Abstract:
A method for providing a compatible interface between a motor control circuit and a system controller is described. The method includes determining which signals in the interface have incompatible voltage requirements, and inserting a series resistance in an interface extending between the motor control circuit and the system controller for each signal determined to have incompatible voltage requirements therebetween.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    This invention relates generally to control of motor operations, and more specifically, to methods and apparatus for providing motor control signals. 
         [0002]    Electronically controlled variable speed motors (VSMs), including electronically commutated motors (ECMs) and induction motors are generally used in air handling (blowers, fans) applications in HVAC (Heating, Ventilation, Air Conditioning) systems. The controllers (e.g., circuit boards) that are utilized in many of these HVAC systems generally switch 115 VAC (furnace) or 230 VAC (air handler, heat pump) voltages for control of the motors operation. These switched voltages are applied to the taps of induction motors to invoke different speeds of operation for the induction motor. Heating, cooling, and continuous fan modes of operation may all be a different speed (motor tap). Installations with higher or lower static pressure, due to restrictions in the ductwork and filters, may require additional choices. 
         [0003]    In systems where VSMs are utilized instead of induction motors, VSM operational selections are typically made with 24 VAC signals that are available in most systems or even with low voltage DC signals developed on the systems control board. To minimize the changes required by an OEM (Original Equipment Manufacturer) to their system controllers when changing from a standard induction motor to a VSM, a way to accommodate the switching and application of the high voltage signals to be compatible with the low voltage signal inputs of the VSM is needed. While active interfaces are known to interface controllers to VSMs, a passive inexpensive solution would also be desirable. 
       BRIEF DESCRIPTION OF THE INVENTION 
       [0004]    In one aspect, a method for providing a compatible interface between a motor control circuit and a system controller is provided. The method comprises determining which signals in the interface have incompatible voltage requirements, and inserting a series resistance in an interface extending between the motor control circuit and the system controller for each signal determined to have incompatible voltage requirements therebetween. 
         [0005]    In another aspect, an interface for utilization between a system controller and a controller for an electronically controlled variable speed motor (VSM) is provided. The interface includes a first plurality of conductors each configured to directly connect an output of the system controller to a corresponding input of the VSM controller, and a second plurality of conductors. Each of the second plurality of conductors includes a series resistor therein, and each of the second plurality of conductors is configured to connect an output of the system controller which operates at a first voltage level through the corresponding resistor to an input of the VSM controller that operates at a second voltage level, where the second voltage level is lower than the first voltage level. 
         [0006]    In still another aspect, a system for controlling an electronically controlled variable speed motor (VSM) including an VSM controller is provided. The system includes a system controller, and an interface electrically coupling the system controller to the VSM controller. The interface includes a first plurality of conductors each configured to directly connect an output of the system controller to a corresponding input of the VSM controller, and a second plurality of conductors each comprising a series resistor therein. Each of the second plurality of conductors is configured to connect an output of the system controller, which operates at a first voltage level, to an input of the VSM controller, that operates at a second voltage level through the resistor. The second voltage level is lower than the first voltage level. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0007]      FIG. 1  is a block diagram of an air moving control system including an interface between a system controller and an electronically controlled variable speed motor (VSM). 
           [0008]      FIG. 2  is a schematic diagram of a passive interface for utilization between a VSM and a system controller. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0009]      FIG. 1  is a block diagram of an air moving control system  10  used to control an electronically controlled variable speed motor (VSM)  12 . One example of such a motor is an electronically commutated motor (ECM). System  10  is an air moving control system, such as a residential heating, ventilation and air conditioning (HVAC) control system, a light commercial HVAC control system, or a clean room filtering control system. System  10  includes an interface  14  providing an electrical coupling between a system controller  16 , for example a HVAC system controller, and a main unit  18 , for example a HVAC unit. Main unit  18  includes components  20  and VSM  12 . In one embodiment, VSM  12  is a blower motor. Motor  12  includes a motor controller  22  including a microprocessor (not shown) and a memory (not shown) containing an VSM program. Specifically, interface  14  provides connection of signals between system controller  16  and motor controller  22 . 
         [0010]    System controller  16  is connected to a thermostat  24  that includes a plurality of settings, or modes, such as low heat, high heat, cooling, dehumidify, and continuous fan. Additionally, thermostat  24  measures a temperature in a predetermined space or location and transmits an electrical signal representing the measured temperature to controller  16 . Controller  16  controls main unit  18  via interface circuit  14 . Interface  14  receives control signals in the form of input voltage signals from system controller  16  and provides signals suitable for controlling operation of VSM  12 . In one known application, interface  14  includes active circuitry for interfacing the signals of system controller  16  with those of VSM  12  (and motor controller  22 ). The translated signals are transmitted to blower motor  12 , and the torque, speed, or air flow of blower motor  12  is varied in accordance with the adjusted voltage outputs. 
         [0011]    In an alternative embodiment, interface  14  is not electrically connected to thermostat  24 , but rather electrically connected to a manual controller  26  and an additional blower motor  28 . When manual controller  26  and blower motor  28  are connected to interface  14 , a user is able to selectively operate blower motor  12  using an on/off switch. In such an embodiment, system controller  16  does not control blower motor  12  operation. 
         [0012]    In one embodiment of the present invention interface  14  does not include active circuitry. Rather, and as described in additional detail below, interface  14  simply includes one or more series resistors to reduce the voltage levels of signals originating at system controller  16  to be compatible with motor controller  22  of VSM  12 . More specifically, in various embodiments, system controller  16  may provide signals substantially equal to 115 VAC or 230 VAC and the resistors within interface  14  are of a resistance such that the signals are reduced to be approximately 24 VAC at motor controller  22 . 
         [0013]      FIG. 2  is a schematic diagram of one embodiment of a passive interface  14  for utilization between a VSM controller  22  (coupled to VSM  12 ) and a system controller  16 . Interface  14  includes a first set of conductors  50  that provide direct connections between system controller  16  and VSM controller  22 . Conductors  50  provide, for example and as illustrated, power connections for VSM controller  22 , specifically, LINE, NEUTRAL, and GROUND connections. Another conductor  50  provides a return signal path for the control signals described below. A return signal is also sometimes referred to as a common. 
         [0014]    As described above, various air moving control system functions utilize different motor speeds. The embodiment of VSM controller  22  illustrated in  FIG. 2  includes a number of motor tap signals, T 1 , T 2 , T 3 , T 4 , and T 5 , which, depending on the signals applied thereto, dictate one or more operational characteristics of VSM  12 . Corresponding signals at system controller  16  include HEAT, COOL, FAN, and may include others not shown in  FIG. 2 , for example a second FAN speed or a second COOLing speed. The embodiment of system controller  16  shown in  FIG. 2  does not include any secondary signals and these are illustrated as no connect or NC. 
         [0015]    Interface  14 , in the embodiment illustrated, includes a plurality of conductors  52  that connect the motor tap signals at VSM controller  22  to the corresponding signals at system controller  16 . Each conductor  52  of interface  14  includes a series resistor  54  which operates to, for example, drop the 115 VAC HEAT signal output by system controller  16  to an approximate 24 VAC level which can be utilized by VSM controller  22 . Motor tap signals at VSM controller  22  are sometimes referred to as selection lines. 
         [0016]    Interface  14  includes a resistor  54  in series with each of the selection lines (or the common (neutral) line in an alternative embodiment) of VSM controller  22  that are used for controlling the VSM. In various alternative embodiments, resistors  54  are part of a wiring harness and are either mounted on a separate circuit board assembly or are added at one of VSM controller  22  or system controller  16 . 
         [0017]    In specific embodiments, for interfacing a nominal 230 VAC signal from system controller  16  to VSM controller  22 , an approximately 22K ohm, 5 watt resistor is utilized for each selection line, while for interfacing a nominal 115 VAC signal from system controller  16  to VSM controller  22 , an approximately 10K ohm, 2 watt resistor is utilized for each selection line. These resistance values provide acceptable drive levels to VSM controller  22  and an acceptable power dissipation rating, at least for the common 115 VAC and 230 VAC signals typically found at system controllers. 
         [0018]    The low voltage interface of VSM controller  22  is a valid and often preferred way to interface with external circuits that are configured to operate with 24 VAC signals. Therefore incorporating resistors internal to VSM controller  22  is not desirable because such a VSM controller would no longer be able to interface to system controllers and thermostats that utilize nominal 24 VAC signals. Additionally, when referring to the 115 VAC and 230 VAC system controllers described herein, resistors within VSM controller  22  are also undesirable because the power dissipated by the plurality of resistors can increase the internal ambient temperature for the control electronics within VSM controller  22  and the high voltage that would need to be input to VSM controller  22  would require safety agency spacing criteria to be met therein, thereby increasing the overall physical dimensions of VSM controller  22 . Additionally, incorporating resistors within a VSM controller would increase the cost. 
         [0019]    Incorporating resistors  54  into a harness assembly, or other embodiment of interface between VSM controller  22  and system controller  16 , minimizes the changes required to either complex electronic assembly. When needed, a different harness assembly (relatively lower cost) can be incorporated into the air moving system, for example, should a change from a VSM to an induction motor occur. Utilization of interface  14  is likely the lowest cost solution for an OEM that has a high mix of products, since it minimizes the inventory of higher cost items such as active interfaces between system controllers and VSM controllers or version of either that are specifically modified for direct connections therebetween. 
         [0020]    Interface  14 , as described herein, provides a high voltage interface technique compatible with low voltage circuitry through use of a resistor in series with each of the I/O (Input/Output) (selection) lines to drop voltage. Interface  14  therefore allows a voltage higher than the motor controller (VSM controller  22 ) is rated for to be used to select operating modes for the motor. 
         [0021]    While the invention has been described in terms of various specific embodiments, those skilled in the art will recognize that the invention can be practiced with modification within the spirit and scope of the claims.