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From: Gary Casey <casey.gary@yahoo.com>
Reply-To: Gary Casey <casey.gary@yahoo.com>
Subject: Re: Contactors vs Solid State Relays
In-Reply-To: <listdigest-5511252@logan.com>
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I'm not an expert in contactors, but there is a reason why the typical airc=
raft "master" contactor and the starter contactor are different, even thoug=
h the starter current goes through both.=C2=A0 The master contactor isn't e=
xpected to open the starter circuit, while the starter contactor is.=C2=A0 =
Therefore, the starter contactor is built with a higher-force return spring=
 to guarantee the contacts won't weld when the starter is shut off.=C2=A0 T=
o energize it they use a coil with fewer turns of larger wire.=C2=A0 Hence =
the coil draws much more current, dissipating more heat than the master con=
tactor.=C2=A0 Works fine for short duty cycles, but will overheat if turned=
 on continuously.=C2=A0 The master contactor, wound with more turns of fine=
r wire and using a lighter spring, will turn on and off more "gently," but =
won't overheat.=C2=A0 So, if the starter contactor (or starter switch) does=
 stick on, will the master be able to break the circuit?=C2=A0 Probably.=0A=
=C2=A0=0AThe starter contactor, though, is designed NOT to be the "weak lin=
k" in the starter circuit.=C2=A0 If it does stick closed, the current draw =
is not "huge," but is probably 100 amps or so, and that won't start anythin=
g on fire.=C2=A0 The starter will definitely burn up before the contactor w=
ill fail.=C2=A0 It's call "intermittent" because it will eventually overhea=
t because of the coil current draw, not because the contacts are weak.=C2=
=A0 Incidently, way back when, car electrical systems were designed so that=
 the battery would go dead before the starter burned up - I don't know=C2=
=A0whether or not=C2=A0that's still true.=C2=A0 Since an airplane battery i=
s maybe a quarter the size of a car battery, I would guess it would go dead=
 in time to save the starter, but I'm not sure.=C2=A0 The worst case is if =
the engine starts, the contactor sticks on=C2=A0and the pilot doesn't know =
the starter is still engaged.=C2=A0 In that case the alternator is charging=
 the battery and then I suppose the starter
 will burn up first.=C2=A0 What to do if you know the starter is still runn=
ing?=C2=A0 I suppose you shut off the master and then turn off the engine.=
=C2=A0 Right?=0AGary=0A=C2=A0=0AFred.=0ASounds like a better choice than we=
=E2=80=99ve been using.  One question comes to mind =E2=80=93 surrounding t=
he idea I have (is it correct?) that the starter solenoid should be intermi=
ttent because you want it to be the weak link in the starter circuit:  if t=
he starter contactors should stick, you=E2=80=99d have a huge amperage draw=
 and maybe start a fire.  If the contactor is intermittent, it will theoret=
ically fail and break the contact before the starter motor burns up and cat=
ches fire or causes other damage.=0AIs this logic correct?  If so, would th=
e specs on this contactor from Cole Hershee still be a good choice for the =
starter circuit with the TSIO 550?
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<html><body><div style=3D"color:#000; background-color:#fff; font-family:bo=
okman old style, new york, times, serif;font-size:12pt"><div>I'm not an exp=
ert in contactors, but there is a reason why the typical aircraft "master" =
contactor and the starter contactor are different, even though the starter =
current goes through both.&nbsp; The master contactor isn't expected to ope=
n the starter circuit, while the starter contactor is.&nbsp; Therefore, the=
 starter contactor is built with a higher-force return spring to guarantee =
the contacts won't weld when the starter is shut off.&nbsp; To energize it =
they use a coil with fewer turns of larger wire.&nbsp; Hence the coil draws=
 much more current, dissipating more heat than the master contactor.&nbsp; =
Works fine for short duty cycles, but will overheat if turned on continuous=
ly.&nbsp; The master contactor, wound with more turns of finer wire and usi=
ng a lighter spring, will turn on and off more "gently," but won't
 overheat.&nbsp; So, if the starter contactor (or starter switch) does stic=
k on, will the master be able to break the circuit?&nbsp; Probably.</div><d=
iv>&nbsp;</div><div>The starter contactor, though, is designed NOT to be th=
e "weak link" in the starter circuit.&nbsp; If it does stick closed, the cu=
rrent draw is not "huge," but is probably 100 amps or so, and that won't st=
art anything on fire.&nbsp; The starter will definitely burn up before the =
contactor will fail.&nbsp; It's call "intermittent" because it will eventua=
lly overheat because of the coil current draw, not because the contacts are=
 weak.&nbsp; Incidently, way back when, car electrical systems were designe=
d so that the battery would go dead before the starter burned up - I don't =
know&nbsp;whether or not&nbsp;that's still true.&nbsp; Since an airplane ba=
ttery is maybe a quarter the size of a car battery, I would guess it would =
go dead in time to save the starter, but I'm not sure.&nbsp; The
 worst case is if the engine starts, the contactor sticks on&nbsp;and the p=
ilot doesn't know the starter is still engaged.&nbsp; In that case the alte=
rnator is charging the battery and then I suppose the starter will burn up =
first.&nbsp; What to do if you know the starter is still running?&nbsp; I s=
uppose you shut off the master and then turn off the engine.&nbsp; Right?</=
div><div>Gary</div><div>&nbsp;</div><div class=3D"yiv1457176076MsoNormal"><=
span style=3D'color: rgb(31, 73, 125); font-family: "sans-serif"; font-size=
: 11pt;'>Fred.</span></div><div class=3D"yiv1457176076MsoNormal"><span styl=
e=3D'color: rgb(31, 73, 125); font-family: "sans-serif"; font-size: 11pt;'>=
Sounds like a better choice than we=E2=80=99ve been using.  One question co=
mes to mind =E2=80=93 surrounding the idea I have (is it correct?) that the=
 starter solenoid should be intermittent because you want it to be the weak=
 link in the starter circuit:  if the starter contactors should stick, you=
=E2=80=99d have a
 huge amperage draw and maybe start a fire.  If the contactor is intermitte=
nt, it will theoretically fail and break the contact before the starter mot=
or burns up and catches fire or causes other damage.</span></div><div class=
=3D"yiv1457176076MsoNormal"><span style=3D'color: rgb(31, 73, 125); font-fa=
mily: "sans-serif"; font-size: 11pt;'>Is this logic correct?  If so, would =
the specs on this contactor from Cole Hershee still be a good choice for th=
e starter circuit with the TSIO 550?</span></div></div></body></html>
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