WO2018092049A1

WO2018092049A1 - Method for controlling an electric drive

Info

Publication number: WO2018092049A1
Application number: PCT/IB2017/057156
Authority: WO
Inventors: Pietro De Filippis
Original assignee: SPAL Automotive SRL
Current assignee: SPAL Automotive SRL
Priority date: 2016-11-16
Filing date: 2017-11-16
Publication date: 2018-05-24
Anticipated expiration: 2019-05-16
Also published as: IT201600115824A1

Abstract

A method for controlling an electric drive comprising a step of measuring the angle φ between the voltage Vf applied at least to a phase of an electric motor forming part of the electric drive and the respective phase current If, a step of indirectly measuring the phase current If in the phase and, given a vector diagram of an equivalent circuit of the phase of the electric motor, a step of calculating the angle γ between the counter- electromotive force Emf and the phase current If defining a reference system comprising an axis X having direction of the current phase If and an axis Y at right angles to the axis X.

Description

DESCRIPTION

METHOD FOR CONTROLLING AN ELECTRIC DRIVE

Technical field

This invention relates to a method for controlling an electric drive and in particular a method for controlling a sinusoidal electric drive (known in the trade as AC) in automotive applications.

By way of non-limiting example, this specification describes an electric drive comprising a three-phase brushless motor with permanent magnets that generates a sine-wave counter electromotive force (c.e.m.f.) for driving electric fans and/or electric pumps. Background art

The electric fan and electric pump applications require, generally, minimum acoustic noise and reduction both of energy consumption and costs.

These requirements have led to the adoption of sine-wave c.e.m.f. brushless motors (AC brushless motors) driven by inverter capable of generating sine-wave currents and making obsolete the use of PWM six- step driven trapezoidal c.e.m.f motors (more commonly known as DC brushless motors).

The sine waveform of the c.e.m.f. and of the related phase current minimizes active torque ripple (virtually zero), thus reducing mechanical vibrations and acoustic noise.

It is also known that it is possible to minimize current draw to generate a certain drive torque, thereby maximizing electromechanical conversion efficiency through an optimum method for control or drive of AC brushless motors which are normally driven by current-controlled, impressed voltage inverters. To obtain this type of drive, the static switches must change state in such a way that, instant by instant, the polar axis of the rotor magnetic field remains at 90 electrical degrees to the polar axis of the magnetic field generated by the current circulating in the stator windings, whatever the torque supplied and the rotation speed.

To obtain continuously information about the angular position of the rotor, costly sensors are normally used, such as, for example, absolute encoders resolvers or Hall effect sensors.

The presence of the position sensors makes the drive relatively expensive and, therefore, various drive strategies have been developed which do not require the sensors, known in the trade as "sensoriessf, precisely to reduce the costs of the drives.

In order for the dynamics of the machine driven not to be too great - and it is the case of electric fans and electric pumps - it is possible to apply an optimum criterion, based on the fact that the polar axis of the rotor magnetic field remains, instant by instant, at 90 electrical degrees to the polar axis of the magnetic field generated by the current circulating in the stator windings: the drive operates in such a way that the c.e.m.f. and the phase current are kept synchronised, in each point in the operating field (torque, rotation speed, Dc supply voltage).

The "sensorless" drives which implement drive strategies based on the above-mentioned criterion are based on the reading of electrical quantities (e.g. voltages at the motor terminals, current circulating in the motor windings) with the aim of:

· detecting the passing through zero of the c.e.m.f. and of the current; calculate the relative phase between c.e.m.f. and current;

implement, lastly, appropriate methods of driving the inverter static switches which tend to keep the two quantities in phase.

Two examples of known methods for controlling a "sensorless" drive are described in patent documents EP2195916 and EP2719071 in the name of the same Applicant and are substantially based, on the one hand, on the measurement of the peak value of the phase current and of the angle Φ between supply voltage and phase current and, on the other hand, on the resistive drop in the phase, to be considered negligible.

More specifically, in the above-mentioned control methods an approximate expression is obtained of the optimum angle δ_opt of advance of the voltage Vs applied to a phase of the motor, with respect to the c.e.m.f. Es in the same phase.

Considering the resistive drop in the phase to be negligible (which is more negligible the higher is the efficiency of the motor) and indicating with KE the c.e.m.f. constant, which, as is known, is a function of the temperature of the magnets comparable with the temperature T inside the motor, measured in V/rpm and p being number of poles, then:

Considering, therefore, the resistive drop Rsls to be negligible with respect to Es and the tangent of the advance angle δ_opt which can be approximated with the angle itself, it is assumed that the advance angle δορΐ depends linearly only on the phase current Is, that is to say:

The above-mentioned values gives the angle δ from an expression which only applies, in an exact fashion, in the operating conditions of maximum efficiency of the motor, that is to say , δ= Φ.

In other prior art solutions, based on the same approximation, that is to say, considering the phase current always in phase with the counter- electromotive force and without the resistive contribution being considered negligible, the angle δ is calculated directly as:

The algorithms used in the above-mentioned drives are therefore approximate and, as mentioned, guarantee an optimized operation only in the so-called "normal operating condition", that is to say, around the optimum operating point of the machine.

As the operation gradually moves away from this ideal condition, the errors deriving from the above-mentioned approximations adopt significant values which no longer allow a knowledgeable control of the motor as the actual operating conditions become unknown.

In this context, the main technical purpose of this invention is to provide a method for controlling an electric drive which is free of the above- mentioned drawbacks.

Disclosure of the invention

An aim of this invention is to propose a method for controlling an electric drive which is more robust and accurate the prior art solutions.

Another aim of this invention is to provide a control method which allows a power and maximum speed control on a minimum supply voltage.

A further aim of this invention is to provide a control method which allows responsiveness and stability of the drive to be obtained even at relatively low sped and which requires a greater precision in estimating the angle γ. Another aim of this invention is to provide a control method which allows stability of the drive to be obtained and the possibility of providing feedback even at a minimum speed lower than the current ones.

The technical purpose indicated and the aims specified are substantially achieved by a method for controlling an electric drive which has the characteristics set out in independent claim 1.

Brief description of drawings

Further features and advantages of the present invention are more apparent in the description below, with reference to a preferred, non- limiting embodiment of an electric drive control method as illustrated in the accompanying drawings, in which:

Figure 1 is a block diagram of an electric drive controlled with a control method according to this invention, with some parts cut away for greater clarity;

Figure 2 illustrates an equivalent circuit of a phase of an AC brushless motor forming part of the drive of Figure 1 ;

Figure 3 illustrates a vector diagram of the circuit of Figure 2. Detailed description of preferred embodiments of the invention

With reference to Figure 1 , the numeral 1 denotes an example of drive which can be controlled with the method according to this invention.

It should be noted that the block diagram of Figure 1 does not show many parts of the drive 1 , in particular those which may operationally be considered as prior art, for example the user control interface for the analogue or digital external inputs, that is, for the commands which control the speed or the frequency of the drive 1.

The electric drive 1 comprises an electric motor 2 with permanent magnets, a three-phase bridge or inverter 3 for powering the electric motor 2, a direct current stage 4 for powering the inverter 3 and a controller 5 for controlling the inverter 3.

Figure 2 illustrates the circuit model of a phase U of the motor 2 and Figure 3 illustrates the vector diagram of the electrical quantities relating to this diagram.

As illustrated in Figure 2, each of the three windings is characterised by the relative resistance R and its synchronous inductance Ls.

If is the phase current, having a sinusoidal form, Vf is the phase or applied voltage and Emf is the counter-electromotive force, which has a sinusoidal form, and is due to the rotation of the rotor with permanent magnets.

In the vector diagram of Figure 3, fully known in the prior art, the vectors and the angles indicated represent respectively: If the phase current;

Vf the applied voltage;

Emf represents the c.e.m.f. induced in the stator winding;

γ is the angle between the counter-electromotive force and current and adopts positive when the current Se is in advance relative to the counter electromotive force Emf;

φ is the angle between the phase current and the applied voltage and it is positive when the applied voltage Vf is in advance relative to the phase current If;

δ is the angle between counter-electromotive force and the applied voltage and it is positive when the applied voltage Vf is in advance relative to the counter-electromotive force Emf.

Conveniently, the modules of the vectors Emf, Vf, If are identified as the peak values of the quantities to which they refer.

The above-mentioned controller 5 is an extremely simple and economical acquisition/processing device, equipped with a data storage memory, and it is described only in terms of the technical features necessary for understanding this invention.

The drive 1 comprises a block 6, preferably analogue, for measuring the peak value of the current If of a first phase U of the electric motor 2, of the low-cost type, and a block 7, preferably analogue, for measuring the passage through zero of the current If of the phase U, both in communication with the controller 5.

In the embodiment illustrated by way of example, the block 6 for measuring the peak value of the phase current If receives as input, in a known manner, a voltage signal extracted from the bridge 3 and returns as output an analogue signal whose level is directly proportional to the amplitude of the phase current If.

The block 6, which is illustrated schematically, is, for example, described in more detail, in patent document EP2195916 which is incorporated herein by reference for completeness of description. The controller 5 comprises a modulator 8, of substantially known type, which generates the signals for controlling the bridge 3.

The controller 5 comprises a digital block 9 which, together with the block 7 defines an analogue/digital stage 10 for measuring the angle φ between the voltage Vf applied to the electric motor and the phase current If.

The digital block 9 is in communication with the above-mentioned modulator 5 to receive as input a digital signal the high-low transition of which identifies the transition through zero of the voltage Vf applied to the first phase U of the electric motor 2.

The digital block 9 measures the duration of the interval of time between the occurrence of the passage through of the voltage Vf applied to the first phase U and the occurrence of the passage through zero of the current in the first phase U and calculates the angle φ.

An example embodiment of the block 10 is described in more detail in patent document EP2719071 which is incorporated herein by reference and for completeness of description.

As schematically illustrated in Figure 1 , the controller 5 comprises a block 11 for calculating the angle γ between the phase current If and the counter-electromotive force Emf in communication with the blocks 6 and 10.

The controller 5 comprises a block 12 which provides to the calculation block 11 the value of ωel, electrical pulsation of the drive 1 , and the value of Emf which is known given the speed of rotation of the motor (or its electrical pulsation) and the temperature of the magnets, which for practical purposes is assumed to correspond to the temperature Tamb_drive inside the motor.

As is known, in fact, Emf=K(T)ωel where K(T) is a machine parameter which is a function of the operating temperature.

In this regard, the controller 5 is in communication with a sensor 13 which provides to the block 12 the value of the temperature Tamb_drive which is assumed to correspond to that of the magnets of the rotor. According to the invention, the calculation of γ occurs by assuming, with particular reference to Figure 3, a reference system comprising an axis X having direction of the phase current If and an axis Y at right angles to the axis X.

Projecting the vectors of the diagram on the axis Y gives:

where:

Preferably, all the values of the variable contained in the above-mentioned expression are estimated at the moment of passage of the phase current through zero.

Advantageously, the value of Vf is known, since it is the voltage applied to the phase of the motor, the values of φ and If are measured, for example as mentioned above, the values of ωel and Emf are known so an exact estimation is obtained of the angle γ between counter-electromotive force and the phase current in the entire interval of operating conditions of the drive 1.

Basically, the factors of the above-mentioned expression are obtained both from the measurement of some physical quantities and from processing based on the values of the characteristic electrical parameters of the motor 2 suitably stored in a memory of the controller 5 which implements the control loops.

The variables are photographed at a predetermined instant, that is, at the passage through 0 of the phase current, which allows the behaviour of the drive in the entire electric cycle to be extrapolated.

The new "observer state", defined by the assumptions of this invention, at the passage through zero of the phase current, allows, as mentioned, the precise estimation of the angle γ. With respect to the prior art solutions, instead of calculating the angle γ presuming a value of δ, on the basis of the approximations adopted in the prior art, this control γ allows the direct calculation of and therefore the control of δ, where δ=γ+φ with φ measured.

The maximum torque developed by the synchronous motor with permanent magnets, once the applied voltage Vf is fixed, reaches the maximum for a predetermined angle

The more accurate estimation of the angle δ compared with the prior art solutions also makes it possible to control the motor bringing it closer to the above-mentioned limit angle.

In one embodiment, for example, the controller 5 imparts a δmax given by Κδlim where K is a constant less than 1 , for example 2/3.

The precise estimation of the angle γ allows, for example, the motor to be accelerated, if required as in the case of particularly rarefied ambient air, even beyond the optimum operational conditions.

The control method is more robust and accurate than the prior art solutions.

The precise estimation of the angle γ allows a power and maximum speed control on a minimum supply voltage.

The precise estimation of the angle γ allows responsiveness and stability of the drive to be obtained even at relatively low speeds and allows feedback to be obtained even at a minimum speed lower than the current ones.

Claims

1. A method for controlling an electric drive comprising an electric motor, the method comprising:

a step of measuring the angle φ between the voltage Vf applied to a phase of the electric motor and the respective phase current If,

a step of indirectly measuring the phase current If in the phase, the method being characterised in that, given a vector diagram of an equivalent circuit of the phase of the electric motor, the angle γ between the counter-electromotive force Emf and the phase current If is calculated defining a reference system comprising an axis X having direction of the phase current If and an axis Y at right angles to the axis X.

2. The method according to claim 1 , wherein the angle γ is calculated by the equation:

where

ω_el is known, being the electrical pulsation of the drive,

Emf is known, given the speed of rotation of the motor and the operating temperature Tamb_drive.

3. The method according to claim 1 or 2, wherein the step of measuring the angle φ comprises a step of measuring the passages through 0 of the phase current If.

4. The method according to claim 2, wherein the values of the variables contained in the expression for calculating γ are estimated at the moment of passage of the phase current through zero.

5. The method according to claim 2, wherein the angle δ between counter-electromotive force Emf and the applied voltage Vf, given by the sum of the angle φ between the voltage Vf applied to a phase of the electric motor and the respective phase current If and the angle γ between the counter-electromotive force Emf and the phase current If, that is to say, δ = φ+γ , is limited above a maximum value δmax given by Κδlim where K is a constant less than 1 , for example 2/3, and δ_lim is given by with Rs (T) expressing the phase resistance as a

function of the temperature T.