Les Diodes Cours ESA-OFPPT-PDF
Data
1 pairing
procedure
The motor is
supplied under standard reduced pressure via an autotransformer which is
switched off when the start is complete.
Start-up
takes place in three stages:
- at head
time, the autotransformer is first coupled in a star, then the motor is coupled
to the network through part of the windings of the autotransformer. Start-up
takes place under reduced pressure which is a function of exchange
compatibility.
The autotransformer
is generally provided with sockets allowing to choose the exchange
compatibility, therefore the value of the reduced pressure best suited,
- before
switching to full pressure coupling, the interlining is open. The winding part
connected to the network then constitutes an inductor in series with the motor.
This operation is performed when the equilibrium speed is reached at the head
time balance,
- the full
pressure coupling takes place after the second time, which is generally very
short (of the order of a part of a second).
The winding
area of the autotransformer in series with the motor is short-circuited, then
the autotransformer is switched off.
The starting
current and torque vary within the same extents.
They are
divided standard (U network / U reduced).
The values
obtained are as follows:
Id = 1.7 to
4 In
Disc = 0.5
to 0.85 Cn
Starting is
carried out without interference from the current in the motor.
Therefore,
the transient phenomena linked to such interference do not exist.
On the other
hand, if certain precautions are not taken, transient phenomena of the same
nature may appear during coupling under full pressure. Indeed, the value of the
inductance in series with the motor, after opening of the interlining, is high
standard affinity to that of the motor. It follows a significant strain drop
which causes a high transient current peak at the second of the coupling under
full pressure. To avoid this drawback, the magnetic circuit of the
autotransformer has an air gap, the presence of which channel reduces the value
of the inductance. This value is calculated in such a way that at the second of
the opening of the interlining at the second stroke, there is no variety of
pressure at the terminals of the motor.
The presence
of the air gap results in an increase in the magnetizing current of the
autotransformer. This magnetizing current increases the current draw in the
network when the autotransformer is put under strain.
This
starting mode is generally used in LV for motors with power greater than 150
kW. But it channels relatively expensive equipment due to the high price of the
autotransformer.
II-2-2-2
Advantages and disadvantages
an)
Advantages
-
Possibility to choose the take-off torque.
- Reduction
of current draw.
- 3-time
start without interruption.
b)
Disadvantage
- High
purchase price of the equipment.
II-2-3
Standard start of the stator resistors.
The
principle consists in starting the motor under reduced pressure by inserting
resistors in series with the windings.
When the
speed settles, the resistances are eliminated and the motor is directly coupled
to the network. This operation is generally controlled by a timer.
II-2-3-1
Pairing and substitution procedure
In this
starting mode, the coupling of the motor windings is not modified. It is
therefore not necessary that the two ends of each winding are forays on the
terminal board.
The
resistance value is calculated according to the starting current peak not to be
exceeded, or the minimum value of the starting torque necessary taking into
account the resistive torque of the driven machine. In general, the values of
starting current and torque are:
- Id = 4.5
In
- Cd = 0.75
Cn
During the
acceleration course with the resistors, the pressure applied to the motor
terminals is not constant. This pressure is equal to the strain of the network
minus the drop in strain in the starting resistance.
The pressure
drop is proportional up to date absorbed by the engine.
As the
current decreases at hide and as the motor accelerates, the same goes for the
pressure drop across the resistor. The pressure applied to the motor terminals
is therefore minimal at the second of starting, and it gradually increases.
The torque
being proportional to the square of the pressure at the motor terminals, it
increases more quickly than in star-delta starting where the strain remains
fixed during all the time of the star coupling.
This
starting mode is therefore well suited to machines with increasing resistance
torque with speed, as standard example fans or axis pumps.
It has the
disadvantage of a relatively large current surge on demurrage. This peak could
be reduced by increasing the resistance value, but this would cause an
additional pressure drop across the motor terminals, and therefore a
significant decrease in starting torque.
On the other
hand, the elimination of the resistance in starting balance is done without
interference from the motor supply, therefore without transient phenomenon.
II-2-3-2
Advantages and disadvantages
an)
Advantages
- The supply
pressure is very greatly reduced at the second start, since the current draw
remains significant.
- Increasing
engine torque during starting and starting in addition to progressive.
b)
Disadvantages
- Current
draw in addition to significant than in star-triangle 4 to 5 times In.
- The
starting torque is reduced by standard compatibility with direct starting 0.75
Cn.
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