Motor control · lesson 10 of 12

Hand-Off-Auto

A Hand-Off-Auto selector lets someone run a motor by hand, stop it, or leave it to an automatic device. In Auto the float switch is in the circuit; in Hand it is bypassed; in Off there is no path at all.

Manuel-Arrêt-Auto avec un flotteur

Activez JavaScript pour appuyer sur les boutons et faire fonctionner ce circuit.

À faire

Manuel la fait tourner tout de suite; Auto laisse le flotteur décider; Arrêt, c’est arrêté.

How the selector is drawn

A selector has contacts that are closed only in certain positions. On a print you will see each contact labelled with the positions where it is closed (the lab writes "HOA (Hand)" and "HOA (Auto)"), or a small truth table beside the switch with an X under each position where a contact is made. Here one contact is closed in Hand and feeds the coil directly; another is closed in Auto and feeds the coil through the float.

The classic HOA trouble call: "the pump runs all the time". Nine times out of ten someone left it in Hand after testing. The pilot light in rung 2 only proves the switch is in Auto — which is exactly why it is there.

Sur le chantier

The HOA is the most-touched device in any plant, which is exactly why it generates so many calls. Most of them are somebody's thumb, not a fault.

Common mistakes

Putting the overload contact where Hand bypasses it. Hand is meant to bypass the automatic control, never the protection. The OL contact goes after the selector, in the common part of the rung.
Landing the pilot device across the wrong contact so it runs in Hand as well. The float should only be in circuit in the Auto position.
Reading the selector's contact table wrong. Each contact is marked with the positions it is closed in, or shown in a small table with an X per position. Guess and you will wire Auto to the Hand contact.
Leaving it in Hand after testing. The classic "the pump runs all the time" call. The Auto pilot light exists to make that visible from the door.
Assuming Off is a disconnect. Off opens the control rung. The panel is still live.

On a real panel

A pilot light that proves "in Auto" is worth fitting even where nobody asked for it. It answers the most common call without a truck.
Maintained selector contacts are not all the same: some are two-position, some three, some have contacts that overlap between positions. The device's own table is the authority.
In Hand, the machine is running with the process control out of circuit. Whatever the float or thermostat was protecting is now your responsibility while you stand there.

Prove it before you leave

In Off, nothing runs and nothing can be started.
In Hand, it runs regardless of the pilot device. Operate the float by hand and confirm it makes no difference.
In Auto, operate the pilot device by hand both ways and watch the motor follow it.
In both Hand and Auto, trip the overload. The coil must drop in both.
Leave the selector in the position the operator expects, and say so to whoever owns the machine.

Questions fréquentes

What does Hand-Off-Auto mean?

A three-position selector. Hand runs the motor now, regardless of the automatic control. Off stops it. Auto hands control to a pilot device such as a float switch, pressure switch or controller.

Is the overload still active in Hand?

In a properly drawn circuit, yes — the overload contact is in series after the selector, so it protects the motor in both Hand and Auto. What Hand bypasses is the automatic control, not the protection.

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