Motor reversing switch

Discussion in 'General Electronics Chat' started by druines, Mar 18, 2009.

  1. druines

    Thread Starter New Member

    Mar 18, 2009
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    I'm new to this but have a question about this topic. I have two 12 v DC motors in the same circuit that need to be able run forwards and backwards simultaneously. My current set up is a DPDT switch - center off, up and down for different directions - going through two SPST relays with a diode across two of the terminals to prevent current flow in the wrong direction. I also need two microswitches to prevent movement beyond a certain point in each direction on one motor which cancel the entire circuit. Total circuit load is 10 amps max. (fuse value). Question is, as long all the parts are rated at >10 amps (e.g. switch, etc) can I replace this set up with a DPDT switch wired to reverse polarity or maybe a switch running a DPDT relay?
     
  2. jpanhalt

    AAC Fanatic!

    Jan 18, 2008
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    This question will probably get moved to its own thread by a moderator.

    As to the question, yes. The motors could be reversed simultaneously with a DPDT switch. I would put the motors in parallel, so each gets 12V from your 12V source. The switch will need to be appropriately rated for the starting current at DC, which can be a lot more than the running current.

    Your present, more complicated design was apparently done to minimize stress on the switch. In other words, the relays take the heavy current loads. That approach is used in many motor controls. One question to consider is why do you want to change?

    John
     
  3. druines

    Thread Starter New Member

    Mar 18, 2009
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    Thanks for the reply. The simple reason for change is that this goes in an experimental aricraft and there is precious little space, plus the wiring between the relays makes an unholy mess which at the moment resides behind the instrument panel, so I was looking to reduce the bulk. I was thinking that in the DPDT relay scenario (which I assume I wire like the DPDT switch) I could locate the relay very close to the motors (one of which is also very close to the microswitches) where there is just room for one relay but not for two plus wiring. Alternatively I could just rely on the switch which is rated at 15amps at 240volts. Is there any weakness in just relying on the switch?
     
  4. beenthere

    Retired Moderator

    Apr 20, 2004
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  5. KMoffett

    AAC Fanatic!

    Dec 19, 2007
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    Yes! :( Since you stated the voltage at "240volts", the "15amps" is most likely a rating for "AC". DC current is more difficult to break than AC, and switches have to be de-rated for DC. This can be a much as 1/5th the AC rating. Pick a switch that has the necessary "DC" current rating. Automotive parts stores are a good bet...since you are talking about "experimental aircraft".

    ken
     
  6. jpanhalt

    AAC Fanatic!

    Jan 18, 2008
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    Are the motors for a critical system, such as flaps or gear? The reason I emphasized the starting current at DC was along the same thought as Ken just mentioned. A gear motor has quite a bit of starting load on it. I would have to check some manuals, but I am not aware of certificated aircraft that use only a switch for gear or flaps. That is, they have relays. Besides, a switch in the panel that operates relays near the motors has several advantages, including less panel space and smaller gauge wiring.

    Can you provide a complete sketch of what you are trying to do. Right now, I am just guessing that the two motors are for redundancy. Hence, the designer used two relays for the same reason.

    John
     
  7. SgtWookie

    Expert

    Jul 17, 2007
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    The diodes absorb the reverse EMF across the relay's coils when the coil is de-energized.
    The SPST relays provide an additional subtle benefit; when both coils are de-energized, the motor's reverse EMF is shorted through the relay contacts, causing a braking effect.

    I do not recommend elimination of the relays.

    If your wiring looks like a rat's nest, fix it. Aircraft wiring should be neat with suitably sized service loops, bundled into harnesses using lacing tape or zip-ties, and anchored to the airframe/bulkheads/panels to prevent deterioration from metal fatigue due to vibration/flexing.

    Don't use "electrical tape" like you find in hardware stores or auto parts stores. It'll fall off after a year or two, and create huge problems. Use self-fusing silicone rubber electrical tape.

    Spend some time with your local EAA chapter.
     
  8. druines

    Thread Starter New Member

    Mar 18, 2009
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    Thanks for the help and advice. I'll stick with the two relays!
     
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