Hi,
A question on the circuit I provided? To me it looks like the fuse or overload protector is in
parallel with the run winding, but other sources tell me it is in series with both windings.
There are three states or conditions that this motor circuit can be in.
AT REST.
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At rest there would be continuity through the overload device, the run winding and the coil of
the special purpose relay.
At rest there would be no continuity through the relay contacts, which would be open, so there
would be no current path for any current to go through the start winding, which would not be
'in circuit' in the at-rest state.
The feed to the motor and overload device may or may not come via the thermostat,
which is shown as 'normally open' so its reasonable to assume the feed to the motor assembly
comes from elsewhere.
STARTING.
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When starting, the special purpose relay would operate from the current going through the run
winding, and its contacts would close.
With the contacts closed, the start winding is effectively in parallel with the run winding.
The coil of the special purpose relay is not normally very many turns of wire and has almost
no effect on the run winding, and remains in circuit throughout.
The starting current is higher, and the special purpose relay responds to the higher current.
During this starting period, the start and the run windings are effectively in parallel,
and as a pair would be considered in series with the overload device.
The overload device and the motor windings would make a series-parallel arangement during
the starting period.
RUNNING.
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Once the motor has run up to its operating speed, the current drawn by the motor reduces and
the special purpose relay drops out, and it's contacts open.
Then the start winding is no longer 'in circuit' and we are left with three items in series,
the overload device, the run winding, and the coil of the special purpose relay.
So the state of the circuit changes from series to series-parallel, and back to series by
the action of the special purpose relay.
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On the other hand if the current were to only go through the run winding and
since current is flowing through the run winding without the aid of the start winding,
wouldn't this also cause the overload to trip
Sometimes yes. These overload devices usually have an interval before they operate, so that
higher initial loads dont necessarily cause them to trip.
If the start relay or the start winding were to fail, then yes that would mean that the current
through the run winding would not reduce, because the motor would not spin up to speed.
This would cause the run winding to get warm, maybe even hot.
If the run winding got too hot, it may short and cause the overload device to trip.
Or maybe the overload device has been chosen to respond if the motor doesn't start within a
given time.
The interval on these devices is usually some hot element made to cause a trip.
They may or may not be re-settable.
There are many types.
Cheers, John