Rockwell Automation Low-Voltage Switchgear and Controlgear - Manual (Page 60)

The effect of the current limitation is reduced with increasing frequency, as at higher frequencies
the peak value of the short-circuit current is already reached during the reaction time of the
switch. In view of the comparatively low short-circuit currents in medium frequency supplies, this
is not relevant in practice. The short breaking times of current limiting circuit breakers are
retained.
With two poles connected in series, circuit breakers with current limitation in single-phase
supplies typically reach up to 400 V the rated breaking capacity at 50/60 Hz. At voltages over
400 V to 690 V a.c. on the other hand, series connection of three poles is required. In single-
phase supplies it must always be ensured that all three poles of thermally delayed overload
releases are in the current loop.
2.4.3.3 Performance of release units at supply frequencies
< 50 Hz and > 60 Hz
Thermal overload releases
Thermally (current-dependent) delayed overload releases and relays operate with bimetal strips.
These are usually heated via a heating coil by the heat losses of the operating current or the
secondary current of a current transformer.
Up to around 400 Hz, the heat losses in the heating coils (ohmic losses) are the main heat
source. The additional inductive heating in the bimetal strips itself is practically negligible up to
these frequencies, so that the tripping characteristic will only be slightly faster than at 50 Hz. At
frequencies over 400 Hz the proportion of inductive heating increases and the ultimate tripping
current falls with increasing frequency.
Overload relays that are connected to main current transformers with a high overcurrent factor
(protective current transformer) or that have integrated current transformers, display a some-
what faster tripping characteristic in comparison to 50 Hz at frequencies over 50 Hz to 400 Hz.
The trip characteristic of relays with a saturation current transformer for heavy-duty starting
becomes considerably faster with frequency increasing up to 400 Hz as the saturation effect will
move proportionally to the frequency towards higher currents.
Electronic overload devices
Due to the variety of principles of operation, no general statement can be made on the perform-
ance of electronic overload relays at frequencies over and below 50/60 Hz. With relays with
current transformers it should be noted that application at low frequencies is limited because of
transformer saturation.
Short-circuit releases
For the activation of electromagnetic overcurrent releases, in addition to the size of the current
also the time is relevant during which the current is applied. At 50/60 Hz the armatures of the
electromagnetic overcurrent releases are activated within around 5 ms. During the half-cycle,
the force is sufficient to pull the armature all the way through to its end position. At higher
frequencies the duration of a half cycle is too short.
The pick-up-threshold of the short-circuit-releases increases over 50/60 Hz and at around
400 Hz approaches 1.4 times of the 50/60 Hz value.
Increased operating frequencies can lead to increased temperature-rise of the releases.
2.4.3.4 Switchgear used with soft starters
Overload protection
Thermal relays and circuit breakers are equipped with thermal overcurrent releases that can be
adjusted to the motor rated current and even during soft starting map the motor heating. The
harmonic components of the currents that also contribute to motor heating are measured.
The performance of electronic motor protective devices with respect to the effect of the harmon-
ics (for example true r.m.s. value measurements) should be obtained from the respective device
documents.
LVSAM-WP001A-EN-P - April 2009 2-32