Electricity: motive power systems – Induction motor systems – Primary circuit control
Reexamination Certificate
1998-10-02
2001-03-13
Martin, David S. (Department: 2837)
Electricity: motive power systems
Induction motor systems
Primary circuit control
C318S805000, C318S812000, C361S086000
Reexamination Certificate
active
06201369
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to a motor controller utilizing SCR protection from stalled motor conditions without current sensing.
BACKGROUND OF THE INVENTION
Solid state starters/controllers have found widespread use for controlling application of power to an AC induction motor. The starter/controller, referred to hereinafter as simply a controller, uses an SCR, such as a triac, for controlling application of AC line voltage to the motor. The controller typically includes heat sinks associated with the SCRs for dissipating heat. When the controller has been running for an extended period of time, the heat sinks and SCRs become warm. If the motor stalls, then the SCRs must carry the motor stall current until protective overload relays trip to stop operation. The size of both the SCR and heat sink must become larger as the test current during stall is increased. In fact, when testing to IEC 947-4-2 standards, the prospective test current is 7.2 times the operational rating of the controller.
Currently controllers must be provided with larger SCRs and/or heat sinks to pass the tests. Alternatively, a current sensor must be used to limit the current during a stalled motor condition. This can increase the cost of the product.
These problems exist because the overload capacity of the overload relay may not match the overload capacity of the controller. At stall currents the overload capacity of the overload relay decreases more slowly than that of the controller. Thus, the controller may not be able to withstand the overload condition of the IEC test for the time that is required for the overload relay to trip.
The present invention is directed to solving one or more of the problems discussed above in a novel and simple manner.
SUMMARY OF THE INVENTION
In accordance with the invention there is disclosed a motor controller that provides protection from stalled motor conditions without use of current sensors.
Broadly, there is disclosed herein a motor controller for a motor operated by an AC line voltage. The controller includes solid state switch means connectable between an AC line and a motor terminal for controlling application of AC line voltage to the motor. Sensing means operatively associated with the solid state switch means sense voltage across the switch means. A control circuit is connected to the sensing means and the switch means for varying relative duration of on time and off time during each cycle of line voltage to control motor voltage at a reference level. The control circuit includes means for determining if a motor stall condition occurs responsive to the sensed voltage across the switch means and, in response to determining that a motor stall condition occurs, reducing motor voltage by decreasing duration of on time to protect the solid state switch means.
It is a feature of the invention that the switch means comprises a silicon controlled rectifier.
It is another feature of the invention that the switch means comprises a triac.
It is a further feature of the invention that the determining means determines that a stall condition occurs if voltage generated by the motor, as sensed by the sensing means, is above a preselect level. The control circuit resumes controlling voltage to the reference level if voltage generated by the motor, as sensed by the sensing means, is below a second preselect level after a stall condition occurs.
It is another feature of the invention that the determining means measures sensed voltage during the off time.
It is an additional feature of the invention that the control circuit reduces the reference level if a stall condition occurs.
It is still a further feature of the invention that the sensing means comprises a comparator.
It is yet another feature of the invention that the control circuit increases off time if a stall condition occurs.
It is yet another feature of the invention that the control circuit decreases off time after a stall condition has terminated as determined by the determining means.
There is disclosed in accordance with another aspect of the invention a motor controller for a motor operated by an AC line voltage. The controller includes solid state switch means connectable between an AC line and a motor terminal for controlling application of AC line voltage to the motor. Voltage sensing means sense motor voltage. Speed sensing means sense motor speed. A control circuit is connected to the voltage and speed sensing means and the switch means for varying relative duration of on time and off time during each cycle of line voltage to control motor voltage at a select reference level. The control circuit includes means for determining if a motor stall condition occurs responsive to the sensed speed, and in response to determining that a motor stall condition occurs reducing motor voltage by decreasing duration of on time to protect the solid state switch means.
It is a feature of the invention that the determining means determines that a stall condition occurs if speed, as sensed by the speed sensing means, is above a preselect level. The control circuit resumes controlling voltage to the reference level if speed, as sensed by the speed sensing means, is below a second preselect level after a stall condition occurs.
It is yet another feature of the invention that the speed sensing means measures sensed voltage during the off time.
More particularly, solid state switch means are protected from stalled motor currents by sensing the EMF voltage generated by the motor during the off time of the solid state switch means. The magnitude of the EMF across the switch means increases as the motor stalls and the motor current increases. The level of over current that trips the circuit can be varied to change the level of EMF required to trip the protective circuit. When this increase is seen above a preselect trip level, then the controller decreases the applied voltage to the motor load. This reduces the stall current. The reduction of stall current allows smaller heat sinks and/or SCRs to be used.
Further features and advantages of the invention will be readily apparent from the specification and from the drawing.
REFERENCES:
patent: 4459529 (1984-07-01), Johnson
patent: 4525660 (1985-06-01), Katto
patent: 5280227 (1994-01-01), Bashark
patent: 5473241 (1995-12-01), Chen et al.
patent: 5682091 (1997-10-01), Williamson et al.
Martin David S.
Siemens Energy & Automation Inc.
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