Initial AC current though a coil

Discussion in 'The Projects Forum' started by jyjohnhenry, Dec 13, 2014.

  1. jyjohnhenry

    Thread Starter New Member

    Nov 16, 2014
    11
    0
    Hi
    I am trying to use a reed switch ( rated 3amp 500v DC) to operate an AC timer relay. Presumably this is a small coil with minimal DC resistance but significant inductive resistance to AC ( probably the wrong terminology but best I can do!!) My query is when the circuit first opens is there a surge of current before the induced resistance kicks in and would it be sufficient to fuse the switch? The timer will operate the starter of a .75kw 3ph fan.
     
  2. MaxHeadRoom

    Expert

    Jul 18, 2013
    10,510
    2,369
    Any AC inductive device presents a low impedance when first energized, usually for less than 1 cycle.
    Max.
     
  3. MikeML

    AAC Fanatic!

    Oct 2, 2009
    5,450
    1,066
    Max, shouldn't that read?

    Any AC inductive device presents a high impedance when first energized
     
  4. crutschow

    Expert

    Mar 14, 2008
    12,991
    3,227
    In general the peak current for an inductor driven by a sine-wave that suddenly starts will be no more than twice the steady-state peak operating current unless the magnetic core saturates which then could increase the current until limited by the coil winding resistance.
     
  5. MaxHeadRoom

    Expert

    Jul 18, 2013
    10,510
    2,369
    Not really, the initial current is governed mostly by the resistive element of the coil, which is very low at that point, this is one thing a AC relay or solenoid has over a DC version, is the High initial current in order to pull the armature over, after this, the DC version has the edge.
    Max.
     
  6. MikeML

    AAC Fanatic!

    Oct 2, 2009
    5,450
    1,066
    Are you saying that the inductance of a AC contactor, relay, solenoid, etc is lower during inrush than in steady state? If so, I need convincing...
     
  7. MaxHeadRoom

    Expert

    Jul 18, 2013
    10,510
    2,369
    In the initial stage, first cycle or so, there is no inductive reactance produced, very low resistance only, it is only when the AC or frequency come into play that the total impedance is the resistance in series with the inductive reactance.
    Frequency is part of the Inductive reactance equation.
    As I mentioned, this is one advantage that an AC relay/solenoid coil has over DC, is the fast initial operation due to this high current, there should be many examples out there that confirm this.
    Why do you think the impedance is higher at switch on?
    One:
    http://www.nhp.com.au/files/editor_upload/File/TNL/NHPTNL54.pdf
    Max.
     
  8. #12

    Expert

    Nov 30, 2010
    16,257
    6,760
    While I am not an expert on this particular, I say that in the first millisecond, the DC resistance is the limiter, much as in a locked rotor motor start. In addition, a moving magnetic metal in most relays changes the inductance in maybe a dozen milliseconds.

    Please don't curse me if I'm wrong.
     
  9. MaxHeadRoom

    Expert

    Jul 18, 2013
    10,510
    2,369
  10. Alec_t

    AAC Fanatic!

    Sep 17, 2013
    5,776
    1,103
    I beg to differ. The coil has inductance from the word go, and therefore current builds up in response to an increase in voltage applied across the coil. Spice confirms this (and we all know Spice doesn't lie ;)). That inductance is the reason that stepper motors and fuel injectors are driven with fairly high voltage pulses; to increase the current rise rate and hence acceleration of their moving parts.
    Coil at switch-on.gif
     
  11. MaxHeadRoom

    Expert

    Jul 18, 2013
    10,510
    2,369
    I did not say it had no inductance, just lower due to the armature or poppet etc being absent, but the Inductive Reactance (Impedance) also has frequency as a part of the equation that does not initially exist, there is a multitude of sites out there that mention this in some form or another, one by a major relay/contactor manufacturer.
    Max.
     
  12. crutschow

    Expert

    Mar 14, 2008
    12,991
    3,227
    Yes, the coil obviously has inductance when an AC voltage is suddenly applied. The problem is that it can take several cycles for the normal AC inductor current to settle to its final value, depending upon where in the waveform the voltage is initially applied.

    Below is a sim of inductor current with a suddenly applied AC voltage. The worst-case is at a 0V start (V1) where the first inductor current peak is near the normal pk-pk steady-state current. When the voltage is applied at the AC peak (V2), there is no transient since the current is zero when the voltage is maximum (which is normal for current and voltage in an inductor with AC applied).

    But as I previously noted, that peak current can go much higher if the magnetic core material saturates during the initial peak.

    Inductor Transient.gif
     
  13. MikeML

    AAC Fanatic!

    Oct 2, 2009
    5,450
    1,066
    I have a different take. The inrush current is limited by the initial inductance. Only if the inductance changes as the armature gap closes (in the case of a relay) does the higher inductance reduce the AC current in the steady-state.

    Look at the following sim. First, I simulate applying the voltage to the inductive load at six different delays with respect to the the AC zero-crossing. Note that the initial current during the first half-cycle is well behaved, and determined by the 100mH initial inductance.

    The final inductance is 1000mH, simulated by a switch that initially shorts out the 900mH inductance and is opened several cycles later at 71ms, so at that point the total inductance increase (cause by the armature closing) and the current decreases.

    Conclusion: The reduction in current is due solely to the inductance change because the reluctance path is effected by the closing armature. If the inductive load has no moving armature, then this effect would not happen.

    relay.gif
     
  14. crutschow

    Expert

    Mar 14, 2008
    12,991
    3,227
    Mike, your inrush current is limited by the 100Ω resistance. If you use a lower resistance to where the inductance is determining the AC current then you will see a change in inrush current with change is starting phase.
     
  15. MikeML

    AAC Fanatic!

    Oct 2, 2009
    5,450
    1,066
    I was trying to model what I remembered a 240V contactor has in the way of coil resistance and inductance.

    I went and measured a motor contactor that just happened on my workbench. It is rated at 30A full-load and 180A locked-rotor. It has a 24Vac coil with a dc resistance of 19.3Ω, an armature-open inductance of 51.6mH and an armature closed inductance of 105.8mH.

    Repeating my sim with the applied voltage of 24Vrms and the measured values yields: Not a very dramatic reduction in current as the armature pulls in... and nothing dramatic during the initial phase.

    relaya.gif
     
    Last edited: Dec 13, 2014
    Alec_t likes this.
  16. jyjohnhenry

    Thread Starter New Member

    Nov 16, 2014
    11
    0
    Wow, I didnt expect my post to start quite such a tecnical hailstorm! but guys - getting back to my original query - is the initial surge of 240v ac current likely to fuse the contacts of my relay together? I havent measured the resistance of the relay coil but similar have DC resistance of ohms in the single figures. I will test the resistance the next time I am in the shed, should be tomorrow.
     
  17. Alec_t

    AAC Fanatic!

    Sep 17, 2013
    5,776
    1,103
    I thought we were discussing possible surge current in the timer relay coil? By 'relay contacts' do you mean those of the reed relay or the timer relay?
     
  18. MaxHeadRoom

    Expert

    Jul 18, 2013
    10,510
    2,369
    I don't think you will have anything to worry about if the timer contacts are picking up a 3ph contactor coil, make sure you put a R/C snubber across the contactor coil.
    You could also use a 24vdc contactor coil?
    The reed switch itself will just be switching into a high impedance input usually.
    On the subject of AC/DC coils, AC solenoids have been replaced to a great degree by DC versions, the draw back to the AC is that if the armature does not fully attract or (more likely) someone pushes the opposite actuator over manually while it is powered it usually result in Blow fuse, Burnt coil, blown semi Cond. if used.

    upload_2014-12-14_10-6-31.png
     
  19. jyjohnhenry

    Thread Starter New Member

    Nov 16, 2014
    11
    0
    My mistake. I meant would the contacts of the reed switch be likely to fuse? I had one apparently fuse and am not sure wether a faulty part or an overload caused it. I could put a, say 40 watt light bulb in series, would this be helpful to protect the reed switch?
     
  20. MaxHeadRoom

    Expert

    Jul 18, 2013
    10,510
    2,369
    Post the number of the timer, but generally a self contained timer relay the switch input is fairly low impedance and is just used to turn the timer circuit on.
    You should still put a snubber across the Cont. coil.
    Also what is the details of the reed switch?
    Max.
     
Loading...