Analyse des circuits pneumatiques Électromécanique des Systèmes Automatisées-ESA-OFPPT-PDF

Analyse des circuits pneumatiques Électromécanique des Systèmes Automatisées-ESA-OFPPT-PDF

Analyse des circuits pneumatiques Électromécanique des Systèmes Automatisées-ESA-OFPPT-PDF

Analyse des circuits pneumatiques    ESA-OFPPT-PDF

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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.


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