Tuesday, July 3, 2012

Design Goals

An alternator attached to a small engine such as used on our Kubota based DC Generator / Water Maker for Viking Star has one major difference compared to alternators used on larger main motors - mainly that in this application the alternator is able to fully load the engine, while in more typical applications an alternator will present at most a small % of the capability of an engines capacity.  In order to keep from overloading the engine one either needs to cap the maximum load by selecting a smaller alternator, or provide some external control to limit the total alternator load placed on the driving engine - 'throttle back' the alternator if you will..  In the case of a combination DC Generator and Watermater, this maximum loading is not constant, and needs to be reduced to account for additional load placed by a watermakers high-pressure pump.

Some external alternator regulators provide a crude method for reducing alternator output (in addition to regulating Voltage) - often called an Amp Manager, or Belt Saver Feature. This capability does give a coarse level of control, at perhaps 10% stepping levels.  I am currently using such a regulator with the Amp manager set at 80% to keep the peak load manageable for the Kubota.  However without reconfiguring the regulator I am unable to operate the alternator while also driving the High Pressure watermaker pump.

This project is to address some of the limitations of most ‘smart’ alternator regulators on the market. The two primary goals I am looking to achieve, which I cannot service at a reasonable cost (under $1,000) using current offerings, are:

  1. Maximize generator output at all points of recharging cycle.
  2. Adjust generator output to accommodate additional engine load of watermaker high pressure pump.

Goal number 1 goes beyond the need to limit the maximum load to the driving engine, but expands to optimize the load to the engine at all points of the battery charging curve.  Today when the house battery is being charged the voltage might start at 13.2v and slowly rise over time until capped at 14.4v. During this time we are delivering around 110A to the battery - limited by the alternators maximum output (combined with the Amp Manager of the current external regulator).  Allowing any more current and the Kubota will start to show signs of being overloaded when voltage is at 14.4v and Amps are also at their max.   This is where the opportunity for the expanded #1 goal has an opportunity:  While charging the greatest load on the Kubota will be at that point when the voltage just reaches 14.4v.  Right when the 'charging' changes from Bulk Charge mode to Absorption Charge mode.  In our case 1,584 watts of energy (or 2.12 HP) is being delivered. Given the ‘approx 50%’ efficiency of alternators, delivering 2.12Hp means we are extracting a bit under the 5HP continuous limit of the Kubota EA300.

However, at the beginning of the Bulk charge cycle the voltage is only 13.2v, while the current is still meing limited to around 110 amps.  This equals 1,452 watts -  which means we are leaving 132 watts of excess capacity ‘on the table’ . A bit less than 10%. If we had a regulator which managed Amps and not just Volts we could increase the Amps to 120a, getting the 'load' on the Kubota back up to its 1,584w point and cut our run-time down by 10%, or about 15 minutes a day. (Hey, every part counts here!  And yes, these are ruff numbers as the benefit scales back as the charging voltage increases)   

And with such a regulator, I could experiment with different alternators - looking for a more efficient solution then the current ~50%   (I have seen some write ups that perhaps using a large 200A alternator, turning at slower speed, might be able to get into the 60-65% efficiency range.   An increase of 10-15% efficiency would be worthwhile.

In order to fully accomplish goal  #1 the controller needs to actively manage energy produced (Watts in this case) and hence needs to sample both Amperage  and Voltage. Goal #2 requires the controller to recognize when the watermaker pump is engaged and further reduce the overall energy produced by the alternator.  All but a few regulators on the market today manage only Volts and rely on the self-limiting factor of the alternator to manage (limit) Amps.  Some more advanced ones will have a crude Amp Management capability, but in every case (under $1000) these are all open-looped and very course in their steps and do not give us sufficient finesse.

The design goals for this intelligent regulator:
  • Alternator Voltage Regulation (Classic ‘regulator’ function)
  • Alternator Amp Regulation (adjust Amps relative to Volts to maximize energy production) 
  • Reduced Watts produced during watermaker operation (Today I have to turn off the alternator all together while making water)
  • Soft-start logic

  • Battery Temperature compensation

  • Alternator Temperature Monitoring
  • Kubota Temperature Monitoring
  • Exhaust Temperature Monitoring (Exhaust Mixer)
  • Low-Oil Pressure monitoring
  • Shut-down and alarm on faulting of any of the above

Optional 2 (Feature Creep)
  • Kubota start / stop control
  • Kubota Throttle (speed) control
  • Maximize Watts produced in response to motor RPMs (Allows for slower / quieter motor RPMs with reduced output)

Stretch Goal (Getting Wide here!)
  • Remote Monitoring and Control Panel
  • Simple ‘Generate’ /  ‘Make Water’ on/off controls.
  • Optional auto-start? (Including Time block-outs, and Safety interlocks for motor servicing)

Required features
  • Good ESD protection
  • Watch-dog function to auto-restart if controller faults
  • Energy efficient design: High efficiency switching DC-Dc converters, Power Down modes
  • Fail-Safe modes (ala, Over-voltage shut down, etc..)

To be honest, as this project progresses the remote operation of the Kubota appeals to me more and more.  Currently all controls are in the lazarette, and hence starting and stopping requires going outside, lifting the hatch and either crawling down or leaning over.  It would be nice to be able to Start and Stop the unit from inside...

The basics: Am looking to use the Arduino Uno microcontroller board and development environment. Am using KiCad software for schematic capture and PCB layout. Looking to use a used automobile ‘Cruise Control’ motor to give me throttle control on the Kubota.   industrial serial interfaces to monitor temperature as well as provide connection to the remote panel.

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