Monday, January 30, 2012

Project ST5000: Report #3

My testing workbench
I have made some progress.
  • The MOV's have arrived (part no. V33ZA70)
  • I have determined that feeding the control inputs to a SSR with reverse voltage results in a 76 mA current flow - probably unacceptable for long-term use.  I got a couple of diodes (Radio Shack, $1.19) which will prevent back-feeding the inputs.
  • The MOV's are temporarily installed - they're the two red disks attached to the terminal strip in the drive... one for the clutch relay, and one for the motor.
  • I have successfully actuated the clutch in the drive unit, using the ST5000+
  • I have successfully activated the drive motor from the ST5000+, using the one SSR that I have. 
  • Irony: reviewing the schematic shows that the output circuit of the ST5000+ is actually an H-bridge.  Which I will be using to feed another H-bridge (albeit one that can handle a lot more current).
Now some thoughts about specifying solid state relays (SSR's) for the H-bridge... One of the figures of merit for solid state relays is its resistance in the "on" state - indirectly, this is used to rate the current carrying capacity of the units.  Using a SSR from Crydom as an example, the D1D40 (40 amps capacity) has an internal resistance of 0.05 ohms.  Using Ohm's law, the D1D40 will be generating heat at a rate of 80 watts (that's a lot - think about how hot a 75 watt light bulb gets) when carrying full load.  Obviously, this would require a heat sink.  The Benmar drive unit has a 10-amp rated motor in it - that's what it says on the motor.  Actual tests with my installed Benmar show that when the motor is stalled (I'm holding the wheel and preventing it from turning), the load is 15 amps.  But under normal operations, the load is something like 5 amps.  So with the D1D40, I could expect to see heat production at something like 1.25 watts under normal operation, and 11 watts in worst-case, locked rotor conditions. 

A Crydom D1D20 (20 amps rated capacity; 0.10 ohms internal resistance) would produce twice the heat.  Both Crydom units far exceed the needed current rating, but considering heat output, I am leaning toward the 40-amp unit.  My plan is to bolt the SSR's to the inside of the cast aluminum drive case cover, using it as a heat sink.  If I stick with 40-amp units, and given the intermittent nature of the motor's duty cycle when the autopilot is in use, heat should not be a problem.

There are a large number of Chinese SSR's on eBay with suitable advertised current carrying capacity.  However, none of these units show internal resistance in their specifications (on purpose?), and many of the ads show the SSR's bolted to large exotic heat sinks.  Tho they are more expensive, I am strongly tempted by the Crydom units.

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    8 comments:

    Anonymous said...

    How have you progressed with this project? I want to something similar - convert a reversing autopilot output from a small pilot to use with a larger reversing motor current. I can't figure out how to do the dynamic braking with Normally open SSRs. I am also scared of putting voltage spikes/shorts through the 12V system. I would appreciate it if you have a diagram. Cheers Simon

    bob said...

    Hi Anon -

    I've made a little more progress, but I was kind of waiting for the SSRs to arrive before I put it up. Now maybe I won't.

    Dynamic braking... Well these drive units are geared down so much that I would think overshoot isnt something to worry about. At least I'm not.

    Shorts: we're all worried about them! Can't say much more than that.

    Spikes: inductors can create damaging voltage spikes when the current is interrupted ( that's how the ignition coil in your car works). To protect the SSRs from these, you can put a backwards diode across the motor terminals... Except that that won't work with a reversing motor. In this case you use an MOV to quench the spikes; they are polarity insensitive.

    Diagram: yeah, I should draw one up. Maybe with that next post I mentioned.

    Thanks for your interest!

    bob

    Anonymous said...

    Thanks for the reply. Are you thinking that the gearing will be enough to prevent backdriving the motor and hold the rudder in place when the control current is off? I came across a diagram which used 4 diodes vertically across the H bridge to provide some dynamic braking through the battery http://www.barello.net/papers/h-bridge.pdf (page 9). I thought I might try that.

    bob said...

    Anon -

    Yes, that was my plan. But thanks for the helpful diagram! If it turns out that the gearing is not enough, adding the diodes across the relay terminals would be easy to do.

    Anonymous said...

    Also did you have a plan for dealing with stalled motor situations. On my X5 controller the only wires from the motor to the controller are the reversing current circuit for power. However the controller has a "stopped drive" and "current limit" alarms (not sure how it distinguishes those two). We have had the stopped drive a few times when the motor has been stalled. Neither of those protections will work in a stalled/short circuit situation with a relay between the motor drive and the controller drive, so the only protection will be the fuse in the motor circuit. Any thoughts?

    bob said...

    Anon -
    The old Benmar controller that used to run the drive had no protection other than the breaker

    Thankfully, the ST5000+ has a rudder position sensor. If it is commanding movement but not seeing any, it alarms and goes into standby

    bob

    Anonymous said...

    Thanks for that - the x5 has an option for a rudder position sensor which I was going to add. Presumably that will solve the problem for me too. It probably also explains the difference between the "drive stopped" and "current limit" alarms I was describing earlier.

    I really appreciate how helpful you have been, and I look forward to your next post.

    Cheers

    Simon

    bob said...

    Thanks Simon -

    I was wrong - diodes *can* be used for spike suppression in an H-Bridge, as the diagram you provided shows. If they are used for dynamic braking they will also serve to protect the SSR's from inductive spikes, eliminating the need for the MOV.

    Since I already have the the MOV, I will use it, and I may also add the diodes - belt and suspenders.

    bob

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