Alternator re-start question

Rick Erwin · #1 08-14-20, 03:04 PM

With two fresh D-Cell batteries in series (3.2 volts total) feeding the alternator re-start circuitry, what voltage is needed at the field connection on the alternator/s to excite the field to re-start the alternator?

I get .8-.9 (8 to 9 tenths) of a volt at the field connection. Is that enough, or is there something amiss?

Thanks,

Rick

DrDave · #2 08-14-20, 04:48 PM

Rick:

I have never spent any time with this particular circuit. Keep that in mind while reading anything I write. The alternator field circuit of the individual alternator should be between 1.6-4ish amps at full load depending on the alternator. You're never going to supply that kind of power from a couple little flashlight batteries. You said that you have 3.2v from the batteries and 0.8v at the alternator (field terminal). I'm certain that 3.2v reading is not at the same time you have 0.8v at the field terminal.

My guess (SWAG) is that the batteries provide just enough power to excite the field circuit and then it becomes "alive" after it gets turning. There might even be a one or two second ramping up as it comes to life. I would only have one alternator on at a time when you try this maneuver.

I would want the best D-cell batteries I could buy.

If you provide your serial number I will look at the alternator system schematic.

Dave

Rick Erwin · #3 08-14-20, 05:01 PM

Hi Dave,

Thanks for looking at this. I have a Reims FT337GP, s/n FP3370017. It is the equivalent of the Wichita built P337G.

Attached is the schematic:

Rick

DrDave · #4 08-14-20, 05:02 PM

I just found the schematic. The alternator restart switch bypasses the regulators and puts power directly to the field terminal of both alternators. It comes off the batteries through a one watt, 12 ohm resistor. It is also interesting that each alternator has it's own regulator.

Dave

DrDave · #5 08-14-20, 05:05 PM

Yes, that is the schematic I have in front of me.

Dave

Rick Erwin · #6 08-14-20, 05:08 PM

I had an alternator shop guy tell me a dead alternator only needs about an amp to excite the field.

Do you think a couple D-Cells could provide that after passing through the 12k resistor and a diode?

Rick

DrDave · #7 08-14-20, 05:13 PM

I would doubt that. Actually the alternator is not dead. The battery field circuit from the main battery is dead. It's a 12 ohm not 12k ohm resistor. Feel free to call me. I will PM my number. I'm working from home today and could use a break.

Dave