Analyse des circuits pneumatiques ESA-OFPPT-PDF
Data
1 Direct
start
It is the in
addition to basic which can only be executed with the asynchronous motor with a
conical rotor. The stator windings are coupled directly to the network
II-1-1
coupling and compensation procedure
This is the
in addition to basic start mode in which the stator is directly coupled to the
network (Fig II.1). The engine starts on its natural characteristics.
At the
second of the setting under strain, the motor behaves like a transformer whose
secondary, constituted by standard the confines of the rotor very little
resistant, is in short-circuit. The current induced in the rotor is
significant. This results in a peak current on the network:
I start = 5
to 8 I ostensible
The starting
torque is on average:
C start =
0.5 to 1.5 C ostensible.
Despite the
advantages it presents (simplicity of the apparatus, high starting torque,
quick starting, low price), direct starting can only be suitable in cases
where:
- the engine
power is low standard compatibility with the power of the network, so as to
limit duty irritations to the current demand,
- the driven
machine does not require dynamic speed setting or includes a damping device
which reduces the shock of starting,
- the
starting torque can be high without frequency on the operation of the machine
or the driven load.
II-1-2
Advantages and disadvantages
an)
Advantages
- Simplicity
of the apparatus.
-
Significant couple.
- Short
start-up time.
b)
Disadvantages
-
Significant current draw
- Merciless
start.
II-2
Starting under reduced pressure
Several
devices make it possible to reduce the pressure at the terminals of the stator
windings during the duration of the starting of the motor which is a means of
limiting the intensity of the starting current. The disadvantage is that the
engine torque is also reduced and that this increases the time before which the
machine reaches the perpetual speed.
II-2-1
Star-delta starting
This process
can only be applied to motors whose all winding ends are fights on the terminal
board, and whose triangle coupling compares to the strain of the network.
Start-up takes place in 2 stages.
- 1 time:
setting under strain and star coupling of the windings.
The engine
starts at reduced strain n
- 2-stroke:
Removal of star coupling, and setting of triangle coupling The motor is
supplied under full strain
II-2-1-1
coupling and reward procedure
This
starting mode (Fig II-2) can only be used with a motor on which the two ends of
each of the three stator windings are brought back to the terminal board.
Standard elsewhere, the winding must be carried out so that the triangle
coupling corresponds to the strain of the network: standard example, for a
three-phase 380 V network, a motor wound in 380 V triangle and 660 V star is
required.
The
principle consists in starting the motor by coupling the star windings under
the network strain, which amounts to dividing the nominal pressure of the motor
in standard star (in the example above, the network strain 380 V = 660 V /) .
The starting
current peak is divided standard 3:
- Id = 1.5
to 2.6 In
Indeed, a
380 V / 660 V motor coupled in star under its nominal strain 660 V spongy a
current times in addition to weak than in triangle coupling under 380 V. The
star coupling being carried out under 380 V, the current is divided a again
standard so all standard 3.
The starting
torque being proportional to the square of the supply strain, it is also
divided by standard 3:
- Cd = 0.2
to 0.5 Cn
The motor
speed balances out when the motor and resistance torques balance, generally
between 75 and 85% of the nominal speed.
The windings
are then coupled in a triangle and the motor returns to its natural
characteristics. The section of the star coupling to the triangle coupling is
controlled by a timer. The delta contactor is closed with a hinder from 30 to
50 milliseconds after the star contactor is opened, which avoids a short
circuit between stages, the two contactors cannot be closed simultaneously.
The current
which directs the windings is interrupted when the star contactor opens. It is
restored when the triangle contactor closes.
This entry
in triangle is accompanied by a very brief but very important transient current
peak, due to the counter-electromotive power of the motor.
Star-delta
starting is suitable for machines which have a low resistance torque or which
start empty (e.g. woodworking machine). To limit these transient phenomena,
variants may be necessary, beyond a certain power. One is a time delay of 1 to
2 seconds at the star-triangle entry.
This delay
allows a decrease in the counter-electr poweromotor, therefore of the transient
current peak.
This can
only be used if the machine has sufficient inertia to avoid too significant
slowing down during the delay time.
Another is
the 3-step start:
star-triangle
+ resistance-triangle.
The cut
remains, but the resistance put in series, for about three seconds, with the
windings coupled in a triangle, reduces the peak of transient current.
A variant is
star-delta + resistance-delta starting without interruption.
The resistor
is placed in series with the windings immediately before the opening of the
star contactor. This avoids any current interference, therefore the appearance
of transient phenomena.
The use of
these variants translates standard the implementation of additional equipment,
which can result in a significant increase in the cost of installation.
II-2-1-2
Advantages and disadvantages
an)
Advantages
- Star
current draw reduced to the levels of its direct value.
- Low
fitting inconvenience.
b) Disadvantages
- Torque
reduced to levels of its live value.
- Cut
between the star and triangle positions or to view transient phenomena.