GEA Grasso Manual page 50

Grasso
Refrigeration Division
2.7.2.3 Motor current limitation
The motor current limitation prevents overloading of the compressor drive motor. It does not become effective
until the start up process has been completed.
If the calculated motor current for the next capacity step reaches the initial limitation value (e.g. 105% of the full
load current), the compressor capacity cannot be increased any further. If the actual motor current reaches this
limit, the compressor capacity is reduced by 1 capacity step immediately.
The signal dominates compared to the capacity control and is effective until the motor current has become less
than the limit value and the limit timer has expired.
The motor current limitation function is illustrated in the following diagram.
I Motor current
mot
A.. Time characteristic of the motor
current:
1 = actual value
2 = calculated value for the next
capacity step
V.. Forced reduction of capacity due too
high motor current
B.. High motor current limitation active
T.. Limit timer active; After that this timer
has expired the capacity control is
enabled again and the compressor
capacity can be increased.
b.. Limit value for disabling capacity
increase (calculated motor current 2)
and for forced reduction of capacity
(actual motor current 1)
W.. Warning limit
Al.. Alarm limit
I Motor current
mot
Cap.. Compressor capacity in %
A.. Theoretical change in motor current in
case the capacity is increased:
1 = actual value
2 = calculated value for next
capacity step
f.. 'Decr factor' in %; If 0 % the
calculated value for the next capacity
step is equal to the actual value. If 100
% the relation between motor current
and capacity is linear. The 'Decr factor'
is used for changing the declination of
line 3. Notice that line 1 — 2 is parallel
with line 3.
b.. Limit value for disabling capacity
increase and for forced reduction of
capacity
Chap. 2 - 14
Figure 2.17, Motor current limitation
Figure 2.18, Determination of the 'Decr factor' for the motor current limitation
Notice that the values mentioned are an example only!
638660E_chapter2.doc
GRASSO SYSTEM CONTROL
01.2004/1