ABB DCS 500B, Description

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Functional Software description
70 DCS 500 Software Description
Qact
time
A
B
C
D
E
F
H
Accel. Constant speed Decel. Standstill Accel.
100%
Motor’s thermal capacity
Qk
Figure 49 Qact as a function of different loads
Previous figure shows how QACT (motor’s thermal capacity) is for-
med of rising and falling sections of the time constant function. On
curve 0-A-C-D an overload trip occurs at point D owing to a too
short time constant even though the heating up clearly does not
reach the limit QACT = 100% which corresponds to the motor’s
thermal capacity.
In point A acceleration does not yet cause a trip because a cold
start was made
On curve 0-B-C-E-F-H the desired loading cycle can be repeated as
many times as necessary because its time constant is sufficiently
long.
The temperature rise of the motor behaves as a time constant
which is proportional to the motor current in power of two.
Iact2
Qact =

* ( 1 - e -t/
τ
) * 100 (1)
Iref2
where:
Qact
thermal value
Iact motor current actual
Iref reference current, normally rated current of motor
τ
temperature time constant.
100 scaling factor
t time