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newSchoolWorks.com › VoTechWorks › Manufacturing & Marine › Industrial Maintenance › Industrial Electrical Systems for Manufacturing and Marine Maintenance › Lesson 7
MFG-INDU16-05 · Lesson 7 · 16.5 clock hours · lab

Motor Control Devices: Push Buttons, Contactors, Overload Relays, and Starters

Objective. The learner will correctly wire and functionally test an across-the-line three-phase motor starter from a ladder diagram, including a momentary start/stop circuit with holding contact and thermal overload relay, with zero wiring errors on inspection.

⚠ Pending professional review

This lesson, its worked examples and its practice questions has not yet been verified by a licensed engineer or certified journey-level tradesperson or by a certification or approval body. It is a study draft and may contain errors. Do not use it as instructions for work on a job site or on live equipment. Always follow your instructor, your supervising professional, and current official references and regulations.

VoTechWorks program approval and accreditation are still in progress. Credentials are issued only by the certifying bodies named in each program, after their own exams.

Motor Control Devices: Push Buttons, Contactors, Overload Relays, and Starters

An across-the-line motor starter is the most common method of starting a three-phase induction motor in manufacturing and marine environments. It connects the motor directly to full line voltage at the moment of starting. Understanding each component — and how they work together in a ladder diagram — is the foundation of industrial electrical maintenance.

Safety First

Before touching any wiring training board, apply lockout/tagout (LOTO) following your facility's energy control procedure. Verify zero energy with your DMM before working. Wear safety glasses throughout the lab. The control transformer in this lab supplies 120 V AC — lethal under the wrong conditions. Never energize the circuit until the instructor has completed the wiring inspection. When fault-injection testing is underway, treat every point in the circuit as live until you have confirmed otherwise with your meter.

Push-Button Stations

Push buttons are momentary-contact devices — they change state only while you press them and return to their normal position when released.

On a ladder diagram, NO contacts are drawn as two parallel vertical lines; NC contacts add a diagonal line through them. Always verify button type with a DMM continuity check before wiring.

Magnetic Contactors

A magnetic contactor is an electromagnetically operated switching device. It has two distinct sections:

1. Main (power) contacts — heavy-duty contacts that open and close the power circuit to the motor. On a three-phase contactor, there are three main contacts, one per phase, rated for the full motor load current.

2. Auxiliary contacts — smaller contacts used in the control circuit. They can be NO or NC and are mechanically linked to the same armature as the main contacts, so they change state whenever the coil is energized or de-energized.

3. Coil terminals — the contactor coil is an electromagnet. When control-circuit voltage is applied across the coil terminals (labeled A1 and A2 on most NEMA devices), the armature pulls in and closes the main contacts simultaneously.

NEMA ratings (such as NEMA Size 1, Size 2) define the horsepower and current the contactor can switch safely. Always match the contactor size to the motor load.

Holding (Sealing) Contact

Because the Start button is momentary, the circuit would de-energize the contactor the instant you release the button — and the motor would stop. The solution is a holding contact (also called a sealing contact or memory contact): one auxiliary NO contact of the contactor is wired in parallel with the Start button. When the coil energizes and the armature pulls in, this auxiliary contact closes and keeps the coil energized even after the Start button is released. The circuit is now "sealed in" through the holding contact.

Thermal Overload Relay

A thermal overload relay protects the motor windings from damage caused by sustained overcurrent (overload). It consists of heater elements sized to match the motor's full-load ampere (FLA) rating, and a set of normally closed contacts wired in series in the control circuit.

When motor current exceeds the rated value for long enough to heat the heaters beyond their trip threshold, a bimetal strip or eutectic alloy mechanism trips, opening the NC contact and breaking the control circuit. The contactor de-energizes, dropping out the main contacts and stopping the motor. After the relay cools, the reset button (manual reset) or automatic reset mechanism restores the NC contact. Select heater elements from the manufacturer's heater selection table based on the motor's nameplate FLA.

Reading the Ladder Diagram

A ladder diagram reads top-to-bottom and left-to-right, like the rungs of a ladder. The two vertical rails represent Line 1 (L1) and Neutral (or L2) of the 120 V AC control circuit supplied by the control transformer.

A typical across-the-line start/stop circuit in one rung reads:

L1 → Stop (NC) → [Start (NO) parallel with Holding contact (NO)] → OL contact (NC) → Coil (M) → Neutral

Trace the path with your finger before you touch a wire. Every component in series must be closed for the coil to energize.

Voltage-Test Fault-Finding Method

The voltage-test method locates an open fault efficiently:

1. With the circuit energized and the fault present, place the DMM on AC volts.

2. Place one probe on the neutral rail (right rail). Place the other probe on the load side of each series component, moving from the neutral side back toward L1.

3. The first point where the meter reads line voltage (120 V AC) instead of 0 V is the component with the open fault — voltage is being dropped across it because no current is flowing.

Record every test point, the voltage reading, and your conclusion in your lab notebook before moving to the next test point.

Assembling the Starter

The thermal overload relay mounts directly below the contactor and its output terminals connect to the motor leads. Together they form a combination motor starter. The overload NC contact is factory-wired or field-wired into the control circuit in series before the coil. Never bypass the overload contact — doing so removes the motor's primary protection.

Key terms9

Magnetic contactorAn electromagnetically operated switch with main power contacts and auxiliary contacts; energizing the coil pulls in the armature and closes the contacts.
Holding (sealing) contactAn auxiliary NO contact of the contactor wired in parallel with the Start push button to keep the coil energized after the momentary Start button is released.
Normally open (NO) contactA contact that is open (non-conducting) in its de-energized or un-actuated state and closes when the device is actuated.
Normally closed (NC) contactA contact that is closed (conducting) in its de-energized or un-actuated state and opens when the device is actuated.
Thermal overload relayA protective device with heater elements and a bimetal or eutectic trip mechanism that opens the control circuit on sustained overcurrent to prevent motor winding damage.
Ladder diagramA schematic drawing style for control circuits in which two vertical power rails represent the rungs of a ladder and horizontal rungs show each control circuit path.
Across-the-line starterA motor starting method that connects the motor directly to full line voltage at start; the simplest and most common starting method.
Voltage-test methodA fault-finding technique using a voltmeter to find an open component in a live series circuit by identifying which component drops full source voltage when no current flows.
NEMA size ratingA standardized designation (Size 00 through Size 9) that defines the maximum horsepower, voltage, and current a contactor or starter is rated to switch safely.

Worked examples4

1. You measure the voltage at the load side of the Stop button (NC) and read 0 V. You then move your probe to the load side of the Start button / holding contact junction and read 120 V AC. What does this tell you, and where is the open fault?
  1. Step 1: Recall the voltage-test rule — full source voltage appears across an open component because no current flows and all voltage drops across the open.
  2. Step 2: At the load side of the Stop button you read 0 V, meaning L1 voltage has already passed through the Stop button without being dropped — the Stop button contact is closed (normal).
  3. Step 3: At the load side of the Start/holding contact junction you read 120 V. This means current cannot flow past this point; the voltage is being dropped across the Start button AND the holding contact simultaneously.
  4. Step 4: Since the motor is not running, the holding contact is open by design (contactor de-energized). The Start button is also open (momentary, at rest). With both parallel paths open, 120 V appears at that node — this is normal when the motor is stopped, not a fault.
Answer: Reading 120 V at the Start/holding contact node with the motor stopped is expected — both parallel contacts are open. No fault is present at this point. Continue probing toward the neutral rail to find an injected fault if one exists.
2. With the motor running (coil energized through the holding contact), you probe toward neutral and read: load side of OL contact = 0 V; load side of coil = 0 V. Then the instructor tells you there is one injected open fault in the circuit. You probe the load side of the Stop button and read 0 V, but the load side of the holding contact reads 120 V. Where is the fault?
  1. Step 1: The motor is running, so the holding contact is closed. With the holding contact closed, the junction between the Stop button and the holding contact should be connected through to L1.
  2. Step 2: Load side of Stop button = 0 V means voltage has passed through the Stop button (it is closed, as expected for a healthy NC stop button).
  3. Step 3: Load side of holding contact = 120 V means current cannot flow past the holding contact even though the coil should be holding it closed — something is wrong.
  4. Step 4: If the motor appeared to be running but the holding contact node reads 120 V, the holding contact itself has an open (broken wire, loose screw, or failed contact). The coil was not truly energized in this fault scenario.
  5. Step 5: The fault is an open in the holding contact branch (or its wiring), preventing the sealing circuit from completing.
Answer: The fault is an open circuit in the holding contact branch. The 120 V reading on the load side of the holding contact while its load side of the Stop button reads 0 V pinpoints the open to that contact or its connecting wire.
3. A motor starter uses a contactor coil rated 120 V AC, 60 Hz. The control transformer secondary is 120 V AC. The installer measures 94 V AC at the coil terminals while the coil is energized and the motor is running. Should this concern the installer, and why?
  1. Step 1: Contactor coil manufacturers specify an operating voltage range, typically 85% to 110% of the rated voltage for reliable operation.
  2. Step 2: Calculate 85% of 120 V: 0.85 × 120 = 102 V. This is the minimum acceptable voltage for reliable pickup and seal-in.
  3. Step 3: 94 V is below 102 V (below 85% of rated). At this voltage the coil may not develop enough magnetic force to hold the armature fully seated.
  4. Step 4: A partially seated armature causes excessive coil current, overheating, buzzing, and premature coil failure. The motor also loses protection because the contacts may chatter.
Answer: Yes, this is a concern. At 94 V the coil is below the typical minimum operating voltage of approximately 102 V (85% of 120 V). The installer must find and correct the cause of the voltage drop — likely undersized control wiring, loose connections, or an overloaded transformer secondary — before the equipment is placed in service.
4. A thermal overload relay heater is selected for a motor with a nameplate full-load ampere (FLA) rating of 8.4 A. The manufacturer's heater table lists Heater H6 for 7.6–8.6 A and Heater H7 for 8.5–9.5 A. Which heater should be installed, and why?
  1. Step 1: The motor FLA is 8.4 A. The overload heater must be selected so the motor's normal running current falls within the heater's rated range.
  2. Step 2: Heater H6 covers 7.6–8.6 A. Check: 8.4 A falls within 7.6–8.6 A — yes.
  3. Step 3: Heater H7 covers 8.5–9.5 A. Check: 8.4 A is below the lower limit of 8.5 A — 8.4 A does not fall within this range.
  4. Step 4: Select Heater H6 because the motor's FLA of 8.4 A falls within its specified range, ensuring the relay will carry normal current without nuisance tripping and will still trip on true overload.
Answer: Install Heater H6. The motor FLA of 8.4 A falls within H6's range of 7.6–8.6 A. H7's lower limit of 8.5 A is above the motor FLA, so H7 would not provide accurate overload protection.

The math in this lesson1

Learners compare 94 V at a coil with its 120 V rating as a percent and select heaters from FLA ranges.
7.RP.3
Brush up in the math halls at newSchoolWorks.com:
In class: Component identification bench exercise: learners disassemble and reassemble a magnetic contactor and thermal overload relay, identifying the main contacts, auxiliary contacts, coil terminals, overload heater elements, and reset button while referencing the manufacturer's datasheet. · Wiring lab: following a provided ladder diagram, learners connect a 120 V AC control circuit with a momentary start push button, stop push button, magnetic contactor coil, holding (sealing) contact, and overload relay normally-closed contact on a wiring training board; instructor inspects wiring before power is applied. · Function test and fault injection: after successful initial test, instructor opens one wire at three different points in sequence; learner must use the DMM to locate each fault using voltage-testing method and restore circuit to operation, recording each test in the lab notebook.
Materials: NEMA-rated magnetic contactors and thermal overload relays (assorted sizes), push-button stations (NO and NC), 120 V AC control transformer, wiring training boards, 18 AWG and 14 AWG THHN wire, wire strippers, screwdrivers, DMM, NCCER Electrical module, ladder diagrams
Check: Instructor wiring inspection checklist (zero wiring errors required) plus functional test: circuit must start, run, stop, and trip/reset on simulated overload correctly; learner must locate all three injected faults using correct voltage-test method within the lab period to pass.

Practice check8

Answer each question, then check it. Nothing is recorded; this is your own practice.

NCCER Industrial Maintenance — Electrical Systems / Motor Controls
1. In a standard across-the-line motor starter control circuit, the Start push button is wired in parallel with the holding contact. What is the primary purpose of this holding contact?
NCCER Industrial Maintenance — Electrical Troubleshooting
2. A technician uses the voltage-test method to find an open fault in a running motor control circuit. Probing from the neutral rail back toward L1, the technician finds 0 V on the load side of the overload relay NC contact and 120 V on the line side of that same contact. What does this indicate?
MSSC CPT — Manufacturing Processes and Production
3. Which of the following describes the correct normal state of the Stop push button in a motor starter control circuit?
NCCER Industrial Maintenance — Safety
4. Before a learner applies power to a wiring training board in this lab, which action is required first?
NCCER Industrial Maintenance — Electrical Systems / Motor Controls
5. On a magnetic contactor, which set of contacts carries the full motor load current?
NCCER Industrial Maintenance — Electrical Systems / Motor Controls
6. A thermal overload relay trips repeatedly without a true motor overload condition. Which is the MOST likely cause?
NCCER Industrial Maintenance — Electrical Systems / Ladder Diagrams
7. A technician reads a ladder diagram and sees the following series path in one rung: L1 → NC contact → [NO contact parallel with NO contact] → NC contact → Coil → Neutral. How many components must be in their closed (conducting) state for the coil to energize?
NCCER Industrial Maintenance — Electrical Troubleshooting
8. After a motor overload relay trips and the motor stops, the technician presses the reset button on the overload relay but the motor will not restart with the Start button. The control voltage at the coil terminals reads 0 V when Start is pressed. Which is the BEST next diagnostic step using the voltage-test method?
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