Gyuláné Vincze, Gergely György Balázs
Budapest University of Technology and Economics Department of Electric Power Engineering
Table of Contents
The induction motor has been known since almost the same time as the DC motor, but its role is increased only recently , since the drive technical features have been optimized and secured with inverter supply and field oriented control. The high power switching elements have enabled the development of the inverter technique, and high speed microcontrollers have enabled the development of the complicated control methods.
Slip-ring induction motors existed also in the past, for example the so-called Italian system vehicles powered by three-phase electrification system operating from 1902 to 1976 with rotor resistance change and mechanical brake. In addition the Ganz-Kandó-type locomotive with phase- and period shifter was a pioneering attempt that was produced with complicated rotating machine converters. The modern induction motor drive technique is a qualitative improvement.
The advantage of the induction motor application prevails at squirrel caged rotor motor without slip-rings. Comparing with the commutated vehicle drives it possesses robust design, smaller space demand and do not need any maintenance. There are some attempts for water-cooled vehicle drive too.
The field oriented control produces a revolutionary change in the improvement of the induction motor drive features.
The basic quantity of the novel control: the rotor flux vector of the induction machine, which is expressed in x-y stationary coordinate system in the following way (Fig.5.1.a):
i.e. it can be represented as a vector with ψr amplitude and α ψ angle, and can be calculated quite complicatedly.
The field oriented control is a current vector control, fixed to the calculated rotor flux direction that is generally represented in α-β coordinate system, fixed to the rotor flux vector as in Fig.5.1.b.
The field oriented control is based on the two component of the motor supplying current vector (α and β), which can be controlled separately. The i α current component is in direction of the rotor flux. The rotor flux amplitude can be controlled by i α , and the motor torque can be controlled by the perpendicular i β current component. The torque of the induction motor is determined by ϑ torque angle (see Fig.5.1.b, p *: number of the motor magnetic pole pairs):
According to the equation, if the magnitude of the rotor flux is constant, the torque depends only on the i β current component. In this case the motor behavior is similar to the behavior of the separately excited DC motor. Negative torque can be developed with negative i β component or ϑ negative torque angle. To realize the control purpose - that is defined in α-β coordinate system – for the i α and i β current components, in x-y stationary coordinate system the corresponding
current vector components ix and iy should be controlled, as in Fig. 5.1.a.
Discussion and equat ion s of the field oriented controlled induction machine
The rotor leakage eliminating modified equivalent circuit is the most suitable for the field oriented control as in Fig.5.2.a (L’ is a so-called transient inductance).
To use voltage equations that contain the stator and rotor quantities, common coordinate system shall be selected rotating with ω k speed. The voltage sources of Fig.5.2.b represent the so-called rotating voltages that depend on the coordinate system selection. With quantities interpreted in common coordinate system – that rotates with ω k speed – the Park-vector, transient equations (valid for instantaneous values) of the induction motor are as follows:
The non-measurable rotor current can be eliminated from the rotor voltage equation by substituting īr from the flux equation :
If a common, fixed to the rotor flux α-β coordinate system is selected, then:
Each quantity has α-β components. According to Fig. 5.1.b the rotor flux is fixed to the real axis, consequently the rotor flux is: , the stator current is: , the terminal voltage is: ū=uα+juβ, etc. The essence of the field oriented control can be presented from the equation (5.3), separated to α-β components. The following equation is valid for the α components of (5.3):
It shows that the rotor flux vector magnitude depends on i α component only, i β has no influence on it. The ψr magnitude can be slowly varied, it follows slowly the change of L m i α with several tenth of seconds T r0 time constant. This feature is similar to the separately excited DC motors, how its flux can be varied by the exciting current.
The torque-producing i β current component can be calculated by the β components of equation (5.3) :
According to equations (5.5), similarly to the separately excited DC motors, the Δω=ω ψ -ω speed drop is proportional to the torque-producing current (i β).
The application of the field oriented control method was difficult for a long time, because the rotor flux vector can be calculated by complicated algorithm and sufficiently high speed microelectronic devices are just recently available to solve the task. Several methods (machine model) exist to calculate ψr, α ψ , ω ψ and the m torque, depending on which quantities are the inputs of the calculations. For example, one method uses the (5.3) stator voltage equation in stationary (ω k=0) x-y coordinate system. The real and imaginary parts of equation (5.3) are:
Fig.5.3 represents the machine model that uses the 5.6 equations and the measured values of i a , i b , i c phase currents and ω rotor speed.
Similarly to the separately excited DC motors, high dynamic drive system can be achieved by field oriented controlled induction motor, if a function similar to Fig.4.6.a is defined for the rotor flux ψr amplitude. The ω≤ω 0n speed range is the normal mode without field-weakening, with nominal rotor flux. The ω>ω 0n range is the field-weakening mode, where the flux decreases hyperbolically with the speed. The ω 0n =2πf n is the nominal synchronous speed that can be reached by nominal flux and nominal voltage, for the induction machine it is: ω 0n ≈ω n. Since more voltage is not available, the flux shall be decreased for further speed increase. Therefore the field-weakening range is for the extension of the speed range that has an important role at vehicle drives. Fig.5.4.a represents the current vector ranges that ensure the maximal utilization of the rotor flux (Capital letters stand for the fundamental amplitudes.)
There are two different control ranges (I and II). The ω≤ω n (I.) range is the constant rotor flux mode: ψr=Ψrn, Iα=Iαn=Ψrn/Lm, the M torque is proportional to the I β current component. I max defines the maximum torque. The ω>ω n (II) range is the field-weakening range. If the inverter output voltage is limited to the maximum transient induced nominal voltage of the motor (nominal value: ωnΨrn=Un), then the speed can only be increased above ω n, if the rotor flux and the current decrease with the ratio of ψr=(ωn/ω)Ψrn and Iα=(ωn/ω)Iαn (minimal value: Iαmin=(ωn/ωmax)Iαn). The torque that can be reached by I max current decreases hyperbolically (Fig.5.4.b). At regenerative brake mode the control ranges are mirrored to the horizontal axis, with the difference that generally at braking mode smaller current maximum is allowed than in motor mode.
The previously described nominal rotor flux mode is sometimes replaced with energy-efficient rotor flux control at ω≤ω 0n speed range. It means that if the load is smaller than the nominal current (I≤I n), the flux is decreased (with I α component) proportionally to I β current, so that the torque angle remains nearly constant (ϑ≈ϑopt). The iron losses can be decreased by the described method, on the other hand the dynamic behaviour of the drive deteriorates. The not a significant energy saving is only justified, if long-term, low-load cruising mode is expected in the vehicle trip. Fig.5.5 presents the energy-efficient control ranges.
In the figure the range I. is the energy-efficient mode with ϑopt torque angle. At the N nominal point ψr=Ψrn, it cannot be increased further. The range II. is the constant rotor flux mode. The range III. is the field-weakening mode.
Summary of the advantages of the field oriented controlled induction motor drives :
The motor flux can be continuously controlled by the i α flux-producing current component.
The motor torque can be continuously controlled by the i β torque-producing current component in the whole speed range, even at standstill. The pull-out torque (specific to induction machines) does not exist.
The motor speed range can be safely extended by the application of the field-weakening range to ω~2ω 0n value, considering that at ~2ω 0n speed the loadability of the motor decreases, e.g. at I n current the developable torque is: M≤M n /2.
The M-ω mechanical characteristic curves of the field oriented controlled induction motor are similar to the curves of the separately excited DC motor (Fig.4.6.b), and possesses similar boundary characteristics.
Those inverters can be applied for the field-oriented control that can achieve the control purposes of the field oriented control. Basically the induction motor dive can be produced with two types of inverters:
with voltage-source inverter and
with current-source inverter.
There was a long-term debate about the advantages and disadvantages of the two solutions. Nowadays, however, almost only the voltage source inverter solutions are widespread; therefore the current source inverter solutions are briefly mentioned only.