Https://www.bueno-electric.com
BUENO ELECTRIC
BUENO ELECTRIC
BUENO ELECTRIC
BUENO ELECTRIC
Contact: [email protected]
Fax: +86-25-51816989
10
Asynchronous motor startup current characteristic
4.2 Overheat protection
The heat effects of motor positive-sequence, negative-sequence current are considered comprehensively, to
provide overheat protection for the overheats caused by various overloads, and as the standby protection for motor
short circuits, overtime startup, rotor-locked, etc.
Equivalent current Ieq is used to simulate motor heating effect, that is:
I
eq
is equivalent current
I
1
is positive-sequence current
I
2
is negative-sequence current
K
1
is heat coefficient of positive-sequence current, and during motor startup, K
1
＝
0.5; after startup completion, it
is recovered to be K
1
＝
1;
K
2
is heat coefficient of negative-sequence current, K
2
＝
3
～
10
，
K
2
＝
6 could be taken.
According to motor heat model, the characteristic curve between motor operation time t and equivalent operation
current I
eq
is given in the following formula.
I
p
：
Load current before overload, if it is at cold state before overload, I
p
I
∞
：
Long-term permitted load current, to protect current limit regulated in requirements of non-operation, which
is “overheat protection startup current”. It could be set based on 1.05
～
1.15 times of rated current I
e.
