Motor controller

Details Motor controller Motor controller

2002-09-16

2004-10-12

Ro, Bentsu (Department: 2837)

Electricity: motive power systems

Induction motor systems

Power-factor control

C318S798000, C318S809000

Reexamination Certificate

active

06803741

ABSTRACT:

CROSS-REFERENCE TO RELATED APPLICATIONS
Not applicable.
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
Not applicable.
BACKGROUND OF THE INVENTION
The field of the invention is motor controllers and more specifically control algorithms for use with thyristor based controllers for balancing the positive and negative current half-cycles within a motor phase.
One type of commonly designed induction motor is a three phase motor having three Y-connected stator windings. In this type of motor, each stator winding is connected to an AC voltage source by a separate supply line, the source generating currents therein. Most utilities which supply power to industrial motors supply well balanced purely sinusoidal three phase voltages (and corresponding currents) that have equal amplitudes and periods and are out of phase by exactly 120°. Nevertheless, for various reasons, switching assemblies and corresponding controllers have been developed to alter the sinusoidal current and voltage waveforms at the point of utilization.
For example, one common three phase switching assembly includes three solid state switching devices, one device for each separate system phase. Each device is positioned in series between the source and the motor phase and can generally be used to control the current passed from the corresponding supply line to the corresponding motor phase. An exemplary switching device may include a pair of separately controllable silicon controlled rectifiers (SCRs) linked together in an inverse parallel relationship so that each SCR is arranged to conduct current in a direction opposite that of the other. Inverse parallel SCRs are commonly referred to as thyristors.
As well known in the art, an SCR does not conduct until after the SCR is triggered (i.e., turned on) and voltage there across is of a polarity that is consistent with the direction in which the SCR conducts. When voltage across an SCR is consistent with the direction in which the SCR conducts, once triggered, the SCR remains in a conductive state until current through the SCR drops to a zero value at which point the SCR turns off. Because the voltage across the SCR is sinusoidal and the current through the SCR is related to (i.e., lags behind) the voltage across the SCR, the SCR turns off within one-half of a line voltage cycle.
According to at least one control scheme, thyristor SCRs are alternately fired to provide alternating positive and negative current half-cycles to a corresponding motor phase. Each pair of consecutive alternating current half-cycles are separated by a “notch” period corresponding to the turn off or shutoff angle of a first SCR in the pair and the turn on or fire angle of the second SCR in the pair. For instance, after a first positive current conducting SCR is fired while the voltage there across is positive, the first SCR remains conducting and current magnitude therethrough rises until the voltage across the SCR reaches a zero value. When the voltage across the first SCR becomes negative, the current magnitude through the first SCR begins to decrease. Eventually, the current through the first SCR drops to a zero value at a first SCR turn off or shutoff angle and the notch period begins.
At the end of the notch period and during the negative half-cycle of the line voltage, the second SCR in the pair is fired, the second SCR begins to conduct and the current magnitude through the second SCR rises until the line voltage again reaches a zero value. When the voltage across the second SCR becomes negative, the current magnitude through the second SCR begins to decrease and eventually drops to a zero value at a second SCR turn off or shutoff angle and a second notch period begins. At the end of the second notch period and during the next positive half-cycle of the line voltage, the first SCR in the pair is again fired, the first SCR begins to conduct and the current magnitude through the first SCR rises until the line voltage again reaches a zero value and the process above is repeated.
Other control schemes call for skipping line voltage cycles to alter motor speed. For instance, according to one scheme, the first SCR in each thyristor is fired during a first line voltage cycle, no SCR is fired during a second voltage cycle, the second SCR in each thyristor is fired during a third line voltage cycle, no SCR is fired during a fourth voltage cycle, the first SCR is again be fired during a fifth line voltage cycle and the pattern is repeated in a continuous fashion. Thus, by controlling the notch periods and, more specifically, the fire times of thyristor SCRs, currents provided to motor phases are controllable.
During a motor starting protocol, after an equipment operator applies a starting signal to the motor controller, the motor controller gradually increases the amount of current applied to the motor by regulating the SCR fire angles. By regulating the fire angles, the controller turns on each thyristor initially for only a portion of each half-cycle of the line voltage for the corresponding motor phase (i.e., the notch periods comprise relatively long portions of each voltage half-cycle). The controller then gradually increases the half-cycle on time of the thyristors (i.e., reduces notch periods), thus gradually increasing stator currents, until the motor is at substantially full speed. This technique reduces the current consumption and torque on the motor during start-up as compared to a hard switching of the full supply line voltage across the motor.
Thus, in thyristor based control systems like the one described above, motor control is premised on a simple algorithm for notch control and more specifically, a simple algorithm for identifying SCR fire times. In essence, according to the simplest fire angle algorithms, the fire angle for one SCR in a thyristor is calculated by adding a desired notch period to a most recent shutoff angle corresponding to the other SCR in the thyristor.
When a simple fire angle algorithm like the one described above is employed, the relationship between the shutoff angle of one SCR in a thyristor pair and fire angle of the other SCR in the pair is wholly a function of the algorithm and does not account for actual motor operating conditions (i.e., fire angle=shutoff angle plus notch period). However, the SCR shutoff angles are directly related to motor operating conditions and are also related to the fire angles at which the SCR is triggered. For instance, all other things being equal, if a fire angle is delayed by 10 degrees the corresponding shutoff angle for the SCR will be expedited by approximately 10 degrees (at least during steady state operation). As another instance, if the power factor PF angle (i.e., the angle corresponding to the delay between the line voltage and the line current) is altered, then the duration between a fire angle and a shutoff angle is also altered. Other changes to the operating conditions that affect shutoff angles are contemplated.
Despite its intuitive form, unfortunately the simple fire angle algorithm described above operates ineffectively under certain circumstances. To this end, there are conditions in which the simple fire angle algorithm causes a system to become “undesirably stable”. Here, the phrase “undesirably stable” is used to refer to conditions wherein the system is stable despite unbalanced positive and negative current half-cycles within each motor phase (e.g., a phase current may include positive half-cycles having a greater magnitude than negative half-cycles or vice versa). For instance, referring to
FIG. 1
, a full cycle of exemplary undesirably stable phase voltage and current waveforms
150
and
152
are illustrated where the notch period is 50 degrees. A first shutoff angle occurs at 50 degrees and thus at 100 degrees (i.e., after a 50 degree notch) a first fire angle occurs and current is conducted through a corresponding SCR. The SCR continues to conduct until current therethrough reaches a zero value at shutoff angle 240 degrees which is 60 degrees after the most recent voltage z

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