ABB 83SR06B-E GJR2395400R1210 New PLC process control board

Description

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The choice of the starting method and also, if necessary, of the type of motor to
be used depends on the typical resistant torque of the load and on the shortcircuit power of the motor supplying network.
With alternating current, the most commonly used motor types are as follows:
– asynchronous three-phase squirrel-cage motors (AC-3): the most widespread
type due to the fact that they are of simple construction, economical and
sturdy; they develop high torque with short acceleration times, but require
elevated starting currents;
– slip-ring motors (AC-2): characterized by less demanding starting conditions,
and have quite a high starting torque, even with a supply network of low
power.
Starting methods
The most common starting methods for asynchronous squirrel-cage motors
are detailed below:
Direct starting
With direct starting, the DOL (Direct On Line) starter, with the closing of line
contactor KL, the line voltage is applied to the motor terminals in a single
operation. Hence a squirrel-cage motor develops a high starting torque with a
relatively reduced acceleration time. This method is generally used with small
and medium power motors which reach full working speed in a short time.
These advantages are, however, accompanied by a series of drawbacks,
including, for example:
– high current consumption and associated voltage drop which may cause
damages to the other parts of the system connected to the network;
– violent acceleration which has negative effects on mechanical transmission
components (belts, chains and mechanical joints), reducing working life.