control and monitoring equipment
The function of this equipment is
threefold:
c energising and stopping (control);
c disconnecting the motor should a
fault occur (protection);
c monitoring the motor (monitoring).
When we talk about monitoring, this
implies the equipment is capable (or
not) of:
c initializing the start (starting sequence
automation);
c acting on motor speed;
c supplying information on motor
electrical status and contributing to
protection.
The monitoring function chiefly relies on
power electronics and low currents
(digital technology); it is currently being
fully developed. Protection of
HV motors will be dealt with in the next
chapter.
electromechanical
solutions
The choice between the various
devices (switch, circuit-breaker or
contactor) depends on:
c operation rate;
c electrical endurance;
c motor power.
The main breaking device
characteristics are summarised in the
table in figure 19.
Fuse-switches
By their very design, the breaking
capacity, mechanical and electrical
endurance of switches is low. This
limits their use to small powers
(In = approx. 50 A - 5,500 V) and
to rates of two to three operations a
day.
Moreover, the low breaking capacity of
these devices makes choice of
protection devices tricky.
Circuit-breakers
Circuit-breakers are generally used for
high motor powers of more than 300 A,
with a small operation rate, and for
operating voltages of more than 6.6 kV.
Their use can naturally be extended to
lower powers, operable by switch or
contactor.
Fuse-contactors
c operation rate
Their simple control mechanism
combined with the robustness and
simplicity of their contacts mean
contactors have a high operation rate.
This rate cannot be withstood by circuitbreakers,
even special ones, and even
less so by switches.
Some installations use contactors with
mechanical latching to do away with
permanent consumption of the closing
electromagnet. This may reduce
endurance as a result of the greater
complexity of the kinematic chain.
c network short-circuit power
This factor does not really affect
contactors thanks to the presence of
fuses placed immediately after the
isolating switch or next to the
disconnecting contacts on the busbar
side. These fuses with their high
breaking capacity, limit the short-circuit
current.
This special feature means that,
if network power is increased, the
motor feeder cubicles can be
maintained. The busbar supports are
strengthened if required.
Fuses
Fuse rating is determined according to:
c rated current In;
c the Id/In ratio (Id = starting current);
c starting time determined using the
chart in figure 20 page 14.
Finally, it should be pointed out that
fuses protect the motor against
overcurrents roughly five times greater
than motor rated current and that they
must be combined with additional
protection devices (thermal relays, etc.
see "protection device" chapter).
Current transformers
The increasing use of digital protection
devices means unconventional current
sensors can be used (e.g. Rogowski
toroids). These sensors have the
advantage of being linear and thus
delivering an accurate signal
throughout the useful current range.
They present no saturation or thermal
problems, as is sometimes the case
with classical Current Transformers
(see "Cahier Technique" n° 112 "The
breaking process with a Fluarc SF6
puffer-type circuit-breaker").
Special features due to use of fuses
or circuit-breakers
c operation in single-phase due to fuse
melt, with the striker not working.
Today striker reliability is such that the
risk is slight. Their dependability may
be increased still further by use of an
additional protection device
(undervoltage or unbalance relay).
c discrimination with upstream
equipment.
This can be hard to achieve when both:
v the fuses used have a high rating
(200 A or 250 A),
v and the feeder protected by these
fuses accounts for a large fraction of
the power supplied by the main circuitbreaker
(see fig. 21 and 22).
However, the high breaking capacity of
the Rollarc contactors combined with
these fuses allows the use of slightly
time delayed overcurrent relays, thus
ensuring discrimination. Discrimination
is easier to achieve if the motor feeder
is protected by circuit-breaker but
since, for high short-circuit currents the
current is not limited, there is an
increase in thermal stresses.
c surge voltages
Some types of device, in particular
vacuum breaking devices, generate
surge voltages on motor energising and
stopping (due to their capacity to break
high frequency currents, resulting for
example from the current pinch-off
phenomenon - see "Cahier Technique"
n° 143).
In order to prevent these surge
voltages progressively damaging motor
insulation, manufacturers place ZnO
type surge voltage limiters in the
equipment if required.
It can be concluded that today's
electromagnetic solutions are reliable,
robust, economic and entirely suitable
for most applications.
electronic solutions
These provide users with additional
possibilities and advantages such as:
c variable speed;
c possibility of speed regulation;
c high operation rate;
c energy savings.
The electronic solution is rarely used
just for starting.
Before dealing with standard cases of
electronic devices, it should be pointed
out that their use calls for a certain
number of constructive precautions at
motor level:
control monitoring and protection of HV motors