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Use of master relay (continuous duty) in place of starter relay (intermittent duty)

FireMedic_2009

FireMedic_2009

Well Known Member
I searched online to see if I can use a continuous duty relay/solenoid/contactor in place of a starter solenoid. I wasn’t able to get a clear answer so what better place to find out the answer than posting it here.

I’m currently using a White-Rogers 120-107112, which is a 14V continuous duty solenoid as my master contactor. It has 4 terminals instead of 3 terminals. https://www.copeland.com/documents/120-series-d-c-contactors-sell-sheet-en-us-1569080.pdf

Recently my starter solenoid failed and had an extra solenoid (4 terminal) I used to replace it. I want to keep an extra one in the plane especially for a cross country trip.

I ordered an extra continuous duty relay to use as a spare with the thinking I can use it to replace either the master and the starter solenoid. I know you can’t use an intermittent solenoid as a continuous duty solenoid but wasn’t absolutely sure if there was some reason why you would not want to use a continuous duty solenoid for the starter.

I do use the diodes for the solenoids. For the starter the band on the diode is connected to the S terminal and the other side is connected to ground. For the master, I have the diode connected between the two smaller terminals which are unlabeled. According to my notes a ground wire is connected to the terminal without the band of the diode. For a 3 terminal solenoid it shows the band on the diode connected to the battery terminal side and the other side of the diode to the small terminal.

In my notes there’s a document showing a blue diode used for the starter solenoid and a yellow diode for the master. Can anyone tell me what the difference is between these two diodes?

Thanks!
Daren
 
The duty isn’t the issue. The fail-op mode can be a big one.

In very short summary, internally powered solenoids that complete the ground for contact are best suited for bus iso. A shorted control wire keeps the bus on-line.

That scenario on a start relay could get very expensive and/or dangerous. Some will argue (and have) the relative importance. Stay safe with a two action control on the hot side of the solenoid for starter engage/control.

Stay safe.
 
Freemasm said:
The duty isn’t the issue. The fail-op mode can be a big one.

In very short summary, internally powered solenoids that complete the ground for contact are best suited for bus iso. A shorted control wire keeps the bus on-line.

That scenario on a start relay could get very expensive and/or dangerous. Some will argue (and have) the relative importance. Stay safe with a two action control on the hot side of the solenoid for starter engage/control.

Stay safe.
Click to expand...
I’m not sure if I follow what you are saying. What do you mean by two action control? I have a push button to engage the starter. If I understand you correctly if the push button wire accidentally shorts in flight it will engage the starter solenoid which certainly would destroy the starter. Are you suggesting to add a device to disable the starter solenoid when the engine is running??? If the starter happened to engage in flight the only way to disengage the starter currently would be to turn off the master but by the time that happens I’m sure the starter would be destroyed.
Thanks
 
FireMedic_2009 said:
I’m not sure if I follow what you are saying. What do you mean by two action control? I have a push button to engage the starter. If I understand you correctly if the push button wire accidentally shorts in flight it will engage the starter solenoid which certainly would destroy the starter. Are you suggesting to add a device to disable the starter solenoid when the engine is running??? If the starter happened to engage in flight the only way to disengage the starter currently would be to turn off the master but by the time that happens I’m sure the starter would be destroyed.
Thanks
Click to expand...
.. but to also prevent accidental engagement when the PP is not running. You'll probably hear contrary anecdote and yes, we're way deep onto the low probability/high risk side of the FMEA. Don't know your wiring scheme but in general:

You want the starter contactor to be controlled from the hot side for the reasons mentioned. If wired on the ground side, a shorted wire will engage the starter. Yes the starter gear would be destroyed if the engine was running but could happen on the ground as well.

You want two actions to engage the starter. An additional "start enable" button/action, the act of intentionally putting a key into the switch before turning or pushing, a spring-loaded switch guard would meet the intent for some. Master switch actuation isn't considered part of the (inadvertent) start process.

Yes you can "what if" the crap out of things but there are many good things to be considered from the certified world. Your build but be cognizant of potential failure modes and consequences. Sometimes that's not enough. Stay safe.
 
That relay has isolated connections to the coil and you can configure it to accept the existing switch logic. I don’t see a problem there.
As to its use in this application… The maximum temperature in continuous mode is 120F which might be exceeded in a Firewall mounted application. There is an asterisk on the 14V version intermittent rating. Note the is a preferred mounting orientation which can be an issue in an airplane. Attached is a drawing of how the unit would be connected and the protection diode polarity in a typical RV

 

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I won’t engage in the conversation about suitability of that particular relay, but there have been horrible stories of starter relays engaging due to G forces during aerobatics.
We know the standard solenoid works.

Also, if you dissect the starter solenoid, you can see in its construction how it works. The physical contact is made by a disk that randomly rotates exposing “fresh” contact material. If the disk stops rotating, which happens occasionally, it can stick in one place and starter operation becomes intermittent. However, the failure mode is open, not shorted. This has been proven many times here on VAF.
I don’t know what the failure mode of the master relay is, but I would not want it to be shorted.
 
Mikeyb said:
That relay has isolated connections to the coil and you can configure it to accept the existing switch logic. I don’t see a problem there.
As to its use in this application… The maximum temperature in continuous mode is 120F which might be exceeded in a Firewall mounted application. There is an astrick on the 14V version intermittent rating. Note the is a preferred mounting orientation which can be an issue in an airplane. Attached is a drawing of how the unit would be connected and the protection diode polarity in a typical RV

Click to expand...
My solenoids are located on the cockpit side so temp isn’t a problem. I can understand if there is an asterisk for continuous duty but why would it be for intermittent if there is no asterisk for continuous?
 
JonJay said:
I won’t engage in the conversation about suitability of that particular relay, but there have been horrible stories of starter relays engaging due to G forces during aerobatics.
We know the standard solenoid works.

Also, if you dissect the starter solenoid, you can see in its construction how it works. The physical contact is made by a disk that randomly rotates exposing “fresh” contact material. If the disk stops rotating, which happens occasionally, it can stick in one place and starter operation becomes intermittent. However, the failure mode is open, not shorted. This has been proven many times here on VAF.
I don’t know what the failure mode of the master relay is, but I would not want it to be shorted.
Click to expand...
What do you mean by “the standard solenoid”? Is there a solenoid that won’t accidentally engage during aerobatics?

You say the failure mode is open and a solenoid has never failed in a closed state, correct? I assume you mentioned this cuz an earlier post someone stated it could enter a fail-op mode?
 
I vaguely recall a key practical difference being speed of engagement and contact pressure. Check me.

Continuous duty contactors have higher coil resistance to keep amps low. Start contactors have lower resistance coils, the result being higher amperage and magnetic force, important because a start contactor is expected to make/break a lot more amps through the primary contacts.

Assuming truth, I would not expect a continuous duty contactor to last a long time when used as a start contactor. However, if I merely wanted a temporary in the tool box to get me home...
 
Keep in mind the temporary in the tool box will not work as a substitute. The master takes a ground wire through a switch to activate it. The starter solenoid takes 12 volts to activate it. If you think you need a spare, carry one of each. My best practice is that I replace them every 1000 hours. Yes, I have had one master solenoid go bad, so that has prompted the preventative maintenance with these items.
 
I just happened to see this which explains the difference between a continuous and intermittent solenoid and why you shouldn’t switch them up

 
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When used to suppress inductive voltage spikes, the banded end of diodes connects to the positive side of a coil.
The other end of the diode connects to the other end of the coil, or to ground if the other end of the coil is connected to ground.
Although the wire does not need to fused between the master contactor coil and the master switch, a 3amp fuse in that wire will prevent damage from a battery that is installed with reversed polarity (provided a diode is installed). Reversed battery polarity has happened to many aircraft owners.
 
vic syracuse said:
Keep in mind the temporary in the tool box will not work as a substitute. The master takes a ground wire through a switch to activate it. The starter solenoid takes 12 volts to activate it. If you think you need a spare, carry one of each. My best practice is that I replace them every 1000 hours. Yes, I have had one master solenoid go bad, so that has prompted the preventative maintenance with these items.
Click to expand...

I swapped one out for that reason -- preventative maintenance -- thinking I was being smart. Ended up AOG because of infant mortality on the new unit. So you just never know.

Thankfully I kept the "old" one and it served me fine for the next 800 hours until I sold the aircraft. It's probably still on the plane, 10+ years later.

--Ron
 
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