Motor overload sensor
Fred Lotte wrote:
In article ,
Robert Baer wrote:
But i know nothing about motors except that they work and have an
initial surge current which probably depends on load.
The initial surge is a magnetic circuit phenomena and does not
depend on the load. You will get precisely the same initial
current with the shaft disconnected from the load or with the
shaft locked. (Assuming the same voltage and initial point of
conduction in the voltage waveform.)
The initial surge is called 'inrush' current and is analogous to
transformer energizing inrush. Once the transient has past, the
current will be dependent on the speed of rotation of the rotor
which is primarily a function of the load inertia and applied
voltage and time. If the rotor doesn't turn the current will be
'locked rotor' current which is nominally about 4 to 10 times the
rated load current. The inrush can be something like 1.6 or 1.7
times that times the ratio of the applied voltage to the rated
voltage.
How long the motor draws 'starting' current does depend on the
load.
The overload device needs to protect the motor from prolonged
starting or locked rotor current. It must ignore the inrush
current. Typically, a separate instantaneous trip device is used
for short circuit protection for currents above about 2-3 times
the maximum inrush current.
Thanks.
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