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OV relay and crowbar

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dballin

Joined: 11 Nov 2007
Posts: 11

Posted: Fri Jan 15, 2021 1:22 pm Post subject: OV relay and crowbar

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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Posted: Fri Jan 15, 2021 1:58 pm Post subject: OV relay and crowbar

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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Ceengland

Joined: 11 Oct 2020
Posts: 391
Location: MS

Posted: Fri Jan 15, 2021 2:12 pm Post subject: OV relay and crowbar

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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_________________
Charlie

Ceengland

Joined: 11 Oct 2020
Posts: 391
Location: MS

Posted: Fri Jan 15, 2021 2:16 pm Post subject: OV relay and crowbar

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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Charlie

user9253

Joined: 28 Mar 2008
Posts: 1921
Location: Riley TWP Michigan

Posted: Fri Jan 15, 2021 2:17 pm Post subject: Re: OV relay and crowbar

Does an internally regulated alternator have a field wire?

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Joe Gores

Ceengland

Joined: 11 Oct 2020
Posts: 391
Location: MS

Posted: Fri Jan 15, 2021 2:41 pm Post subject: OV relay and crowbar

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?)

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Charlie

user9253

Joined: 28 Mar 2008
Posts: 1921
Location: Riley TWP Michigan

Posted: Fri Jan 15, 2021 4:50 pm Post subject: Re: OV relay and crowbar

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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Joe Gores

johnbright

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

Posted: Sat Jan 16, 2021 5:49 am Post subject: Re: OV relay and crowbar

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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