Page 613 - Motor_protection_and_control_Manual_motor_starters_ contactors

Surge suppressors for contactor coils

Description
(V)
1000 The operation of inductive circuits causes overvoltages, in particular on opening of the contactor coil.
T (s)
0 100 The electromagnetic energy stored in the coil during contactor closing is restored on opening in the form
1000 of surges, the slope and amplitude of which may rise to several kilovolts. A number of drawbacks are
observed ranging from interference on the electronic devices to breakdown of insulators and even destruc-
0 100 tion of certain sensitive components.
The graph opposite reproduces the oscillogram showing voltage discharges at the terminals of a 42 V / 50 Hz
A 884D coil without peak clipping. The coil was switched by 8 series-connected poles of a contactor relay.
Following a burst of discharges with a very steep slope a damped oscillation emerges with a peak value of
3500 V.
Overvoltage Factor
The overvoltage factor k is defined as the ratio of the maximum overvoltage peak value Ûs to the peak
A078 value Ûc of the coil rated control voltage Uc:

k = Ûs max. in DC: k = Ûs max. or in AC: k = Ûs max.
RC-EH Ûc Uc Uc√2 5

For example the following is obtained for the above graph: k = 3500 ≈ 60
42 √2
To reduce the harmful effects of these overvoltages, ABB has developed a range of surge suppressors de-
signed to reduce the k factor defined above and to limit or even completely eliminate the high pre-damping
voltage frequencies.
Each case is different, but the technical data tolerances and the generous sizing of parts have enabled us
to reduce the number of variants.
We have chosen the following solutions: transil diodes, varistors and RC blocks.
Note: A varistor is a resistor whose value decreases to a very large extent when a certain voltage is ap-
plied at its terminals.

Ordering details
For contactors Rated control circuit Type Order code Pkg Weight
voltage qty (1 pce)
Uc
V AC DC kg
EK550, EK1000 48...110 ● – RC-EH800/110 SK829007-C 1 0.015
EK550, EK1000 24...125 – ● RC-EH800/110 SK829007-C 1 0.015
EK550, EK1000 220...600 ● – RC-EH800/600 SK829007-D 1 0.015

1SFC101223C0201

ABB | 5/413