Thursday, January 19, 2012

Project ST5000: Early findings

Findings so far:
  • The Benmar clutch relay draws 100 mA.  Plenty of headroom within the ST5000's spec of 2 amps.
  • Yes, PWM* is used for the output in all operating modes.   When set up to drive a hydraulic pump, the duty cycle is higher, but still uses PWM
  • When you press the +1 button (adjust the course 1°), the PWM duty cycle is about 10%.  There is no chance this will operate a relay coil.  The mechanical relays in the Benmar drive will need to be replaced.
  • The PWM frequency is 48 Hz - well within the operating range (multiple kHz) of almost any solid state relay
  • An H-bridge of 4 solid state DC relays can operate a reversing DC motor and will pass thru the PWM.
  • Having soft start and low speed runs for small rudder movements will be a huge boon.  I welcome the PWM overlords. 
  • Update:  I have ordered Allied Electronics part No. 70184710 as the MOV - it will shunt 20 amps at 58V, and cost a thundering $0.67.
There are no barriers to proceeding.   Next step:  get 4 suitable solid state relays and a big MOV (to quench the magnetic field collapse back currents - the motor is a very inductive load).

* PWM: an acronym standing for Pulse Width Modulation.  The DC power to the motor is not continuous, but rather is rapidly switched off and on.  The duty cycle is the fraction of the time that the power is on, and can vary from 0% (no power at all) to 100% (full power).  For a familiar example, your microwave uses PWM when you select anything less than full cook power.  When cooking at 80%, it will run the magnetron at full power for 8 seconds, and then turn it off for 2 seconds, repeating (you would measure a frequency of roughly 0.1 Hz).

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    3 comments:

    Dougm said...

    Bob,
    I am very intersted in your project, since I have a Benmar drive unit AND a ST4000 in my basement. I'm not nearly as electrical-engineering savvy as you are though, and am hoping to take a lesson (read: "copy") from what you are doing. Judging by the comments to your earlier post, I don't think I'm the only one.
    Maybe you could snap a few photos when you put it all together?

    Thanks, and keep those posts coming!

    Doug Mentuck
    Perelandra, DE 38

    bob said...

    Hi Doug -

    Will do on the photos.

    Is yours a ST4000 or a ST4000+? If it is a 4000+, can you put it into dealer setup mode and see if you can turn it into an ST5000+?

    bob

    Chuck and Jackie on SV Aria said...

    On my ST4000+ I can switch between tillerpilot and wheelpilot, with the difference being the DC motor on the wheelpilot is physically longer and can draw more amps than the smaller tillerpilot motor turning a jackscrew. I knew this from the manual so I bought a used tillerpilot head to replace my older defective ST4000 head.

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