Motor control system for variable speed induction motors
First Claim
1. A control system for an induction motor, comprising:
- control means for providing a variable frequency signal for controlling the speed of said motor, for providing phase signals from said variable frequency signals, and for a period of time, when said motor is started, for keeping said variable frequency signal and said motor speed at low levels;
switch means for applying power from a power source to said motor;
driver means for applying said phase signals to actuate said switch means; and
power sensing means for sensing the power drawn by said motor, said control means limiting said power drawn by said motor when said power exceeds a predetermined level by ramping down said variable frequency signal to a lower frequency to lower the speed of said motor and the power drawn by said motor.
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Accused Products
Abstract
A control system for a multiphase induction motor includes microcontroller control means providing a variable frequency signal and deriving high side/low side phase signals from the variable frequency signal. Power is applied from an AC/DC converter to the motor by an output drive bridge with three pairs of high side/low side semiconductor switches, one pair for each phase. The high side and low side phase signals are applied through high side/low side driver means to control the switches and effectively apply a quasi-square waveform to the motor windings. The motor and the control system electronics are protected from excessive current levels by current sensing means. When the current exceeds a predetermined level, the microcontroller limits the power drawn by the motor by ramping down the variable frequency to lower the speed of the motor. In case the motor or control electronics overheat, a temperature sensor is monitored by the microcontroller to shutdown the motor. When the motor is started, current inrush is avoided by modulating the phase signals during a start up period. The duty cycle of the modulation is gradually increased in steps until a duty cycle of 100% is reached at the end of the starting period. The voltage level applied to the motor may be reduced. In one embodiment, a voltage controller generates a pulse of fixed width which controls the duty cycle of the converter to lower the voltage level to a particular voltage level when enabled by an enabling signal from the microcontroller. In a second embodiment, a binary signal from the microcontroller to the voltage controller selects a particular pulse width to select the voltage level applied to the motor when enabled by the enabling signal.
161 Citations
19 Claims
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1. A control system for an induction motor, comprising:
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control means for providing a variable frequency signal for controlling the speed of said motor, for providing phase signals from said variable frequency signals, and for a period of time, when said motor is started, for keeping said variable frequency signal and said motor speed at low levels; switch means for applying power from a power source to said motor; driver means for applying said phase signals to actuate said switch means; and power sensing means for sensing the power drawn by said motor, said control means limiting said power drawn by said motor when said power exceeds a predetermined level by ramping down said variable frequency signal to a lower frequency to lower the speed of said motor and the power drawn by said motor. - View Dependent Claims (2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18)
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19. A motor control system, comprising;
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switch means to connect a motor across a power supply; frequency control means providing switching signals to said switch means at a variable frequency, the speed of said motor being related to said frequency being kept at a low level for a period of time when said motor is started; and power limit means responsive to power drawn by said motor which exceeds a predetermined level for controlling said frequency control means to reduce said frequency and said speed of said motor to reduce the level of power drawn by said motor.
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Specification