tailieunhanh - Chapter 5: Scalar control methods

This chapter introduces two-inductance, T and T', per-phase equivalent circuits of the induction motor for explanation of the scalar control meth-ods. The open-loop, Constant Volts Hertz, and closed-loop speed control methods are presented, and field weakening and compensation of sup and stator voltage drop are explained. Finally, scalar torque control, based on decomposition of the stator current into the flux-producing and torque-producing components, is described. | 5 SCALAR CONTROL METHODS This chapter inttoduces two-inductance r and r per-phase equivalent circuits of the induction motor for explanation of the scalar conttol methods. The open-loop Constant Volts Hertz and closed-loop speed control methods are presented and field weakening and compensation of slip and stator voltage drop are explained. Finally scalar torque control based on decomposition of the stator current into the flux-producing and torqueproducing components is described. TWO-INDUCTANCE EQUIVALENT CIRCUITS OF THE INDUCTION MOTOR As a background for scalar conttol methods it is convenient to use a pair of two-inductance per-phase equivalent cữcuits of the induction motor. They differ from the three-inductance cữcuit introduced in Section which can be called a T-model because of the configuration of inductances see Figure . Introducing the transformation coefficient y given by y ệ- 93 94 CONTROL OF INDUCTION MOTORS the T-model of the induction motor can be transformed into the so-called r-model shown in Figure . Components and variables of this equivalent circuit are related to those of the T-model as follows 1. Rotor resistance referred to stator Rr y2Rr 2. Magnetizing reactance Xs. 3. Total leakage reactance XL yX s 7 . 4. Rotor current referred to stator Ir ị y 5. Rotor flux Ar yAr The actual radian frequency Op of currents in the rotor the induction motor is given by Or 5 0. This frequency subsequently called rotor frequency is proportional to the slip velocity Osl as wr Pp osl. FIGURE The r equivalent circuit of the induction motor. CHAPTER 5 SCALAR CONTROL METHODS 95 Taking into account that R s 7 Rw j p current R in the T-modei can be expressed as ÍR I rI Ă A x Kr V Tr Or Z 1 where Tr Ll Rr. Symbol denotes the total leakage inductance Ll XL o . The electrical power Pelec consumed by the motor is f elec 3 4 and the mechanical power Pmech can be obtained from Pelec by subtracting the .