For industrial applications of the typical single-phase AC induction motor, or for any application where the motor is located remote from supervision, the simple toggle switch is generally replaced with a fractional-horsepower (FHP) manual motor starter to protect both the motor windings and the supply-circuit conductors from overheating due to mechanical overloading of the electric motor. The FHP motor starter is a form of manual controller that combines the overload-overcurrent protection of a thermal overload relay with the control of a simple toggle switch to provide a means of manually controlling the run status of the motor.

Thermal overload element

The thermal sensing element (heater element) is made integral with the toggle switch in the FHP manual motor starter. The overload relay consists typically of a single overload heater element, which holds a latching mechanism for the toggle switch. If the overload relay is reset, the toggle switch latches when the switch is flipped or snapped on. The switch contacts are held closed against spring tension. If a sustained motor-overload should occur, the higher-value motor overload current will cause the solder to melt or the bimetallic strip to bend, which will trip the latch and release the switch contacts.

Installation of the FHP manual motor starter consists of terminally connecting the line (supply) and load (motor) conductors and then mounting the switch. The overload-protection latching mechanism is designed in such a manner that power cannot be applied to the motor through the switch unless the overload heater element is installed. Installation of the heater element typically consists of removing two screws and mounting the element to the terminals with the same two screws. The heater element terminals are usually located between the two wire terminals.

FHP motor-starter operation

The FHP motor starter can be used for both manual and remote operation. The branch-circuit supply power is normally run to the motor-starter enclosure. The grounded supply conductor is spliced, without connection to the motor-starter assembly, and extended to the motor terminal housing (enclosure). The ungrounded supply conductor is wired in series with the FHP motor-starter across its line and load terminals (normally L1 and T1, respectively). At the motor, the leads are connected to their respective supply conductors (grounded and ungrounded). For manual operation, when the latch switch in the FHP motor-starter is closed, power is applied directly to the motor for a run condition. The motor stops when the switch is opened.

This type of limited manual control has its uses. For example, an exhaust fan is to be used for air exchange in a remote part of the building. The branch circuit is sourced from a load center within the local area of the fan installation. Serving as the required motor disconnecting means that the FHP motor starter must be readily accessible and within line-of-sight of the single-phase AC induction motor. Once installed, the electric motor is allowed to run continuously without direct supervision. Should the motor fall victim to an overload, the heater element will trip the latch switch within the FHP motor starter and remove power from the motor.

FHP motor-starter as a disconnect

For the FHP motor-starter to serve as the disconnecting means for the motor: The NE Code requires the FHP motor-starter switch to be readily accessible and within line-of-sight of the single-phase AC induction motor. To be readily accessible, the switch handle, when in its highest (on) position, cannot be more than 6 ft, 7 in (2 m) above the finished floor, finished grade (outside) or working platform. To be within line-of-sight, the position of the handle (up or down ― on or off) must be visible; the switch cannot be located more than 50 ft (15 m) away from an unobstructed view of the electric motor or the equipment it serves.

To protect the operator or other persons from exposure to live wire terminals when operating the switch of an FHP motor starter, the manual starter assembly must be installed in a dead front enclosure. When installed in a standard device or junction box, a lighting switch cover is installed to close the box opening. When installed in a discrete motor-starter enclosure, the enclosure sides usually are cut away to facilitate the wiring of the FHP motor-starter switch. The motor-starter enclosure is fitted with a wrap-around front cover, which is normally mounted across the strap of the FHP motor starter to completely close the enclosure opening when installed.

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