6.1 Contactors, Relays & Other Control Devices

What Is a Contactor?

A contactor is an electrically operated switch designed to make and break a power circuit — most commonly the three-phase line feeding a motor, but also lighting banks, heating elements, capacitor banks, and other heavy loads. Like the control-circuit elements you met in Chapter 5, a contactor is built around an electromagnetic coil. When the coil is energized, it creates a magnetic field that pulls in an armature, which in turn closes (or in some designs opens) a set of contacts. When the coil is de-energized, a spring returns the armature and contacts to their resting position. This makes a contactor fundamentally a remote-controlled switch: a small control-circuit signal at the coil operates a much larger power-circuit connection at the contacts.

Because a contactor's main job is switching motor and other heavy loads, its main contacts are built for that duty — heavy silver-alloy contact tips, robust spring pressure, and, on larger contactors, arc chutes that stretch and extinguish the arc that forms as the contacts open under load. A contactor by itself provides no overcurrent protection; that role belongs to the overload relay and the branch-circuit fuse or breaker covered in Chapter 5. The contactor's job is purely to connect and disconnect the circuit on command.

Contactor vs. Relay: The Key Distinction

A control relay (often just called a "relay") is built on the exact same electromagnetic principle as a contactor — a coil pulls in an armature to operate a set of contacts. The difference between the two devices is one of scale and purpose, not principle:

Because a control relay never switches motor current, it needs no heavy contacts or arc-quenching hardware — its contacts only ever see the small current drawn by another coil, a pilot light, or a low-power control input. A useful rule for the exam: if it switches the motor, it's a contactor; if it switches a signal to control other devices, it's a relay.

Contact States: Normally Open and Normally Closed

Every contact on a contactor or relay is described by its resting (de-energized) state:

• A normally open (NO) contact has no continuity when the coil is de-energized, and closes when the coil is energized.
• A normally closed (NC) contact has continuity when the coil is de-energized, and opens when the coil is energized.

This NO/NC labeling always refers to the coil's de-energized state, never the "as found" condition of the equipment. A device wired with an NC contact to shut something down on a fault — such as an overload relay's trip contact — is deliberately relying on the fact that the contact stays closed, and the circuit stays complete, until something forces it open.

Auxiliary Contacts: Sealing and Interlocking

In addition to its main power-circuit contacts, most contactors carry one or more auxiliary contacts — small NO and/or NC contacts, mechanically linked to the same armature, but wired entirely into the control circuit. Auxiliary contacts never carry motor current; their only job is signaling and logic. Two uses come up constantly in trade practice:

• Sealing (holding) the coil circuit — an auxiliary NO contact wired in parallel with a momentary START pushbutton, so that once the contactor picks up, the auxiliary contact holds the coil circuit energized after the operator releases the button.
• Interlocking — an auxiliary NC contact from one contactor wired into the coil circuit of a second contactor, preventing both from being energized at the same time. This is standard practice on reversing starters, where the forward and reverse contactors must never close together, since that would short line-to-line through the motor windings.

Because auxiliary contacts share the same armature as the main contacts, they change state in lockstep with the power contacts, giving the control circuit a reliable, physically linked confirmation of whether the contactor is actually picked up — not just commanded to be.

Timing Relays: On-Delay and Off-Delay

A timing relay (or "timer") adds a deliberate time interval between a triggering event and a change in its output contacts. Two basic types cover most field applications:

• On-delay (delay-on-energization) timing relay — when the coil is energized, the timer begins counting; its output contacts change state only after the set delay elapses. If the coil de-energizes before the delay finishes, the timer resets immediately with no output change.
• Off-delay (delay-on-de-energization) timing relay — the output contacts change state the instant the coil is energized, but when the coil is de-energized, the timer holds the contacts in that state for the set delay before they return to normal.

Timing relays are the device behind the star-to-delta transition timing referenced in Chapter 5's star-delta starter: an on-delay timer starts counting the moment the starter is commanded to run, holding the motor in the reduced-voltage star connection for the programmed interval before its contacts trigger the switch to the full-voltage delta connection. Other common uses include delaying the start of a second pump until a first pump is confirmed running, or holding a cooling fan on for a set period after a heated process shuts down — an off-delay application.

Other Common Pilot and Control Devices

Beyond pushbuttons, a master electrician regularly wires several other pilot devices that convert a physical condition into an NO or NC contact signal for the control circuit:

• Limit switch — a mechanically actuated switch that changes contact state when a moving part (a door, a conveyor guard, a machine slide) reaches a physical limit of travel. Commonly used to stop a motor when equipment reaches an end-of-travel position, or as a safety interlock on a guard door.
• Float switch — actuated by the rising or falling level of a liquid, typically through a sealed float that tilts or rises to operate an internal contact. Used to start/stop sump and lift-station pumps, or to signal high- or low-level alarms on tanks.
• Pressure switch — actuated when system pressure crosses a set point, commonly used to start/stop air compressors, control well-pump systems, or shut down equipment on loss of pressure, such as a low-oil-pressure safety shutdown.

From the control circuit's perspective, a limit switch, a float switch, and a pushbutton are interchangeable in principle — each is simply another source of an NO or NC contact feeding into the circuit, differing only in what physically operates it.

Reading a Ladder Diagram

Control circuits are almost always drawn as a ladder diagram: two vertical "rails" representing the two sides of the control voltage supply, connected by a series of horizontal rungs, each rung representing one complete control-circuit path. Standard conventions include:

• Each rung is read left to right, showing the devices in series that must all be satisfied to complete that path.
• An NO contact is drawn as two short parallel lines with a gap between them; it must close to complete the rung.
• An NC contact is drawn the same way with a diagonal slash through it; it is already complete and must be forced open to break the rung.
• Coils — contactor coils, relay coils, timer coils — are typically drawn at the right-hand end of a rung, shown as a circle.
• Rungs are numbered down the ladder, and contacts belonging to a given coil, such as a seal-in auxiliary contact, are cross-referenced back to that rung number.

Being able to trace a rung — identifying every device that must be satisfied for a coil to energize, and every auxiliary or interlocking contact tied back to it — is a core practical skill tested on the RME exam and used daily in field troubleshooting.

Key Takeaways

• A contactor switches power/motor loads with heavy-duty contacts; a control relay switches lower-power control-circuit signals, often through multiple contact pairs
• A normally open (NO) contact is open when de-energized and closes when the coil is energized; a normally closed (NC) contact does the opposite
• Auxiliary contacts carry no motor current — they seal in the coil circuit and interlock with other devices
• On-delay and off-delay timing relays introduce a controlled time interval, such as the star-delta transition timer from Chapter 5
• Limit switches, float switches, and pressure switches convert a mechanical, level, or pressure condition into an NO/NC signal for the control circuit