Parameter Estimation of an High Performance I.M. Drive for Sensorless Operation

Parameter Estimation of an High Performance I.M. Drive for Sensorless Operation

Monika Dixit (Sagar Institute of Research, Technology & Science, India) and Smita Shandilya (Sagar Institute of Research, Technology & Science, India)
DOI: 10.4018/978-1-4666-9911-3.ch016
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Abstract

A modern AC motor drive is a very intelligent system which covers a wide range of different electro technical apparatus and a wide scope of electrical engineering skills. A modern AC motor drive consists of four closely acting main parts: the AC machine, the power electronics, the motor control algorithm and the control hardware, i.e. the signal electronics. The advances in semiconductors and microelectronics have made the rapid development of AC motor drives possible. Semiconductors used in the switching converters provide the electric energy processing capability and microcontrollers and digital signal processors provide the data processing power for complex control algorithms.
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1. Introduction

In previous days 1950s nearly DC motors were used in all industrial applications because of its easy speed control. The use of induction motor is very limited normally these motors have a constant speed depending on the frequency of the supply and the no of windings. In the past it was not possible to control the speed of the induction motors according to the need. That’s why their use was limited and despite having many a motors they advantages over dc motors they could not be used because of this disadvantage. But at the field of drivers have improved due to the availability of thyristor or SCRthyristors, power transistors, IGBTs and GTOs the variable speed induction motor drives have been invented. Though the cost of these drivers are more than dc driver, but still the use of induction motors are increasing and they are replacing dc motors because of their advantages. Now days with increase in technology modern AC motor drives performance is increases comparable to DC motors. This improvement in performance of AC motor drive is achieved due to the advancement of power electronics and microprocessors.

Takahashi and Nagouchi have reported a new type of control technique for ac motor drives. This technique controls both the stator flux linkage and the torque directly, and is different from the vector control technique in which, these variables are controlled via the stator currents. Takahashi and Nagouchi have named this new control technique as direct torque control (DTC). Almost simultaneously, Depenbrock has also proposed the same type of control technique and named it as direct self-control (DSC). The DTC method was developed as an alternative and fast control method for an induction motor drive.

In a direct torque control induction motor drive, the basic concept is to control both stator flux and electromagnetic torque of the machine simultaneously with the help of torque control loop, flux control loop and speed control loop. In comparison with the conventional Vector-controlled drives, in a DTC-based Drive the only requirement is the knowledge of stator resistance, and thus the sensitivity associated to parameter variations is reduced. Moreover, the DTC-based drives do not require fulfilling the coordinate transformation between stationary frame and synchronous frame. Thus the complexities of the algorithms involved in a Field Oriented control are minimized. DTC algorithm is more efficient and easiest technique among all available techniques.

For the implementation of DTC algorithm, requires an independent control of electromagnetic torque and flux with two independent control loops. Hence the parameters which are to be measured or estimated for the implementation of DTC algorithm are,

  • 1.

    Stator flux (estimated)

  • 2.

    Electromagnetic torque (estimated/measured)

  • 3.

    Rotor speed (estimated/measured)

  • 4.

    Motor phase currents (measured)

  • 5.

    Stator phase voltage (estimated/measured)

  • 6.

    DC link voltage (measured)

There is a strong interdependency among these variables. For example the flux of an induction motor drive can be estimated with the help of motor back emf, which in turn depends upon stator phase voltage, phase current and stator resistance. The mathematical expression for stator flux in stationary reference frame can be given by (1) – (3) and (1) where

(2)
(3)

Thus for the estimation of stator flux in the required variables are phase voltage, phase current and stator resistance.

Similarly, the electromagnetic torque of an induction motor drive can be either sensed or can be estimated. Estimation of electromagnetic torque rather than its measurement by any torque transducer makes a DTC drive more cost effective and less complex. The electromagnetic torque in stator reference frame is given by (4)

(4)

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