A current limit for an AC motor drive that is operative whenever motor speed change is commanded and motor current exceeds the current limit value. The rate of change of motor speed is controlled to maintain maximum motor current, i.e., motor current substantially equal to the current limit. This is achieved by moderating the rate of change command to the motor drive circuit in accordance with the amount of motor current in excess of the current limit.
A hermetic box contains a motor and a self reset type protector which interrupts the supply of electric power to the motor when an abnormally heated condition takes place in the hermetic box and automatically supplies again the electric power when the abnormally heated condition ceases. The motor is energized by an inverter having AC output terminals connected to the motor outside of the hermetic box. A current detector which detects that no current is flowing into the motor is provided on the output side of the inverter outside for a hermetic box. Provision is further made of speed settor for setting the speed of the motor, a lump circuit which receives the output of the speed setting and produces a signal obtained by loosening the sudden change of the output of the speed setting, inverter output control which so controls the inverter that it produces an output voltage and an output frequency depending upon the output of the lump circuit, and a reset circuit which decreases the output of the lump circuit when the current detector has detected that no current is flowing into the motor.
The invention relates to a speed control system which includes an inverter circuit for converting a direct current into a polyphase alternating current in the form of a rectangular wave by the switching action of transistors, and an inverter drive circuit adapted to apply control signals to the inverter circuit to bring its output frequency to a desired value, an induction motor being rotated by the alternating current supplied by the inverter circuit. The system features current detection means for detecting current that flows through the output side of the inverter circuit, and decision means for examining the output of the current detection means for a fixed period at a frequency higher than that at which the induction motor is driven. The decision means interrupts the operation of the inverter circuit when it has been judged that the current flowing through the outside thereof is greater than a prescribed value, and actuates the inverter circuit when the current flowing through the output side thereof is less than the prescribed value. The result is that the inverter circuit is allowed to operate intermittently at a current which does not exceed a prescribed value, whereby the induction motor runs continuously without any observable halt in operation.
A D.C. motor control system of the closed loop regulating type is disclosed. The system includes a timed speed reference circuit with means for clamping the same to hold the speed reference voltage change at a rate that corresponds to the rate of motor speed change when armature current limit action occurs during acceleration and deceleration periods of the motor. A speed error spill-through circuit is also provided to increase the proportional speed gain of the system when the speed error increases beyond a given percentage of base speed to afford improved damping characteristics and reduce overshoot or undershoot of the overall motor drive.
An induction motor controller with a scaling convertor for both providing selective alteration of the overload sensitivity characteristic of the controller and converting the input current to a proportional control voltage. The feedback circuit includes a plurality of feedback resistors which are selectively connectable between the output of the current-to-voltage convertor and its input to selectively change its gain and thus the sensitivity of the controller.
In a motor control apparatus arranged to drive a motor by rectifying a.c. voltage and smoothing a rectified voltage by a capacitor, a voltage across the capacitor is detected to see if the voltage is below a reference voltage or not by a first comparator (24), and if so, an output voltage (V) is produced. This output voltage (V) is used to change another reference voltage of another comparator (21) used for detecting if current to the motor (5) or a motor drive circuit (4) is greater than the reference. When the current is larger than the reference, the comparator (21) produces a low level output causing a control circuit (23) to control the drive circuit (4) to reduce the current to motor (5).
