Electricity: motive power systems – Limitation of motor load – current – torque or force
Reexamination Certificate
1999-07-07
2001-03-06
Masih, Karen (Department: 2837)
Electricity: motive power systems
Limitation of motor load, current, torque or force
C361S030000, C361S031000, C361S033000
Reexamination Certificate
active
06198241
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates, in general, to motor protection circuits and, more particularly, the instant invention relates to motor protection for transit vehicle door systems and/or elevator doors.
BACKGROUND OF THE INVENTION
Powered door systems for use on transit vehicles, elevators and the like typically experience very heavy usage which may place severe demands on the motors that open and close the doors. Such motors are generally undersized due to size and weight constraints. Consequently, motor protection circuits are generally necessary to prevent burn out of such motors.
The friction which the doors experience when they are moved between open and closed positions may increase due to a number of factors including temperature, loss of lubricant, galling of load bearing surfaces, etc. In such applications, the doors may be quite massive and may be required to move rapidly between open and closed positions. This places a heavy demand on the motors to overcome the inertia of the doors when accelerating and decelerating the doors during opening and closing strokes of the doors. The doors may also encounter obstructions that require rapid braking which may be done dynamically, again placing heavy loads on the motors. All of these factors cause heat buildup in the windings of the motors.
In order to prevent such motors from overheating, one prior art approach is to place a temperature sensing element in the winding of the motor. The temperature sensing element may include a switch to open the circuit of the motor winding in the event that a dangerously high temperature is encountered. Another approach is to place a temperature sensor in the windings which has signal connection to a controller for the motor to turn off the current to the motor in the event of a dangerously high temperature. Such approaches generally respond slowly to motor overload conditions due to the time needed for heat to penetrate the insulation on the temperature sensor.
SUMMARY OF THE INVENTION
In one aspect, the invention is a motor protection circuit for an electric motor driven by a pulse width modulated motor drive amplifier. It has a capacitor having a first terminal and a second terminal and a first conduction path connected to the first terminal, the first conduction path being for connection to a first conductor. It also has a second conduction path connected to the second terminal, the second conduction path being for connection to a second conductor. There is a DC voltage difference between the first conductor and the second conductor. There is also a third conduction path connecting the first terminal and the second terminal. The third conduction path having a third conduction path resistance which is lower than the resistance of the first conduction path plus the resistance of the second conduction path. The third conduction path passes through a capacitor discharge switch. The capacitor discharge switch having a switch signal input connection to be opened and closed thereby. The switch signal input connection is for receiving a first digital signal indicating a motor overload condition, whereby the capacitor discharge switch is closed when the first digital signal indicates the motor overload condition and the capacitor discharge switch is opened when the first digital signal does not indicate the motor overload condition, so that a charge on the capacitor increases when the first digital signal does not indicate the motor overload condition and so that the charge on the capacitor decreases when the first digital signal indicates the motor overload condition. The circuit also has a voltage comparator connected to receive a capacitor voltage from the capacitor. The capacitor voltage being due to the charge on the capacitor. The voltage comparator produces a second digital signal indicating whether the capacitor voltage is below a predetermined voltage level, the second digital signal being for signalling a shut down switch to shut down the motor.
In another aspect, the invention is a motor protection circuit for an electric motor driven by a pulse width modulated motor drive amplifier. It has a capacitor having a first terminal and a second terminal and a first conduction path connected to the first terminal. The first conduction path being for connection to a first conductor. It also has a second conduction path connected to the second terminal. The second conduction path being for connection to a second conductor. There is a DC voltage difference between the first conductor and the second conductor. There is also a third conduction path between the first terminal and the second terminal. The third conduction path having a third conduction path resistance which is lower than the resistance of the first conduction path plus the resistance of the second conduction path. The third conduction path passes through a capacitor discharge switch. The capacitor discharge switch having a switch signal input connection to be opened and closed thereby. The switch signal input connection is for receiving a first digital signal indicating a motor overload condition, whereby the capacitor discharge switch is closed when the first digital signal indicates the motor overload condition and the capacitor discharge switch is opened when the first digital signal does not indicate the motor overload condition, so that a charge on the capacitor increases when the first digital signal does not indicate the motor overload condition and so that the charge on the capacitor decreases when the first digital signal indicates the motor overload condition. The circuit also has a voltage comparator connected to receive a capacitor voltage from the capacitor. The capacitor voltage being due to the charge on the capacitor. The voltage comparator produces a second digital signal indicating whether the capacitor voltage is below a predetermined voltage level. The circuit also has a shut down switch connected to shut down the motor based on the second digital signal.
In another aspect the invention provides an electric motor and motor control system having overload protection. It has an electric motor driven by a pulse width modulated motor drive amplifier. It also has a capacitor having a first terminal and a second terminal and a first conduction path connected to the first terminal. The first conduction path being for connection to a first conductor. It also has a second conduction path connected to the second terminal. The second conduction path being for connection to a second conductor. There is a DC voltage difference between the first conductor and the second conductor. There is also a third conduction path between the first terminal and the second terminal. The third conduction path having a third conduction path resistance. The third conduction path resistance being lower than a resistance of the first conduction path plus a resistance of the second conduction path. The third conduction path passes through a capacitor discharge switch. The capacitor discharge switch having a switch signal input connection to be opened and closed thereby. The switch signal input connection is for receiving a first digital signal indicating a motor overload condition from the pulse width modulated motor drive amplifier, so that the capacitor discharge switch is closed when the first digital signal indicates the motor overload condition and so that the capacitor discharge switch is opened when the first digital signal does not indicate the motor overload condition. In this arrangement, a charge on the capacitor increases when the first digital signal does not indicate the motor overload condition and the charge on the capacitor decreases when the first digital signal indicates the motor overload condition. The system also has a voltage comparator connected to receive a capacitor voltage from the capacitor. The capacitor voltage being due to the charge on the capacitor. The voltage comparator produces a second digital signal indicating whether the capacitor voltage is below a predetermined voltage level. There i
Fletcher Marlon
James Ray & Associates
Masih Karen
Westinghouse Air Brake Company
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