An AC-AC power converter, such as a motor soft starter, includes an input connectable to an AC source with a disconnect switch, an output connectable to an AC load, and phase lines connecting the input and output to transmit power. In-line solid-state switching blocks are connected between line terminals and load terminals of the AC source and AC load, respectively, such that each phase line includes a solid-state switching block connected thereto. Free-wheeling solid-state switching blocks are connected to the load terminals at one end and together at a common connection at another end, such that each phase line includes a free-wheeling solid-state switching block connected thereto. Each of the in-line and free-wheeling solid-state switching blocks comprises a bi-directional switching block that selectively controls current and withstands voltage in both directions. The switching blocks also provide soft-starter functions, variable speed control, and integrated circuit breaker protection capability.

An electronic overload relay leverages a ratio metric current design to permit size and cost optimized circuit components that can be used to sense current for purposes of protecting motors by detecting overcurrent conditions in branch motor applications in lieu of thermal overload devices. A current divider is used to significantly reduce the current that must be sensed by a magnetically coupled toroid to permit its components to be size and cost optimized and to be implemented easily on a printed circuit board. The DC resistance can be used to provide a coarse design that can be calibrated pre-manufacture to establish the accuracy required in sensing motor load current by adjusting the value of the burden resistor. Precision printed circuit board traces can be used to ensure repeatability during manufacturing. A sweepable trigger value generator can permit operation over the entire range of FLC and the threshold can be calibrated into the device.

Detection of a locked rotor condition of an electric motor during startup herein uses a virtual speed switch. The methods and systems herein determine whether the motor is in a starting state, and the monitor the motor current to determine if it is greater than a predetermined threshold, and that the motor current decrease exceeds a predetermined threshold after a predetermined amount of time. If the decrease in motor current is not less than a predetermined threshold, then a speed switch bit is set, indicating that the rotor is turning. The methods and systems herein further determine whether a physical speed switch is faulty, and determine a locked rotor condition using the virtual speed switch signal, a physical speed switch signal, and whether the physical speed switch is faulty.

In the coupling of bi-directional, non-freewheeling-type DC motors (24,25) to the pulling spool (23) of a canopy (11) deployment strap (21,22) and to a take-up roller (15), ratchet gears (26,27) assure automatic alignment and synchronization of the deployment and retraction mechanisms. The reverse direction rotation of the unwinding motor can be uninhibited through the slipping of its ratchet gear when the winding motor is slow to take up the pulling strap or the tarp (11) being wound thereon. The slipping provides sufficient slack to accommodate changes of spool and roller diameters as more material is successively wound thereon. A friction brake acting against a flange of die spool keeps it from spinning freely when not engaged by the motor. Winding motors can be protected against excessive power use by measuring the level of drawn current with a digital ammeter. If the current exceeds a threshold level of operation, a timer is started. As soon as a given absolute safe period of operation is surpassed, a breaker switch is tripped interrupting the motor power supply. A plural number of trip points can be programmed into the device.

A hybrid air-gap/solid-state device protection device (PD) for use in an electrical power distribution system includes an air-gap disconnect unit connected in series with a solid-state device, a sense and drive circuit, and a microcontroller. Upon the sense and drive circuit detecting an impending fault or exceedingly high and unacceptable overvoltage condition in the PD's load circuit, the sense and drive circuit generates a gating signal that quickly switches the solid-state device OFF. Meanwhile, the microcontroller generates a disconnect pulse for the air-gap disconnect unit, which responds by forming an air gap in the load circuit. Together, the switched-OFF solid-state device and air gap protect the load and associated load circuit from being damaged. They also serve to electrically and physically isolate the source of the fault or overload condition from the remainder of the electrical power distribution system.

A switching device protects an electrical load against overcurrent. A load current path from a current source to the load is connected through a semiconductor switch. In the on-state of the switch, the current is permitted to flow. In the off-state, the load current is blocked. A control circuit is configured to switch the semiconductor switch in response to a switching signal. The current in the load current path is measured. A controller execute an individual function or a combination of two or more predefined functions depending on an operating mode of the electrical load. The functions compare a current intensity and/or a temporal current profile of the current through the load current path with defined threshold values. The control circuit is driven by the switching signal that depends on one or more results of the executed functions to selectively switch off the semiconductor switch.