Abstract:
Methods and apparatus for minimizing electrical machine vibration are described. In an exemplary embodiment of the method, power is applied to the motor under microprocessor control such that a pulse modulated current profile is applied to the motor which in turn controls the amount of torque generated by the motor. By adjusting current profiles, torque generation is controlled, and vibration and noise are eliminated.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates generally to dynamo-electric machines and, more particularly, to processes for reducing vibration in motors during start-up and operation. 
     Electric motors are used in countless varieties and applications worldwide. Typically, torque generated at a rotor, which supplies the rotational force, is the product of current applied to the motor and an electromotive force generated by the application of a voltage to the coils of the motor. Motors generate torque in order to do work, that is, typically to drive a load. 
     In some applications, depending upon motor mounting or other factors, the generated torque, together with the load, may cause a motor to vibrate and generate noise as the motor begins to move its load. One example of such an application is where the motor is driving a fan as its load. Imbalances in the fan combined with torque pulses produce vibrations which are conducted to the motor and fan mounting, producing undesirable noise. Damping systems are typically employed to minimize the effects of the vibrational energy induced into the motor and fan system. Such damping system are expensive and tend to deteriorate over time due to exposure to the elements and continued exposure to vibrational energy, leading to loosened motor and fan assemblies, potentially leading to failures of the motor or the fan. 
     In some applications, the problem is most prevalent at startup. However, once the motor is up to speed however, the noise and vibrations lessen or disappear. In many applications, the motor generated noise and vibrations at startup are undesirable. In other applications, such as the fan example described above, the noise and vibration problems are always present. It would be desirable to control motor startup and operation to eliminate the problem of high torque vibration and noise, allowing the possibility of eliminating damping systems, and reducing costs. 
     BRIEF SUMMARY OF THE INVENTION 
     In an exemplary embodiment, a motor control system includes a microprocessor programmed to apply current to an electrical motor. The microprocessor pulse width modulates the current so that reductions in peak currents and the filtering of fundamental frequencies reduce the amount of torque generated by the motor. Reductions in torque reduce vibrations and noise of the motor thereby allowing reductions or elimination of damping systems. The method for minimizing electrical machine vibration includes the steps of applying power to the motor under microprocessor control such that a pulse width modulated current profile is applied to the motor. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is an exemplary embodiment of an electric machine vibration reduction system; 
     FIG. 2 is a chart showing back electromotive force, current, and torque waveforms for one known motor control system; 
     FIG. 3 is a chart showing back electromotive force, current, and torque waveforms for a motor control system according to one embodiment of the present invention; 
     FIG. 4 is a chart showing back electromotive force, current, and torque waveforms for a motor control system according to a second embodiment of the present invention; 
     FIG. 5 is a chart showing back electromotive force, current, and torque waveforms for a known motor control system; 
     FIG. 6 is a chart showing back electromotive force, current, and torque waveforms for a motor control system according to a third embodiment of the present invention; 
     FIG. 7 is a chart showing back electromotive force, current, and torque waveforms for a motor control system according to a fourth embodiment of the present invention; and 
     FIG. 8 is a chart showing back electromotive force, current, and torque waveforms for a motor control system according to a fifth embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 shows a block diagram of a motor control system  10 , according to one embodiment of the present invention. System  10  includes a motor  12 , a microprocessor  14 , a memory  16 , which can be internal or external to microprocessor  14 , motor driver circuits  18 , level shifting logic  20 , a hall effect sensor  22 , and a strobe circuit  24 . As used herein, microprocessor  14  refers to controllers and processors, including microcontrollers, programmable logic controllers, input/output controllers, reduced instruction set circuits, application specific integrated circuits, logic circuits, and any other circuit, processor or microcomputer capable of processing the embodiments described herein. System  10  further includes an undervoltage reset circuit  26 , a power supply circuit  28 , and an input circuit  30 . As described in more detail below, memory  16  is configured with an algorithm, described in more detail below, which when executed by microprocessor  14 , control the time and duration which driver circuits  18  supply current in a current profile to motor  12 . Hall effect sensor  22  and strobe circuit  24  provide feedback to microprocessor  14  on the rotor position of motor  12  for controlling the algorithm. 
     The processes described below alternatively can be implemented, for example, in a personal computer programmed to execute each described step. The processes, however, can be implemented in many different manners and are not limited to being implemented and practiced on a personal computer. For example, the processes could be implemented in a server and accessed via a network, such as a local area network and/or a wide area network. 
     Motors typically are configured to satisfy specific performance requirements measured at several steady state operating points. The performance requirements include rated operating point torque, current, slip, power factor, and efficiency, pullout (breakdown) torque, locked rotor torque and current, and no-load current. 
     Torque of brushless DC motors and electronically commutated motors (ECM) is equal to the product of motor current and a back electromotive force (EMF) multiplied by a constant which represents losses present in the motor. FIG. 2 illustrates motor torque, motor current, and a back EMF of one known motor control system using waveforms  50 . As shown in FIG. 2, a torque waveform  52  is the product of a motor current waveform  54  and a back EMF waveform  56 . As torque reaches a particular level, which is different in each motor and for each motor application, shown as peaks  58  in torque waveform  52  in FIG. 2, vibrations in the motors or the mountings are induced. 
     FIG. 3 shows waveforms  70  where current has been limited using a motor control system  10  (shown in FIG. 1) configured with an algorithm to limit motor current during periods of peak back EMF. Back EMF waveform  72  shows a typical back EMF sinusoid and its associated peaks  74 . Current waveform  76 , which is generated using system  10  is pulse width modulated so that periods of current reductions  78  occur during periods of peak back EMF. As used herein pulse width modulation includes repetitive pulse control, missing pulse waveforms, and variable width pulse waveforms. A torque waveform  80  is shown which represents a product of back EMF waveform  72  and current waveform  76 . As a result of current reductions  78 , the torque waveform  80  becomes more constant than torque waveform  52  (shown in FIG. 2) since a fundamental frequency of the torque is reduced and therefore motor vibrations are decreased. 
     FIG. 4 shows waveforms  90  depicting another embodiment of the present invention. A current waveform  92 , generated by system  10  (shown in FIG. 1) is shown as leading a back EMF waveform  94  by n electrical degrees. Leading angle n is determined by the configuration of system  10  and by inductance of the motor. By changing the timing of current waveform  92  with respect to back EMF waveform  94  shaping and timing of torque waveform  96  is accomplished. In the embodiment depicted in FIG. 4, advancing the current waveform  92  causes more motor torque to be generated at higher speeds, but also generates negative torque pulsations. 
     FIG. 5 shows waveform  100  of a known system configured so that current is removed from a motor at  135  electrical degrees of a 180 degree back EMF waveform  102 . Current pulses are as shown by current waveform  104 . By controlling an amount of time a current pulse is applied to a motor, a torque waveform  106  is controlled. 
     FIG. 6 shows waveform  110  including a torque waveform  112  according to another embodiment of the present invention. Instead of having a fixed time without current, as described by the system depicted in FIG. 5, for a portion of a back EMF waveform  114 , system  10  is configured to provide multiple current pulses  116  during the back EMF waveform  114 , and also multiple no current times  118 , resulted in a reduced peak torque. In addition the amount of current in pulses  116  control torque level. As shown in FIG. 6, current pulses  116  are lower in amplitude during periods of higher back EMF, as shown on waveform  114 , thereby resulting in uniformity in amplitude of torque pulses  112 . 
     FIG. 7 shows one embodiment including waveforms  120  where system  10  is configured to provide a pulse width modulated current waveform  122  that is on for a longer period of a back EMF waveform  124 , thereby resulting in a torque waveform  126  with a high peak value. As shown in FIG. 7, waveform  122  includes multiple current pulses  128  during a period of positive back EMF, shown on waveform  124 . 
     Referring now to FIG. 8 waveform  130  include a current waveform  132  constituting current pulses  134 . As shown in FIG. 8, system  10  has been configured so that one of the modulated current pulses  134  is missing from waveform  132 . By removing a current pulse  134  from waveform  132 , the current waveform can be on for a longer portion of back EMF waveform  136  while still reducing an amount of peak torque as shown in torque waveform  138 . 
     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.