S

SebsRV7A

Well Known Member
Sponsor
There was an earlier discussion Ironflight started concerning installing an alternator field switch in his RV-3. I found it interesting and decided to leave my alternator switches on for startup. The other day, on my 3rd flight since my new methodology was started, I got no power from the SD-8 alternator. I've had this before, and it was always the field CB was blown. This time, it was the 15A inline fuse coming from the power supply line directly from the SD-8 to the regulator.
My question is: Do permanent magnet alternators not like to have their field on for startup, or is this a fluke unrelated to the switch position. I was just working on the annual, so it's quite likely it was not a function of switch position.
I looked in the Aeroelectric Connection and it didn't have any discussion I found regarding leaving the field switch on or not.

Thanks!

Seb Trost
RV-7A
4th Annual complete
Boulder City, NV
 
The "Field" control in the SD8 is not really a field as in normal alternators.

Because of the perm magnets, the SD8 will output power anytime is is turned.

The "Field" control is actually controlling the regulator, and not the "alternator".

IIRC, as the rpm goes down, the voltage also goes down, and if there is a constant load, (load being figured as wattage), then the wattage requirements will drive the amperage toward infinity.

Someone else out there with less than 40 years of time lag from learning this stuff should probably chime in.
 
I stay in a lot of hotels (holiday inn et al)

I am not a double E but I do know the SD-20 is the same rotor and guts as the B&C 40A belt drive alternator. It's just cased in a different casting for vac pad mounting. The 20A rating is all you can get out of the 40A windings at the slow vac pad rpm. You should be able to treat the SD-20 just as you would the belt drive 40A.

If memory serves, my SD-8 Dynamo breaker does not "control" the SD-8 voltage regulator. It is used to hold an S-700 relay closed that is between the regulator and my aircraft buss. The OV senses over voltage and pops that breaker and the S700 opens and the SD-8 and its regulator are disconnected from the aircraft buss. The SD-8 will STILL be chugging away making too much power since it is a permanent magnet device.
 
rzbill said:
If memory serves, my SD-8 Dynamo breaker does not "control" the SD-8 voltage regulator. It is used to hold an S-700 relay closed that is between the regulator and my aircraft buss. The OV senses over voltage and pops that breaker and the S700 opens and the SD-8 and its regulator are disconnected from the aircraft buss.
Click to expand...

That is what I was trying to say......thanks for clearing it up.
 
Output Current Surges

I have an SD-20 on my RV-10, and I found that it generates
an output current surge on engine start. Cranking the
engine pulls the battery voltage down. When the engine
catches, the alternator/regulator sees the low voltage
and puts out as much current as it can to try to bring
the voltage back up. During this brief surge, the output
current can go well above the rated output of the alternator.
On the SD-20, it will easily blow 20A slow-blow fuses in
the output line. I suspect this is also what you are seeing
with the SD-8.

My solution was to use a larger fuse value. You could also use a
breaker, which is usually more tolerant than a fuse of brief
over-current conditions. B&C also sells "current limiters" which
are like fuses but can handle short over-current conditions.
Unfortunately these don't come in the right current ranges
for the SD-8.

- Dan Benua
Repeat Offender
EAA Technical Counselor
 
Sounds like a probable scenario

danbenua said:
I have an SD-20 on my RV-10, and I found that it generates
an output current surge on engine start. Cranking the
engine pulls the battery voltage down. When the engine
catches, the alternator/regulator sees the low voltage
and puts out as much current as it can to try to bring
the voltage back up. During this brief surge, the output
current can go well above the rated output of the alternator.
On the SD-20, it will easily blow 20A slow-blow fuses in
the output line. I suspect this is also what you are seeing
with the SD-8.

My solution was to use a larger fuse value. You could also use a
breaker, which is usually more tolerant than a fuse of brief
over-current conditions. B&C also sells "current limiters" which
are like fuses but can handle short over-current conditions.
Unfortunately these don't come in the right current ranges
for the SD-8.

- Dan Benua
Repeat Offender
EAA Technical Counselor
Click to expand...

That sounds like the reasonable situation. It's never happened before, though like I said, the 'field' fuse had blown several times, likely when I was doing a hot start with a lot of cranking (though I have 2 buses and the SD-8 is not on the start bus). I think having the switch on is what blew the big fuse this time. I'll leave it off like I did before until the engine's running.

Seb Trost
RV-7A
Boulder City, NV
 
Yes but the fuse is there to protect the wire, nothing more, nothing less. If you increase the value of the fuse please make sure the wire is suitable for the size fuse you've decided to use

my .02

Ken
 
Bill Bainbridge, the "B" in B&C, is the most knowledgable person on the SD-8 and I would recommend giving him a call. I just happen to live in Newton where B&C is located and I have never hesitated dropping by or calling Bill whenever I had a question. He has always been more than eager to talk to me to help resolve my problem/questions.
 
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