Method of braking a vector controlled induction machine, control device for carrying out the method and storage medium

• US 6,326,762 B1
• Filed: 08/18/2000
• Issued: 12/04/2001
• Est. Priority Date: 08/18/1999
• Status: Expired due to Term

• Current Assignee: Weg Automacao S/A (Wpa Participações e Serviços SA)
• Original Assignee: Weg Automacao S/A (Wpa Participações e Serviços SA)
• Inventors: Holtz, Joachim, Jiang, Jinsheng
• Primary Examiner(s): IP, SHIK LUEN PAUL
• Application Number:
  US09/641,533
• Time in Patent Office: 473 Days
• Field of Search: 318/799-812, 318/720, 318/701, 318/798, 318/759, 318/375, 318/370, 318/757, 180/65.1, 180/65.6, 363/34, 363/37, 363/41, 363/36, 363/35
• US Class Current: 318/811
• CPC Class Codes: H02P 21/36 Arrangements for braking or slowing; Four quadrant control
• CPC Class Codes: H02P 3/18 for stopping or slowing an ac motor

Abstract

A method and a control device for braking a variable speed vector controlled induction machine are driven by a pulse width modulation (PWM) inverter, in which the q-current component and the d-current component of the stator current are controlled independently from one another in accordance with a first reference signal (i_q*) and a second reference signal (i_d*), respectively. For braking, high frequency components are superimposed on the second reference signal (i_d*), and the root-mean-square (rms) value (i_{d rms}) and the average value (i_{d av}) of the resultant second reference signal are controlled independently from one another such that the field requirements are met by the average value and high machine losses are produced by the rms value.

Claims

1. A method of braking a variable speed vector controlled induction machine driven by a pulse width modulation (PWM) inverter, wherein the q-current component and the d-current component of the stator current are controlled independently from one another in accordance with a first reference signal (i_q*) and a second reference signal (i_d*), respectively, wherein the method comprises steps that perform the acts of:

(a) measuring or estimating actual rotor speed (ω

) of the induction machine,

(b) obtaining a flux value (Ψ

_r) representing an estimation of either actual stator flux or actual rotor flux,

(c) receiving a braking signal indicating a need for braking torque if the actual rotor speed (ω

) is higher than a reference speed (ω

*),

(d) superimposing, in response to the braking signal, high frequency components on said second reference signal (i_d*), and

(e) controlling the root-mean-square (rms) value (i_{d rms}) of said second reference signal obtained from step (d) in accordance with a difference between the absolute value of said reference speed (ω

*) and that of the speed obtained in step (a), while

(f) controlling the average value (i_{d av}) of said second reference signal obtained from step (d) independently from step (e) in accordance with a difference between a reference flux (ψ

_r*) and the flux value (Ψ

_r) obtained in step (b).

2. The method of claim 1, wherein step (e) comprises limiting said rms value to a predetermined maximum (i_max) as determined by the current limit of the PWM inverter.

3. The method of claim 2, wherein step (d) comprises generating a square wave carrier signal of a period smaller than the rotor time constant (τ

_r), and steps (e) and (f) comprise modulating the pulse width and pulse amplitude of said carrier signal such that the amplitude becomes i_{d rms} and the duty ratio d becomes $d=\frac{1}{2}<br><br>\left(1+\frac{i_{d<br><br>\text{\hspace{1em}<br><br>}<br><br>\mathrm{av}}}{i_{d<br><br>\text{\hspace{1em}<br><br>}<br><br>\mathrm{rms}}}\right)$

wherein i_{d av} represents the average value and i_{d rms} represents the rms value of the said modulated carrier signal.

4. The method of claim 1, wherein step (d) comprises generating a square wave carrier signal of a period smaller than the rotor time constant (τ

_r), and steps (e) and (f) comprise modulating the pulse width and pulse amplitude of said carrier signal such that the amplitude becomes i_{d rms} and the duty ratio d becomes $d=\frac{1}{2}<br><br>\left(1+\frac{i_{d<br><br>\text{\hspace{1em}<br><br>}<br><br>\mathrm{av}}}{i_{d<br><br>\text{\hspace{1em}<br><br>}<br><br>\mathrm{rms}}}\right)$

wherein i_{d av} represents the average value and i_{d rms} represents the rms value of the said modulated carrier signal.

5. The method of any one of claims 1 through 4, further comprising:

(g) controlling said first reference signal (i_q*) in such a way that the DC voltage (u_d) at the DC side of said PWM inverter does not exceed a predetermined maximum voltage (u_{d max}).

6. A control device for carrying out a method of braking a variable speed vector controlled induction machine, comprising:

a rectifier having an output,

a direct current (DC) link capacitor coupled to the output of said rectifier,

a pulse width modulation (PWM) inverter coupled between said DC link capacitor and said induction machine,

a controller for said PWM inverter, said controller having a first input terminal for receiving a first reference signal (i_q*) for the q-current component of the stator current, and a second input terminal for receiving a second reference signal (i_d*) for the d-current component of the stator current,

a terminal for receiving a first signal (ω

) representing actual rotor speed of the induction machine or an estimation thereof,

an observer for obtaining a second signal (ψ

_r) representing an estimation of either actual stator flux or actual rotor flux of the induction machine,

a speed controller for generating said first reference signal (i_q*) in response to a difference between a reference speed (ω

*) and said first signal,

a flux controller for generating a third reference signal (i_{d av}) in response to a difference between a reference flux (ψ

_r*) and said second signal (ψ

_r),

a terminal for receiving a braking signal indicating a need for braking torque if the actual rotor speed (ω

) is higher than a reference speed (ω

*),

a loss controller for generating a fourth reference signal (i_{d rms}) in response to a difference between the absolute value of said reference speed (ω

*) and that of said first signal (ω

), and

a signal generator responsive to said braking signal for generating said second reference signal (i_d*) including high frequency components, and having its rms value (i_{d rms}) controlled in accordance with said fourth reference signal, and its average value (i_{d av}) independently controlled in accordance with said third reference signal.

7. The device of claim 6, wherein said signal generator comprises:

a pulse width and pulse amplitude modulator responsive to said third and fourth reference signals for generating a square wave carrier signal having a period smaller than the rotor time constant (τ

_r) and being pulse width modulated to have a duty ratio d of $d=\frac{1}{2}<br><br>\left(1+\frac{i_{d<br><br>\text{\hspace{1em}<br><br>}<br><br>\mathrm{av}}}{i_{d<br><br>\text{\hspace{1em}<br><br>}<br><br>\mathrm{rms}}}\right)$

 

wherein i_{d av} represents said third reference signal and i_{d rms} represents fourth reference signal, such that the average value of the pulse width modulated carrier signal corresponds to said third reference signal and its rms value to said fourth reference signal, and

a switch responsive to said reference speed (ω

*) and said first signal (ω

) for providing, as said second reference signal (i_d*), either said modulated carrier signal or said third reference signal (i_{d av}), said modulated carrier signal being provided when the absolute value of said reference speed (ω

*) is lower than that of said first signal (ω

), while, otherwise, said third reference signal (i_{d av}) is provided.

8. The device of claim 7, further comprising a first limiter for limiting the magnitude of said fourth reference signal to a predetermined maximum (i_max) as determined by the current limit of the PWM inverter.

9. The device of claim 8, further comprising:

a second limiter for limiting said first reference signal (i_q*) output from said speed controller to a positive and a negative limit value, said second limiter being responsive to a control signal for adjusting said negative limit value in accordance with said control signal, and

a voltage controller receiving a reference voltage (u_{d max}) representing the maximum allowable value of the DC link voltage across said DC link capacitor and the actual value (u_d) of the DC link voltage, and generating said control signal such that negative values said first reference signal are limited by said second limiter such that the actual value of the DC link voltage (u_d) never exceeds said reference voltage (u_{d max}).

10. The device of claim 9, further comprising a third limiter for limiting said control signal to a predetermined maximum (i_{q max}).

11. The device of claim 7, further comprising:

a second limiter for limiting said first reference signal (i_q*) output from said speed controller to a positive and a negative limit value, said second limiter being responsive to a control signal for adjusting said negative limit value in accordance with said control signal, and

a voltage controller receiving a reference voltage (u_{d max}) representing the maximum allowable value of the DC link voltage across said DC link capacitor and the actual value (u_d) of the DC link voltage, and generating said control signal such that negative values said first reference signal are limited by said second limiter such that the actual value of the DC link voltage (u_d) never exceeds said reference voltage (u_{d max}).

12. The device of claim 11, further comprising a third limiter for limiting said control signal to a predetermined maximum (i_{q max}).

13. The device of any one claims 7 through 12, wherein said pulse width modulator comprises:

an oscillator for generating a triangular signal (u_Δ

) of said predetermined frequency,

a first multiplier for multiplying said triangular signal with said fourth reference signal (i_{d rms}) thereby generating a modulated triangular signal,

a first adder for adding said third reference signal to the output signal of said first multiplier,

a comparator receiving the output signal of said first adder and outputting a positive signal of unity magnitude in response to a positive input signal, and a negative signal of unity magnitude in response to a negative input signal, and

a second multiplier for multiplying the output signal from said comparator with said fourth reference signal.

14. The device of claim 13, wherein said switch comprises:

a first limiter receiving said fourth reference signal and having a transfer function y=f(x), $\begin{array}{cc}y=0& \mathrm{for}<br><br>\text{\hspace{1em}<br><br>}<br><br>x><br><br>0,\\ y=-x& \mathrm{for}<br><br>\text{\hspace{1em}<br><br>}<br><br>0\ge <br><br>x><br><br>x_{\max }\\ y=x_{\max }& \mathrm{for}<br><br>\text{\hspace{1em}<br><br>}<br><br>x\le <br><br>x_{\max },\end{array}$

 

wherein x_max represents a predetermined maximum (i_max) of said fourth reference signal as determined by the current limit of the PWM inverter,

said first multiplier multiplying the output signal of said first limiter with said triangular signal (u_Δ

),

a second adder for obtaining the difference between the output signal of said first limiter and said third reference signal (i_{d av}),

a fourth limiter receiving the output signal from said second adder, said fourth limiter having a transfer function y=f(x)

y=0 for x<

0

y=x, otherwise, and

a third adder for adding said third reference signal (i_{d av}) to the output signal of the third limiter, the output signal of the third adder being applied to said second multiplier to be multiplied with the output signal of said comparator.

15. The device of claim 14, wherein said speed controller, flux controller, loss controller, voltage controller, pulse width and pulse amplitude modulator, and switch are implemented by a program controlled microprocessor.

16. The device of claim 13, wherein said speed controller, flux controller, loss controller, voltage controller, pulse width and pulse amplitude modulator, and switch are implemented by a program controlled microprocessor.

17. The device of claims 7 or 8, wherein said speed controller, flux controller, loss controller, pulse width and pulse amplitude modulator, and switch are implemented by a program controlled microprocessor.

18. The device of any one of claims 9, 10, 11, or 12, wherein said speed controller, flux controller, loss controller, voltage controller, pulse width and pulse amplitude modulator, and switch are implemented by a program controlled microprocessor.

19. A machine readable medium carrying a program of instructions for execution by a device to perform a method for braking a variable speed vector controlled induction machine driven by a pulse width modulation (PWM) inverter, wherein the q-current component and the d-current component of the stator current are controlled independently from one another in accordance with a first reference signal (i_q*) and a second reference signal (i_d*), respectively, wherein the method comprises steps that perform the acts of:

(a) measuring or estimating actual rotor speed (ω

) of the induction machine,

(b) obtaining a flux value (Ψ

_r) representing an estimation of either actual stator flux or actual rotor flux,

(c) receiving a braking signal indicating a need for braking torque if the actual rotor speed (ω

) is higher than a reference speed (ω

*),

(d) superimposing, in response to the braking signal, high frequency components on said second reference signal (i_d*), and

(e) controlling the root-mean-square (rms) value (i_{d rms}) of said second reference signal obtained from step (d) in accordance with a difference between the absolute value of said reference speed (ω

*) and that of the speed obtained in step (a), while

(f) controlling the average value (i_{d av}) of said second reference signal obtained from step (d) independently from step (e) in accordance with a difference between a reference flux (ψ

_r*) and the flux value (Ψ

_r) obtained in step (b).

20. The machine readable medium of claim 19, wherein step (e) comprises limiting said rms value to a predetermined maximum (i_max) as determined by the current limit of the PWM inverter.

21. The machine readable medium of claim 20, wherein step (d) comprises generating a square wave carrier signal of a period smaller than the rotor time constant (τ

_r), and steps (e) and (f) comprise modulating the pulse width and pulse amplitude of said carrier signal such that the amplitude becomes i_{d rms} and the duty ratio d becomes $d=\frac{1}{2}<br><br>\left(1+\frac{i_{d<br><br>\text{\hspace{1em}<br><br>}<br><br>\mathrm{av}}}{i_{d<br><br>\text{\hspace{1em}<br><br>}<br><br>\mathrm{rms}}}\right)$

wherein i_{d av} represents the average value and i_{d rms} represents the rms value of the said modulated carrier signal.

22. The machine readable medium of claim 19, wherein step (d) comprises generating a square wave carrier signal of a period smaller than the rotor time constant (τ

_r), and steps (e) and (f) comprise modulating the pulse width and pulse amplitude of said carrier signal such that the amplitude becomes i_{d rms} and the duty ratio d becomes $d=\frac{1}{2}<br><br>\left(1+\frac{i_{d<br><br>\text{\hspace{1em}<br><br>}<br><br>\mathrm{av}}}{i_{d<br><br>\text{\hspace{1em}<br><br>}<br><br>\mathrm{rms}}}\right)$

wherein i_av represents the average value and i_{d rms} represents the rms value of the said modulated carrier signal.

23. The machine readable medium of any one of claims 19 through 22, wherein the method further comprises:

(g) controlling said first reference signal (i_q*) in such a way that the DC voltage (u_d) at the DC side of said PWM inverter does not exceed a predetermined maximum voltage (u_{d max}).