An interlocking control for a three-phase induction motor which by monitoring the voltage on the three-phase supply lines detects the presence of motor rotation and determines the direction thereof. The circuit includes operational amplifiers which monitor the voltage present on two of the three-phase supply lines with respect to the third phase, this monitored voltage being dependent upon generator action of the motor while coasting. The presence of a signal at the output of the operational amplifiers indicates the presence of motor rotation. The output signals of the operational amplifiers are coupled into logic circuitry to determine the direction of rotation, the output of the logic circuitry serving to drive silicon controlled rectifiers which in turn control interlocking relays. The contacts of these interlocking relays are disposed in the motor control circuit so as to prevent the sudden reversal of a rotating motor before it has come to rest, while allowing reenergization of a rotating motor in the same direction.
A motor current sensing circuit using a single sensing resistor in the motor current path to sense the current from the voltage source. The circuit includes an amplifier and a unity-gain inverter that translate the current across the sensing resistor into a voltage analog of the current in the motor. At any instant in time, the magnitude of the voltage across the sensing resistor is proportional to the magnitude of the motor current. During the forward portion of the duty cycle, a high-speed switch connects the sensor output to the amplified sensed bus current analog. During the reverse portion, the sensor output is connected to the inverted bus current analog. The average value of the sensor output is the voltage analog of the average value of the motor current.
A control circuit is connected between a three-phase power supply and a motor and ensures the correct application of phase signals to the motor, regardless of the way in which the control circuit is connected to the power supply. A first output signal of the supply is connected directly to an input of the motor via the circuit. Supply of power to the other two inputs is controlled by four switches, two of which receive signals from the second supply output and have their outputs connected to respective motor inputs, and the other two of which receive signals from the third supply output and have their outputs connected to respective motor inputs. The switches are controlled such that two switches are on, and the other two switches are off if the third output signal is positive with respect to the first output signal when the second signal passes through zero with respect to the first signal in a positive sense.
A motor switching and control circuit is disclosed in which the direction of transport of web material transferred between two reels is reversed by detecting a condition requiring reversal, generating a reversing signal in response to the detected condition, and switching the direction of rotation of an AC motor in response to the reversing signal. Both motor direction switching and reversing condition detection switching are accomplished at substantially zero motor current flowing through and zero motor voltage across the contacts of any circuit switches, thereby eliminating radio frequency interference and logical noise generation during switching. The circuit is particularly useful in connection with the ribbon control drive in an impact printer such as a matrix printer.
A device for controlling an AC motor for a fork lift truck comprises a detector to detect direction of rotation of an AC motor. The direction of rotation thus detected is compared with a desired direction of rotation of the AC motor, and the AC motor is controlled such that the AC motor operates in a power running mode when the actual direction of rotation of the AC motor is equal to the desired direction of rotation of the AC motor, but it operates in regenerative braking mode when said actual direction of rotation of the AC motor fails to be equal to the desired direction of rotation of the AC motor. The desired direction of rotation of the AC motor may be set manually by an operator or a driver of a vehicle drien by the AC motor.
A method and system for determining zero current level occurrences in a reversible power converter without requiring additional component complexity and costs. A digital controller selectively determines the line to line voltage for the most recently fired thyristor pair. The selected line to line voltage is identified as the bridge reconstruction voltage and is compared against the actual bridge output voltage for the conducting bridge. The difference between the two voltage signals is identified as the bridge error voltage and the sign of its magnitude is indicative of a load current zero level occurrence. A zero current level occurrence happens whenever the bridge error voltage drops below zero. This indication is positive and substantially instantaneous and safely enables the reversal of power flow without the risk of line faults due to cross-bridge short circuits.
