Starting of an AC induction motor produces an inrush current in windings of the motor. Magnitude of the inrush current is 5-10 times greater than nominal current.
Base method to make inrush current lower is to limit power, which the motor is able to receive during starting period, so that to make possible desired starting process. AC induction motor starters are used to change characteristics of electric current supplied to the motor for realizing desired starting.
It is sometimes necessitated that a stop process of the motor will be within definite characteristics, for example, smooth or slow stopping.
In starters used in modern industry: star-delta starter, auto-transformer starters, some types of soft starters change of characteristics of electric current supplied to an AC induction motor is fulfilled by steps. Moments of step changing as a rule are predetermined. In some cases the moments are defined during start process.
It is known method of controlling an induction motor and apparatus thereof (PCT WO 00/77919 A1 from 21 Dec. 2000), which based on reference values that are set according to characteristics of an induction motor by user, controlling currents inputted to induction motor and comparing the currents and reference values to discriminate a trip conditions of induction motor. Among reference and control parameters are driving current value, peak current value, delay time period and Y-starting time period. Reference parameters are introduced in controller and corrected when operating mode is manual to compensate motor parameters changing. They are used by controller, when operating mode is automatic.
Y-starting time period introduced and corrected in controller, when operating mode is manual, does not react on changes in supplied to motor voltages and charge of the motor, when operating mode is manual or automatic in future starts.
A method is provided for controlling the starting of an AC induction motor with a soft starter (U.S. Pat. No. 6,380,708, from 30 Apr. 2002). The windings are connected in a delta configuration and each winding has a corresponding thyristor switch connected in series therewith such that each winding and thyristor switch combination is connected between two corresponding terminals of the AC induction motor. Alpha control of the thyristor switches is utilized to start the AC induction motor and bring the AC induction motor to near full operating speed. As the AC induction motor nears full operating speed, gamma control of the thyristor switches is utilized to bring the AC induction motor to full operating speed.
The method include steps: repeating for a predetermined time period the steps of calculating a new alpha firing angle and sequentially firing each thyristor switch; upon completion of the predetermined time period, sequentially firing each thyristor switch at a user selected alpha firing angle after occurrence of zero volts supplied by the AC input source; and monitoring motor current in the AC induction motor and if the motor current is greater than a predetermined value, repeating the step of sequentially firing each thyristor switch at a user selected alpha firing angle. New alpha firing angle is calculated according to the initial firing angle, integral of the monitored motor current over a conduction time interval and a user selected preset current limit.
In soft starter it is possible to change firing angle every half period. In the discussed invention it is selected to work at least predetermined time period with the same firing angle and time of firing at each user selected firing angle equals one or some predetermined time periods depending of relations between integral of the monitored motor current over a conduction time interval and reference parameter—a user selected preset current limit. Step difference of user selected firing angles also depends from these parameters.
It is necessary to say that motor current is not enough suitable parameter for selection of motor starting step duration because the current depends of parameters of power supply, motor and charge. In conditions of low level of power supply and big charge even large motor current will be only enough to rotate motor with existing low speed without acceleration. Prolongation of starting with such power level increases starting time.
This is a list of mostly used methods of power consumption changing in start or stop process of an AC induction motor:                1. Changing connection of motor windings. In case of three windings, the change of winding connection from star to delta configuration increases motor power consumption three times.        2. Changing voltage of power supply to an AC induction motor, for example, by changing transformation ratio of transformer used in power controller.        3. Changing time of power supply to an AC induction motor by some type of modulation. Here is phase modulation, which uses in thyristor switches, pulse-time modulation in solid state relays and etc.        4. Changing motor current, for example, the rotor current by means of changing of resistors in circuit of phase rotor in motor with collector.        
Each step of starting is characterized with changing of motor power consumption, higher at beginning and lower at end of the step. This motor power consumption depends of parameters of power supply, motor and charge.
Power controller output defines power level limit, which the motor may reach in definite conditions on each step. It is possible only to establish relationship between power level limits of the steps in identical conditions and to give to each step a number in increasing or decreasing row of power level limits.
Number of power level limits and a magnitude of each power level limit or defined by power controller construction as in case of star-delta starter or transformer starter, and consist of limited number of power level limits. In case of soft starter larger number of power level limits may be chosen before or during starting for more smooth process.
If number of changing of power level limits and magnitude of each power level limit are definite, an optimal sequence of time periods of selected power level limits may be found for each starting condition. Changing of power level limits in shorter time intervals brings to increasing of start current. Prolonged time intervals between changes of power level limits are resulted in prolongation of start period with current greater than nominal current.
However, the conditions of each motor start are changing from start to start because of changing of motor charge, voltage of AC source, temperature, aging of motor and starter and so on. Power, received by an AC induction motor at the same power level limit may be in different cases sufficient or insufficient to produce breakaway torque or acceleration. If load can be changed during start process, the same one step changing of power level limit increases or does not increase the speed of motor rotation. The last case may be realized when increase of load needs greater torque increase than the motor possible to produce after a stepped changing of power level limit.
So, as the result of changing start conditions, AC induction motors starts as a rule or with increased current, or with prolonged start time, or with both phenomena.
For fulfilling desired start of an AC induction motor we need, at first, to supply power, which can produce breakaway torque, and further to add power, which increases speed of motor rotation on each step so, that transient conditions will be reasonable or desired.
Therefore, it is a primary object and feature of present invention to provide an improved method for controlling of an AC induction motor.
It is a primary object and feature of present invention to provide an improved method for controlling the starting and stopping of an AC induction motor in stable manner.
It is a primary object and feature of present invention to provide an improved method for controlling rotation speed of an AC induction motor.
It is still a primary object and feature of present invention to provide an improved method for controlling an AC induction motor, which utilizes traditional and less expensive components.