Checking The Need For Speed Switch; Cumulative Start-Up Time Counter - ABB REM 610 Technical Reference Manual

REM 610
5.1.3.2.
5.1.4.
120
Motor Protection Relay
Technical Reference Manual
For instance, if the start-up current of the motor is 6.2 x FLC and the start-up time
11 seconds, I > = 6.2 and t
s

Checking the need for speed switch

When protecting motors of ExE-type, for instance, the safe stall time may be shorter
than the start-up time of the motor, which is why a speed switch on the motor shaft
is required to give information on whether the motor is accelerating during motor
start up. In this case, the start-up time is set to slightly below the safe stall time.
The speed switch should be open at standstill and closed during acceleration. When
the input is activated, stage I
2
accelerate, stage I x t
s s
start-up supervision is based on overcurrent protection, stage I
expiration of the set operate time
However, if the safe stall time is longer than the start-up time of the motor when no
prior load, a speed switch will not be required.

Cumulative start-up time counter

The cumulative start-up time counter functions as backup to the thermal overload
protection and prevents too frequent motor start ups, i.e. ensures that the
recommendations from the manufacturer are followed.
There are two values to set: the restart inhibit value in seconds, Σt
countdown rate of the start-up time counter, ∆Σt
The restart inhibit value is calculated as follows:
Σt ( ) t ×
= n 1 – + margin
si
where
n = allowed number of motor start ups
t = start-up time of the motor (in seconds)
margin = safety margin (~10... 20%)
The countdown rate is calculated as follows:
t
∆Σt ---------- -
=
s
t
reset
where
t = start-up time of the motor (in seconds)
t = time during which the maximum number of motor start ups stated by the
reset
manufacturer can be made (in hours)
If the motor manufacturer has recommended a maximum of three start ups in four
hours and a start-up time of 60 seconds/start up, the restart inhibit will be activated
when the third motor start up has been initiated, thus preventing a fourth motor start
up. Consequently, the restart inhibit value is to be set to 130 seconds.
> = 11 s x 1.1 = 12 s.
s
2
x t /I > will be blocked. If the motor does not start to
s s s
will trip when the reference value, I
s
1MRS 752263-MUM
2
x t , is exceeded. If
s s
> will trip on
s
, and the
si
/∆t.