Direct Torque Control Essay

Torque ripple reduction for Direct Torque Control of voltage source inverter fed Induction motor drive
Rajasekaran.P1, V.Jawahar Senthilkumar2.
1 Research Scholar, Jawaharlal Nehru Technological University Hyderabad-85 and Associate Professor,Department of EEE, Aarupadai Veedu Institute of Technology,Paiyanoor-603104.
2Asst.Professor, Department of ECE, College of Engineering, Guindy, Anna University, Chennai-25
Corresponding Author e-mail: [e-mail]

ABSTRACT: This paper presents analysis of recently used direct torque control (DTC) techniques for voltage source inverter-fed induction motors to overcome torque ripple reduction in motor steady state operation. In classical Direct Torque Control (DTC), the torque and stator

One of the approaches is to increase the number of voltage vectors applied in a sampling period, using some sorts of pulse width modulation (PWM). Among the various techniques, a simpler one is to use two voltage vectors: a nonzero one, applied for a portion of the sampling period, and the null vector for the rest. The duty ratio must be calculated each sample period, and by varying it between its intense values, it is possible to apply more voltage levels to the motor, according to the desired torque variation. In [5], an analytical online algorithm calculates the optimum duty ratio each sampling period, by using a torque ripple minimization condition, which is based on ripple equations. However, this algorithm requires high computational effort and additional motor parameters to be known. In [6] the duty ratio value is provided by a new fuzzy logic module, whose inputs are the stator flux position, the electromagnetic torque and an input defining the motor operating point, given by the speed and the torque values. This algorithm involves expert knowledge and needs the rotor speed
This paper proposes a simple solution for reducing the torque ripple in classical Direct Torque Control, while preserving the good dynamic and structural simplicity of this scheme. The proposed method consists in the modulation of the nonzero voltage vector duration

The instantaneous values of the stator flux and torque are calculated from stator variable by using a closed loop estimator [1]. Stator flux and torque can be controlled directly and independently by properly selecting the inverter switching configuration.

Figure 1.Basic direct torque control Scheme for AC motor Drives
III.CLASSICAL DIRECT TORQUE CONTROL SCHEME

In a symmetrical three phase induction machine, the instantaneous electromagnetic torque is proportional to the cross vectorial product of the stator flux linkage space vector and stator current space vector. Te=3/2ps*is (1)  - stator flux linkage space vector Is – stator current space vector
By considering that s-I s exp(js) (2)
s- angle of the stator flux linkage space vector with respect to the direct –axis of the stator reference frame . Is=I Isexp(js) (3)
In eqn Te=3/2 p I si I Is I sin(s-s)n (4) =3/2 p I s I I Isi