Frequencies And Direct Current (Dc) - ABB MS116 Manual

4.3 Frequencies and direct current (DC)

The magnetic trip values for manual motor starters are valid for frequencies from 50 to 60 Hz. Frequencies other
than 50/60 Hz will have an impact on the instantaneous short-circuit release. In the range from 45 to 66 Hz, the oper-
ating values of the instantaneous short-circuit release are within tolerance. For frequencies above 60 Hz, or for direct
current (DC), the operating value of the instantaneous short-circuit release is increased; for frequencies below 50 Hz,
the operating value of this magnetic release is decreased. Correction factors for these applications are shown below.
2
1,8
1,6
DC Value for MS132
1,4
DC Value for MS165
1,2
1
0,8
50
Thermal tripping characteristics for manual motor starters are valid for DC and AC with frequencies from 16.66 Hz to
400 Hz. For three-pole loads and currents of between 3 - 13 times the set current, the tolerance of the tripping time is
±20 %.
In contrast to alternating current (AC), no current passes take place with direct current (DC). Since current-limiting
circuit-breakers do not rely on a current zero crossing to extinguish the arc like contactors, they have hardly any re-
duced switching capacity in the case of DC short-circuits compared to alternating current.
In the case of a DC short circuit, the current increase in the first phase is quite comparable to that in the case of al-
ternating current (depending on the time constant of the DC circuit). Only the current increase (the steepness of the
short-circuit current) is crucial for the opening of the contacts in current-limiting circuit-breakers. As soon as the arc
is actively conducted from the blowing field into the quenching chamber and extinguished there, the short circuit is
switched off, regardless of when and if a current zero crossing follows.
For symmetrical heating of the thermal overload releases it is also mandatory for circuit-breakers to connect all poles
in series (mandatory for differential tripping, analogue thermal overcurrent motor protection relays). The series con-
nection also leads to a faster shutdown.
APR I L 19
MA N UA L MOTOR START ER G UI D E
100 150 200
Frequency in Hz
Figure 25: Diagram of different frequencies and direct current (DC).
250 300 350
MS132
MS165
400
45 /80