Motor Control Components Electricians Use: Contactors, Starters, Relays, and Interlocks

1) Identify and Function-Check Common Motor Control Components

Motor control circuits are built from a small set of repeatable components. Your job in the field is to (a) identify what each device is supposed to do, (b) confirm it is wired and rated correctly, and (c) verify it changes state when the process or operator demands it.

Contactor (Power Switching Device)

A contactor is an electrically operated switch used to connect/disconnect motor power. It has a coil (control voltage) and main power contacts (line/load). Many contactors also have auxiliary contacts used in the control circuit for seal-in and interlocks.

• Identify: Look for L1/L2/L3 (line) and T1/T2/T3 (load) terminals, plus coil terminals (often A1/A2).
• Function-check (de-energized): Verify main contacts are open; check coil resistance (not open/shorted) with a meter.
• Function-check (energized): Apply correct coil voltage; confirm audible/physical pull-in; verify continuity across main contacts and correct auxiliary contact state change.

Overload Block (Overload Relay Assembly)

The overload block mounts to the contactor or starter and provides a control-circuit trip contact that opens the coil circuit when an overload condition is detected. In typical circuits you will see an NC overload contact in series with the contactor coil (often labeled 95-96), and sometimes an NO auxiliary for indication (97-98).

• Identify: Mounted under/behind the contactor; has reset (manual/auto) and trip indicator.
• Function-check: Use the test/trip mechanism if present; confirm the NC contact opens and requires reset as designed.
• Wiring check: Ensure the overload NC contact is actually in series with the coil circuit, not only feeding a pilot light.

Control Transformer (Control Power Source)

A control transformer provides a safer or standardized control voltage (commonly 24 VAC, 120 VAC) from a higher line voltage. It isolates the control circuit and reduces the voltage present at pushbuttons and sensors.

• Identify: Primary terminals marked H1/H2 (and possibly H3/H4) and secondary X1/X2.
• Function-check: Measure primary voltage and secondary voltage under load; confirm secondary is fused/protected as required by the design.
• Common field clue: If the contactor coil is 120 V but the control transformer secondary is 24 V, the coil will never pull in (or will burn up if reversed).

Pushbuttons (Momentary Operator Inputs)

Pushbuttons are typically momentary and come in NO (start) and NC (stop) versions. The stop button is usually NC so a broken wire stops the motor (fail-safe behavior).

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• Identify: Terminal markings often show NO/NC blocks; some have separate contact blocks stacked.
• Function-check: With power off, meter continuity while pressing/releasing; confirm stop is closed at rest and opens when pressed; confirm start is open at rest and closes when pressed.

Selector Switches (Maintained Operator Inputs)

Selector switches provide maintained positions like HAND-OFF-AUTO or FWD-OFF-REV. Internally they may switch multiple contacts at once (multi-pole) and may have different contact states in each position.

• Identify: Note the legend plate and number of contact blocks.
• Function-check: Verify which contacts are made in each position using the switch’s contact chart (often printed on the block) and a continuity test.

Auxiliary Contacts (Control Logic Contacts)

Aux contacts are small contacts mechanically linked to a contactor or relay. They are used for seal-in (holding), status indication, and interlocks.

• Identify: Markings like 13-14 (NO) and 21-22 (NC) are common conventions.
• Function-check: Energize the parent device and verify the aux changes state reliably; check for welded/sticky contacts if behavior is inconsistent.

Time-Delay Relays (TDRs)

Time-delay relays change contact state after a set time. Two common behaviors are on-delay (contacts change after coil energizes and time elapses) and off-delay (contacts change after coil de-energizes and time elapses). They are used for staged starts, purge cycles, and proof timing.

• Identify: Dial or digital setting; terminals labeled for supply/trigger and timed contacts.
• Function-check: Confirm supply voltage matches rating; apply control signal and time the contact change with a stopwatch; verify correct mode (on-delay vs off-delay).

Pressure Switches (Process-Proof Devices)

A pressure switch changes contact state when pressure crosses a setpoint. In motor controls it often proves airflow (fan) or proves pump discharge pressure.

• Identify: Mechanical device with setpoint adjustment; contact type may be SPDT (common, NO, NC).
• Function-check: Use a manometer or known process condition; verify contact changes at the expected pressure and returns with hysteresis (differential).

Float Switches (Level-Proof Devices)

Float switches open/close based on liquid level. They may be used to start a sump pump at high level, stop at low level, or provide high-level alarm interlocks.

• Identify: Cable float or mechanical float in a tank; may be NO/NC depending on orientation.
• Function-check: Simulate level change (lift/lower float) and meter the contact; confirm the intended fail-safe state (for example, a high-level safety might be NC so a broken wire trips).

Thermostats (Temperature-Proof Devices)

Thermostats are temperature-actuated switches used for heating/cooling enable, fan control, or safety limits. They can be line-voltage or low-voltage control devices.

• Identify: Setpoint dial or fixed limit; terminals often labeled R/W/Y/G in HVAC low-voltage systems, or simple two-wire contacts in industrial controls.
• Function-check: Adjust setpoint across ambient temperature and verify contact change; confirm correct contact type (NO/NC) for the intended action.

Practical Component Verification Checklist (Field Routine)

1. Read device ratings: coil voltage, contact ratings, and any required supply (AC/DC).
2. Confirm control power source: measure transformer secondary or control supply at the source and at the load.
3. Continuity test inputs: pushbuttons, selector switches, safeties (pressure/float/thermostat) in their normal state.
4. Verify outputs: contactor coil energizes when commanded; aux contacts change state; timed contacts operate in the correct mode.
5. Document terminal numbers: write down what is landed where before moving wires.

2) Read Ladder Diagrams: Seal-In Circuits and Interlocks

Ladder diagrams show control logic as rungs between two rails (often L1 and L2 for AC control, or + and − for DC). Each rung typically represents a function: a coil, a pilot light, or a timer. Understanding seal-in and interlocks lets you troubleshoot most motor control problems quickly.

Basic Start/Stop with Seal-In (Holding) Contact

A classic three-wire control uses an NC stop, NO start, and an NO auxiliary contact from the contactor to “hold” the coil after the start button is released.

L1 ──[ STOP (NC) ]──[ OL (NC) ]──+──[ START (NO) ]──( M COIL )── L2
                                |
                                +──[ M AUX (NO) ]──+

• How it works: Press START to energize M coil. When M pulls in, M AUX closes and provides a parallel path around START. Releasing START does not drop the coil because M AUX maintains the circuit.
• What drops it out: Pressing STOP opens the NC stop contact, or an overload trip opens OL (NC), removing coil power.

Interlocks: Preventing Unsafe or Conflicting Commands

An interlock is a contact used to prevent two incompatible states from happening at the same time (for example, forward and reverse contactors both energized). Interlocks can be electrical (NC auxiliary contacts in the control circuit) and/or mechanical (a physical linkage preventing both contactors from closing).

Rung FWD: L1 ──[ STOP NC ]──[ REV AUX NC ]──[ FWD START NO ]──( FWD COIL )── L2
Rung REV: L1 ──[ STOP NC ]──[ FWD AUX NC ]──[ REV START NO ]──( REV COIL )── L2

• Electrical interlock idea: If FWD is energized, its NC aux opens in the REV rung, blocking REV coil.
• Verification step: With FWD pulled in, meter the REV rung at the REV coil terminal to confirm the interlock contact is open where expected.

Reading Ladder Diagrams Efficiently (Step-by-Step)

1. Find the load: locate the coil (M, CR, TR) or output device at the right side of the rung.
2. Trace left: identify every series condition that must be true to energize it (stops, safeties, selector position, interlocks).
3. Identify parallel branches: look for seal-in contacts or alternate permissives (HAND vs AUTO paths).
4. Check contact “normal” state: NO/NC is shown in the diagram’s normal (de-energized) condition, not the running condition.
5. Map devices to terminals: match rung labels (M AUX, OL, PS) to actual terminal numbers (13-14, 95-96, COM/NO/NC).

3) Typical HVAC Control Sequences Electricians See

HVAC motor controls often add proof devices and auto sequences. Two common examples are a fan with proof-of-airflow and a pump controlled by a float switch. The exact voltages and controller types vary, but the logic patterns repeat.

Fan with Proof-of-Airflow (Pressure Switch Proving)

Goal: When the fan is commanded ON, the fan contactor energizes. A pressure switch must prove airflow within a time window; otherwise the system shuts down or alarms.

Typical devices: HOA selector (HAND-OFF-AUTO), fan contactor (M), pressure switch (PS), time-delay relay (TDR), and an alarm relay/light.

Rung 1 (Fan enable): L1 ──[ STOP NC ]──[ HOA HAND/AUTO permissive ]──( M COIL )── L2
Rung 2 (Proof timer): L1 ──[ M AUX NO ]──────────────────────────────( TDR COIL )── L2
Rung 3 (Proof OK):    L1 ──[ PS NO (closes on airflow) ]─────────────( PROOF RELAY )── L2
Rung 4 (Trip/Alarm):  L1 ──[ TDR timed contact ]──[ PROOF not made ]─( ALARM )── L2

Sequence (step-by-step):

1. Operator selects HAND or controller calls in AUTO; STOP and safeties are closed.
2. M coil energizes; fan starts; M AUX closes.
3. M AUX energizes the TDR coil to start the proof timing window.
4. If airflow is adequate, PS changes state (often closes) and energizes a proof relay or satisfies a permissive input.
5. If PS does not prove before the timer expires, the timed contact changes state to trigger an alarm and/or drop out the fan (depending on design).

Field checks: Verify PS tubing is connected and not kinked; confirm PS contact type (NO vs NC) matches the ladder; confirm timer mode (on-delay) and setpoint.

Pump with Float Switch (Automatic Level Control)

Goal: Pump starts when level rises to a high point and stops when level falls to a low point. Some systems use two floats (start/stop), others use one float with internal differential, and many include a high-high alarm float.

Two-float example (start at high, stop at low):

L1 ──[ STOP NC ]──[ OL NC ]──[ HOA in AUTO ]──[ LOW LVL (NC) ]──+──[ HIGH LVL (NO) ]──( P COIL )── L2
                                                             |
                                                             +──[ P AUX (NO) ]──────+

• LOW LVL (NC): opens at low level to stop the pump and prevent dry-run.
• HIGH LVL (NO): closes at high level to start the pump.
• Seal-in: P AUX holds the pump on after HIGH LVL returns to normal, until LOW LVL opens.

Sequence (step-by-step):

1. In AUTO, with adequate level above low cutoff, LOW LVL (NC) is closed.
2. When level rises, HIGH LVL closes and energizes the pump contactor coil.
3. P AUX closes to seal in the circuit so the pump continues running as the level drops.
4. When level reaches the low cutoff, LOW LVL opens, dropping out the coil and stopping the pump.

Field checks: Confirm float orientation and tether length; verify which float is wired to which terminals; test by lifting floats and observing coil voltage and contactor pull-in.

4) Common Wiring Errors and Verification Steps

Most “won’t start” or “won’t stop” problems in motor control panels come from a short list of wiring and interpretation errors. Use targeted measurements to avoid guesswork.

Error: Coil Voltage Mismatch (Wrong Coil or Wrong Control Power)

Symptoms: Contactor never pulls in, chatters, overheats, or coil burns out quickly.

Verification steps:

1. Read the coil nameplate: confirm rated voltage and AC/DC type (example: 24 VAC, 120 VAC, 24 VDC).
2. Measure actual coil voltage: meter across A1-A2 when the circuit is calling for ON.
3. Check control transformer taps: confirm primary is landed on the correct line voltage tap and secondary is what the circuit expects.
4. Check for voltage drop: if coil voltage is low, measure upstream across each series device to find where voltage is being lost (high resistance contact, loose terminal).

Error: NO/NC Contact Confusion (Especially on Pushbuttons and Process Switches)

Symptoms: Motor starts only when STOP is pressed, runs only while START is held, won’t seal in, or trips immediately when it should run.

Verification steps:

1. Do not trust wire color alone: identify the actual contact block terminals (NO vs NC markings).
2. Continuity test in the “normal” state: with power off, confirm which terminals are closed at rest.
3. Confirm ladder symbol meaning: ladder shows contacts in de-energized state; a pressure switch may be drawn as NO but in the field it might be wired to the NC side of an SPDT switch.
4. Prove seal-in path: with the contactor pulled in, verify the aux contact used for holding is actually closed and wired in parallel with START.

Error: Miswired Seal-In Circuit (Holding Contact Not in Parallel)

Symptoms: Motor runs only while START is held; releasing START drops out immediately.

Verification steps:

1. Identify the intended holding contact: usually M AUX NO (13-14).
2. Confirm parallel branch: one side of START and one side of M AUX must land on the same two nodes (electrically common points).
3. Measure at the coil: if coil voltage disappears when START is released, check whether M AUX is actually closing and whether it is landed correctly.

Error: Interlock Wired Backwards or Missing (Forward/Reverse, Lead/Lag, Safety Chains)

Symptoms: Both contactors can energize, nuisance trips, or one direction never works.

Verification steps:

1. Check for both electrical and mechanical interlocks: confirm the mechanical kit is installed where required.
2. Confirm NC aux contacts are used for interlock: interlocks typically use NC contacts that open when the opposite contactor energizes.
3. Functional test: energize FWD and confirm REV coil circuit is interrupted at the interlock contact (meter for open circuit at the expected point).

Error: Control Transformer Secondary Not Referenced or Fusing Issues

Symptoms: Intermittent control power, blown control fuse, strange readings to ground, or multiple devices dropping out together.

Verification steps:

1. Check secondary protection: confirm correct fuse size/type and that it is not open.
2. Verify secondary wiring: measure X1-X2 voltage and then measure from each leg to the intended reference (if the design bonds one side).
3. Inspect for shorts: isolate downstream control wiring and reconnect one branch at a time to find the faulted leg.

Error: Misinterpreting Field Devices (Pressure/Float/Thermostat) as “Bad” When the Process Condition Isn’t Met

Symptoms: Fan won’t run in AUTO, pump won’t start, or unit locks out after a delay.

Verification steps:

1. Confirm the process condition: is there actually airflow, pressure, level, or temperature demand?
2. Bypass only for testing and only if permitted: temporarily jumper the switch contact to confirm the rest of the circuit works, then remove the jumper immediately after the test.
3. Check setpoints and differential: a pressure switch set too high will never prove; a float tether too short may never actuate.

Targeted “Voltage Tracing” Method for Control Circuits

When a coil is not energizing, voltage tracing finds the open contact quickly.

1. Command ON: put the circuit in the state where it should energize (START pressed, AUTO call present, etc.).
2. Measure across the coil: if rated voltage is present and the coil doesn’t pull in, suspect the coil/contactor. If voltage is not present, continue.
3. Measure from L1 to each node along the rung: work left-to-right (or right-to-left) to find where voltage stops.
4. Confirm the specific device state: once you find the open point, test that contact directly (continuity or voltage drop) and verify it is the correct NO/NC terminal.