Electricity: motive power systems – Induction motor systems – Primary circuit control
Reexamination Certificate
1998-12-03
2001-06-12
Nappi, Robert E. (Department: 2837)
Electricity: motive power systems
Induction motor systems
Primary circuit control
C318S778000, C318S807000, C318S438000, C318S254100, C318S801000, C318S802000
Reexamination Certificate
active
06246207
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates to induction motors, and particularly to a method and apparatus for controlling an induction motor.
Previous methods and apparatus of controlling an induction motor incorporate the changing of speed taps or the use of triac controls, which simply provide an effective reduction in voltage or flux so as to cause the motor to run at a reduced speed by the nature of slip. Slip is generally a measurement of how much the movement of the rotor follows the excitation field, and is defined as the difference between the frequency of the excitation energy and the speed of the motor. While these controls provide adequate speed control, they do so at the expense of efficiency as the motor runs at a higher slip which is proportional to rotor conduction loss.
U.S. Pat. No. 5,252,905 a shows and describes such a controller for a motor. The controller uses a single phase pulse width modulated inverter to control the speed of the motor at speeds that are less than the full operating speed of the motor. The voltage applied to the motor is adjusted in direct proportion to the frequency output from the pulse width modulator. That is, a constant voltage to frequency ratio is maintained. This constant voltage to frequency ratio results in a constant torque output regardless of the speed of the motor.
SUMMARY OF THE INVENTION
It has been determined that the reduction of input power to a blower motor has a dramatic effect on increasing the Seasonal Energy Efficiency Ratio or (“SEER”) of heating, ventilating, and air conditioning (“HVAC”) equipment. It has also been determined that blower applications exist that do not require the full torque output (typically measured in cubic feet per minute (“CFM”)) that an induction motor excited by sixty cycle voltage would provide. The purpose of the invention is to provide a low cost, “power efficient” way of reducing the indoor blower speed, and consequently, the indoor blower CFM and input power so as to realize an increase in overall system efficiency.
Accordingly, the invention provides a controller having a variable speed drive that provides an optimum reduced speed setting for the operation of the blower motor when less than full speed and full torque output are required. The controller includes an inverter and pulse width modulator connected to the inverter to control the inverter. The inverter includes a microprocessor to calculate a quadratic relationship between applied voltage and frequency rather than the constant voltage to frequency ratio of the prior art. The use of a quadratic control relationship between the applied voltage and the frequency reduces the torque output matching the fan law torque curve, resulting in a more efficient controller that requires fewer and lower cost, lower power rated parts.
The controller also incorporates the use of relays local to the controller. The relays allow the motor to be run using the variable speed excitation voltage (for reduced speed operation) and allow the inverter to be bypassed to excite the motor using sixty cycle line excitation voltage (for full speed operation). The ability to run the motor at full speed directly from the line voltage further reduces the power requirements of the components in the inverter. Because set reduced speed operating points are selected using a quadratic voltage to frequency relationship, the electronic components of the inverter can be sized for approximately ¼ the full rated horsepower.
This control concept is particularly effective in applications which follow a “cubed-law” power characteristic such as fans and pumps. In these applications, the power demanded by the load is a cubed function of the speed, that is: y=CX
3
; wherein y is the power, and X is the speed of the motor. By reducing the speed in half, you reduce the power required by ⅛.
Various other features and advantages of the invention are set forth in the following drawings, description and claims.
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Dellinger Stephen J.
Rudiak Jerry A.
VanSistine Thomas G.
A. O. Smith Corporation
Michael & Best & Friedrich LLP
Nappi Robert E.
Smith Tyrone
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