9.1 Magnetic Contactors, Starters & Relays

9.1 Magnetic Contactors, Starters & Relays

In commercial and industrial facilities, electric motors power critical mechanical systems, including HVAC chillers, air-handling fans, heavy manufacturing conveyors, and process pumps. Controlling these rotating machines safely requires specialized switching equipment capable of handling high inductive inrush currents, isolating line voltages, and protecting equipment windings from sustained thermal degradation. Journey-level electricians must master the internal operating principles, rating standards, and selection criteria governing magnetic contactors, magnetic motor starters, and overload relays under the National Electrical Code (NEC) and industrial equipment standards.

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Magnetic Contactor Anatomy & Operating Principles

A magnetic contactor is an electrically actuated, multipole switch designed to repeatedly establish and interrupt electrical power circuits. Unlike manual knife switches or circuit breakers, contactors allow high-power, high-voltage circuits (e.g., 480VAC three-phase) to be controlled safely from remote locations using low-voltage pilot control circuits (e.g., 24VDC or 120VAC).

The Electromagnetic Assembly

The mechanical movement of a contactor relies on an electromagnet consisting of three core parts:

1. Operating Coil (Solenoid): An insulated copper wire winding wound around a nonmagnetic bobbin (terminals designated A1 and A2). When control voltage energizes the coil, it produces an intense magnetic field proportional to coil turns and operating current ($NI$).
2. Stationary Core: A fixed magnetic core constructed of thin, laminated silicon steel sheets insulated from one another to minimize parasitic eddy current and hysteresis losses. The core is commonly manufactured in an "E" shape.
3. Movable Armature: A laminated steel bar (often "I"-shaped) mechanically attached to the contact carrier assembly. When the coil energizes, magnetic pull overcomes the resistance of internal stainless steel return springs, snapping the armature tightly against the stationary core faces.

The Shading Coil & Anti-Chatter Physics

In alternating-current (AC) contactor coils operating on a standard 60 Hz electrical supply, line current passes through zero 120 times per second. Because the magnetic force generated by a coil is proportional to the square of instantaneous current ($F \propto i^2$), the magnetic holding force collapses to zero at every current zero-crossing:

• The Chatter Problem: Without intervention, the mechanical return springs begin pulling the armature open during each zero-crossing. The armature rebounds as current rises, slamming back into the core 120 times every second. This violent mechanical oscillation—termed contact chatter—destroys contact surfaces through severe arcing, causes contact welding, and produces an unbearable 120 Hz acoustic hum.
• The Engineered Solution: To solve this problem, manufacturers embed a shading coil into the pole faces of the stationary E-core. A shading coil is a heavy, single-turn loop of copper or brass pressed into a slot cut across approximately one-third of each pole face.
• Phase-Shift Mechanism: As alternating magnetic flux from the main coil expands and collapses through the pole face, it induces a heavy circulating current in the shorted shading coil. By Lenz's Law, this induced current creates an auxiliary magnetic flux that lags behind the main coil flux by approximately $90^\circ$. When the main core flux drops to zero during line zero-crossings, the out-of-phase flux from the shaded portion of the pole is near its maximum. Consequently, the net magnetic force holding the armature never drops below the mechanical counter-force of the return springs, ensuring whisper-quiet operation and eliminating chatter.

              Stationary Laminated E-Core
      ===========================================
      |   Unshaded   | Shading |   Unshaded   |
      |  Pole Face   |  Coil   |  Pole Face   |
      |  (Main Flux) | [====]  |  (Main Flux) |
      ===========================================
                 ^ Induced Lagging Flux ($90^\circ$)

Contact Construction & Arc Quenching

Contactors make and break high-magnitude inductive circuits, generating severe plasma arcs during contact separation:

• Contact Materials: Power contacts are formed from silver-cadmium oxide ($\text{AgCdO}$) or silver-tin oxide ($\text{AgSnO}_2$). Fine pure silver provides excellent conductivity but softens and welds under high inrush; adding cadmium or tin oxide prevents arc welding, resists erosion, and breaks apart surface oxidation.
• Double-Break Contacts: Industrial contactors utilize a double-break design, in which two contacts on a movable crossbar bridge across two stationary contacts. Opening the crossbar creates two arcs in series across each phase. Splitting the arc doubles the total arc resistance, cuts recovery voltage in half, and rapidly deionizes the plasma channel.
• Arc Chutes & Blowout Coils: Power poles on larger contactors are enclosed within arc chutes constructed of ceramic or magnetic steel splitter plates. When contacts open, magnetic blowout coils wired in series with the contacts generate a transverse magnetic field that physically drives the plasma arc upward into the metal splitter plates. The plates divide, cool, and extinguish the arc within milliseconds.

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Magnetic Motor Starters vs. Contactors

A widespread source of confusion among apprentice electricians is the distinction between a contactor and a motor starter:

• Contactor: An electrically controlled switch rated for switching power circuits. It provides no running thermal overload protection for the connected equipment. Contactors are utilized for resistive heating banks, commercial lighting panels, capacitor banks, and transformer primary switching.
• Magnetic Motor Starter: Defined as the integrated assembly of a magnetic contactor combined with an overload relay (OLR). The contactor provides the mechanical switching capacity, while the overload relay provides ongoing thermal protection against motor overloads and single-phasing.

$\text{Magnetic Motor Starter} = \text{Contactor} + \text{Overload Relay (OLR)}$

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Power Contacts vs. Auxiliary Contacts

A magnetic starter incorporates two completely distinct sets of electrical contacts mechanically operated by the same moving armature:

1. Main Power Poles

• Function: Carry full branch-circuit line current to the motor stator windings.
• Designation: Line input terminals are stamped L1, L2, L3; load output terminals connected to the motor are stamped T1, T2, T3.
• Ratings: Engineered for 600VAC service, rated to carry continuous full-load amperage (FLA) and interrupt locked-rotor current (LRC), which typically reaches 600% of FLA.

2. Auxiliary Contacts

• Function: Pilot-duty contacts mechanically linked to the armature crossbar, used exclusively to control pilot lights, relays, holding circuits, and electrical interlocks.
• Ratings: Standardly rated for NEMA pilot duty (e.g., NEMA A600: 10A continuous, 60A make / 6A break at 120VAC).
• Normally Open (NO): Open when the contactor is de-energized; closes when the armature pulls in. Standardly utilized as the holding (seal-in) contact across a momentary START pushbutton, or to energize an amber "RUNNING" pilot light.
• Normally Closed (NC): Closed when the contactor is de-energized; opens when the armature pulls in. Standardly utilized for electrical interlocking between forward and reverse starters to prevent simultaneous energization, or to illuminate a green "STOPPED" pilot light.

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NEMA vs. IEC Starters: Sizing, Design & Standards

Commercial and industrial facilities utilize two competing engineering frameworks for motor controllers: North American NEMA (National Electrical Manufacturers Association) and European/International IEC (International Electrotechnical Commission).

Standard NEMA Starter Sizes (at 460V, 3-Phase)

• Size 00: 2 HP (9A rating)
• Size 0: 5 HP (18A rating)
• Size 1: 10 HP (27A rating)
• Size 2: 25 HP (45A rating)
• Size 3: 50 HP (90A rating)
• Size 4: 100 HP (135A rating)
• Size 5: 200 HP (270A rating)

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Overload Relays (OLRs): Thermal vs. Solid-State

Motor branch-circuit overcurrent protective devices (fuses and circuit breakers per NEC 430 Part IV) are sized to protect conductors against short circuits and ground faults; they are set far too high (175% to 300% FLA) to protect motor windings from gradual overheating. The Overload Relay (OLR) monitors motor current and disconnects the contactor coil when current exceeds 115% to 125% of FLA over a sustained interval.

                 Motor Current Spectrum & Protection Zones
0% FLA ------------ 100% FLA ------------ 125% FLA ---------------- 600% FLA ++
  [ Normal Operation ]   [ Continuous Run ]   [ Overload Relay Zone ]  [ Short-Circuit OCPD ]
                                              (Sustained Heating)      (Instantaneous Magnetic)

1. Thermal Overload Relays

Thermal OLRs convert motor branch-circuit current into heat through dedicated resistive heater elements:

• Eutectic Alloy (Solder Pot) Relays: Motor line current flows through an interchangeable heater element surrounding a small brass pot containing a specialized eutectic alloy (a metal mixture engineered to melt instantly at a precise temperature) and a ratchet shaft. Under sustained overload, thermal energy melts the alloy, freeing the spring-loaded ratchet wheel to spin. The spinning shaft snaps open a set of Normally Closed (NC) auxiliary contacts (terminals 95-96) wired in series with the contactor coil, dropping out the starter. After cooling, the alloy resolidifies, allowing the relay to be manually reset. Eutectic relays do not offer ambient temperature compensation.
• Bimetallic Overload Relays: Current heats a bimetallic strip fabricated from two dissimilar metals (such as brass and invar) bonded together. Because the two metals expand at different rates, heat causes the strip to warp mechanically, pushing a trip slide bar that forces open the NC pilot contacts. Modern bimetal relays feature an ambient temperature compensation strip that bends in the opposite direction in response to ambient air temperature, preventing nuisance tripping in outdoor motor control centers (MCCs) exposed to high summer heat.

2. Solid-State Electronic Overload Relays (SSOLR)

Modern industrial installations predominantly utilize electronic solid-state overload relays. SSOLRs do not use resistive heater elements; instead, they measure motor current directly using internal current transformers (CTs) or Hall-effect sensors on each phase.

• Digital Thermal Modeling: A built-in microprocessor tracks real-time current and calculates motor heating based on an internal algorithmic $I^2t$ thermal model.
• Advanced Diagnostic Protections: SSOLRs provide capabilities impossible with thermal heaters:
  • Phase Loss (Single-Phasing) Protection: If one phase of a three-phase supply opens (e.g., a blown utility cutout fuse), current in the remaining two phases spikes by approximately $173\%$, rapidly destroying stator windings. An SSOLR detects phase loss and trips in under 3 seconds.
  • Phase Unbalance Protection: Trips when current unbalance across phases exceeds 20% to 30%, preventing rotor hot-spot damage.
  • Ground-Fault Protection: Detects low-magnitude leakage currents to ground before catastrophic phase-to-phase flashovers occur.
  • Wide Current Adjustment Dial: Eliminates physical heater replacement; an electrician simply turns a calibrated dial to match the motor's exact nameplate FLA.

Overload Relay Trip Classes (NEMA / UL 508)

Trip classes designate the maximum time in seconds that an overload relay will take to open under a locked-rotor overload of 600% of rated full-load current:

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Manual vs. Automatic Reset Safety Protocols

Most industrial overload relays feature a two-position selector switch or adjustment dial marked MAN (Manual) and AUTO (Automatic):

• Manual Reset: When the relay trips, the internal NC pilot contacts latch open mechanically. After the thermal element cools, a qualified worker must physically press the external reset plunger on the starter enclosure to re-close the contacts.
• Automatic Reset: As soon as the bimetallic strip or electronic thermal model cools down below its reset threshold, the NC pilot contacts re-close automatically without human intervention.