Improved Direct Torque Control Induction Motor Drive · PDF file Direct torque control for...

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  • rrr r

    ss s

    s

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    Improved Direct Torque Control Induction Motor Drive

    Dr. M. V. Aware Assistant Professor,

    Electrical Engineering Department,

    VNIT, Nagpur (India).

    Dr. S. G. Tarnekar Principal, GHRCE,

    Nagpur (India).

    Jagdish G. Chaudhari Lecturer, Electrical

    Department,GHRCE, Nagpur (India).

    jagdish_chaudhari123@rediffmail.com

    Abstract-This Paper describes sinusoidal pulse width modulation of the voltage impressed to the stator of an induction motor for direct control of torque and stator flux. Instantaneous voltage vectors applied by an inverter have redundancy characteristics which provide some flexibility for selecting the inverter switching modes. By Using this switching freedom, control is achieved according to the following properties; high speed torque control, regulation of the primary flux, minimization of the inverter switching frequency. This utilizes a constant hysteresis band for both developed torque and stator flux and indirectly maintains the switched stator voltage waveforms averaged over a switching period to sinusoidal as in SPWM technique. It improves the dynamic performance of the machine compared to the conventional speed control of induction motor drives. A simulation programme has been developed to verify the results. The inverter duty cycle can then be calculated using the space vector PWM technique. The proposed method is very promising for rapid torque control which is quite different from FOC (Field Orientation Control). These instructions give you basic guidelines for preparing papers for conference proceedings.

    I. INTRODUCTION

    High dynamic performance of induction motor drives is indispensable in many applications of today’s automatically controlled machines. Induction motor control has attracted much attention recently in the power electronics field. Field- oriented control has been developed, enabling an ac motor to attain dynamic responses as rapid as for dc motor [2]. The principle of field-oriented control is based on Fleming’s law, which describes the interaction force between fluxes and currents. Many papers have reported the problems associated with compensating various parameters. The current-controlled inverters typically used in the field- oriented drive system develop output waveforms which do not compare favorably with those of the voltage-controlled inverter. The current controlled inverter often causes increased motor harmonic losses and acoustic noise during steady-state operation [5].

    This paper proposes new control schemes based on the principle of direct torque control, which can be considered a basic law of torque generation in the induction motor. It makes possible both fast torque response and high- efficiency control at the same time.

    II. NEED FOR DIRECT TORQUE CONTROL

    Inverter fed induction ,motors are increasingly being used in general applications varying the input voltage to the motor with frequency on open loop is one of the very simple and

    popular methods of speed control. In this method V/F is held constant. In steady state operation, the machine air gap flux is approximately related to V/F. As the frequency approaches zero near zero speed, the magnitude of the stator voltage also tends to zero and this low voltage is absorbed by the stator resistance. Therefore at low speed of operation the stator resistance drop is compensated by injecting an auxiliary voltage so that rated air gap flux and full load torque becomes available up to zero speed. At steady state operation, if the load torque is increased, the slip will increase within the stability limit and a balance will be maintained between the developed torque and the load torque. However, if the supply voltage to the inverter (which is obtained by rectifying the input AC supply) fluctuates, the air gap flux will vary. Also, increase in the stator resistance with temperature results in the variation of air gap flux. Hence, in constant V/F control scheme the air gap flux may drift and as a result the torque sensitivity with slip frequency (or stator current) will vary. If the correct V/F ratio is not maintained the flux may be weak (or may saturate). If the air gap flux decreases, slip frequency )1( sω will increase for the same torque demand. Also response of the machine detoriates, hence a speed control scheme with independent control of torque and flux loop is desirable. DTC is one such method of speed control.

    III. INDUCTION MOTOR MODEL

    (1)

    (2)

    Induction motor is modeled by its voltage equations in stator co-ordinates for both stator and rotor as follows:

    where,

    is the stator voltage

    is the stator flux linkage

    is the stator current � �

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    � �

    � � �

    � =

    � �

    � � �

    � =

    sq

    sd s

    sq

    sd s

    q

    d s

    i

    i i

    V

    V V

    ψ ψ

    ψ

    1-4244-0549-1/06/$20.00 ©2006 IEEE.

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    t sL

    EV Is Δ

    ′ −=Δ

    is the rotor flux linkage

    is the rotor current

    rω is the rotor speed in electrical radian/sec. Rs is the stator resistance Rr is the rotor resistance

    Equation (1) is for stator voltage and (2) for rotor voltage. As the rotor winding is short circuited, the rotor voltage is zero in magnitude [3]. The induced emf is compensated by the resistive drop and the rotational voltage. Again, stator and rotor flux linkages are related to the stator and rotor currents as follows:

    (3)

    (4)

    Ls is the stator self inductance Lr is the rotor self inductance Lo is the mutual inductance

    These two phase variables are obtained from three phase values using the following matrix:

    (5)

    Torque developed within the machine can also be expressed as a cross product of stator flux and current as following:

    (6)

    Considering stator flux vector and rotor flux vector to be independent state variable, equation (3) and (4) can be modified as following:

    where,

    The torque expression can also be written in terms of these two flux signals as following:

    (7)

    (8)

    where, � is the space angle between these two flux vectors.

    IV. DIRECT TORQUE CONTROL PRINCIPLE

    Induction motor torque control has traditionally been achieved using Field Oriented Control (FOC). This involves the transformation of stator currents into asynchronously rotating d-q reference frame that is typically aligned to the rotor flux. In this reference frame, the torque and flux producing components of the stator current are decoupled. A PI controller is then used to regulate the output voltage to achieve the reputed stator current and therefore torque. This PI controller limits the transient response of the torque controller.

    Direct Torque Control (DTC) uses an induction motor model to achieve a desired output torque. By using only current and voltage measurements, it is possible to estimate the instantaneous stator flux and output torque [1]. An induction motor model is then used to predict the voltage required to drive the flux and torque to demanded values within a fixed time period. This calculated voltage is then synthesized using space vector modulation (SVM).

    The stator flux vector, sψ , and the torque produced by the motor, Tem , can be estimated using (9) and (10) respectively. These only require knowledge of the previously applied voltage vector, measured stator current, and stator resistance.

    (9)

    (10)

    Once the current stator flux magnitude and output torque are known, the change required in order to reach the demand values by the end of the current switching period can be determined. An equivalent circuit of the induction motor in a stationary d-q reference frame is shown in Fig. 1. over a short time period, the change in torque is related to the change in current and from the equivalent circuit equation (11) can be obtained. The voltage E can also be determined by using the stator flux and current vectors.

    (11)

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    =� �

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    c

    b

    a

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    d

    f

    f

    f

    f

    f

    2/32/30

    2/12/11

    3

    2

    � �

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    � =

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    rd r

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    i

    i i

    ψ ψ

    ψ

  • ( )( ) ( ) tVtrV

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    tP T

    sIsss

    s s

    em

    Δ⋅=Δ−=Δ

    −× ′

    Δ=Δ

    ψ

    ψ 22

    3

    Figure 1. Equivalent circuit of an induction motor in a d-q reference frame.

    By combining (10) and (11), an expression for the change in torque can be obtained as shown in (12). Equation (9) can also be rewritten a