Communications: electrical – Condition responsive indicating system – Specific condition
Reexamination Certificate
1999-08-13
2001-04-03
Trieu, Van T. (Department: 2736)
Communications: electrical
Condition responsive indicating system
Specific condition
C340S662000, C340S663000, C318S800000
Reexamination Certificate
active
06211792
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to detecting a failed thyristor, and more particularly to detecting a failed thyristor in reduced voltage solid-state motor starters or controllers.
2. Description of the Related Art
Electric motors often use “thyristors,” which are also known as “silicon controlled rectifiers” (“SCRs”), as part of the motors' control circuitry. A thyristor can be thought of as a switchable diode with three terminals: a gate, an anode, and a cathode. If a supply voltage that is less than a breakdown voltage is applied across the anode and cathode of the thyristor, and no “trigger” current or voltage (trigger signal) is applied to the gate, the thyristor is “off,” i.e., no current flows from the anode to the cathode. If a trigger signal is applied to the gate, the voltage across the anode and cathode of the thyristor drops to a very low value in comparison to the supply voltage, and the thyristor turns “on,” i.e. current flows through the thyristor from the anode to the cathode. Once on, the thyristor can remain on, provided the current through the thyristor remains above a holding current, regardless of the trigger signal at the gate. For the thyristor to turn off, the anode to cathode current must be reduced to a level below the holding current value for the device.
As is well known in the art, solid state starters, or controllers, control electric current flow from a power supply to the motor while the motor is starting. These starters have thyristor switches that gradually increase the current delivered to the motor. Using the thyristor switches, the starter regulates the time period that the thyristors conduct electricity and pass current. In other words, the starter controls when the current from the power supply is delivered to the motor. By controlling the current supplied to the motor during startup, the motor is gently brought up to full operating speed.
When an electric motor is started without such a starter, current drawn by the motor can be excessive, typically six times the steady state current, i.e., the current once it reaches full operating speed. This large current inrush can cause a voltage drop in the power distribution system, causing lights to dim and flicker and disturbing nearby equipment. In addition, the motor torque may rise quickly and oscillate, which can adversely affect the mechanical components of the motor or anything coupled to it.
Failure of a thyristor in the starter may also result in poor motor functioning. Thyristor failures generally result in unbalanced power supply conditions, which may lead to large torque oscillations that can damage mechanical couplings and gears driven by the motor.
Therefore, there is a need to be able to detect a failed thyristor during operation of a motor.
SUMMARY OF THE INVENTION
Methods and systems consistent with this invention detect a failed short thyristor in a solid-state controller for delivering power to a load during reduced-voltage operation. Methods consistent with this invention detect a voltage across the thyristor, and indicate a failed short thyristor when the absolute value of the voltage across the thyristor remains below a threshold value during a predetermined period of time. Systems consistent with this invention comprise a voltage detector circuit comprising a resistor, a capacitor, and a light emitting diode for detecting a voltage across the thyristor; and a microprocessor coupled to the voltage detector circuit for indicating a failed short thyristor when the absolute value of the voltage across the thyristor remains below a threshold value during a predetermined period of time.
Methods and systems consistent with this invention detect a failed open thyristor in a solid-state controller during full-voltage operation. Methods consistent with this invention detect a voltage across the thyristor, and indicate a failed open thyristor when the absolute value of the voltage across the thyristor exceeds a threshold value. Systems consistent with this invention comprise a voltage detector circuit comprising a resistor, a capacitor, and a light emitting diode for detecting a voltage across the thyristor; and a microprocessor optically coupled to the voltage detector circuit for indicating a failed open thyristor when the absolute value of the voltage across the thyristor exceeds a threshold value.
The summary and the following detailed description should not restrict the scope of the claimed invention. Both provide examples and explanations to enable others to practice the invention. The accompanying drawings, which form part of the detailed description, show one embodiment of the invention and, together with the description, explain the principles of the invention.
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Beaverson Gregory K.
Jadrić Ivan
Schnetzka Harold R.
Finnegan, Henserson, Farabow, Garrett & Dunner, L.L.P.
Trieu Van T.
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