Sincap kafesli asenkron makinalara yumuşak yolverilmesi

Abstract

Bu çalışmada, kullanılagelen yolverme yöntemleriyle karşılaştırmalar yapılarak, sincap kafesli asenkron makl- naya yumuşak yolverme incelenmiştir. Büyüyen makina güçle rinde ve şebeke kısa devre gücünün küçük olmasında önem kazanan yolverme, düzgün hızlanmanın da gerektiği endüstri yel uygulamalarda yeralmaktadır. Ayrıca, anma çalışmasına göre düşük yüklü ya da boşta çalışan asenkron makinalarda; gerilimin azaltılmasıyla, demir kayıplarının (gerilimin karesiyle orantılı) azalması, enerjinin verimli kullanılma sının önem kazandığı günümüzde, yumuşak yolvericilerln bu yönüyle de gözönüne alınmasını sağlamıştır. Gerçekleştirilen yumuşak yolvericilerle ve kullanıla gelen yolverme yöntemleriyle elde edilen gerilim, akım ve hızın zamanla değişimleri; kalkış sırasındaki enerji kaybı elde edilmiştir. Asenkron makinanin miline bağlı yük momentinin devir hızıyla değişimi bilindiğinde? düzgün bir hızlanma için rotor, stator akımı ve stator gerilimin etkin değerinin zamanla değişim ifadesi verilmiştir. Kullanılan sistemdeki asenkron makinaya ait parametreler ve yük momenti ele alın dığında, düzgün hızlanma için bulunan stator akımı ve geri liminin zamanla değişimiyle, ölçülenler uyuşmaktadır. Düşük yüklü ya da boşta çalışmalarda, stator gerilimi nin düşürülmesiyle elde edilen enerdi tasarrufu da veril mektedir.

Whenever a squirrel cage induction motor is started, the electrical system experiences a current surge and the mechanical system experiences a torque surge. With line voltage applied to the machine, the current can be anywhere between four and seven times the machine full-load current at a low power factor. The magnitude of the torque that the driven equipment will see could be in excess of 200 percent of the machine full-load torque. These current and torque surges can be reduced substantially by reducing the voltage supplied to the machine during starting. If the impedance of the supply system is low, the volt-drop caused by the large current will be negligible. Other consumers on the same supply will be unaffected. In contrast, when the supply impedance is high, an appreciable volt-drop will occur every time the machine is started, causing lights to dim and interfering with the apparatus on the same supply. The mechanical stress due to the sudden torque that has to be absorbed by the gearbox, belts, drive shafts and the driven equipment is another problem. Machines which start without significant load torque or inertia can accelerate very quickly, so high starting current is only drawn for a short period. Conversely, if the load torque and/or inertia are high, the starting may take many seconds. There are some conventional starting methods which are extensively used in large applications. Their principle is to start the squirrel cage induction machine by reducing its voltage in the beginning and to increase the magnitude of the voltage through the acceleration period. The simplest and most widely used method is Star/Delta (Wye/Mesh) starting. It ensures the windings of the machine to be connected in star in the beginning, thereby vi reducing the voltage applied to each phase by approximately 60 percent of Its direct-on-line value. Then, when the machine speed approaches its running value, the windings are switched to delta connection. The main advantage of the method is its simplicity, while its main drawback is that the sudden transition from star to delta gives rise to a second disturbance. With a star-delta starter the current drawn from the supply is approximately one third of that drawn in a direct-on-line start, but at the same time the starting torgue is also reduced to one third of its direct-on-line starting torgue. When this method is applied, the reduced torgue should be sufficient to accelerate the load, and bring it up to a speed at which it can be switched to delta without an excessive jump in the current. One of the starting methods of the sguirrel cage induction machine is to use an autotransformer. A three- phase autotransformer is usually used where it is necessary to reduce the starting current to a value less than that necessary with star-delta starting. If the reduced voltage is chosen so that a fraction % of the line voltage is used to start the machine,- the starting torgue is reduced by x2 times its direct-on-line value, and the current drawn from the lines is also reduced to x2 times its direct value. As with the star-delta starter, the torgue per ampere is the same as that for a direct start. The switchover from the starting tap to the full voltage results in mechanical and electrical stresses to the machine. In large machines the transient overvoltages caused by switching can be enough to damage the insulation. By inserting three resistors or inductors of appropri ate value in series with the machine, the starting current can be reduced by any desired value. With large machines, this approach using resistors will be very undesirable in respect to energy losses. Besides these conventional starting methods? there is an important method known as "The solid state soft- starting". In this method, there are some arrangements on the AC power set. The most widely-used arrangement includes three pairs of back-to-back thyristors connected in series with the three phase supply lines. The basic principle of the operation is as follows. When the power supply is connected to the thyristor bridge and when a run command is given to the control circuit, the triggering angle is gradually reduced by the control vi i voltage applied to the firing circuit. In this basic method, the current and speed of the machine are not taken into consideration. R»- So- To- Figure 1. Basic soft-start system In this study three modes of soft start control were realized. The first one is the voltage ramp control with current limit level. The ramp time is adjustable. In the beginning of the starting, the voltage must be in a value that can give the breakaway torque required by many mechanical systems. This starting value of the voltage on induction machine is known as "The pedestal voltage". v" Pedestal Voltage ? Figure 2. Voltage ramp vi i In this method, the machine current is taken into consideration in addition to the voltage. The information on the values of the stator current and the stator voltage are used in a closed loop control. As the stator voltage is increasing, the stator current can be limited to 2 to 5 times the machine full-load current. The second method is the current ramp control. This mode of operation is similar to that of the voltage ramp mode, but the stator current is increased gradually instead of the stator voltage. The third type of the implemented soft-starter is known as the speed control. To use a soft starter in the speed control, it is necessary to fit a tachogenerator to the machine shaft to provide a speed feedback to a control loop to maintain a linear rate of change of speed during machine starting, independent of the load. Also, by controlling the input of the ramp circuit, the machine can be made to run continuously at any speed between standstill and full speed, but this method of speed control has a low efficiency. "tc^9 »Speed Controller.V *o- current Controller Speed feedback Firing Circuit Current / -i- T- J fceAbccA '" V -J* 3~ Tacho Figure 3. Soft starter by controlling the speed The linear acceleration mode is widely used in some applications as textile, special material handling and conveyors. In some cases the replacement of a DC motor- drive by a squirrel-cage motor is allowed. It is also used in process machines which must be accelerated in a precise time, such as wire drawing machines. The required waveform of the rotor and stator current and the required waveform of the stator voltage for a linear acceleration are given in chapter 6. The rate of energy losses in all three types of soft starters and in the conventional methods of the direct-on line and wye-delta are given in chapter 6. One can see that the current has a peak in the beginning of starting in direct-on-line method. In Wye-Delta starting there are current peaks in the beginning and when it has been switched from wye to delta connection. Solid state starters have the capability to reduce no- load or light-load machine losses by sensing the machine load and reducing the voltage applied to the machine during these conditions. Without this feature the machine would draw the maximum magnetising current regardless of load. For the "machine used in this thesis, the energy which can be saved when the machine is at no-load or light load is given in chapter 6.