Direct-on-line starter
In the simplest case the motor is
connected directly with a contactor. The
combination of motor protection and
cable protection (fuse) is called a motor
starter (MSC = Motor Starter
Combination).
By applying the full mains voltage to the
motor windings, DOL starting may
produce large starting currents which
may result in troublesome voltage
changes. Direct-on-line starting
three-phase motors must not cause
interference voltage changes in the
public utility grid. This requirement is
generally fulfilled if the apparent power
of a three-phase asynchronous motor
does not exceed 5.2 kVA or its startup
current does not exceed 60 A.
With a mains voltage of 400 V and 8 times
the starting current, this corresponds to
a rated motor current of around 7.5 A and
thus a motor rating of 4 kW.
The motor rating denotes the mechanical
output of the motor at the shaft.
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