Pumps – Condition responsive control of pump drive motor – Responsive to accumulation of pumped liquid in receiver
Reexamination Certificate
1999-05-17
2001-07-24
Walberg, Teresa (Department: 3742)
Pumps
Condition responsive control of pump drive motor
Responsive to accumulation of pumped liquid in receiver
C417S019000
Reexamination Certificate
active
06264431
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates generally to liquid pump systems and, more particularly, to circuitry for controlling a variable-speed motor drive thereof.
2. Description of the Related Art
Domestic water systems typically include a line-operated (i.e., 60 Hz, single-phase power) motor for driving a pump-motor assembly to retrieve water from a well. The pump-motor assembly is generally submerged in the well at the end of a drop pipe. A less efficient alternative configuration is a surface-mounted jet pump and submerged ejector nozzle.
To maintain an approximately constant supply pressure, these water systems also typically include a pressurized storage tank and a pressure sensing switch (hereinafter “pressure switch”) that causes the pump-motor assembly to run when pressure is low (i.e., when the tank water level is low) and stop when pressure is high (i.e., when the tank water level is high). However, starting single-phase motors is inefficient and cannot be repeated too frequently or the motor will overheat. As a result, typical domestic water systems use a relatively large pressure tank (e.g., 20 gallons) and a relatively large pressure differential (e.g., 20 psi) to limit the frequency of motor starting.
Recent progress in power electronics has resulted in the incorporation of a frequency changer (i.e., variable-speed motor drive) in the pump-motor assembly. Frequency control has allowed the motor to be operated at higher speeds than the 3450 rpm typical of 60 Hz line-operated motors, in turn allowing the pump to be physically smaller. For submersible pumps, this advantage is realized in a reduction in the number of stages, inasmuch as the size of each stage in a multi-stage submersible pump is restricted by the well bore diameter. Another advantage of a variable-speed pump-motor assembly involves “soft starting,” or ramping up the speed of the motor during starting to provide a more efficient startup procedure.
In one such variable-speed motor-pump assembly, the variable-speed motor drive was included as part of a submerged pump-motor assembly unit. A pressure switch located near the pressure tank was utilized to cut power to the pump-motor assembly once the pressure reached a predetermined high level. Cutting power to the pump-motor assembly, however, would also de-energize the variable-speed motor drive. As a result, a capacitor bank in the variable-speed motor drive had to be recharged during every startup. Recharging the capacitor bank, particularly if done quickly, places extreme stresses on the rectifiers that supply current to the capacitor bank. Reducing the recharge rate to avoid damage to the rectifier, however, undesirably decreases the responsiveness of the pump-motor assembly. A lower responsiveness translates into a larger tank.
Another variable-speed system similarly cut the power to the pump-motor assembly via a control unit disposed near the tank. In this system, a pressure switch near the control unit coupled a pair of signal conductors to the control unit. These signal conductors then led to the variable-speed motor drive via a cable separate from the power conductors carrying the 60 Hz power.
Other variable-speed systems have replaced the on/off signal of a pressure switch with an analog control signal developed by a pressure transducer. The pressure transducer signal has also been delivered as a digital signal to the embedded microcontroller, which can then decide to start, stop, or adjust: the speed of the motor. Generally, these systems relying upon a pressure transducer have often severely limited the closed loop bandwidth in order to maintain system stability. This bandwidth limitation significantly decreases the responsiveness of the system. Other systems have had to utilize a flow sensor in addition to the transducer to determine when the pump-motor assembly should completely shut down.
SUMMARY OF THE INVENTION
In accordance with one aspect of the present invention, a pump system is useful for providing liquid to a tank. The pump system includes a variable-speed motor drive and a pair of conductors coupled thereto to supply power to the variable-speed motor drive. The pump system further includes a switch disposed in a state in accordance with an amount of liquid in the tank. The switch is coupled to at least one conductor of the pair of conductors for control of the variable-speed motor drive and power is supplied to the variable-speed motor drive via the pair of conductors regardless of the state of the switch.
According to a preferred embodiment, the pump system further includes a third conductor coupling the switch to the variable-speed motor drive. Alternatively, the pump system further includes a bypass circuit connected to at least one of the pair of conductors in parallel with the switch. The bypass circuit may include a rectifying element such that the pump system further includes a sensor coupled to the variable-speed motor drive that detects whether the pair of conductors are supplying half-wave rectified power.
According to another preferred embodiment, the pump system further includes a transmitter coupled to the switch and the pair of conductors to generate a high frequency signal on the pair of conductors representative of the state of the switch. The pump system may further include a detector coupled to the pair of conductors and the variable-speed motor drive that is responsive to the high frequency signal to control the variable-speed motor drive.
In accordance with another aspect of the present invention, a pump system powered by a power source for providing liquid to a tank includes a variable-speed motor drive and a control circuit coupled to the power source and the variable-speed motor drive. The control circuit, in turn, includes a switch disposed in a state in accordance with an amount of liquid in the tank, and a bypass circuit connected in parallel with the switch that allows power to be supplied to the variable-speed motor drive from the power source regardless of the state of the switch.
In accordance with yet another aspect of the present invention, a pump system useful for providing liquid to a tank includes a variable-speed motor drive and a pair of conductors coupled to the variable-speed motor drive to supply power thereto. The pump system further includes a transmitter inductively coupled to the pair of conductors to generate a high frequency signal representative of an amount of liquid in the tank. A detector of the pump system is coupled to the variable-speed motor drive and inductively coupled to the pair of conductors to receive the high frequency signal therefrom.
The pump system preferably includes a mechanism coupled to the transmitter for indicating the amount of liquid in the tank. This mechanism may include a pressure switch disposed in a state in accordance with the amount of liquid in the tank.
According to a preferred embodiment, the pump system further includes a pair of capacitors connected across the pair of conductors. The transmitter and the detector may then be coupled to the pair of conductors via a respective transformer such that each transformer, together with a respective capacitor of the pair of capacitors, couples the pair of conductors together.
According to another preferred embodiment, the transmitter includes an oscillator. The transmitter may further include an amplifier coupled to the oscillator via a switch that is disposed in a closed state when the amount of liquid in the tank reaches a low level.
In accordance with a still further aspect of the present invention, a control circuit is useful in a pump system having a tank for storing liquid and a variable-speed motor drive. The control circuit includes a switch disposed in a state in accordance with an amount of liquid in the tank and a bypass circuit connected in parallel with the switch. As a result, the variable-speed motor drive is supplied with power when the switch is disposed in an open state. The control circuit further includes a detector responsive to the power to generate
Fastovsky Leonid
Franklin Electric Co. Inc.
Marshall O'Toole Gerstein Murray & Borun
Walberg Teresa
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