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Compatibility of Selected UPS Device to Handle Inductive Loads

  1. Dec 13, 2013 #1
    Dear Forum Members,

    Related to this forums area of expertise, I have a question I would like to ask. Any input would be greatly appreciated!

    First, some background:
    My church's bells are rung by three separate Rabbit 2000 Embedded Processors. The combined wattage output of the processors, when not activating solenoids to ring the bells, is only six watts. Two of the bells are rung with a single solenoid which, at activation, draws a peak current of 250 watts. The big bell uses two solenoids which has a peak current of 500 watts. The activation periods for the solenoids is between 60 and 90 milliseconds. The only situation in which more than one bell is rung at the same time is when regular AC current is available and then manually by pushing a button. Lately, our town has had a slew of power outages. During the last outage, one of the processor's flash drives was corrupted (i.e. power surges, other current irregularities). We will have the system designer reload the program and plan on purchasing a good surge protector designed for electronics. As a result of the outages, we are considering the purchase of a line interactive, pure sine wave UPS that is rated at 1500 VA and 1050 watts. I have been told and have read that lower priced simulated sine wave UPS will usually overload when they encounter inductive loads. Based on our system requirements, do you believe the selected device should be adequate to handle the relatively infrequent inductive loads (bell ringing cycles) without overloading? Of course, if we know the power is out, we will have the church office delete any programmed rings until the power is restored. That way, the UPS will only have to handle the very small power load of the powered up processors.

    Again, any input is greatly appreciated.


  2. jcsd
  3. Dec 15, 2013 #2
    Most inverters have a peak rating and a continuous rating. The peak rating usually 50-100% more than the continuous rating. If your inverter is rated @ 1050W continuous then you should have no problem.

    I'm not sure that you really need to have a sine inverter for this application. There is usually no problem with modified sine inverters driving inductive loads such as solenoids. The main problem with the modified sine is that there is a lot of harmonic noise in the power provided. The MOSFETs controlling the inverter output are switched fully on and off so the voltage essentially goes from 0 to 120V to 0 to -120V and on like that. Hardly a sinewave but close enough for most line powered electronics devices. This fast switching is very noisy and can interfere with the operation of noise sensitive electronics. Most computer equipment is run from switching power supplies that also have a lot of switching noise and therefore filter the DC outputs very well to get rid of that switching noise. So the expense of the sine inverter will probably be a waste.

    Three Rabbit 2000's seems a bit excessive for this application as well. One Rabbit should be able to ring 20 bells with microsecond accuracy. I know it seems a lot for one Rabbit, but the little guy can do it. I can't imagine that this application is much more complicated than "ring bell one, wait for 750 milliseconds, ring bell 2, wait for 500 milliseconds, ring bell 1... The hardware interface should be nothing more than a few solid-state relays to drive the solenoids. This controller could also easily be made "line aware" so it could cancel bell ringing automatically when the power was lost. This should actually eliminate the need for an inverter all together since the computer is running on DC and could be run directly from a 12V battery (I am assuming that the Rabbit 200 development board is in use here). You could put a trickle charger on a cheap lawnmower battery.

    Hope that helps a bit.
    Last edited: Dec 15, 2013
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