DIRECT TORQUE CONTROL OF AN INDUCTION MOTOR

INTRODUCTION

• Three phase Induction motors are simple,rugged,low cost and easy to maintain.• They run at essentially constant speed from zero to full load.Therefore they are the most frequently encountered in the industry• A 3-phase induction motor has two main parts: A stator – consisting of a steel frame that supports a hollow,cylindrical core of stacked laminations. Slots on the internal circumference of the stator house the stator winding. A rotor – also composed of punched laminations, with rotor slots for the rotor winding.

OPERATING PRINCIPLE•Operation of 3-phase induction motors is based upon the application of Faraday’s Law and the Lorentz Force on a conductor.

•Consider a series of conductors (length L) whose extremities are shorted by bars A and B. A permanent magnet moves at a speed v,so that its magnetic field sweeps across the conductors.

•Now,close the ladder upon itself to form a squirrel cage, and place it in a rotating magnetic field .This is an Induction Motor.

The following sequence of events takes place:

1.A voltage E = BLv is induced in each conductor while it is being cut by the flux (Faraday’s Law).

2. The induced voltage produces currents which circulate in a loop around the conductors (through the bars).

3. Since the current-carrying conductors lie in a magnetic field, they experience a mechanical force (Lorentz force).

4. The force always acts in a direction to drag the conductor along with the magnetic field.

OPERATING PRINCIPLE

Locked rotor: When the rotor is stationary, the field rotates at afrequency (relative to the rotor) equal to the supply frequency. Thisinduces a large voltage – hence large currents flow within the rotor,producing a strong torque.Acceleration: When released, the rotor accelerates rapidly. As speedincreases, the relative frequency of the magnetic field decreases.Therefore, the induced voltages and currents fall rapidly as themotor accelerates.Synchronous speed: The relative frequency of the rotating field is zero,so the induced currents and voltages are also zero. Therefore, thetorque is zero too. It follows, that induction motors are unable toreach synchronous speed due to losses such as friction.Motor under load: The motor speed decreases until the relativefrequency is large enough to generate sufficient.

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TORQUE-SPEED CHARACTERISTICS

1. The induced torque is zero at synchronous speed. Discussed earlier.

2. The curve is nearly linear between no-load and full load. In this range, the rotor resistance is much greater than the reactance, so the rotor current, torque increase linearly with the slip.

3. There is a maximum possible torque that can’t be exceeded. This torque is called pullout torque and is 2 to 3 times the rated full-load torque.

4. The starting torque of the motor is slightly higher than its full-load torque, so the motor will start carrying any load it can supply at full load.

5. The torque of the motor for a given slip varies as the square of the applied voltage.

6. If the rotor is driven faster than synchronous speed it will run as a generator, converting mechanical power to electric power.

Contd...

COMPLETE TORQUE-SPEED CHARACTERISITCS OF INDUCTION MOTOR

MOTORING MODE: 0<S<1For this range of slip,torque developed is in the direction in which the rotor rotates. Torque is zero at s=0. The torque has maximum value,called the breakdown torque at slip Smax. The motor halt if it is loaded with more than the breakdown torque. At S=1,the torque corresponds to the starting torque.

GENERATING MODE:S<0Negative slip implies rotor running at super-synchronous speed.

BRAKING MODE:S>1The motor runs in opposite direction to the rotating field,absorbing mechanical power which is dissipated as heat in the rotor copper

VARIABLE FREQUENCY ELECTRIC DRIVESControls the rotational speed of an alternating current (AC) electric motor by controlling the frequency of the electrical power supplied to the motor.

sourcePower

modulatormotor load

controllersensors

INVERTERS•An inverter is an electrical device that converts direct current (DC) to alternating current (AC)

•There are 2 modes of operation in a three phase inverter:

•18o degree

•120 degree

VECTOR TORQUE CONTROL

•Vector control (also called Field-Oriented Control FOC) is one method used in variable frequency drives to control the torque (and thus finally the speed) of three-phase AC electric motors by controlling the current fed to the machine.

•In 1972,Blaschke introduced the principle of field orientation to realize dc motor characteristics in an induction motor drive.

•In the vector control mode, the cage motor drive is linearised and it behaves like a fully compensated separately excited dc motor.

•Stator currents are resolved into two orthogonal components which produce the flux and the torque in the motor.

Direct torque control (DTC) is one method used in variable frequency drives to control the torque (and thus finally the speed) of three-phase AC electric motors.

This involves calculating an estimate of the motor's magnetic flux and torque based on the measured voltage and the current of the motor.

DIRECT TORQUE CONTROL

METHOD OF DTC..•Stator flux linkage is estimated by integrating the stator voltages.

•Torque is estimated as a cross product of estimated stator flux linkage vector and measured motor current vector.• •The estimated flux magnitude and torque are then compared with their reference values.

• If either the estimated flux or torque deviates from the reference more than allowed tolerance, the transistors of the variable frequency drive are turned off and on in such a way that the flux and torque will return in their tolerance bands as fast as possible

• Thus direct torque control is one form of the hysteresis control.

•The direct torque method performs very well even without speed sensors.

CONTINUED…

DISADVANTAGES…

VECTOR CONTROL• Greater parameter

sensitivity

• Commissioning problems with the setting up of current control loops.

• Complex architecture

DIRECT TORQUE CONTROL• Possibility of loss of flux

control at low speeds/loads

• Higher torque/current ripple

• Uncontrolled current transients

Comparision b/w DTC & VC

PROBLEMS WITH DTC•The problems associated with the DTC drives are:-

•Variable switching frequency due to the hysteresis comparators used for the torque and flux comparators•This problem can be easily done away with adopting space vector modulation.•This technique is adopted to maintain a constant average switching frequency.

•Inaccurate stator flux estimations which can degrade the drive performance.•The inaccurate flux approximation can be avoided by using a low pass filter instead of using an integrator because of which the problem of drift arises.

STATE SPACE MODULATIONThe topology of a three-leg voltage source inverter is as shown-.

• A Voltage Source Inverter can assume only eight distinct topologies.• Six out of eight topologies produce a non-zero output voltage and are known as zero switching states.

Topologies…

VOLTAGE SPACE VECTORSSVM for three-leg VSI is based on the representation of the three phase quantities as vectors in a two-dimensional plane.Considering topology 1 line voltages Vab ,Vbc, and Vca are given by-

The effective voltage vector generated by this topology is represented as V1

•Similarly the six non-zero voltage vectors (V1 - V6) can beshown to assume the positions . The tips of these vectors form regular hexagon.

•These last two topologies are represented as vectors which have zero magnitude and hence are referred to as zero-switching state vectors or zero voltage vectors.• The vectors V1-V8 are called the switching state vectors.

The desired three phase voltages at the output of the inverter could be represented by an equivalent vector V rotating in the counter clock wise direction.

The magnitude of this vector is related to the magnitude of the output voltage.

THEORETICAL BACKGROUND IM PHASE EQUATIONSBy neglecting ,hysterisis,eddy currents and saturation of the magnetic circuit,the IM equations are-

Where, k = the winding p=pole-pair number

IM SPACE VECTOREQUATIONSThe IM flux voltage equations,written in terms of space vectors related to rotating d,q reference frame common to stator and rotor,become

DTC operation is usefully analysed by writing the stator equations in a d,q reference frame fixed to the rotor,i.e by

BASIC PRINCIPLE OF DTC

• The only quantity driven by inverter is the stator voltage space vector.

• The stator voltage space vector commands practically the variation of stator flux space vector.

• Any variation of the stator flux space vector leads to a variation of the torque due to both the amplitude variation and the phase angle variation between the stator and rotor flux space vectors.

BASIC DTC BLOCK DIAGRAM

Dq0 transformationDirect–quadrature–zero (or dq0 or dqo) transformation or zero–direct–quadrature (or 0dq or odq) transformation is mathematical transformation used to simplify the analysis of three-phase circuits. In the case of balanced three-phase circuits, application of the dqo transform reduces the three AC quantities to two DC quantities.

Definition:The dqo transform applied to three-phase currents is shown below in matrix form:

The inverse transform is:

Matlab Model till now…..