2 Functions
144
7UM61 Manual
C53000-G1176-C127-3
2.24 Motor Starting Time Supervision (ANSI 48)
When the 7UM61 is used to protect a motor, the startup time monitoring feature sup-
plements overload protection (see Section 2.9) by protecting the motor against ex-
tended startup durations. In particular, rotor-critical high-voltage motors can quickly be
heated above their thermal limit if multiple consecutive startup attempts are made. If
the durations of these starting attempts are prolonged, e.g. by excessive voltage
surges during motor startup, by excessive load torques, or by blocked rotor conditions,
a tripping signal will be initiated by the protective relay.
2.24.1 Functional Description
Motor Startup As a criterion for a motor startup, transgression of a (settable) current threshold I
MOTOR START is assessed and used for enabling calculation of the tripping time.
The protection function consists of one definite time and one inverse time tripping
characteristic.
Inverse Time Over-
current Character-
istic
The inverse time tripping delay time operates only when the rotor is not blocked. With
decreased startup current resulting from voltage dips when starting the motor, pro-
longed starting times are calculated properly and tripping can be performed in time.
The tripping time is calculated based on the following formula:
with
t
TRIP
Actual tripping time for flowing current I
t
Startmax
Tripping time for nominal startup current I
A
(Param. 6503, STARTING
TIME)
I Current actually flowing (measured value)
I
StartCurr.
Nominal starting current of the motor (Parameter 6502, START.
CURRENT)
I
MotorStart.
Pickup value for recognition of motor startup (Parameter 6505, I
MOTOR START)
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