GB2417623A

GB2417623A - Dead-time compensation in a voltage source inverter

Info

Publication number: GB2417623A
Application number: GB0501299A
Authority: GB
Inventors: Robert Carter
Original assignee: Siemens AG
Current assignee: Siemens AG
Priority date: 2004-08-28
Filing date: 2005-01-21
Publication date: 2006-03-01
Also published as: GB0501299D0; GB0419214D0

Abstract

In a system of controlling an AC motor, said system including a pulse generator and an inverter circuit and means to insert dead-time to protect said circuit, a method of compensating for dead-time error on the output comprising measuring the output current and modifying the pulse width generated from the pulse generator dependent on said error.

Description

24 1 7623 A Method Of Dead-time Compensation in a Voltage Source Inverter

In electrical and electronic control of 3 phase AC and other such motors, a pulsewidth modulating voltage source inverter is commonly used, where voltages on each line are switched in polarity; in other words terminals switch from negative to positive and vice versa. This is usually done by inverter bridge pole. It is very important to avoid switching at the wrong time, i.e. switching such that both switches in a pole are switched on at the same time, or a destructive short circuit event results. A known solution to this Is to insert "dead time" between the "switched on" periods of the upper and lower transistors in an inverter bridge pole. This is so as to prevent the aforementioned conduction causing a destructive short circuit of the DC link as well as reduce losses in the active devices caused by charging currents of the output line capacitance.

The problem with dead time insertion Is that it results in output inaccuracy. The voltage error caused by this is a function of the output current, output hue capacitance, switching frequency, dead time and DC link voltage. During operation of a drive (motor) the output current Is the most important variable factor., being typically sinusoidal in waveform.

Minimising dead-tme gives best output waveform but must be undertaken carefully to avoid destruction or excessive heating in the inverter due to simultaneous conduction as described above. Furthermore dead time errors are also a major cause of the low frequency open loop instability apparent in many drives.

It is an object of the invention to reduce and overcome the aforementioned problems.

The invention comprises, in a system of controlling an AC motor, said system including a pulse generator and an inverter circuit and means to insert dead-time to protect said circuit, a method of compensating for dead-time error on the output comprising measuring the output current and modifying the pulse width generated from the pulse generator dependent on said error.

The basis of the invention is to predict the voltage error on the basis of current flowing in and out of an inverter. During dead time when current flows into an inverter the error voltage is positive and when current flows out of the inverter error voltage is negative.

The error in the voltage is the difference between the desired voltage and actual voltage in the motor leads. As it is an AC system, output current is whistling past zero many times a second; overall on average when there is no error the current is zero. Thus a measure of voltage error is obtained by output current The invention will now be described with reference to the following figures of which: Figure 1 shows a view of the actual voltage error against the output current on an I S mverter pole bridge.

Figure 2 shows the pulse width correction to be applied against the current measured.

The invention measures the output current in the motor leads in order to obtain a measure of the voltage error and therefore the correction to be applied, This correction is apply by varying the pulse width of the corresponding PWM pulses Figure I shows a view of the actual voltage error against the output current on an inverter pole bridge. The error function has a linear portion at low currents where the load current only partially charges the output capacitance during the dead time and an asymptotic portion where the line is fully charged.

In a preferred embodiment of the invention the figure I characteristics are approximated to a voltage slope as shown in figure 2. This figure actually shows the pulse width correction to be applied against the current measured; the pulse width correction is proportional to the voltage; hence voltage against current would look exactly the same shape.

A modulator provides pulses to an inverter circuit which in turns puts a pulsewidth modulated AC voltage output to the motor line(s).The modulator also adds dead time into its output. The current on the inverter is measured. From this is determined the pulse width correction to be added to the modulator. The embodiment is thus a feedback circuit.

to In a preferred embodiment of the invention the current is measured by circuitry already present m the motor control for other purposes.

The following are two more detailed alternative preferred embodiment of the invention.

Example 1

In a system where analogue to digital converters are used for current output the following an ADC for phase output current running with say 1 conversion per PWM pulse edge; the current must be continuously converted so the controller can continuously control it.

A digital multiplier computes the product of the current magnitude and a slope parameter and the resultant correction limited to a maximum value determined by a "maximum correction" parameter. This correction is applied to the mverter PWM output by delaying either rising or falling edges dependent on current sign.

Example 2

Where a system which uses sigma delta converter for its output current measurement.

The sigma delta converter is run at lOMHz and a protection filter converts the sigma delta modulated bit stream into an analogue value of current magnitude of 8 bits plus sign. A FIR f lter (or IIR) f lter may be used. A digital multiplier computes the product of current magnitude with a "slope parameter". The resultant correction is limited to a maximum value determined by a "maximum correction" which is then applied to inverter PWM output by delaying either the rising edges or falling edges depending on the current sign.

Often circuitry for AC drives include current measuring devices for other purposes and in a preferred embodiment of the invention this is also used to measure current for the purpose of determining pulse width correction.

When an inverter contains only 2 current sensors the filtered value of the third phase may be calculated thus: It = -(Ir + Is) It, Ir and Is are the 3 motor current in the 3 phases. s

Claims

Claims 1. In a system of controlling an AC motor, said system including a

pulse generator and an inverter circuit and means to insert dead-time to protect said circuit, a method of compensating for dead-time error on the output comprising measuring the output current and modifying the pulse width generated from the pulse generator dependent on said error.
2. A method as claimed in claim 1 wherein said inverter circuit includes output bridge circuit switching elements.

3A method as claimed in claim
3 wherein said measure of voltage error is determined by measuring output current.
4. A method as claimed in any preceding claim wherein the pulse width is modulatedladjusted m proportion to voltage error.
5. A method as claimed In claim 4 wherein said pulse width is modulated in proportion to the voltage error up to a maximum.
6. A method as claimed in any preceding claim wherein the pulse width modulation correction comprises delaying the rising edges or falling edges of said input signal dependent on the current sign and/or magnitude. 2:S
7. A system for controlling an AC motor including a pulse generator, an inverter circuit, associated with said inverter circuit means to insert dead-time and wherein said system also includes means to compensate for errors resulting in dead-time comprising means to measure the output of said inverter, and means to modulate the pulse width of the signal input to said inverter, dependent upon said measurement.
8. A system as claimed in claim 7 wherein said invertcr includes output bridge circuit switching elements.
9. A system as claimed in claim 7 or 8 wherein said measure of voltage error is determined by measuring output current.
10. A system as claimed in claims 7 to 9 wherein means to modulate the pulse width of the output of the pulse generator comprises means to modulate the pulse width in proportion to voltage error.
11. A system as claimed in claim 10 wherein means to modulate the pulse width from the pulse generator comprises means to modulate the pulse width in proportion to voltage error up to a maximum.
12. A system as claimed of claims 7 to 11 wherein pulse width modulation correction means includes means to effectively delay the rising edges or falling edges of said input signal dependent on the current sign and/or magnitude.