GB2426393A

GB2426393A - An AC motor drive current measurement system

Info

Publication number: GB2426393A
Application number: GB0510318A
Authority: GB
Inventors: Robert Carter; Richard Kenney
Original assignee: Siemens AG
Current assignee: Siemens AG
Priority date: 2005-05-20
Filing date: 2005-05-20
Publication date: 2006-11-22
Anticipated expiration: 2025-05-20
Also published as: GB2426393B; GB0510318D0

Abstract

In an AC motor drive system (1) a motor current to be applied to a motor (2)is derived by providing shunts (10-12) and A to D converters (15-17), and processor (18), to determine the current flowing through the shunts. The average or mean current is determined for an integral number of pulse widths of the switching elements. This enables more accurate determination of the current flowing to motor (2). Embodiments provide closed and open loop control.

Description

An AC motor drive current measurement system This invention relates to

vector controlled AC motor drive system and, in particular, to an induction motor drive system.

In order to perform vector control of an induction motor it is necessary to perform closed loop current control. This requires measurement of the three current phases applied to the motor. Unfortunately, this is difficult to achieve because the voltage levels on the phases have Pulse Width Modulated waveforms. This means that the voltage level varies over a wide range requiring expensive circuitry to measure successfully.

In an open loop drive arrangement it is necessary to accurately determine currents to ensure motor and installation protection.

It is known to provide shunts between a -ye DC link line and the inverter switches. These allow the current to be determined by measurement of the voltage drop across the shunt. This requires a sampling system whilst the connection to the motor is made. Sampling systems require accurate synchronisation to measure the applied current at a centre of the duration of the current. Then the transient effects, in particular, for narrow pulses cause difficulty in determining the actual current level.

According to the invention there is provided an AC motor drive system comprising: shunts coupling a DC link to a plurality of respective switching elements; outputs to, in use couple the switching elements to an AC motor which drive system characterised by means to determine across an integral number of pulse widths for each of the switching elements a mean voltage across the respective shunts; means to derive from the determined mean voltage a mean current through the shunt.

In a closed loop system there is preferably provided means to derive a motor current by dividing the mean current by the duty cycle of the switch.

Other preferred features in a closed loop system include the following.

Preferably, means is provided to derive a third phase of the motor current from a first and a second phase determined by the means to derive a phase.

Preferably, selection means is provided to select the two widest pulse widths for the determination of the mean voltage and hence mean current and means for determining from the resultant derived phases the third phase of the motor current.

Advantageously, for a closed loop system, the voltage applied to the motor is biased to make the most negative of the three phases substantially equal to the -ye DC link voltage. This is achieved in the one embodiment by a particular method of space vector modulation. (As disclosed in US 4480301 by Wick). It concentrates all the zero vectors to the "000" case. This gives us the maximum possible duty cycle on the lower switch element in the preferred embodiment which gives the best results. (This means that the noise is not multiplied).

There is a further advantage, in that, for one of the phases the duty cycle is 1.0 that is to say one phase of the motor current is measured directly.

Further advantages are enabled by a reduction in processing required.

A specific embodiment of the invention will now be described, by way of example only, with reference to the drawings of which: Figure 1 shows an AC motor drive system in accordance with the invention used in an open and a closed control loop embodiment of the invention; Figure 2 shows voltage waveforms applied to the motor of figure 1; Figure 3 shows three phases of voltage waveforms applied to the motor of figure 1; and Figure 4 provides a comparison of inverter waveforms applied to open control loop and closed control loop embodiments of the invention.

As is shown in figure 1, a vector motor controller 1 provides three outputs Ir, Is and It to an induction motor 2. The letters r, s and t denote the S, R and T phases. The controller 1 includes an inverter 3 having six switching transistors 4 to 9. Three shunts 10 to 12 connect the lower switches 7 to 9 to the lower -DC link 13. The upper switches 4 to 6 connect to the upper +DC link 14.

Analogue to Digital converters 15 to 17 are connected across the shunts to 12 to determine the mean current flowing through them over an integration period. The integration period is an integral number of pulse widths. They are fonned from sigma delta converters in this embodiment but alternative converters may be used. They provide signals to a processor 18 which performs the required analysis and control operations.

The same apparatus will be used to illustrate a first open control loop embodiment and a second closed control loop embodiment.

in the open control loop embodiment, the three currents ler, les and let (the three emitters for the phases r, s and t) are determined for a particular phase of one rotation of the motor. This is done use of the converters referred to above. The measurement of these currents yields a non- sinusoidal three phase waveform. The deviations from the sinusoidal form become greater as the depth of modulation (output voltage) increases.

In a first step, the processor 18 calculates the DC link current, Idc, from the relationship Idc ler + les + Jet.

The average current, lay, is calculated lay = Idc I 3 The root mean square motor current is then estimated from Irms = K J((ier - Jay)2 + (Jes lay)2 + (Jet - Jav)2) where K is a scaling constant K= 1.1635O. O852*Moddepth.

Moddepth is the depth of modulation for the PWM scaled 0 to 1.

The Irms value will contain a strong third harmonic of the motor frequency ripple component which is filtered out using a motor frequency dependent filter within the processor 18. The real part of the motor current is derived from Idc. The result is filtered as before for Irms to ensure consistency.

The derived values of the currents may then be used by the processor 18 to protect the motor via control signals to the inverter 3 along a control line 19.

The inverter 3 provides the varying phase signal required to drive the motor 2 by selectively closing the switches 4 to 9. The technique involves the more usual symmetrical space modulation for example that disclosed in US 4480301 by Wick.

Figure 2 shows the PWM controls applied to the switches 4 to 9. The "active" time in the PWM period is when a combination of the upper and lower switches are closed. The "inactive" time is when either all the upper switches or all the lower switches are closed.

In a closed control loop embodiment, the closing of the switches is controlled in a novel manner. In normal symmetrical space vector modulation, the time period when the three upper switches are closed is made equal to the time period when all the three lower switches are closed. In this particular modulation scheme, the time period when all the three upper switches are closed is set to zero which results in the applied voltage being zero volts. This is referred to in this specification as fixed lower side bias modulation (FLB).

Figure 3 shows the three phase voltages using FLB modulation applied to the motor. If the lower switch is on for U% of the time, the average current ler is measured across the shunt; then the motor current must be Ier* 1 00/U.

At any one time one phase has U= 100% whilst another phase has U= 100Msin(phase) where M is the modulation depth. This calculation is carried out in the fastest part of the control code and is very straightforward involving a single sin (x).

The third phase is then derived from the first two. Preferably, the two phases used for deriving the third are those with the greatest U% values to ensure greatest accuracy.

Figure 4 shows the waveforms used for the two embodiments. Waveform is that used in the open loop embodiment and waveform 41 is that used in the closed loop embodiment.

Claims

Claims 1. An AC motor drive current measurement system comprising: shunts

coupling a DC link to a plurality of respective switching elements; outputs from the switching elements to, in use, couple phases of motor current to an AC motor to drive the motor; which drive system characterised by means to determine across an integral number of pulse widths for each of the switching elements a mean voltage across the respective shunts; means to derive from the determined mean voltage a mean current through the shunt.
2. A system as claimed in claim 1 further comprising: means to derive a motor current to be coupled to the output by dividing the mean current by a duty cycle of the switching elements.
3. A system as claimed in claim 2 wherein means is provided to derive a third phase of the motor current from a first and a second phase determined by the means to derive a phase.
4. A system as claimed in claim 3 further comprising: selection means to select the two widest pulse widths for the determination of the mean voltage, and hence the mean current; and means for determining from the resultant derived phases the third phase of the motor current.
5. A system as claimed in claims 2 to 4 comprising biasing means to make the most negative of the three phases to be coupled in use to the motor substantially equal to a -ye DC link voltage.
6. A system as claimed in claim 3 wherein the biasing means performs space vector modulation.
7. Apparatus substantially as hereinbefore described with reference to and as illustrated by the drawing.