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Automotive/heater blower motor- 6 volt to 12 volt conver.

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Expert: - 10/28/2007


Question
I converted my 47 Buick to 12 volt.  Used wire wound resistors on heater and defroster motors.  Defroster works fine.  Heater motor runs slow.  Only 3.99 volts at the motor.  Resistor to hot to touch. Motor and wire both new.  Single speed switch.  Blower motor works fine when connecterd to a 6 volt battery.  No heating of the wire.  Tried a different resistor with same results. The printing on the resistor says: Dale RH-50-50W; 1 ohm 1%   MO733
Any help or suggestions would be appreciated.
                         Thanks,  John

Answer
Too much resistance for the current drawn if voltage is dropped to 4 volts; an overly hot resistor would echo this finding.

If you can measure the current drawn from that six-volt battery when running the motor, you can calculate both the effective resistance and the power dissipated by the motor.  That data will enable you to specify a resistor or resistance wire of the same resistance and power handling capability as the motor, to be put in series with the motor in order to also drop six volts (actually a range covering  6.4 to 7.1V or so) so as to connect to your 12V system.  

That would be the traditional, albeit somewhat wasteful, approach.

In the interest of efficiency and better blower performance, I would be inclined to use a small switching regulator, something designed around the National SemiConductor "Simple Switcher" series, readily available in a five-pin variant of the the industry TO-220 package, plus a small inductor.

Switching convertors can be considered to be the DC equivalent of transformers, trading off voltage for current when used in the step-down mode.

Do a google search for "National Semiconductor LM2575 Simple Switcher".  This will provide data and also an on-line design guide.  Note that you will need to interface this regulator to your blower motor using a higher-current transistor to handle the several amps required by the motor, and will also require a "free-wheeling" diode to protect that transistor from back-EMF transients from the motor.

Good luck ... EGK

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Ernest (Ernie) Kenward

Expertise

The challenges I most enjoy are thoughtful technical questions of a trouble-shooting nature in both electrical, power electronic and mechanical systems, mainly automotive but also machine control and small-machine PLC applications. Please note, however, that I am NOT a walking shop manual! I DO, however, make it a point to have those manuals and other service literature for those vehicles I DO own, and highly recommend that anyone serious about maintenance or modification of their vehicles do the same; MOST of your answers WILL be found there. For that matter, I do NOT go out of my way to acquire shop manuals for any vehicle I do NOT own! That being the case, any general query to me along the lines of "What is the meaning of this code read from the ECU of my 2015 XYZ?" or "Where is the fuse for the windshield washer pump found?" (try your car's electrical distribution panel for a start!) will not go far. What I do offer is a pretty good collection of literature, insights and hands-on experience with 1950s to 1980's Ford products (plus a developing database of information and practice with the Mercedes diesel cars), along with an engineering perspective and the ability to design and implement custom control, electrical and mechanical subsystems for vehicles. For that reason, I am happy to make my thoughts and efforts available to those who are of like mind and/or are seriously making a point of learning about their vehicles. Use the Opportunity to Learn!

Experience

A key skill in my work and hobby pursuits both is STRATEGIC TROUBLESHOOTING. I am a senior instructor in Electrical Engineering Technology at a leading Canadian polytechnic, my areas being Electrical Power and Industrial Control, electrical and electronics design and manufacturing, and AutoCAD and related CAD/CAE software - plus equipment problem-solving and new equipment design and prototyping. Hobby-wise, I have 30-plus years of experience in auto restoration, mostly in electrical and mechanical systems. Ongoing projects include a 1959 Edsel Corsair, my 1978 Ford E250 class-B motorhome conversion, and the care and upkeep of my Mercedes 300CD. My vehicles become engineering test beds for electrical and mechanical upgrades as ideas present themselves. This includes the design and production of circuit boards to restore or enhance features for which no OEM replacement parts are obtainable, or where better specifications or reliability can be had via newer concepts. Regarding the E250 RV conversion, I designed and continue to revise a custom power distribution system, managed by a Programmable Controller (PLC); this has made most revisions as easy as uploading new firmware as I develop it. The "mini" PLC is a powerful device for custom automotive control systems. One good example (there are many) would be the Moeller "Easy Relay"; these offer a wealth of control, monitoring and variable-and-status display options for such projects. A good example project which has worked well is that one for my RV noted above, which has been on the job - revised in firmware only - for a decade now. It is a load management and charging control system to avoid the sulfation-induced early failure that often befalls deep-cycle batteries used in RV power applications. The battery installed in 2003 lasted long enough to more tnan pay for the PLC that contributed to its longer life ... and the PLC will be there for the next battery as well!

Organizations
IEEE - senior member ... past WCC Student Activities; SME - senior member ... past chair, greater Vancouver chapter chair 318; Edsel Owners' Club - have served in various capacities on chapter executive during seventies; have been Power and Driveline resource on the Edsel Owners' Club "E-team" for more than a decade.

Education/Credentials
Graduate of UBC

Awards and Honors
Certificates of appreciation from IEEE and SME for work in student and chapter activities

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