Angle control of modulating wave to reduce reflected wave...

Details Angle control of modulating wave to reduce reflected wave... Angle control of modulating wave to reduce reflected wave...

2001-11-20

2003-04-01

Leykin, Rita (Department: 2837)

Electricity: motive power systems

Positional servo systems

Pulse-width modulated power input to motor

C318S808000, C318S811000

Reexamination Certificate

active

06541933

ABSTRACT:

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH
Not applicable.
CROSS-REFERENCE TO RELATED APPLICATIONS
Not applicable.
CROSS-REFERENCE TO RELATED APPLICATIONS
Not applicable.
BACKGROUND OF THE INVENTION
The present invention relates to motor controllers and more particularly, to a method and an apparatus for altering stator winding voltages to eliminate greater than twice over voltage as a function of modulating wave angle.
Many motor applications require that a motor be driven at various speeds. Motor speed can be adjusted with an Adjustable Speed Drive (ASD) which is placed between a voltage source and an associated motor that can excite the motor at various frequencies. One commonly used type of ASD uses a three-phase Pulse Width Modulated (PWM) inverter and associated PWM controller which can control both voltage and frequency of signals that eventually reach motor stator windings.
A three-phase PWM controller receives three reference or modulating signals and a triangle carrier signal, compares each modulating signal to the carrier signal and generates firing signals consisting of a plurality of pulses corresponding to each modulating signal. When a modulating signal has a greater instantaneous amplitude than the carrier signal, a corresponding firing signal is high producing a pulse on-time. When a modulating signal has an instantaneous amplitude that is less than the carrier signal, a corresponding firing signal is low producing a pulse off-time.
The firing signals are used to control the PWM inverter. A three-phase PWM inverter consists of three pairs of switches, each switch pair including series arranged upper and lower switches configured between positive and negative DC power supplies. Each pair of switches is linked to a unique motor terminal by a unique supply line, each supply line is connected to a node between an associated pair of switches. Each firing signal controls an associated switch pair to alternately connect a stator winding between the positive and negative DC power supplies to produce a series of high frequency voltage pulses that resemble the firing signals. A changing average of the high frequency voltage pulses over a period defines a fundamental low frequency alternating line-to-line voltage between motor terminals that drives the motor.
Insulated Gate Bipolar Transistors (IGBTs) are the latest power semiconductor switches used in the PWM inverter, IGBTs have fast rise times and associated switching speeds (e.g. 50-400 ns) that are at least an order of magnitude faster than BJTs and other similar devices. At IGBT switching speeds, switching frequency and efficiency, and the quality of terminal voltages, are all appreciably improved. In addition, the faster switching speeds reduce harmonic heating of the motor winding as well as reduce audible motor lamination noise.
While IGBT PWMs are advantageous for all of the reasons identified above, when combined with certain switch modulating techniques (i.e. certain on/off switching sequences), IGBT fast dv/dt or rise times can reduce the useful life of motor components and/or drive to motor voltage supply lines. In particular, while most motors and supply lines are designed to withstand operation at rated line voltages for long periods and to withstand predictable overvoltage levels for short periods, in many cases, fast switch rise times causes over voltages that exceed design levels.
For a long time the industry has recognized and configured control systems to deal with twice overvoltage (i.e. twice the PWM inverter DC power supply level) problems. As well known in the controls art, twice overvoltage levels are caused by various combinations of line voltage rise time and magnitude, imperfect matches between line-to-line supply cable and motor surge impedances, and cable length. Line voltage frequency and switch modulating techniques have little effect on twice overvoltage levels.
One common way to cope with twice overvoltage levels has been to reduce reflected voltage by terminating the cable supply lines at the motor terminals with a cable to motor surge impedance matching network. Resistor-Inductor-Capacitor or R-L-C filter networks mounted at the drive output are also used to change and reduce the slope of the voltage pulses (i.e. the turn on times) as they arrive. This network increases the cable distance where twice voltage in the motor terminals is developed to a length outside the application distance of interest. In addition, to reduce the possibility of damage from periodic twice overvoltage levels, most cable supply lines and motors are insulated to withstand periodic twice overvoltage levels. Thus, the industry has developed different system configurations for dealing with twice overvoltage.
Unfortunately, there is another potentially more damaging overvoltage problem that has not been satisfactorily dealt with. The second overvoltage problem is referred to herein as greater than twice overvoltage. Unlike twice overvoltage, greater than twice overvoltage is caused by faster IGBT switching frequencies and faster IGBT dv/dt rise times interacting with two different common switch modulating techniques, that result in overvoltage problems referred to as “double pulsing” and “polarity reversal”.
Referring to
FIG. 1
, double pulsing will be described in the context of an IGBT inverter generated line-to-line voltage V
i
applied to a line cable and a resulting motor line-to-line terminal voltage V
m
. Initially, at time T
1
, the line is shown in a fully-charged condition (V
i
(T
1
)=V
m
(T
1
)=V
DC
). A transient motor voltage disturbance is initiated in
FIG. 1
by discharging the line at the inverter output to zero voltage, starting at time T
2
, for approximately 4 &mgr;sec. The pulse propagation delay between the inverter terminals and motor terminals is proportional to cable length and is approximately 1 &mgr;sec for the assumed conditions. At time T
3
, 1 &mgr;sec after time T
2
, a negative going V
DC
voltage has propagated to the motor terminals. In this example, a motor terminal reflection coefficient ┌
m
is nearly unity. Thus, the motor reflects the incoming negative voltage and forces the terminal voltage V
m
to approximately negative bus voltage:
 
V
m
(
T
3
)=
V
m
(
T
1
)−
V
DC
(1+┌
m
)≈
−V
DC
  Eq. 1
A reflected wave (−V
DC
) travels from the motor to the inverter in 1 &mgr;sec and is immediately reflected back toward the motor. Where an inverter reflection coefficient ┌
i
is approximately negative unity, a positive V
DC
pulse is reflected back toward the motor at time T
4
. Therefore, at time T
4
the discharge at time T
2
alone causes a voltage at the motor terminal such that:
V
m
(
T
4
)=
V
m
(
T
1
)−
V
DC
(1+┌
m
)−
V
DC
┌
i
┌
m
(1+┌
m
)≈
V
DC
  Eq. 2
In addition, at time T
4
, with the motor potential approaching V
DC
due to the T
2
discharge, the inverter pulse V
i
(T
4
) arrives and itself recharges the motor terminal voltage to V
DC
. Pulse V
i
(T
4
) is reflected by the motor and combines with V
m
(T
4
) to achieve a peak value of approximately three times the DC rail value:
V
m
(
T
4
+)=
V
m
(
T
1
)−
V
DC
(1+┌
m
)−
V
DC
┌
i
┌
m
(1+┌
m
)+
V
i
(
T
4
)(1+┌
m
)≈3
V
DC
  Eq. 3
Referring to
FIG. 2
polarity reversal will be described in the context of an IGBT inverter generated line-to-line voltage V
il
, and a resulting motor line-to-line voltage V
ml
. Polarity reversal occurs when the firing signal of one supply line is transitioning into overmodulation while the firing signal of another supply line is simultaneously transitioning out of overmodulation. Overmodulation occurs when a reference signal magnitude is greater than the maximum carrier signal magnitude so that the on-time or off-time of a switch is equal to the duration of the carrier period. Polarity reversal is common in all types of PWM inverter control.
Initially, the inver

Affiliated with

Also associated with

Rating

Angle control of modulating wave to reduce reflected wave... does not yet have a rating. At this time, there are no reviews or comments for this patent.

If you have personal experience with Angle control of modulating wave to reduce reflected wave..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Angle control of modulating wave to reduce reflected wave... will most certainly appreciate the feedback.

Rate now

Comments { 0 }

Profile ID: LFUS-PAI-O-3012535