Electricity: motive power systems – Limitation of motor load – current – torque or force
Reexamination Certificate
2003-02-12
2004-11-16
Ro, Bentsu (Department: 2837)
Electricity: motive power systems
Limitation of motor load, current, torque or force
C318S810000, C318S811000
Reexamination Certificate
active
06819070
ABSTRACT:
CROSS-REFERENCE TO RELATED APPLICATIONS
Not applicable.
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
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.
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 overvoltages 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.
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”.
Each of the double pulsing and polarity reversal problems are described in detail in U.S. Pat. No. 5,912,813 (hereinafter “the '813 patent”) which issued on Jun. 15, 1999, is entitled “Method and Apparatus for Controlling Reflected Voltage Using A Motor Controller”. The '813 patent is incorporated herein by reference for its teachings regarding double pulsing and polarity reversal.
One way to mitigate the adverse effects of rise time induced motor overvoltages has been to design and construct relatively complex passive filter networks. Unfortunately addition of passive filter networks increases overall system design costs and implementation, requires excessive relatively expensive panel space within a system housing or cabinet and can lead to heating and other operating problems. In addition, unfortunately, passive filters limit carrier frequency selection.
One other solution to mitigate the adverse effects of rise time induced motor overvoltages has been to modify modulation and commutation software. Some of the more sophisticated techniques of this type include providing a motor controller that modifies firing pulses that are provided to an inverter in a manner calculated to eliminate greater than twice overvoltage switching sequences. When the period between two voltage changes is less than the period required for a substantially steady state voltage near zero to be reached, the period between the two voltage changes is increased. Where switching sequence results in greater than twice overvoltage due to polarity reversal, the switching sequence is altered to eliminate the possibility of greater than twice overvoltage.
Software correction solutions generally contemplates two different methods of altering the switching sequence referred to as the Maximum-Minimum Pulse Technique (MMPT) and the Pulse Elimination Technique (PET) methods. According to the MMPT method, when a PWM pulse has characteristics which could generate greater than twice overvoltage, the pulse width is altered so that its duration is set equal to or between the minimum and maximum pulse times allowed (i.e., the carrier period less a dwell time where the dwell time is the minimum period required to avoid overvoltage). Importantly, only pulses that cross the threshold level for double pulsing induced motor voltages greater than twice overvoltage and during polarity reversal periods are altered so that the resulting terminal voltage magnitude is only minimally affected.
According to the PET method, instead of only limiting pulses to within the maximum and minimum pulse times, some of the pulses having characteristics which could generate greater than twice overvoltage are eliminated. In other words, some of the positive pulse durations during positive half cycles are increased and set equal to the carrier period and some of the negative pulse durations during negative half cycles are increased and set equal to the carrier period. The result is a terminal voltage magnitude which is essentially unaffected by pulse alterations.
Unfortunately each of the MMPT and PET methods alone do alter the resulting terminal voltages. For example, when an MMPT method is employed the terminal voltage magnitude is noticeably reduced as some positive pulse durations during positive half cycles and some negative pulse durations during negative half cycles are reduced. Similarly, when a PET method is employed the terminal voltage magnitude is noticeably increased as some positive pulse durations during positive half cycles and some negative pulse durations during negative half cycles are increased.
One way to deal with errors caused by MMPT and PET methods is to provide feedback loops in the control system. For example, one control system including a feedback loo
Kerkman Russel J.
Leggate David
Quarles & Brady LLP
Ro Bentsu
Rockwell Automation Technologies Inc.
Walburn William R.
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