Abstract:
A control method and a control system for single phase induction motors driven by two-power electronic switch inverter are disclosed. The system fulfills two main tasks i.e. precise motor speed control and maximum motor efficiency control over wide ranges of motor load and speed command without a motor speed feedback.

Description:
FIELD OF THE INVENTION 
   The present invention relates to motor control systems and more particularly to the control of single phase alternating current induction motors. 
   BACKGROUND OF THE INVENTION 
   A Single phase induction motors (SPIM) need an auxiliary winding in parallel with motor main winding for starting. Also, there should be a current phase difference between the main and auxiliary windings of SPIMs. This is usually facilitated by inserting a capacitor in series with the auxiliary winding. The auxiliary winding and the capacitor may also improve the motor performance if they are not disconnected from the supply after starting. Such a capacitor is preferred to be adjusted to provide a close to 90° phase differences between currents of the two windings. This results in a balanced motor operation. However, in variable speed drives, the 90° can not be maintained at non-rated speeds unless the capacitor impedance is readjusted to an optimized value at each supply frequency by additional means. Therefore, the balanced operation can not be provided and the motor efficiency reduces without capacitor readjustment. 
   Variable speed drives improve energy efficiency. If they are implemented with efficiency optimizing schemes, even more energy saving is gained. Various variable speed drives and efficiency optimization schemes have been proposed for SPIMs. U.S. Pat. No. 4,455,521 has presented a circuit for controlling the operation of a SPIM to obtain maximum motor efficiency over an entire range of load conditions. It has used a well known stator voltage control method to reduce motor flux when motor load decreases. This is a constant frequency control and doesn&#39;t provide variable speed capability for motor. Similar methods of stator voltage control for constant speed applications have been used in U.S. Pat. No. 4,409,528 and U.S. Pat. No. 5,670,858. 
   A single phase sinusoidal shaped variable voltage variable frequency waveform has been proposed in U.S. Pat. No. 4,706,180 for driving a single phase ac induction motor by employing a sinusoidal pulse width modulated signal to switch a pair of solid state power switches. In U.S. Pat. No. 5,252,905 a variable frequency driving system has additional options: an ac power line for providing single phase ac power line voltage at a fixed frequency to drive the motor at an adjustable speed. The last two patents concentrate on the improvement of power supply specifications. However, control of motor and improvement of its performance are not considered. In U.S. Pat. No. 4,566,289, a refrigerator control system has been implemented by a change-over device to drive a motor both through an inverter at reduced speed and directly by a commercial power supply at a nominal speed. As a result, the inverter loss is eliminated when it is bypassed and the total efficiency of the refrigerator increases. This idea has been used in U.S. Pat. No. 6,570,778 B2 in a different way such that the main winding of the motor is supplied directly from ac power line to develop start-up torque; while in lower speed values the motor is supplied by an inverter. In the last two patents, motor efficiency may decrease substantially at non-rated frequencies when the motor is supplied from the inverter or under non-rated loads. 
   In the present invention a low cost variable speed drive for SPIMs is proposed to control motor speed and maximize motor efficiency over the entire speed range. 
   SUMMARY OF THE INVENTION 
   A low cost variable speed single phase induction motor drive is much needed for many applications. The proposed invention provides such a drive with a minimum number of components and without a mechanical sensor. The drive is controlled by a simple and effective control system which optimizes motor efficiency at every operating point regardless of its controlled speed and load. 
   This invention uses the fact that the phase difference of the main and auxiliary winding currents is a motor variable which influences major motor characteristics and can be used in the determination of motor performances. It is claimed in this invention that this variable is independent of motor load and depends only on motor supply frequency. It is proposed in the present invention to control this angle. 
   The invention makes it clear that there is an optimal motor slip, at each motor frequency, corresponding to a maximum motor efficiency. However, using motor slip as a means of efficiency optimization requires an expensive speed sensor which is not favored in SPIM drives. It can be shown that the motor slip is a function of the currents phase angle. As so, in every frequency there is a unique optimal currents phase angle corresponding to a maximum efficiency. 
   Therefore, it is proposed in this invention to control the phase angle as a means of efficiency optimization control of SPIMs. On the other hand, by controlling the phase angle, the motor slip, and in turn the motor speed is controlled. This results in a precise motor speed at every specific supply frequency despite load variation. Therefore, the present invention materializes a variable speed control together with an efficiency optimization control by a simple and low cost system. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is an overview of a system that controls the SPIM according to the invention. 
       FIG. 2  is a schematic representation of the control system circuit diagram of the present invention that connects to a SPIM. 
       FIG. 3  is the terminal box of the SPIM and its connections to internal and peripheral elements. 
       FIG. 4  is a block diagram of the control system that divides into two main blocks, the efficiency control block and the speed control block. 
       FIG. 5  is a curve fitted by a first order function on the experimental values of optimal windings currents phase difference. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   With reference to the drawings, a low cost drive system for single phase induction motors is illustrated as  10  in  FIG. 1 . The drive  10  receives electric power from a single phase ac power supply  11 . Two input lines  12  and  13  connect ac power supply  11  to a rectifier  14  and a capacitor filter  15  which provide DC power through DC bus lines  16  and  17 . The DC voltage on  16  and  17  is connected to an inverter  18 . The inverter  18  can provide a variable voltage/variable frequency sinusoidal pulse width modulation supply, and supplies a single phase induction motor  19  through an output line  20  that is connected to the motor terminal box  21  through the motor supply cable  22 . The motor ground cable  23  is connected to the null of the ac power supply  24  through a line  25 . The motor  19  is coupled to a load  26  through a coupler device  27 . Two motor signals  28  and  29  that are in proportion to the motor winding currents are delivered to a control block  30  which commands the inverter  18  according to a user speed reference  31 , through two control lines  32  and  33 . 
     FIG. 2  illustrates the rectifier  14 , the capacitor filter  15 , the inverter  18  and the motor  19  in more details. The rectifier  14  includes two diodes  35  and  36  connected together at a node  37  to which the ac supply line  13  is connected to the pair of diodes  35  and  36  across DC bus lines  16  and  17 . The capacitor filter  15  includes two capacitors  38  and  39 , connected together at a node  40  to which the ac null line  12  is connected. The pair of capacitors  38  and  39  also is connected across DC bus lines  16  and  17  to filter the rectified voltage by the rectifier  14  to provide fairly constant DC bus voltage across the lines  16  and  17 . The inverter  19  is implemented by power electronic switches, such as IGBTs  41  and  42  shown in  FIG. 2  that are driven by gate drivers  43  and  44  which receive control signals from the control block  30  through the control lines  32  and  33 . The pair of IGBTs  41  and  42  are connected across DC bus lines  16  and  17 , and they are connected together at a node  45  which the inverter output line  20  is connected to and extends to the motor terminal box  21  through the motor supply cable  22 . The single phase induction motor  19  includes a main winding  46 , an auxiliary winding  47 , a phase shifting capacitor  48  and a terminal box  21  that are shown in  FIG. 2  by  19  and in  FIG. 3  by  19 . The main winding circuit includes the main winding  46  and a very small resistor  49  in series with  46  for main winding current sampling. The auxiliary winding circuit includes the auxiliary winding  47  and another small resistor  50  the same as resistor  49  in series for auxiliary winding current sampling. In  FIGS. 2 and 3  it is shown that the motor supply cable  22  is connected to the first terminal  51  of auxiliary winding  47  and the first terminal  52  of main winding  46 . The main winding is connected to the resistor  49  at terminal  53  and through that resistor  49  is connected to a null connection at terminal  54 . The auxiliary winding  47  is connected to capacitor  48  at terminal  56 , and the capacitor  48  is connected to resistor  50  at terminal  55  and through that resistor  50  is connected to the null connection at terminal  54 . The terminal  54  is connected to the null of ac power supply through the line  25 . Two output signal lines  28  and  29  are in proportion to windings currents and are extended to control block  30 . 
     FIG. 4  shows the details of control system  30 . It is divided into two interrelated control subsystems  60  and  61 , where the former is a speed control subsystem  60 , and the latter is an efficiency control subsystem  61 . The speed control subsystem  60  receives a frequency input value  63  as an input of a SPWM wave generator  66 . Another input of the SPWM wave generator  66  is the amplitude of voltage and is provided by multiplying  63  to a gain  64  that is a rated V/f value; therefore, corresponding value of motor voltage  65  is obtained. Then, SPWM wave generator  66  according to frequency value  63  and voltage value  65  generates a SPWM wave on  67 . Phase inverter  68  provides another SPWM wave that is an inverted signal of  67 . According to SPWM waves  67  and  69 , a block  70  provides two SPWM signals  32  and  33  with considering dead time between them to be applied to inverter  19  and its switches&#39; drivers  43  and  44 . Without the efficiency control subsystem and the proposed inter-related speed control subsystem, the speed control could have been a feed-forward speed control by which the motor speed changed with load changes. In addition it would have not had any control to improve efficiency. With utilizing the efficiency control subsystem  61  and applying its output  62  to speed control subsystem  60 , both efficiency maximization and steady-state speed control are obtained. User speed reference  31  enters to efficiency control subsystem  61  and is transformed to a frequency value  71  by block  72 . The relation of speed and frequency is a definite curve that can be obtained from experimental tests or simulation results. Because of little motor slip, it can be estimated as a first order function with very low error. So, it is applied in block  72 . This relation is satisfied when windings currents phase difference  73  is adjusted to a constant value commanded by  75  and through controller  76 . This controller maintains it at a constant value in steady state mode. The constant value of windings currents phase difference is selected according to efficiency maximization curve presented in block  74 . We proved analytically for single phase induction motors with an auxiliary winding that at each motor supply frequency, the motor efficiency relates to windings currents phase difference only. The efficiency maximization curve embedded in block  74 , presents the corresponding windings currents phase difference that maximizes the motor efficiency for various motor supply frequencies. It is based on a first order function that is fitted to the desired values of windings currents phase differences under various load conditions at various motor supply frequencies obtained by experiments as in  FIG. 5 . So, we can maximize the motor efficiency in various load conditions if we control the windings currents phase difference at various frequencies according to this curve. Therefore, block  74  obtains the desired value for windings currents phase difference  75  from frequency  71  and a Proportional-Integrator controller  76  controls the actual windings currents phase difference  73  at desired value  75 . The output of PI controller  77  is deducted from the voltage value  65  and the result is applied to the SPWM generator  66 . The actual windings currents phase difference  73  is obtained by the phase difference detection block  78  which detects phase difference of two signals  28  and  29  that are in proportion to motor windings currents. As the efficiency control is a steady-state control, it may not control the motor transient behavior well. Therefore, a transient state detection block  79  is considered that neutralizes the effect of PI control output by adding a desired value  80  to PI output  77  in transient state. Transient state of motor performance is detected with peak detection of signal  28  that is in proportion to main winding current by a peak detector  81 . For tailoring the peak detector output value to the desired value  80 , a feed-forward controller  83  is used which includes a low pass filter and a Proportional-Deriver controller. Before the PD controller, the low pass filter eliminates high frequency noises to prevent amplifying them by PD controller. 
   It is understood that the invention is not confined to the particular embodiments set forth herein as illustrative, but embraces all such forms thereof as come within the scope of the following claims.