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3/228 ABB MOTOR PROTECTION AND CONTROL

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Contactors for DC switching applications
DC-1, DC-3, DC-5 applications according to IEC 60947-4-1

The circuit switching on DC is more difficult than on AC, as alternating current go to zero according to the frequency of the
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supply source while DC current has a continuous value.
The main parameters to be considered for selecting a contactor are the current, the voltage and the L/R time constant of the
controlled load.
Time constant and utilization categories
In DC applications, the nature of load to switch (resistor, inductance or a combination) is characterized by the ratio of the in-
ductance to the resistance (L (inductance of operated circuit) / R (resistance of operated circuit) = mH/Ω = ms)
This ratio L/R is called the time constant of the circuit.

DC current utilization categories are defined according to IEC 60947-4-1:
• DC-1 non inductive or slightly inductive loads, resistance furnaces (L/R ≤ 1 ms)
• DC-3 shunt motors: starting, plugging, inching, dynamic breaking of DC motors (L/R ≤ 2 ms)
• DC-5 series motors: starting, plugging, inching, dynamic breaking of DC motors (L/R ≤ 7.5 ms).

The higher the time constant value is, the more difficult it is to break the arc.
The addition of a resistor in parallel with an inductive winding helps in the elimination of the arcs, by reducing the time con-
stant.

Operational voltage
• The higher the operational voltage value is, the more difficult it is to break the arc.
• The use of main poles connected in series will allow to increase the value of switched voltage.
However, the maximum switched voltage must be within the max operational voltage of the contactor.
All the poles required for breaking must be connected in series between the load and the source polarity not linked to earth
(or chassis) (see recommended connection diagrams).

ABB offer a large choice of possibilities for DC switching applications (see selection tables):
• Standard 3-pole or 4-pole contactors with either 1- pole breaking or breaking with poles connected in series.
• Special contactors designed for DC breaking with permanent magnets fitted on the main poles for use with the 3 poles
connected in series and considered as 1-pole devices:
- GA75 and GAE75 contactors: the 3 poles are connected in series via two supplied and fitted insulated connections (25
mm²)
- GAF145 … GAF2050 contactors: the 3 poles must be connected in series by the user according to conductor cross-sec-
tional area (refer to main pole technical data) or by using LP connection bars to be ordered separately.

Selection tables
The enclosed selection tables will guide your choice through all contactor variants according to utilization category, for op-
erational voltage up to 1000 V DC-1 and operational current up to 2050 A in ambient temperatures from -25 °C up to 40 °C.
For higher values of current or voltage or heavy DC switching applications see bar mounted R contactors.