Electricity: motive power systems – Synchronous motor systems – Armature winding circuits
Reexamination Certificate
2001-01-17
2001-11-06
Ro, Bentsu (Department: 2837)
Electricity: motive power systems
Synchronous motor systems
Armature winding circuits
C318S801000, C363S040000
Reexamination Certificate
active
06313599
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a method for controlling an inverter which comprises the switching components to be controlled, the method comprising the steps of determining a current vector for the inverter output; determining a flux vector for the inverter load; determining an angular speed for the inverter load; and determining a resistance for the inverter load.
Inverters are used for transferring power between DC and AC circuits. In a voltage-controlled inverter, DC voltage is typically converted to AC voltage by using semiconductor power switches. Depending on the structure of the inverter employed, AC voltage consists of voltage pulses of one or more levels because the switches have only two states, a conductive one and a blocking one. The output voltage of the inverter is generated by applying modulation, and a modulator is a device used for forming the inverter switch position commands at a given moment.
Most AC applications have an active voltage source on the AC voltage side, the voltage source usually comprising one or more rotating electric machines. An electric network can also be described using an equivalent generator, because the voltage in the network consists of voltage generated by several generators together. The operation of rotating electric machines is based on a magnetic flux generated in a stationary stator and a rotating rotor. The term often used in connection with electric machines is “coil flux”, fluxes being usually generated, irrespective of the type of the machine, from currents flowing in coils. Alternatively, either the stator flux or the rotor flux can also be generated using permanent magnets. The electromagnetic state of electric machines is determined by the flux and the torque of the machine.
A multi-phase system is currently analysed almost without exception with space vectors. A vector formed of phase variables and presented as a complex number allows the machine to be controlled as an entity, instead of separate phase-specific adjustments. Space vectors can be shown using either their real or imaginary portions, or, alternatively, their length and phase angle. The vectors rotate in relation to a stationary reference coordinate system, i.e. the real and imaginary axes of a complex plane, at an angular speed &ohgr; determined by frequency. Instead of stationary reference coordinate system, the calculation is often carried out using a coordinate system rotating at a specific speed. For example, in a coordinate system rotating at the angular speed &ohgr;, ordinary AC variables are shown as direct components, the control thus being simple to implement.
The control or adjustment of rotating machines, i.e. those forming a rotating field, is based on making the control of torque and flux independent of each other. The control of the flux aims at maintaining a desired length for the flux vector of the air gap formed by the stator, the rotor, or a combination thereof in the air gap between the rotor and the stator, i.e. at a constant in a normal situation. The torque, in turn, is proportional to the angle between the stator and rotor flux vectors.
The main purpose of network inverters is to maintain the power transferred between DC and AC circuits at a desired level by controlling the effective power and the idle power separately. The network inverter is controlled in order to provide sinusoidal phase currents, which reduces harmonic waves in the network current.
The modulation of inverters is most often based either on pulse width modulation (PWM) or on two-point control. In pulse width modulation the switch position commands are formed using a separate PWM modulator, which receives the amplitude and frequency command for the output voltage from a higher control level of the inverter. In its simplest form, pulse width modulation can be carried out in a phase-specific manner on the basis of a sine-triangle comparison. By comparing sinusoidal phase voltage commands of a desired frequency and amplitude with a triangular carrier wave, an output voltage formed of voltage pulses is obtained, the average of the voltage changing sinusoidally. Instead of a phase-specific implementation, the PWM modulator can also be implemented using vector modulation based on the space vector of the output voltage. However, the main weakness of a separate PWM modulator is that the control is slow.
Another commonly applied principle is to modulate the inverter by using two-point control. When two-point controllers are used, the modulation is formed as a by-product of the control algorithm to be used, without a separate modulator. Depending on the variable to be controlled, each turn of the switch is made either on the basis of a real value calculated directly on the basis of either a measured variable or measured variables. The operational principle of the two-point control is to keep the real value always close to the reference value. Whenever needed, the switches are turned so that the real value again starts to approach the reference value. In the simplest case the switches are turned directly on the basis of whether the real value is lower or higher than the reference value. In addition, a specific variation range, or hysteresis, accepted for the real value is often determined around the reference value, the switches being then turned only when the deviation of the real value from the reference value exceeds the accepted range of variation. Hysteresis thus allows the number of switch turns needed to be reduced. The only requirement set to two-point control is that every switch control action changes the value of the variable to be controlled to the right direction.
The currently employed inverter modulation methods based on the use of two-point controllers can be divided into different two-point controls of current and to direct control of torque. The most simple method to implement two-point control of current is a phase-specific control. The controllers are typically provided with sinusoidally varying phase current commands with which the measured phase currents are then compared. The main problem in phase-specific implementation is that the star point is usually not connected in applications employing inverters. In other words, the phase currents are not independent of each other, and the phase current may change even if the switches associated with the phase concerned were not turned. For this reason, current control is preferably carried out by controlling the space vector formed of phase currents because the currents can then be controlled as an entity.
In direct control of torque the flux of the machine and its torque are controlled directly in a stationary coordinate system associated with the stator coils. The flux control aims at maintaining the absolute value of the stator flux at a constant, and the torque is controlled by controlling the speed of rotation of the stator flux. Each turn of the switch is performed on the basis of the instantaneous values of the flux and the torque. Next, the voltage vector to be used, i.e. a specific combination of the inverter switch positions, is selected using the two-point controllers such that the stator flux and the torque remain within the hystereses defined around the reference values.
The change in the magnetic flux with respect to time is known to induce voltage. In connection with electric machines, a change in the flux vector thus induces in the coils a counter-electromotive force {overscore (e)}, which can be mathematically expressed as a time derivative of the flux vector &ohgr; using the following equation:
e
_
=
ⅆ
ψ
_
ⅆ
t
.
(
1
)
As already stated above, space vectors can be represented with their length and phase angle. The following equation is obtained for the flux vector:
{overscore (&psgr;)}=&psgr;e
j&ohgr;t
, (2)
where &psgr;=flux length, i.e. an absolute value
&ohgr;=angular speed
t=time
e=neper (≈2.718)
j=imaginary unit (={square root over (−1+L )})
The vector length and the phase angle ar
Heikkila Samuli
Norrkniivilä Mikko
ABB Industry OY
Dykema Gossett PLLC
Ro Bentsu
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