Problems reversing a field-wound motor...

Discussion in 'The Projects Forum' started by Dodgy Geezer, Oct 26, 2012.

  1. Dodgy Geezer

    Thread Starter Member

    Nov 30, 2009
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    Hi,

    Sorry about the long preamble, but I think it's better than no background at all....

    I run a simple web site about the Taycol series of model boat motors - popular amongst modellers in the UK in the 1950s. (well, someone's got to do it...) It's HERE if anyone's interested...

    These are field-wound motors, so if you reverse the polarity they continue to run the same way. In the 1950s they were reversed by using a DPDT switch or relay to reverse the armature polarity while leaving the coil polarity the same.

    Modern radio control speed controllers automatically reverse the feed polarity, and it would be nice to be able to use them in a vintage boat with a Taycol. There is a technique which can achieve this - using a rectifier to maintain original polarity to the field coils while the armature is reversed. I cover this on the web-site. But there are some issues with this approach - it drops a bit of voltage and has safety issues if the feed to one coil set fails. So I looked for another way...

    What I have done is make a simple circuit using an op-amp as a voltage comparator. This sits on the feed lines, and when the voltage changes it turns on and works a relay through a transistor switch. At least - that's the theory. I have had a lot of difficulty making it work.

    What seems to be happening is that the op-amp works OK and switches the transistor off, but the voltage at the transistor emitter does not drop to 0v - it drops to about 2v according to my multi-meter. This is not enough to throw a 12v relay, but it can be enough to keep a relay closed if it's closed already. I'm not skilled at electronics, and can't understand where the voltage is coming from. Is there something simple that I'm doing wrong? Or is there a better way of performing this job?
     
  2. crutschow

    Expert

    Mar 14, 2008
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    The output of a 741 can not go to the negative rail. Typically it stays about 1.5-2V above the rail. You need a single-supply type op amp, such as an LM324.
     
  3. Dodgydave

    Distinguished Member

    Jun 22, 2012
    4,998
    745
    Why dont you just use a relay without the op amp like this to reverse the motor.
     
  4. Dodgy Geezer

    Thread Starter Member

    Nov 30, 2009
    10
    0
    I had thought of that (with a transistor) first, and was advised that the signal from the ESC is a complex one. It will be pulse-modulated with a varying duty cycle. SO I wouldn't necessarily get a crisp clean throw at the null point when the voltage changes. If that happens, a boat would travel fast, slow down, stop, start to speed up again in a forward direction as the ESC starts to raise voltage on the opposite polarity and then slam into reverse when the voltage is high enough to switch the transistor - which would probably break the propshaft.

    Also, with a diode, the reverse voltage won't be enough to pull the relay in until it hits 6v or so - again resulting in a sudden motor reverse at the wrong time...

    I was told that an op-amp comparator would clean that signal up. I note that Crutschow's recommended LM324 is an 8-pin - does anyone know if a 4-pin exists?

    P.S. - Nice to meet another Dodgy!
     
    Last edited: Oct 26, 2012
  5. Dodgydave

    Distinguished Member

    Jun 22, 2012
    4,998
    745
    Try putting the relay in the collector load like this circuit
     
  6. Dodgy Geezer

    Thread Starter Member

    Nov 30, 2009
    10
    0
    Ok - give me 15 mins or so...

    You have lost the diode and put the transistor to the ground side of the relay as opposed to the positive side. Originally I didn't have a diode at the op-amp output, and only put it in when trying to get the transistor to switch off completely. I'm not sure why altering the transistor position should have an effect, but I'll give it a go...
     
  7. Dodgy Geezer

    Thread Starter Member

    Nov 30, 2009
    10
    0
    Hmm.... after a quick cut'n'paste on an increasingly scrappy bit of veroboard, I can report the following.

    1 - the relay stays on ALL the time
    2 - because the output from the op-amp does not go to 0, but switches between +12 and +8 v

    3 - so I must be doing something wrong somewhere, and will make up another test piece. This one is becoming impossible to solder...
     
  8. crutschow

    Expert

    Mar 14, 2008
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    I assume by 4-pin you mean an 8-pin DIP with 4 pins on a side. For that you could use a dual LM358.
     
  9. Dodgy Geezer

    Thread Starter Member

    Nov 30, 2009
    10
    0
    Thanks - you are right. I am simply looking for the smaller package, since I want the whole assembly to fit on a small relay. Hence the small bit of veroboard, which has curtailed my work for this evening...
     
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