Abstract:
A test setup for a permanent magnet motor provides a method for identifying symmetry or asymmetry in the magnetic fields of the motor&#39;s permanent magnets. The setup comprises a test circuit that includes a fixed reference node that provides a stable reference to which the motor&#39;s common node can be compared. Observing the waveform of the voltage between the two nodes while the motor is running helps identify an imbalance in the magnetic fields of the motor&#39;s permanent magnets.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This is a continuation of application Ser. No. 12/321,451 filed on Jan. 21, 2009 by the present inventors. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The subject invention generally pertains to permanent magnet motors and more specifically to a setup and method for testing and/or monitoring such a motor. 
       BACKGROUND OF RELATED ART 
       [0003]    Permanent magnet motors, such as brushless DC motors and stepper motors, typically include multiple permanent magnets each having a magnet field that interacts with multiple electrical coils to rotate a rotor within a stator. Some permanent magnet motors have the magnets on the rotor with the coils in the stator, and others have the magnets in the stator with the coils being part of the rotor. In either case, it may be desirable to check the symmetry of the magnetic fields of the permanent magnets to ensure proper, smooth operation of the motor. Checking magnetic field symmetry; however, can be difficult to do, particularly after the motor is assembled and operating. 
       SUMMARY OF THE INVENTION 
       [0004]    It is an object of the present invention to provide a setup and method for testing and/or monitoring a permanent magnet motor. 
         [0005]    Another object of some embodiments is to check the symmetry or asymmetry of the magnetic fields provided by a plurality of magnets of a permanent magnet motor. 
         [0006]    Another object of some embodiments is to check the symmetry or asymmetry of the magnetic fields as the motor is energized and running. 
         [0007]    Another object of some embodiments is to check the symmetry or asymmetry of a permanent magnet motor&#39;s magnetic fields by comparing the voltage, current, and/or waveform of the motor&#39;s common node relative to a fixed reference node of an external test circuit. 
         [0008]    Another object of some embodiments is to measure the imbalance of a motor&#39;s magnetic field, wherein the measured imbalance can be used, depending on rotor design, to monitor rotor conditions due to variations in mechanical, thermal, or magnetic operating parameters. The imbalance may be deliberately introduced in the rotor design to allow this monitoring. 
         [0009]    One or more of these and/or other objects of the invention are provided by a motor test setup comprising a test circuit and a permanent magnet motor. The test circuit includes a fixed reference node that provides a reference to which the motor&#39;s common node can be compared. An instrument sensing the voltage, for example, between the two nodes helps identify an imbalance in the magnetic fields of the motor&#39;s permanent magnets. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]      FIG. 1  is a schematic diagram illustrating a method and setup for testing and/or monitoring a permanent magnet motor. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0011]      FIG. 1  schematically illustrates a motor test setup  10  with one example of a permanent magnet motor  12  connected to a test circuit  14  for detecting and evaluating an imbalance or asymmetry in the magnetic field of the motor&#39;s permanent magnets. The expression, “permanent magnet motor” means any electromotive rotational machine that includes at least one permanent magnet having a magnet field interacting with the magnet field of an electric coil. The permanent magnet could be attached to the rotor with the coil being stationary, as shown in the example, or the magnet could be stationary with the coil rotating with the rotor. Examples of a permanent magnet motor include, but are not limited to, a brushless DC motor and a stepper motor. More specific examples of motor  12  include TK-85 and TK-106 series motors by PHASE Motion Control of Italy. For the example illustrated in  FIG. 1 , motor  12  happens to be a brushless DC motor with a rotor  16  having a plurality of permanent magnets  18 ; however, other types of permanent magnet motors with any number of magnets and any number of coils can also be tested using the method described and illustrated here. 
         [0012]    Referring to  FIG. 1 , motor  12  comprises a rotatable shaft  20  supporting rotor  18  within a stator  22 . Magnets  18  of rotor  16  are elongate in a direction generally parallel to shaft  20 . The magnetic poles of magnets  18  run generally radially with the north/south poles alternating from one magnet to the next. Stator  22  comprises a plurality of coils  24  that when sinusoidally or strategically energized by a polyphase power source  26  provide an electrically induced magnetic field that interacts with the magnet fields of magnets  18  urge rotor  16  to rotate within stator  22 . The phrase, “strategically energized,” refers to a waveform that is something other than purely sinusoidal. Such strategic energizing of a motor can be provided, for instance, by a PWM electronic power converter. Power source  26  can be a 3-phase system or some other polyphase power source (e.g., 2-phase, 4-phase, 5-phase, etc.). 
         [0013]    To connect motor  12  to power source  26 , the coils of stator  22  include a first electrical lead  26 , a second electrical lead  28  and a third electrical lead  30  that can be wired to power source  26 . Also, coils  24  share a common node  32 . Under normal, non-test operation, node  32  might be grounded or left as a floating ground. 
         [0014]    To test or monitor the magnetic field imbalance of motor  12 , test circuit  14  comprises a first resistor  34 , a second resistor  36  and a third resistor  38  that are wired respectively to leads  26 ,  28  and  30 , whereby power source  26  energizes motor  12  and test circuit  14  in a similar manner. Resistors  34 ,  36  and  38  share a reference node  40  that provides a reference point to which common node  32  can be compared when power source  26  energizes motor  12  and circuit  14 . Resistors  34 ,  36  and  38  preferably are of equal resistance with a resistance value suitable for the power source. An appropriate resistor size for a 480-volt system, for example, would be about 1 megohm. 
         [0015]    An instrument  42  connected to reference node  40  and common node  32  is able to detect a differential electrical signal  46  between nodes  32  and  40 , wherein signal  46  reflects how well the plurality of magnet fields of magnets  18  are balanced or similar to each other. Instrument  42  can be any device capable of sensing an electrical difference between nodes  32  and  40 . Examples of instrument  42  include, but are not limited to, an oscilloscope, a voltmeter, an ammeter, etc. Examples of differential electrical signal  46  include, but are not limited to, voltage between nodes  32  and  40  or electrical current between nodes  32  and  40 . 
         [0016]    A displayed reading  44  of instrument  42  can show the actual waveform of signal  46 , wherein signal  46  varies cyclically as rotor  16  rotates, or reading  44  can perhaps be a substantially constant output value such as an RMS value of signal  46 . Instrument  42  displaying a sinusoidal waveform having a peak voltage or current amplitude that is substantially constant (i.e., each peak has substantially the same amplitude) can indicate that the magnetic fields of rotor  16  are substantially symmetrical. If a generally sinusoidal waveform of differential electrical signal  46  has a peak amplitude that varies cyclically as rotor  16  rotates, that could indicate an asymmetry or an imbalance in the rotor&#39;s magnetic fields. 
         [0017]    In the proposed possible case where instrument  42  displays an RMS value of signal  46 , an RMS value of zero could indicate that the rotor&#39;s magnetic fields are substantially symmetrical or balanced. An RMS value other than zero could indicate appreciable asymmetry with the amplitude of the RMS value reflecting the magnitude of the asymmetry. The motor system can include triplen and other harmonics depending on motor design which result in nonzero differential signals and can be used for monitoring. In some cases, a predefined acceptable range of differential electrical signal  46  would be where the asymmetry is appreciably greater than zero (i.e., not perfectly symmetrical) but less than a predetermined upper limit. In such cases, motor  12  would be considered acceptable for normal use if signal  46  was within the predefined acceptable range, but motor  12  would be considered unacceptable for normal use if signal  46  was beyond the predefined acceptable range (i.e., greater or less than the predefined acceptable range). The expression, “normal use” refers to a motor fully functioning as it was originally designed to operate. It should be noted that the terms “symmetrical” and “balanced” are used interchangeably throughout the description of the invention. Likewise, the terms, “asymmetrical” and “imbalanced” are also used synonymously. 
         [0018]    Although the invention is described with respect to a preferred embodiment, modifications thereto will be apparent to those of ordinary skill in the art. The scope of the invention, therefore, is to be determined by reference to the following claims: