Realisation of the mechanical characteristic of a series excited DC machine in the drive with converter – fed induction machine

A control algorithm for converter-fed induction machines, which allows the operation of the drive along of mechanical characteristic similar to the characteristic of series-excited direct current machine especially suitable for adhesion traction drives, is presented.


Introduction
Converter-fed induction machines are at present time in the modern energy economical traction drives widely used. In that application, which is still in continuous development are problems in the mechanical part of the drive observed, [1,2,3,4], [8], which are connected with well-known problems of slip and slide dynamic states. The problems are still waiting for final solution. The comparison of properties of old fashioned drives with series-excited DC motor characteristic allows showing of the principal difference between the series and the shunt or separately excited motors. The induction machine belongs to the second group of electrical machines. The rapid change of the developed torque on the mechanical characteristic of induction machine in the region of normal operating points, generates an enormous dynamical stress, not only for electrical, but for the mechanical part of the traction channel, too. Especially difficult, for the control circuits, is to try to avoid, the suddenly change from motoring operation into the braking operation, due to change of wheel speed of the drive. The suddenly sign change of the developed torque, reduce the traction force of the drive and cause overload and fast destruction and devastation of the mechanical parts of the drive.

Presentation of the "series excitation" control method of induction machine
The analytical description of the relations used in the control method resulting in creation of series-excited machine properties is based at modified equivalent circuit of induction machine. The classical Kloss equivalent circuit doesn't specially support the main control quantities allowing simple derivation of the relevant relations which are valid for a series-excited mechanical characteristic of induction machines. The modified equivalent circuit of the machine is shown at  Presented equivalent circuit is used to emphasise the main role of the flux linkage  of the stator winding in the analytical description of induction machines. The value of stator flux linkage) which is a function of converter output voltage fundamental harmonic and pulsation in stator and rotor, allows simple description of voltage drops in rotor equivalent circuit. In the rotor part of modified equivalent circuit, a voltage sink has been introduced. The voltage sink allows explaining the energy flow in the induction machine. The power consumed by the voltage sink is equal to the mechanical power of the machine. The voltage sink can be understood as the equivalent source of the mechanical power, current I2 in the voltage sink as equivalent of torque and voltage UZ2 as equivalent of angle velocity of the machine shaft. The equivalent circuit in Fig. 1 can be analytically described with following U1/ I1 immittance. 1 1 The stator susceptance is described by equation: Admittance of the rotor equivalent circuit is described by equation: Three important voltage drops in the equivalent scheme of induction machine depends in very characteristic way of the stator flux linkage  The voltage on magnetic part of equivalent circuit, without voltage drop on stator resistance R1 depends on flux and additionally of the stator voltage pulsation  Voltage on the rotor equivalent voltage sink depends of angle velocity of the machine shaft : Voltage drop on rotor passive elements depends on the slip pulsation: Stator current can be described by equation: And rotor current achieves the value: The only chance to improve the behaviour of induction motor drives is to approach of the characteristic of series-excited direct current machine. In that machine, the increase of the angle speed of rotor does not lead to the change of torque sign. Such DC drives do not lose the ability to pull. The solution of the presented problems, observed in converter traction drive systems, seem to lie in the change of the present time used control algorithm of converter-fed induction machines. The mechanical behaviour similar to the series-excited DC machines can be generated by converter-fed induction motor drives too. There is also the important, additional possibility to change the inclination of the mechanical characteristic in any way. The equation [6] describing the change of the torque along the "series" characteristic of a converter-fed induction machine, as a function of stator and rotor frequency, by constant value of stator voltage is as follow: The position of the induction machine operating point is controlled by |I1|=const., =const. or  and |I1| = var: • |u| -the module of amplitude of stator voltage; • p -number of magnetic poles of stator windings. Comparison of the characteristics || = const. and |I1| = const. carried out on the Figure 4 highlights  Properties of the mechanical characteristic of induction machine driven with constant value of flux linkage: • torque reversal possible even with small change of wheel speed; • high dynamic loading of the mechanics; • sudden medium frequency relief and load the elastic elements of the coupling and transmission; • great steepness of the mechanical characteristic; • significant reduction of the force transmitted from the motor to the rails in slip and slide condition; • lower load and minor wear on the wheels of the vehicle; • large values of derivation  Properties of the mechanical characteristic of the induction machine operated along a series mechanical characteristic of DC motors: • no change in the sign of the drive torque even with a larger increase of the rotor angular speed; • even in the slip or slide conditions, the drive, which is in the driving mode remains in the driving and the drive which is in the braking operation remains in the brakes; • reduction of the dynamic stress of elastic elements in power transmissions channel; • small steepness of the mechanical characteristic; • larger wear of the wheels, but the vehicle can pull better; • small values of derivation 0,1 2 Ms

Mn -Nominal value of torque
Stator frequency in Hz 1·Mn 0.5·Mn 0·Mn -1·Mn -0.5·Mn Nm  Fig. 8. Slip conditions in motoring state of the traction drives. The reduction of the torque starts as reaction on the rotor angular velocity increase of the induction machine. The change of the torque with control method which is keeping constant value of the rotor frequency is presented. The frequency controller increases the stator frequency to avoid suddenly change of rotor frequency. The controller adds the previous constant value of rotor frequency to the mechanical frequency of the rotor. Due to the simultaneously occurring reduction of stator current in function of stator frequency, the torque decreases faster than by constant current control method.

Conclusion
The described control method can be implemented in vehicles with multi converter-fed drive systems with induction machines. Such drive systems are commonly used in traction system with adhesion power transmission.
The control method allows the realisation of mechanical characteristic of separately-fed induction machine drives similar to the mechanical characteristic of series-excited direct current motor.
During occurring of fast slip or slide condition in the drive, the sign of the torque developed through the machine does not change.
The drive can stationary be operated in the range of macro slip.
The presented method is based on common control of the stator voltage fundamental harmonic which is equal for all motors of the vehicle. The value of the amplitude of the voltage is derived in suitable way from the value of vehicle speed.
The method uses the well-known concept of operating point control trough appropriate control of stator frequency, but the frequency control is made for each motor completely separate.
Each drive is running with own mechanical velocity and with own separately controlled stator frequency.
The stator frequency control circuit can, as far as possible, change the stator frequency with use of one the algorithms listed below: 1) with constant value of stator current module, 2) with constant value of rotor frequency, 3) with custom defined simultaneously change of stator current value or/and rotor frequency.
The described control method is based on fast measurement of 3 phase current module and of the mechanical frequency which is commonly used for description of the velocity of machine shaft.
In multi drive systems the recognition of slip and slid conditions of individual axes is not necessary.
Only in the drive with single motor or in the case of slip of all wheels in multi drive systems it is necessary to recognise on time the hazardous dynamical state.
In the starting state of the drive and in very low stator frequencies where f1 < ~4·f2N, the value of stator current and the value of the torque is controlled directly by stator voltage.
The traction properties of the adhesion drive which are controlled by the presented algorithm will be essentially improved in comparison to the control methods used today.
The state of development in the field of traction drive control is displayed in the specified references.