[TR] TR2 - Rope Fan Belt and Non-stopping Generator

[Dave1massey at cs.com]

"Randall" <tr3driver at ca.rr.com> wrote:

>> The cutout relay opens up in response to large currents flowing into the
>> generator when it comes to a stop.  Since the generator never came to a
>> stop there was no large current.  And the lower lever of current flowing
>> into the generator to spin it was not sufficient.  I suspect there is
>> nothing wrong with the control box.
>> 
>> The same thing happened to my MG back in the 70's.  New belt and all was
>> fine.
>
>Could be, I guess, but it shouldn't take much current for the cutout to drop
>out.  It has two windings, one for voltage and one for current; the current
>has to only match the voltage (and be opposite sign) for the cutout to open.
>
>It will actually stay closed if you mash it closed with the engine stopped,
>because the current is enough larger than the voltage to hold the armature
>down.
>
>Randall

True but the voltage driven pull-in coil has many windings but the current driven drop-out coil is only two or three.  So it seems normal that it would take a fair amount of current to counter the voltage coil.  And if the generator is already spinning the current to maintain it may not be enough.  But when the generator is connected to the engine via the belt the mechanical loading on the "motor" is virtually infinite and pulling locked rotor torque is easy.

Then it is a race condition.  Will the contacts open before the current rises so high that the magnetic field from the current coil will overwhelm the field from the voltage coil enough to pull in the contact?  (Which is what happens when you mash the contacts closed)

Dave