Figure 3-56: Circuit With A Varistor (Ac/Dc Operation); Figure 3-57: Circuit With An Rc Element (Ac/Dc Operation) - Siemens sirius System Manual

3RT1/3RH1 contactors
Varistor
RC element
3-74
Circuit with a varistor
(AC/DC operation)
U [ V ]
Sp
200
0
0 20

Figure 3-56: Circuit with a varistor (AC/DC operation)

This is what happens when a coil is disconnected that is connected to a
varistor (voltage-dependent resistor):
Voltage peaks still occur. They are cut off at approximately 400 V and have a
shorter overall duration (approximately 50 microseconds).
(Note: The oscillogram is cut off, and the voltage is reduced to zero after
approximately 3 ms.)
A varistor is suitable for AC and DC operation.
The off-delay of the contactor is extended by approximately 2 to 5 ms.
Circuit with an RC element
(AC/DC operation)
U [ V ]
Sp
200
0
0 5

Figure 3-57: Circuit with an RC element (AC/DC operation)

This is what happens when a coil is disconnected that is connected to an RC
element:
The amplitude and rate of rise of the switching overvoltage are reduced by
the capacitor. Showering arcs no longer occur. The voltage swings briefly to
400 V and then slowly drops down. This represents ideal damping.
Disadvantage: The component is larger and generally more expensive.
RC elements are suitable for AC and DC operation.
Only a minimal off-delay occurs (under 1 ms).
U
Sp
t [ µs ]
U
Sp
t [ m s ]
GWA 4NEB 430 0999-02b
SIRIUS System Manual