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A New Soft Starting Method for Wound–Rotor Induction Motor

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Journal of ELECTRICAL ENGINEERING, VOL. 62, NO. 1, 2011, 31{36 A NEW SOFT STARTING METHOD FOR WOUND{ROTOR INDUCTION MOTOR Mohammad Bagher Bannae Sharifian | Mohammad Reza Feyzi ∗ Mehran Sabahi | Meysam Farrokhifar Starting of a three-phase Induction motor using a starter rheostat in rotor circuit has some disadvantages. A new method for starting of a three-phase motor by using a parallel combination of resistors, self-inductors and capacitors in rotor circuit is proposed in this paper. The proposed method ensures the soft and higher starting torque as well as limited starting current as compared to shorted rotor method. The characteristic curves for both methods (shorted rotor and rotor with added elements) are provided. The mathematical model based on the steady-state equivalent circuit of the induction motor is expanded in frequency domain and the required computer program is prepared using an optimization method. The values for added elements to rotor circuit are calculated in such a way that minimum starting time considering current and torque limitations are achieved. K e y w o r d s: induction motors, starting methods, rotor impedance, starting torque Nomenclature equal to unity by considering the equivalent circuit of an induction motor, it is clear the input impedance which is seen by supply side, has the lowest value causes to create VLL { Line to line voltage high current which is drawn by motor during the starting fS { Line frequency period. Therefore, using the drivers to limit the starting FR { Rotor current frequency current is recommended [5{7]. Such drivers which pro- Rt { Stator resistance vide the above mentioned goals are required in order to R2 { Rotor resistance improve the performance of the induction motors. Xt { Stator reactance Xt { Rotor reactance In this paper a new method for soft starting of an in- Vt { Input phase voltage duction machine is presented. By using a parallel combi- Ra { Starting resistor nation of resistors, self-inductors and capacitors in rotor Xca { Starting capacitor reactance circuit soft and higher starting torque as well as limited XLa { Starting inductor reactance starting current as compared to shorted rotor method are Td { Induced electrical torque archived, without any external driver requirements. tst { Motor starting time !l { Steady state angular speed of the rotor N { Per-unit speed of the rotor 2 STARTING OF A SQUIRREL CAGE MOTOR S { Slip 2.1 On-line direct starting Sm { The slip at maximum torque Sl { Steady state slip A { Acceleration torque In this method stator is directly connected to the util- t ity. The current drawn by motor, depending on its design class, will be from 5 to 7 times the nominal current rating. Since this amount of current flows only for a short period 1 INTRODUCTION of time, it would not damage the squirrel cage motor, but it may cause undesirable drop in supply voltage and The starting torque which is of great importance in subsequently affects the performance of other equipment case of driving high-inertia loads is proportional to ro- connected to the same supply. tor resistance. Several studies have been done to improve starting properties of an induction motor [1{4]. It is rea- 2.2 Starting with a resistor or an inductor in sonable to have a high resistance during starting period, stator circuit then to decrease it and finally remove that from rotor circuit when motor reaches to its steady state condition. In this method a resistor or reactor is used between On the other hand, since during starting period, slip is supply and motor. In starting instant, some voltage is Faculty of Electrical and Computer Engineering, University of Tabriz, Tabriz, Iran, sharifi[email protected], [email protected], [email protected] ,[email protected] DOI: 10.2478/v10187-011-0005-3, ISSN 1335-3632 ⃝c 2011 FEI STU 32 M. B. B. Sharifian | M. R. Feyzi | M. Sabahi | M. Farrokhifar: A NEW SOFT STARTING METHOD FOR WOUND-ROTOR ::: b) Increases the starting torque; c) Improves the starting power factor. But this method has some disadvantages as follows a) Mechanical switches and corresponding problems; b) Discontinuity in starting torque; c) Sudden changes in supply current In order to improve the wound-rotor induction motor con- ditions and overcome the above-mentioned problems, us- ing a parallel combination of resistors, self-inductors and Fig. 1. Equivalent circuit of an induction motor with added ele- capacitors in rotor circuit is proposed in this paper. The ments rotor frequency given by fr = s fs is high at starting in- stant, therefore, the maximum current flows through ca- pacitor and resistor at starting instant and this increases dropped across starter resistor or reactor and only a frac- the starting torque and improves the starting power fac- tion of supply voltage is present across motor terminals, tor. As the speed of the motor increases, the rotor fre- thus the starting current is decreased. As speed of the mo- quency decreases, thus the impedance of self inductor sig- tor increases, the amount of resistor or reactor is manually nificantly decreases and effectively shorts the resistor. In decreased and when reaches its nominal speed rating the this condition, the capacitor acts as an open circuit which resistor or reactor is completely shorted out. The disad- leads to an improved operating condition. During start- vantages of this method are the need for extra equipment ing, the effective rotor resistance, is gradually decreased in order to gradually remove the resistor or reactor from which ensures the smooth starting of the motor. the circuit and low starting torque due to low starting voltage across the motor. 4 MOTOR EQUIVALENT CIRCUIT 2.3 Autotransformer starting method Since the transient components of the starting cur- rent are quickly damped as compared to starting period In this method, a fraction of supply voltage is applied and considering the fact that starting impact torque on to stator using an autotransformer. This approach de- shaft has no effect on the average accelerative torque [1], creases the current drawn by motor and the supplied cur- so using the steady state equivalent circuit is acceptable. rent. When the motor approaches to its nominal speed, Of course, the equivalent circuit is modified according to autotransformer is removed from the circuit and entire new method theory. Numerical optimization method is voltage of the supply is applied to the induction motor. used to calculate the value of added elements in order to In this method, quite less current is drawn from supply as achieve minimum starting time considering rotor current compared to previous method, but the extra equipment and torque limitations. The equivalent circuit of an in- is still required. On the other hand, the starting torque duction motor is shown in Fig. 1. In this circuit Rt , Xt is small as a result of low amount of voltage at starting and Vt are the parameters of the Thevenin's equivalent instant, so this method is not useful for high inertia loads. circuit for stator and R2 and X2 are rotor parameters. Other parameters Ra , Xla and Xca represent the added 2.4 Y{∆ starting method resistor, self inductor and capacitor respectively. In this circuit, the impedance of rotor equivalent circuit is R+jX This method is used for motors that are designed to which is calculated as follow operate with ∆ connection. The phases of stator are X2 X2 R initially Y connected using a TPTD switch and when R = ca la a ; (1) S5D motor reaches its steady state, the stator winding change ( )( ) 2 Xca − to ∆ connection. In this method the starting voltage XlaXcaR 2 Xla X = a S ; (2) across each phase is VpLL and thus the starting current S4D 3 ( )2 is lower which leads to a smaller starting torque. The ( )2 XlaXca XlaRa XcaRa extra equipment and TPTD switch are also required. D = + − ; (3) S4 S S3 Vt I2 = q (4) 3 STARTING OF WOUND{ROTOR 2 2 Xeq + Req INDUCTION MOTOR where, Xeq = Xt + X + X2 and Req = Rt + R + R2=2. The simplest and most inexpensive method for start- In order to determine the air gap and produced torque, ing of wound-rotor motor is adding a resistance to rotor the rotor current can be used as follows ( ) circuit and applying the nominal voltage to stator. The R2 2 R + s added resistance to rotor circuit Td = 3I2 : (5) !s a) Decreases the starting current; Journal of ELECTRICAL ENGINEERING 62, NO. 1, 2011 33 5 ESTIMATION OF OPTIMIZED 2) The value of the integral function is calculated for a VALUES FOR ELEMENTS wide range of Ra and Xca . This is done by using trapezoidal numerical integration. The main object of adding resistor, capacitor and self 3) The values of Ra and Xca are calculated in such a inductor to rotor circuit is improving the starting perfor- way that the integral function has its maximum value mance. Therefore the values of elements should be deter- without exceeding of current and torque limitations. mined in such a way that the best starting conditions are 4) The performance of the motor under normal operation achieved. One of the important parameters is the start- conditions with the added elements is evaluated by ing time, tst, which has to be calculated considering the checking the slip value. If any of these variables is not following conditions satisfactory the value of Xla is changed and the above 1) The starting current should never exceed the values mentioned stages are repeated until the desired results limited by utility or thermal capabilities of motor. are achieved. 2) The starting torque should never exceed the limits de- termined by the type of the load or maximum allowed shaft torque.
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