Les Diodes Cours ESA-OFPPT-PDF
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
- Possibility to choose the take-off torque.
- Reduction of current draw.
- 3-time start without interruption.
- 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
- 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.
- 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.