Rpm/load detector

Discussion in 'General Electronics Chat' started by snav, Sep 14, 2014.

  1. snav

    Thread Starter Active Member

    Aug 1, 2011
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    I need to detect that a motor has achieved/is maintaining a certain RPM before connecting or automatically disconnecting the load. Rpm would need to have hysteresis to permit drop from load but needs to have min setpoint where load gets dumped. Original circuit was very basic permanent magnet, pickup coil, relay. Parts not available. RPM is not listed but output of 3600 cfm is measured by floating ball indicator.
     
  2. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    Those are all very common parts. Unless you're an extra-terrestial, how come they're not available to you? ;)
     
  3. snav

    Thread Starter Active Member

    Aug 1, 2011
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    The coil and relay are bad and only marked with mfg PN, no longer avail. We don't know what the power supplied by coil-PM or draw at requisite RPM was.
     
  4. MaxHeadRoom

    Expert

    Jul 18, 2013
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    Any details on the equipment it hooks up to?
    Or a schematic of the 'Very Basic Circuit'?
    If simple as suggested, then it should be fairly simple to duplicate them?
    Max.
     
  5. snav

    Thread Starter Active Member

    Aug 1, 2011
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    Basically imagine a magneto as used on a gasoline lawnmower and a small disc cap across coil of relay, Cap is broken but looks to be a .1 to.22 ceramic. I assume that even though the motor is reversed it still produces a DC supply whichever polarity. No diode was seen. This is/was a mine ventilator.
     
  6. faley

    Member

    Aug 30, 2014
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    Kind of vague. What's the application? What's the nature of the load. Some of the basic options are: tach generator w/control circuit, current sensing relay (given that your target speed is somewhere near the top of the ramp), or a centrifugal switch, to name a few.
     
  7. sirch2

    Well-Known Member

    Jan 21, 2013
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    Slightly off topic but I'm intrigued as to why a mine ventilator motor would need to disconnect its load if not running at the correct speed. I can see that you would want a warning if it slowed down and so wasn't ventilating but surely running slow (or fast) is better than not ventilating at all?
     
  8. snav

    Thread Starter Active Member

    Aug 1, 2011
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    The centrifugal switch is broken. A 'universal' electronic switch is available but the original switch had a secondary set of contacts once speed was attained.
     
  9. snav

    Thread Starter Active Member

    Aug 1, 2011
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    I wasn't provided with all the details but what I surmise is that it is reversible and can be used to supply heat. Will ask someone for details.
     
  10. faley

    Member

    Aug 30, 2014
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    Can you rig up a control circuit that will mimic the second set of contacts and use the universal switch as a control?
     
  11. snav

    Thread Starter Active Member

    Aug 1, 2011
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    I am thinking that's the best way as the start circuit would be dropped when RPM is reached. Just not sure about the hysteresis, I guess the old switch wouldn't have been any better though.
     
  12. faley

    Member

    Aug 30, 2014
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    Sorry- supper called. You really don't have a whole lot of options without going to extremes in both time and money. Perhaps you could identify a happy medium. It wouldn't be perfect but it might do the trick.
     
  13. shortbus

    AAC Fanatic!

    Sep 30, 2009
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    Couldn't you use a refrigeration compressor type 'potential relay', to do this? They use the back EMF from the motor to shut off the start windings when the motor reaches ~75 - 80% of run speed. It could also be used to switch something else if that's what you need.

    A link to how they work - http://www.industrial-electronics.com/Electricity-Refrigeration-Heating-Air-Conditioning_10-3-6.html
     
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