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PLAN
Introduction
I- General principles setting
II- Control of stator voltages
II-1 Principle of command BO
II-2 Control Strategy of PWM converter
II-3 Production of velocity control
III- control the stator currents
III-1 Control Act
III-2 speed servo Realization
III-3 reversible Converter
IV- asynchronous machine Association - current switch
IV-1 Power Circuit
IV-2 Velocity Control command converter
Conclusion
Introduction
Recent advances in the fields of power and digital control electronics have allowed recently the development of variable speed drives for AC machines. Today AC machines can replace DC machines in most electrical variable speed drives.
In many industries, we must expect the progressive disappearance of the use of dc machines, the collector still constituting a magnificent "Achilles heel."
I- General principles of setting:
The power of asynchronous machines with variable frequency is done using a converter generally alternating continuous (Figure 1). The input source may be of the current source or voltage source. Output converters are controlled amplitude voltages or stator currents and their frequency fs.
Figure 1
To highlight the general principles of the electromagnetic torque of the asynchronous machine, we will work from the model in Figure 2, valid in sinusoidal steady state. The machine is considered receiver convention.
Figure 2
The elements of the model are:
Ls: cyclic inductance of the stator
Mgr: cyclic mutual inductance stator-rotor
Rs: resistance of the stator windings
Rr: resistance of the rotor windings
ωs: pulsation of the stator currents
ωr angular frequency of rotor currents
estate total leakage inductance reduced to the rotor
g: slip g = ωr / ωs
Io: magnetizing current representative of the stator flux.
To further simplify the model (Figure 3), the resistance Rs is generally overlooked. Assumption is valid only near the nominal operating point of the machine.
The R'r Ns and elements are defined from the model in Figure 2 by the following equations:
From the model of Figure 3 we will calculate the electromagnetic power and then deduct the expression of it provided by the asynchronous machine.
Figure 3
electromagnetic power:
Power is transmitted to the rotor:
with
The joule losses in the rotor:
electromagnetic power:
And
electromagnetic torque:
Let p be the number of pole pairs
So :
So
The voltage and stator flux are linked by the relationship:
Thus the electromagnetic torque can be expressed as:
(*)
To control the electrical torque of the asynchronous machine, we see from the relationship (*), the need to control the stator flux and pulsation ωr of the rotor currents (magnitude which is not directly accessible). A constant stator flux, we can draw (fig 4 ') from the expression (*) curve Cem = f (ωr)
4 '
The curve has a maximum Cemax for pulsation ωrmax noted:
ωrmax = R'r / Ns (1) and Cemax = 3p Φs 2 / 2Ns (2)
When the pulse is low (ωr-> 0: low slip), the electromagnetic torque is proportional to the pulsation ωr.
Cem = 3p Φs 2 ωr / R'r
From the knowledge of the law Cem = f (ωr) different control strategies of asynchronous machines have been developed.
II- Control of stator voltages:
II-1 Principle of command BO:
A constant flow, the electromagnetic torque of the asynchronous machine depends only on the pulsation ωr. Thus, for different values of the pulsation of the stator ωs quantities, a family couples speed characteristics is obtained, Cem = f (ω) 4, which can be built from Figure 3 as ωs = ω + ωr.
Figure 4
In the linear region (low slip), this family of characteristics is similar to that of a DC machine where the induced voltage is the speed adjustment setting. And to vary the speed of an asynchronous machine BO supply frequency must be varied to the stator maintaining constant flux.
The voltage settings and frequency of the stator of the asynchronous machine is achieved through a PWM voltage inverter (Figure 4). The fundamental components of the stator voltages are balanced three-phase system. Their effective value Vs must be adjusted to maintain a constant stator flux not to downgrade the torque machine. So to get a constant flow must be proportional to Vs Vs = ωs because jωsФs
C-ie Фs = Vs / ωs
Figure 4
However, this relationship is not valid for weak pulses ωs because the voltage drop RSIS due to the resistance of the stator windings is negligible before the term: LsωIo. Also being considered on most drives a compensation of this voltage drop by increasing the amplitude of the stator voltages for low pulsation ωs (Figure 5) to maintain constant Фs.
On the other hand if an overload operation of the machine is considered, it is not possible to exceed the rated stator voltage (breakdown of the insulation). Фs the flow is reduced as the maximum electromagnetic torque (Figure 5). The command BO does not allow to control the rotational speed of the machine since constant ωs heartbeat, the speed of rotation depends on the load torque of the driven load (Figure 6).
Figure 5
Figure 6
II-2 converter control PWM Strategy:
The structural diagram of Figure 7 describes the working principle of the PWM inverter. V1Mréf, V2Mréf and V3Mréf are sinusoidal. It is generated by a voltage controlled oscillator whose output frequency is proportional to the input frequency ωsréf. The amplitude of Vs V1Mréf tensions V2Mréf V3Mréf and is controlled through the multipliers, and takes into account the law Vs = f (ωs) described in Figure 5.
Figure 7
The complete converter generally uses a diode rectifier to power the PWM inverter from the network. Because the diode rectifier, this structure is not reversible and it is necessary to provide a dynamic braking device when the asynchronous machine functions as a generator
II-3 Realization of the servo speed
In order to slave the rotation speed. an outer loop is added that, from the speed error, increases the frequency of the stator voltages so as to speed error due to sliding (figure8).
Figure 8
The corrector, usually PI, estimates the rotor pulsation. The corrector rated output frequency ωr is added to the image frequency ω of the rotational speed and this in order to get the proper value for the stator pulsation. Thereof is calculated by the equation:
= ωs + ωr ω. This is the frequency autopilot. This relationship is necessary for the existence of an electromagnetic torque to average non-zero value.
The output of the PI controller is provided with a limiter device so as to limit the value ωr. Thus the amplitude of the stator currents is indirectly limited (Figure 9)
Figure 9
Control of the amplitude of stator currents is here based on the simplified model of the asynchronous machine which is only valid in continuous operation. During transients, the instantaneous current is not controlled.
To avoid large-current transient, the integral gain of the PI controller should be increased. Therefore, the rapid variations of ωr are avoided but at the expense of the dynamics of the controlled system. Also it is preferred in most cases to enslave the inverter output current PWM so as to perfectly control the instantaneous values of stator currents of the asynchronous machine.
III- control stator currents:
III-1 Control Act:
A current loop controls the output current of each arm of the PWM inverter.
To control the electromagnetic torque of the asynchronous machine must maintain constant stator flux and control the pulsation ωr. Since the machine is powered by current and not voltage, it is necessary to determine the law s = f (ωr) which maintains the constant flow Фs.
The simplified expression of the law s = f (ωr) can be found from the equivalent diagram of Figure 3. To maintain the constant flow requires that the magnetizing current Io is constant (= Фs LSIOs). According to the equivalent circuit diagram of Figure 3, we have:
Either module, knowing that ωr = gωs:
In graphing, Is = f (ωr) is given by the following figure:
Figure 10
III-2 speed servo Realization
The principle of the speed of the asynchronous machine is servo décri by the structural diagram of Figure 11.
Figure 11
The rotor pulsation ωr is estimated using the PI controller. To determine the angular frequency of the stator currents must add the image of the rotation speed and the image of the rotor pulsation. This verifies the equation ωs = ω + ωr and realize the frequency autopilot. Since ωr is negligible ωs, the speed sensor used must be digital (incremental encoder) to have high accuracy in measuring ω.
Is = the law f (ωr) described above allows to set the amplitude of the currents of Is1réf references Is2réf Is3réf and which are generated by a voltage controlled oscillator. The control 11 is complex and is generally performed by using digital techniques.
III-3 reversible converter:
With the GTO, the PWM inverter enables today variable speed asynchronous machine order MW.
In these cases, it is necessary to provide braking of the asynchronous machine by returning energy to the power supply. For this, it is possible to replace the diode rectifier by a PWM inverter structure which functions as a rectifier when the machine operates as a motor and inverter when the asynchronous machine operates as a generator (Figure 12).
Figure 12
The network side connected chokes smoothing line currents. Network side, the PWM inverter structure allows the collection of quasi-sinusoidal currents in phase with the voltages: it optimizes the inverter power factor. Note that this is the control of the PWM network side inverter that regulates the continuous voltage E. device of this type, very recent, is used in particular for the North TGV that uses asynchronous motors. The PWM inverter connected network is expected next phase.
Before the GTO will allow the realization of PWM inverter high power, it was necessary to use thyristor converters to power variable frequency induction motor.
Association IV-current-switch asynchronous machine:
IV-1 power circuit:
The simplified diagram is given in Figure 13. The network side converter is a thyristor rectifier. By cons, regarding the UPS feeding the asynchronous machine, it is impossible to use thyristors running in natural commutation because the currents are still behind the tensions and whatever the mode of operation
Thus, you must use controlled switches blocking to realize the inverter current. It is bidirectional switches unidirectional in voltage and current: thyristor-diode-kind dual
Figure 13
To provide for the realization of switches thyristor_diode-dual-type with thyristors, there must be a forced switching circuit for locking. This leads to the structure of Figure 14 called current switch to isolating diodes. The capacitors used to apply a negative voltage across the thyristor at the time of locking.
Figure 14
The major drawback of this assembly is the presence of voltage to the stator of the asynchronous machine at the time of switching (Figure 14). These overvoltages, caused by opening of an inductive circuit (blocking control) are limited by the forced commutation capacitors. It is therefore necessary for this type of food, oversize the insulation of the stator windings.
The structure of Figure 13, however, has the advantage of being naturally reversible, when the asynchronous machine operates as a motor the thyristor bridge functions as a rectifier and the controlled bridge inverter in blocking. When the asynchronous machine operates as a generator (ωr <0), the thyristor bridge operates as an inverter and the controlled bridge blocking functions as a rectifier.
The stator currents are not sinusoidal. We must plan a downgrade of the machine (usually 10% of rated power) because of additional losses due to current harmonics. In addition, the electromagnetic torque will here also have a significant ripple (Figure 15) which can be annoying if one drives a load with low inertia.
Figure 15
IV-2 Servo control of speed- control thyristor:
The controlled current blocking inverter switches the current in the phases of the asynchronous machine. The amplitude of the currents in the phases of the machine is imposed by the current source Io realized with the thyristor valve. The converter control principle with autopilot frequency is identical to that described in paragraph III. The structural diagram of the servo is given in Figure 16.
Figure 16
Conclusion
The principles of controlling the electromagnetic torque of the asynchronous machine that just described have all been developed from the static model (Figure 2) valid only in sinusoidal steady state. This has the consequence that the electromagnetic torque is not controlled during transient.
Bibliography
Websites :
http://www.electrotech.fr.st/
http://www.cours.com
http://hydro.marseille.free.fr/liens_electricite2.htm