Page 1 of 4 123 ... LastLast
Results 1 to 10 of 36

Thread: Any time delay relay experts here (Ideally with boiler knowledge)?

  1. #1
    Join Date
    Nov 2014
    Location
    Miami, Florida, USA
    Posts
    55

    Any time delay relay experts here (Ideally with boiler knowledge)?

    Gentlemen, I need your advice on an electrical solution to solve a problem I am having. While the problem is more mechanical in nature I can’t address it that way.

    I have a float switch that is mounted on a boiler. This float switch sends 120 VAC to the coil of a motor starter. As the water level starts to rise in the boiler the float starts bouncing, causing the motor starter to chatter badly. At times the contacts in the motor starter are welding themselves together. There are things that have changed on the boiler(s); new (excessive) loads, old floats switches were mercury bulbs and new ones are snap switches, water chemistry (well let’s not go there). While it isn’t the right way I need to consider an electrical solution, provided it is safe/legal.

    I’m proposing to install a time delay relay across float switch contacts. This way the float switch would still operate feed water pump motor starter if time delay relay failed. It looks like an off delay, with power trigger (S break), is what I need. I would want the time delay quite short, just long enough to cover any bounce in the float switch.

    Just to be clear, assume float switch calls for the pump to start every sixty seconds. Pump runs an average of 11 seconds. During the last two to three seconds of pump operation, motor starter drops in an out repeatedly. I’ll install the time delay relay in parallel with float switch. Input voltage will be a continuous 120 VAC. The float switch will provide a 120 VAC power trigger to the time delay relay. Time delay will be set rather short, say 0.25 seconds. Upon closure of float switch, trigger is energized and relay contacts close. Once float switch opens and trigger is removed, and relay contacts start timing open…..unless trigger is closed again before timing out, in which case time delay is reset and relay contacts stay closed until trigger is removed for time exceeding the time delay setting.

    So, what do you guys think? Is it safe/legal? Am I picking the right time delay relay type? If said relay is a “universal” voltage (24-240 VAC) will the bouncing float switch let voltage drop enough to reset time delay or should I look for something that only uses a 120 VAC trigger? Any better (electrical) ideas on how to solve my problem?

    BTW, pump is 3 phase, 480 VAC, size one NEMA motor starters with electronic overload relay.

  2. #2
    Join Date
    Jul 2013
    Location
    Arcata, CA
    Posts
    393
    I had a similar problem with a storage tank on a RO system. I used an on-delay relay, set for 1 second or so, which was on the output of the float switch. It closed an ice cube relay that then operated the pump motor. This way, the switch had to quit oscillating before the pump would come on.

  3. #3
    Join Date
    Aug 2004
    Location
    Northern illinois
    Posts
    16,534
    You might be able to make this work with an off delay timer. You will want to wire the switch to the timer and use the timer contacts to control the pump.

    Best bet is a switch to turn it on and another one to turn it off. Often you can get such an arrangement in one package.

    You might also be able to set the deadband on your switch. A few come with such an adjustment but usually not the lower cost units.
    Bob

  4. #4
    Join Date
    Jul 2006
    Location
    San Francisco Bay Area, CA, USA
    Posts
    7,635
    Although it could be, this is not typically done with a timer, it's done with the hysteresis (difference between On and Off) of the float switch. Old mechanical float switches would have an adjustment for this. But what happens sometimes is that an old rod or chain operated float goes bad and a replacement can't be found, so it is replaced with a mercury or some other type of simpler "tilting" type of float switch, and that's where this sort of issue happens. When using a tilting float switch system, you should use TWO of them, one set as the On, one set as the Off level.

    But assuming that's not an option for you now, yes it can be done with a timer. You would use what's called an "Off Delay" timer, also known as "Delay On De-Energization", abbreviated as DOD (as opposed to DOE). On that type of timer, energizing the coil changes the state of the contacts immediately, just like any relay. So in your application you would use the Normally Open Timed-to-Open (N.O.T.O.) contacts and feed the circuit through those to yopur starter coil. But unlike a relay, when the coil is dropped out the timer KEEPS the contacts from opening again, until AFTER the time expires. In that scenario if the coil is re-energized before time expires, it simply resets the time and the contacts stay closed. So it acts as a "minimum run timer" for the pump.

    The tricky part in using a timer is that you MUST ensure than at no point can the time value exceed the amount of time it takes the pump to go dry. That's where the second tilting float switch is better.
    __________________________________________________ ____________________________
    Many people are shocked when they discover I am not a good electrician...

  5. #5
    Join Date
    Apr 2008
    Location
    Ann Arbor, Michigan
    Posts
    6,780
    180410-1457 EDT

    Russs57:

    You want what what I would describe as an off delay relay. You need to study how a particular relay works with a particular name label.

    What I mean by an off delay here is that ---

    1. The relay output contact closes immediately on application of the input control signal to the timing relay.

    2. The relay output contact remains closed so long as the input control signal is present, and the output contact remains closed for a predetermined time after the input control signal is removed.

    3. If the input control signal is reapplied during the timing out time, then the timing circuit is reset so that full time out time exists for next loss of the input control signal.

    .

  6. #6
    Join Date
    Feb 2009
    Location
    Durango, CO, 10 h 20 min without traffic from winged horses.
    Posts
    9,449
    I'm not understanding the bouncing float. I understand how it could happen but float switches are designed to prevent this.
    If you go and decide to dance with a gorilla the dance ain't over till the gorilla decides it's over.

  7. #7
    Join Date
    Jan 2016
    Location
    PA
    Posts
    5,299
    I would use a latching arrangement
    as float hits the on level it energizes a relay that latches itself in
    this relay also starts the pump, it runs until the float switch hits the off level and interrupts/breaks the latch
    Attached Images Attached Images  
    Last edited by Ingenieur; 04-10-18 at 05:58 PM.
    The difference between genius and stupidity is that genius has its limits.

  8. #8
    Join Date
    Jul 2006
    Location
    San Francisco Bay Area, CA, USA
    Posts
    7,635
    Quote Originally Posted by Ingenieur View Post
    I would use a latching arrangement
    as float hits the on level it energizes a relay that latches itself in
    this relay also starts the pump, it runs until the float switch hits the off level and interrupts/breaks the latch
    Right. But we can assume, since it IS bouncing, that he does not have a hysteresis in the float switch. So he either needs a different floatie, a second one like the one he has, or the off delay timer. Were it me, I would use the second floatie.
    __________________________________________________ ____________________________
    Many people are shocked when they discover I am not a good electrician...

  9. #9
    Join Date
    Jan 2016
    Location
    PA
    Posts
    5,299
    Quote Originally Posted by Jraef View Post
    Right. But we can assume, since it IS bouncing, that he does not have a hysteresis in the float switch. So he either needs a different floatie, a second one like the one he has, or the off delay timer. Were it me, I would use the second floatie.
    most I've worked with are 3 w, form c
    otherwise upon fall/low it would make starting the pump
    as soon as it was elevated a small amount it would turn off
    fall again, repeat, short cycle, it may though, 60 sec off, 11 sec on
    50 starts an hour
    but who knows
    this sounds like a control strategy issue requiring more than a patch
    sounds like boiler feed water, definitely not a system to cobble together lol

    one sketch
    one data sheet
    and one sentence would give us far more info than the long op
    Last edited by Ingenieur; 04-10-18 at 07:12 PM.
    The difference between genius and stupidity is that genius has its limits.

  10. #10
    Join Date
    Dec 2007
    Location
    NE Nebraska
    Posts
    36,267
    How large is this boiler. One I am familiar with holds enough water that when feed pump runs it maybe runs for about 30 seconds, but typically won't run again for at least 15 - 20 minutes. To start every 60 seconds and only run for about 11 seconds sounds like maybe too large of a pump for the application and is possibly adding to the issue. May be hard on pump motor to have that many starts per hour also.

    That said a fairly inexpensive delay timer is ICM 203. You may be able to find one for under $10, works on 18-240 volts, adjustable from 1.8 seconds to 10 minutes. Only a two terminal encapsulated device with time adjust knob. Place in series with your motor controller coil. Any time the power has been removed it won't let it run again until the time delay is up.

    Sold at HVAC supply houses as an anti short cycle timer for HVAC equipment if you can't find it elsewhere.
    I live for today, I'm just a day behind.

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •