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Example 3
Protecting a
direct started
motor with a low
safe stall time
In many applications, e.g. ExE-type drives, the
motor is not allowed to be in a stalled condi-
tion as long as its own start-up time. To find
out whether the motor is speeding up or not, a
speed switch on the motor shaft is used. The
switch should be open at standstill and close
when the motor speeds up.
The speed switch should be open at standstill and close when the motor speeds up.
Fig. 7. Protection of a directly started motor. A speed switch on the motor is used to produce a
secure stall protection even though the maximum safe stall time of the motor is less than the start-
up time.
Note that because of the fact that only two phase CTs are used, the third phase is reconstructed by
summing the two monitored currents through the third winding.
The speed switch information is used to control
the start-up stress monitor and the setting ts is
set a little shorter than the maximum allowed
jam time t
. If the motor starts accelerating, the
e
speed switch will inhibit the start-up supervision
unit trip and leave the protection to the thermal
unit. If the motor does not speed up, tripping
will be carried out after the time t
= t
.
s
e
11
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