GB2418993A

GB2418993A - A method for determining parameters of an induction motor

Info

Publication number: GB2418993A
Application number: GB0422446A
Authority: GB
Inventors: Robert Carter
Original assignee: Siemens AG
Current assignee: Siemens AG
Priority date: 2004-10-09
Filing date: 2004-10-09
Publication date: 2006-04-12
Anticipated expiration: 2024-10-09
Also published as: GB0422446D0; GB2418993B

Abstract

A method of determining parameters of an induction motor comprises: applying a first voltage to an induction motor system for a first time period, then applying a second (different) voltage for a second time period and analysing the transient current behaviour during the said periods to determine parameters of the induction motor. The parameters determined may include stator resistance, leakage inductance, magnetising inductance, rotor resistance, magnetising current and the voltage lost in associated power control semiconductors. The transient current curve characteristics over the voltage step periods mentioned above may be analysed to derive the above mentioned parameters. Various current, voltage and resistance inter-relationships are used to determine these paramenters.

Description

24 1 8993

A METHOD FOR DETERMINING PARAMETERS OF AN

INDUCTION MOTOR

This invention relates to a method and a controller for s determining parameters of an induction motor.

Induction motors have been adopted for a wide-range of applications requiring motive power. Their advantages over DC motors is that they are brushless and hence have a longer life. One to drawback is that they require more complicated control arrangements if the speed of the motor is to be varied.

Recently, vector control theory has been applied to the control of induction motors. Vector control theory allows the separate control Is of the magnetic flux and a torque component of the motor using a speed sensor and a suitably programmed microprocessor. The control results in a motor which is more efficient than a DC motor. In a way, the control is similar since it is usual for the magnetic flux to be controlled to provide a fixed value and to vary the torque producing current value.

In order to successfully apply vector control to an induction motor the drive system must know key data about the behaviour of the motor.

A prior art method for determining the motor data has employed an AC drive, including a voltage source inverter with motor current sensing, to stimulate the motor, to measure its response to the stimulus and to derive much of the required data.

The parameters of the motor that are required are: stator resistance Rs, leakage inductance Lsig, magnetising inductance Lmag, rotor resistance Fir, magnetising current Imag and the power lost in the power semiconductors Vigbt. A complex sequence of stimuli is required to determine all these parameters.

to The present invention arose in an attempt to provide a simpler way of determining the parameters.

According to the invention there is provided a method for determining parameters of an induction motor comprising: Is determining a voltage drop Vigbt lost in associated power control semiconductors by: applying a first voltage for a first time period; applying a second voltage for a second time period; interpolating the curves to derive Vigbt.

The invention also provides apparatus.

A specific embodiment of the invention will now be described, by way of example only, with reference to the drawing in which: Figure 1 shows a vector controlled inverter motor control system in accordance with the invention; - 2 Figure 2 shows a control circuit used in the system shown in figure 1; and Figure 3 is an explanatory diagram.

s As is shown in figure 1, an induction motor control system 1 comprises a power supply 2 connected to mains 3 and providing three phase power via lines 4 to an induction motor 5. The system 1, further includes a control circuit 6 coupled by lines 7 to an inverter 8 of the power supply 2. The control circuit 6 is also coupled to current to measurement sensors 9, adjacent the lines 4, to detect the current flowing therein.

The power supply 2 also includes a rectifier 10 and a capacitor 11 and it is controlled by the control circuit 6 in accordance with Is vector control theory in a manner well known in the art.

The control circuit 6 is shown in greater detail in figure 2. It includes a processor 12 and associated memory 13. The processor 12 is programmed to provide the required functionality to perform the go vector control and, in addition, the functionality required by the method in accordance with the invention.

In order to apply the vector control, the control circuit 6 has to stimulate the motor 5 in a predetermined manner and to monitor the us motor's response. This takes place during a test phase which is periodically carried out. - 3

The test method carried out will now be described with reference to figure 3. In a first step 30, the control circuit 6 instructs the power supply 6 to apply a DC voltage step to the motor 5 via lines. This results in a current in the lines which is detected by the current sensors 9.

The motor current applied is a two stage waveform by two exponential functions held in memory. Figure 4 shows the waveform for an ideal motor whose T1 and T2 are the first and the second to exponential functions. Figure 5 shows a typical current waveform for a typical induction motor rather than the ideal one depicted in figure 4. It will be seen that T2 is corrupted. The departure function ideal is caused by the magnetic saturation of the motor.

Is From the first current level 11, the processor 12 is able to determine Rr + Rs where Rr is the resistance of the rotor of the motor and Rs is the resistance of the stator. From the final current level 12, the processor is able to determine Rs using the relationship V=IR.

The first time constant of the function T1 is used to determine Lsig where Lsig is the leakage inductance, by the relationship Lsig = T1 * (Rr + its). - 4

Thus in step 31 the parameters Rr, Rs and Lsig are determined.

The next step 32 is to apply zero volts to the motor 5. This reduces the previously saturated current level swiftly to an s unsaturated level. In step 33, the time constant for the current decay is determined by the processor 12 monitoring, the current sensor output during the fall from the saturated condition to the unsaturated level. From the time constant in step 34, the processor 12 determines Imag.

The process in more detail requires 1) application of 2 volts for 5 sees resulting in the current level rising to 11.

2) Application of 4 volts for 5 sees resulting in the current Is level rising to 12.

3) Interpolating the curves to derive the voltage drop IGBTs (Vigbt). Having derived Vigbt in step 3 this voltage is applied to the motor for 1 ms which ensures the upper gate drive bootstrap capacitors are fully charged.

so 4) Sufficient voltage V is applied for the motor nominal current to pass through the required measurement phase in voltage steps Vstep = I nominal * Rs. The current level s is thus high enough for accurate measurement and ensures magnetic circuit saturation which enables an accurate Imag reading.

5) dl/dt is determined for successive samples and after the s current has reached a maximum the time for dl/dt to reduce to l/e of the maximum value determined from the number of samples. Interpolation is used to do this as T1 is only four or five samples long.

6) A time of five times T1 is waited for the first time constant to to time out. An interpolation process is then carried out to determine the current 11 at T1.

7) Rr + Rs is then determined from the relationship Rr + Rs = Vstep/11.

8) Lsig is determined utilising the relationship Lsig = T1 * (Rr Is + its) .

9) When the rate of change of current dl/dt reaches zero the current level has reached 12. Rs is then determined from the relationship Rs = Vstep/12.

10) A voltage equal to that lost in the semiconductors Vigbt is go then applied. A time period equal to or exceeding five times T1 is waited and an interpolation carried out to determine Imag at T1 from the relationship Imag = I * (Rr + Rs)/Rr using the determined values of Rr, Rs.

11) The current is monitored until l=lmag/e which is achieved us at time T2. Lmag is then determined from the relationship. Lmag = T2*Rr*Rs/(Rr + Rs) and the process is terminated. - 6 l

The standard deviation of the results for two tested motors is given to show the repeatability of the measurements achievable by using the invention.

400 w motor 1.5 kw motor Rs 0.9% 0.42 % Rr 6.1 % 4.3 % Lsig 9.9% 2.3 % Lmag 3.4 % 2.6 % Imag 5. 1% 2.6 % s - 7 - i I

Claims

1. A method for determining parameters of an induction motor comprising: s determining a voltage drop Vigbt lost in associated power control semiconductors by: applying a first voltage for a first time period; applying a second voltage for a second time period; interpolating the curves to derive Vigbt.

2. A method as claimed in claim 1 comprising applying to the motor the determined voltage Vigbt and applying the voltage steps Vstep = Inominal * Rs where Rs is the stator resistance, and Inominal the nominal current.

3. A method as claimed in claim 2 wherein the rate of change in current through the motor dl/dt is determined for successive samples and after the current has reached a maximum the time for dl/dt to reach l/e of the maximum value determined.

4. A method as claimed in claim 3 wherein a first time constant of a current rise function of the motor is allowed to time out and an interpolation process carried out to determine the current 11 at the first time constant T1. 1 1

5. A method as claimed in claim 4 comprising determining from the relationship Rr + Rs = Vstep/11 the value of Rr + Rs.

6. A method as claimed in claim 5 comprising determining a leakage inductance Lsig of the motor by the relationship Lsig = T1 * (Rr + its).

7. A method as claimed in claim 6 wherein when the rate of change of current dl/dt reaches zero determining the current 12 to and determining a stator resistance Rs from the relationship Rs = Vstep/12.

8. A method as claimed in claim 7 comprising applying the voltage Vigbt, waiting a time period equal or greater than five times the first time constant of the current rise function, determining a magnetising current Imag from the relationship Imag = I * (Rr + Rs) Rr go

9. A method as claimed in claim 8 comprising monitoring the current until I = Imag/e and hence determining T2, determining Lmag the magnetising inductance from the relationship Lmag = T2 * Rr * Rs/(Rr + Rs)