. The situation I had is very confussing too. Any-way I am also trying to contact the relay manfacturer (which is GE) to resolve this issue.

RE: DC relay problem

Is there any special purpose to having the resistor in series.  I've done it to reduce current coil but not to this extent.  Was the old relay modified so that mechanically the core was never allowed to close, like in the old voltage regulators on generators.  Is the indicator neon or does this also put a load on the circuit?  Given that a resistor in series with the relay would give less reliable operation, there must be a purpose.  A mechanical shock could cause this relay to drop out. Could you indicate what this relay controls.  I've seen pull in currents for even the same relay vary more than 40% in the same lot.  You may have just gotten lucky.

RE: DC relay problem

(OP)
Hi OperaHouse
This arrangment is to check "breaker trip circuit healthy" indication. The Indication lamp (Incandescent Type) along with resistance is connected in series with trip coil (20 ohm resiatance). In parallel to trip coil auxiliary relay is connected. These relays shall pick-up only when there is trip signal (from relay etc). Recently auxiliary relay was replaced. The orignal relay (4.5W) was replaced by new 7.8W relay

RE: DC relay problem

My 2 cents in response to Buzz' question: yes current determines pickup for a given relay. But when comparing two different relays, all bets are off.

It sounds to me like you need to add more series resistance to keep your coil from picking up when you don't want it to. Maybe relay manufacturer can tell you at what current/voltage pickup is expected.  Or you can measure it on the bench.  Or if you tell us the part number maybe someone will look it up on the internet.


RE: DC relay problem

Check the circuit.  Soudnd like your running a coil through a GE ET-16 indicating lamp.  I would guess there is a wiring error and the lamp is supposed to energize through a contact on the relay.  The coil should energize via some other path.

RE: DC relay problem

I guess I do have a question about the circuit as described by the original poster. Under what circumstances can the aux relay ever deenergize?

Here's the way I picture it

    Vin
     |
   -------
   |      |
   RL    TripContact
   |      |
   R      |
   |      |
   -------
   |      |
   TC     AuxRelay
   |      |
   -------  Ground.

Where:
Input voltage applied between Vin and Ground
RL = Red Light
R = resistor
TripContact - the contact that closes to initiate trippign
TC = Trip Coil
Aux = Aux Relay to sense tripping

Maybe there are some tc b contacts in series with TC.

Is this right or missing something?

RE: DC relay problem

I withdraw the question at beginning of previous post. It was a moment of stupidness.

Assuming the drawing is correct:
- Normal operation with TripContact open the aux relay is deenergized since current is limited.
- When TripContact closes the aux relay picks up.

My proposed fix as mentioned is to increase resistance.
I don't understand why BJC suggests there is a miswiring.

RE: DC relay problem

(OP)
I will re-write the whole question:
+ VE
 ---------------
   |      |
   RL  Trip Contact
   |      |
   LAMP   |
   |      |
   -------
   |      |
Bkr NO    |
Contact   |
   |      |
   TC   Aux.Relay
   |      |
   ---------------  -VE



RL-Resistance 2000 ohm
Lamp- Indication Lamp
TC - Trip Coil
Aux. Relay- Orignal relay has 4.5W & new relay has 7.8W coil
(Trip & Aux. relay rated for 120 V DC)

The problem is as Under:

Under Normal conditions the indication lamp is ON (Shows bkr trip circuit is healthy) & aux. Relay & trip relay will not pick-up. In case of fault the trip contact (from protective relay) will provide 125 V DC & the trip relay & aux. Relay will pick-up. The problem is after replacing Aux. Relay  with new relay, the new aux. relay is picking-up under normal condition (without  trip signal).
I feel to pick-up a relay, a potential difference is required (Which should be rated voltage) & this will cause flow of current. With the circuit I had the 2000-ohm resistor is giving me sufficent voltage drop that the relay should not pick-up. But actually it is picking-up.
I could not understand why it is picking up?

RE: DC relay problem

Looks like the same diagram.  It still sounds like you need to increase the series resistance to get this relay to work as desired (or go back to the original coil style).   What other choice do you have?

With some very simple measurements you can determine the coil pickup current (or voltage), confirm the coil resistance and other component resistances, and select resistance accordingly. Or take a shot in the dark and add another 2000ohm resistor in series and see if it works.

RE: DC relay problem

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.

RE: DC relay problem

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.

RE: DC relay problem

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.

RE: DC relay problem

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.

RE: DC relay problem

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.

RE: DC relay problem

... 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.

RE: DC relay problem

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.  

RE: DC relay problem

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)?

 

RE: DC relay problem

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.

RE: DC relay problem

(OP)
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.

RE: DC relay problem

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.

RE: DC relay problem


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?
  

RE: DC relay problem

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.

RE: DC relay problem

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.

RE: DC relay problem

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.

RE: DC relay problem

(OP)
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.   

RE: DC relay problem

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

RE: DC relay problem

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.

RE: DC relay problem

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?\\\


\\\   

RE: DC relay problem

I'm with you, Dan.

RE: DC relay problem

Jattiw,
  did you ever find the problem?

Regards,
A.H.

RE: DC relay problem

(OP)
Hi A.H.,
I don't have confiremed answer, but as soon I get some thing I will post the solution.
Regards

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