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OV relay and crowbar

 
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dballin



Joined: 11 Nov 2007
Posts: 11

PostPosted: Fri Jan 15, 2021 1:22 pm    Post subject: OV relay and crowbar Reply with quote

I have an internally regulated Denso alternator on a Carbon Cub. I'd
like some sort of OV protection and was looking at installing a
Crowbar and OV relay. My question is do I need the relay? If I
install the OV module (crowbar) on the Field wire (and ground), seems
like when the voltage spiked, I'd cut the field current and hence the
alternator output. Am I missing something???
Dan Ballin


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jluckey(at)pacbell.net
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PostPosted: Fri Jan 15, 2021 1:58 pm    Post subject: OV relay and crowbar Reply with quote

Dan,
As you may already know, the Crowbar circuit is intended to blow the circuit breaker that feeds the regulator. The Crowbar fires, shorts the 5 amp breaker to ground, pops the breaker and thus turns-off the regulator/alternator.
What you are suggesting is interesting but will the field circuit in your regulator tolerate being shorted to ground for a long period of time?
-Jeff

On Friday, January 15, 2021, 01:34:52 PM PST, Dan Ballin <dballin(at)gmail.com> wrote:




--> AeroElectric-List message posted by: Dan Ballin <dballin(at)gmail.com (dballin(at)gmail.com)>

I have an internally regulated Denso alternator on a Carbon Cub. I'd

like some sort of OV protection and was looking at installing a

Crowbar and OV relay. My question is do I need the relay? If I

install the OV module (crowbar) on the Field wire (and ground), seems

like when the voltage spiked, I'd cut the field current and hence the

alternator output. Am I missing something???

Dan Ballin
===================

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http://www.mat====


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Ceengland



Joined: 11 Oct 2020
Posts: 378
Location: MS

PostPosted: Fri Jan 15, 2021 2:12 pm    Post subject: OV relay and crowbar Reply with quote

On 1/15/2021 3:17 PM, Dan Ballin wrote:
Quote:


I have an internally regulated Denso alternator on a Carbon Cub. I'd
like some sort of OV protection and was looking at installing a
Crowbar and OV relay. My question is do I need the relay? If I
install the OV module (crowbar) on the Field wire (and ground), seems
like when the voltage spiked, I'd cut the field current and hence the
alternator output. Am I missing something???
Dan Ballin
Hi Dan,


Unfortunately, yes.
Most IR alternators place the regulator between the 'ground end' of the
field winding and the ground itself. The 'high side' of the field
winding is tied to the B lead output. That means that if the regulator's
final 'output' transistor fails shorted, or any of the drive circuitry
in the regulator fails in a manner that drives the output transistor to
full conduction, there is no control over the alternator's output.

The 'field' terminal on IR alternators is not a positive (meaning
'certain') control; think of it as more of a 'suggestion box' to the
regulator. It is not connected directly to the field winding.

So if you want positive control over the B lead output, you need the
relay. There are internal mods you can do to the alternator to allow
positive control, if you are motivated and are willing to develop the
required skills. While I have the requisite skill-set, my personal
preference for my IR alternator is to just use a B lead relay; this
allows for quick/easy swap-out of the alternator, sourced from a
convenient auto parts store, if I have a failure while away from home.

Charlie

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Ceengland



Joined: 11 Oct 2020
Posts: 378
Location: MS

PostPosted: Fri Jan 15, 2021 2:16 pm    Post subject: OV relay and crowbar Reply with quote

On 1/15/2021 4:09 PM, Charlie England wrote:
Quote:
On 1/15/2021 3:17 PM, Dan Ballin wrote:
>
>
> I have an internally regulated Denso alternator on a Carbon Cub.  I'd
> like some sort of OV protection and was looking at installing a
> Crowbar and OV relay.  My question is do I need the relay?  If I
> install the OV module (crowbar) on the Field wire (and ground), seems
> like when the voltage spiked, I'd cut the field current and hence the
> alternator output.  Am I missing something???
> Dan Ballin
Hi Dan,

Unfortunately, yes.
Most IR alternators place the regulator between the 'ground end' of
the field winding and the ground itself. The 'high side' of the field
winding is tied to the B lead output. That means that if the
regulator's final 'output' transistor fails shorted, or any of the
drive circuitry in the regulator fails in a manner that drives the
output transistor to full conduction, there is no control over the
alternator's output.

The 'field' terminal on IR alternators is not a positive (meaning
'certain') control; think of it as more of a 'suggestion box' to the
regulator. It is not connected directly to the field winding.

So if you want positive control over the B lead output, you need the
relay. There are internal mods you can do to the alternator to allow
positive control, if you are motivated and are willing to develop the
required skills. While I have the requisite skill-set, my personal
preference for my IR alternator is to just use a B lead relay; this
allows for quick/easy swap-out of the alternator, sourced from a
convenient auto parts store, if I have a failure while away from home.

Charlie
followup:

Is this a 'homebuilt' install, or a Plane Power-branded Denso
alternator? If it's a Plane Power, it likely has built-in OV protection,
but as far as I know, Plane Power has never revealed their actual OV
circuit.

Charlie

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user9253



Joined: 28 Mar 2008
Posts: 1906
Location: Riley TWP Michigan

PostPosted: Fri Jan 15, 2021 2:17 pm    Post subject: Re: OV relay and crowbar Reply with quote

Does an internally regulated alternator have a field wire?

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Ceengland



Joined: 11 Oct 2020
Posts: 378
Location: MS

PostPosted: Fri Jan 15, 2021 2:41 pm    Post subject: OV relay and crowbar Reply with quote

On Fri, Jan 15, 2021 at 4:31 PM user9253 <fransew(at)gmail.com (fransew(at)gmail.com)> wrote:

Quote:
--> AeroElectric-List message posted by: "user9253" <fransew(at)gmail.com (fransew(at)gmail.com)>

Does an internally regulated alternator have a field wire?

--------
Joe Gores
 
No. (But you knew that, didn't you?) Wink


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user9253



Joined: 28 Mar 2008
Posts: 1906
Location: Riley TWP Michigan

PostPosted: Fri Jan 15, 2021 4:50 pm    Post subject: Re: OV relay and crowbar Reply with quote

Yes, the wire from a switch to an internally regulated alternator is a control
wire. It enables the alternator. A crowbar module could be connected to the
control wire. But if the internal regulator has failed, how do know that the
control circuit has not also failed? A relay in series with the alternator output
will open the output no matter what has failed inside of the alternator.


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johnbright



Joined: 14 Dec 2011
Posts: 165
Location: Newport News, VA

PostPosted: Sat Jan 16, 2021 5:49 am    Post subject: Re: OV relay and crowbar Reply with quote

I don't know about all internally regulated alternators but it is common for the control wire to turn the alternator on but have no ability to turn it off.

In any case, as stated in AeroElectric Connection book: The '‘control” input to the alternator has no direct ability to open the field supply circuit and halt a runaway condition.

Ref attached image snip from AeroElectric Connection book.


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