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Battery Charging Generator Starter  (February 26, 2011)

Wily was clearing his driveway of snow when he got a call.  It came from a company who makes custom propane fueled electrical generators.  Their devices were modified RV generators specifically designed for backup power for homes off the utility grid.  Instead of producing AC power, as most emergency power generators, the company’s generator was designed to charge up a large lead-acid battery bank.  When the battery voltage reached a critical point, the generator automatically kicked into operation and ran until the battery was fully charged. 

The company said that for the most part their machines worked fine but they were having problems with their engine starting mechanism.  This engine was started with a conventional starter motor, which was engaged to a gear on the engine shaft using a solenoid.  The solenoid and motor were powered by a small 12 volt battery.  The caller said that the engine often didn’t start properly. They wondered if Wily had any suggestions.

Wily asked a bunch of questions before deciding to take on the project.  The alternator used very powerful permanent magnets in the rotor. The three phase stator windings were connected to a bridge rectifier.  The output voltage was designed for large 48v battery banks.  The alternator’s speed was controlled by a circuit to first limit the battery charging current and then later to limit the charging voltage.  A 12v starter motor and solenoid were engaged when starting the engine. A separate 12v battery was mounted next to the engine and was trickle charged from the main battery bank.  The main problems seemed to involve the starter motor and the solenoid, which engaged the starter motor to the engine shaft.

  Propane Powered Generator   

Wily gave some thought to this system and wondered if they needed a starter motor and solenoid at all.  He thought that since the starter was causing so many headaches that perhaps they could do away with the starter motor, 12v battery, solenoid and gears entirely.   

As the drawing below illustrates, the alternator was wired in a three phase “Y” configuration.  By making a few changes, the alternator could be turned into a very powerful motor.  The motor had the advantage of being able to draw current from the main battery bank.  Using this method, the company’s control system would just need to activate a relay to switch the battery between the rectifier circuit and a brushless DC motor configuration.

Alternator's Wye CircuitWily Widget

Alternator’s Wye Circuit

To drive the motor, three powerful MOSFET transistors would be needed.  The drive signals to the FETs would come from three Hall Effect switch devices, mounted in the motor stator and positioned so the magnets in the rotor would switch the Hall sensors.  The hall sensors would then commutate the motor properly during engine starting.  They only had to apply power to the starter relay for a few seconds to almost guarantee an engine start. 

After a few round of prototypes, the company told Wily that the new starting system worked well.  Starting was smooth and reliable. The company saved some money by removing the starter motor, solenoid and battery.  They kept the pull starter, which allowed the engine to be started without a battery.

Alternator/Motor Switching CircuitWily Widget

Alternator/Motor Switching

Hall Sensors used as FET DriversWily Widget

Hall Sensors Used as FET Drivers

March 2011     Issue 15

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Good Idea
gone Badly
New Products What the World
needs Now
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