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BLD-ELEC05-06 · Lesson 5 · 24 clock hours · lab

Contactors, Starters, and Overload Relays — Installation Lab

Objective. Students wire a full-voltage magnetic starter with a thermal overload relay to a three-phase motor using a provided schematic, then verify correct operation and overload trip.

Contactors, Starters, and Overload Relays — Installation Lab

What a Magnetic Starter Does

A magnetic starter is the standard device for switching a three-phase motor on and off from a remote location while protecting it from overload damage. It combines two parts: a contactor and a thermal overload relay. The contactor is an electromagnetically operated switch. When its coil is energized, it pulls in a set of main contacts that connect line voltage to the motor. When the coil de-energizes, a spring opens those contacts and the motor stops. The overload relay is wired in series with the motor leads and monitors current continuously; if current exceeds the relay's trip setting for long enough, a bimetal element bends and opens a normally closed contact in the control circuit, dropping out the coil and stopping the motor before the windings are damaged.

NEMA (National Electrical Manufacturers Association) sizes the starter by the horsepower and voltage rating of the motor it controls. A NEMA Size 1 starter handles motors up to 7.5 HP at 230 V or 10 HP at 460 V — common in training panels and light commercial applications.

Control Circuit Fundamentals — Three-Wire Control

The power circuit (line to motor) and the control circuit (coil logic) are separate. In this lab the control transformer steps 480 V or 208 V down to 120 V for the control circuit, limiting shock hazard at push buttons and pilot devices.

The three-wire control scheme uses a momentary Start push button (normally open) and a momentary Stop push button (normally closed), plus a holding contact — a small auxiliary contact on the contactor that is wired in parallel with the Start button. When you press Start, current flows through the coil; the contactor pulls in, closes the main contacts to run the motor, and simultaneously closes the holding contact. When you release Start, current continues through the holding contact, keeping the coil energized. Pressing Stop breaks the circuit; the coil drops out, the holding contact opens, and the motor cannot restart until Start is pressed again. This provides low-voltage protection: a power interruption drops the contactor out, and the motor will not restart automatically when power returns.

Overload Relay Trip Class and Setting

The thermal overload relay has an adjustment dial calibrated in amperes. You set it to match the motor's Full-Load Amperage (FLA) shown on the motor nameplate. The relay is rated by trip class (commonly Class 10 or Class 20), indicating how many seconds at 600 % of FLA the relay takes to trip. Class 10 protects motors with short acceleration times; Class 20 is typical for general-purpose motors. In the lab you will read actual running current with a clamp ammeter and compare it to the relay setting — they should be close under no-load or light-load conditions.

Reading the Ladder Diagram

A ladder diagram shows the control circuit as horizontal rungs between two vertical power rails (L1 and L2 of the control circuit, or X1 and X2 of the transformer secondary). Each rung is read left to right. The first rung typically shows: Stop (NC) — Start (NO) with holding contact in parallel — Coil (M). The second rung may show an indicator light or alarm. You must wire exactly what the schematic shows; any deviation is a wiring fault.

Safety Rules for This Lab

No energizing without instructor sign-off. Before anyone applies power, the instructor performs a 100 % wiring inspection. This is not optional.

Wiring Sequence

1. Mount the starter in the training enclosure per the instructor's direction.

2. Run and terminate the three-phase power conductors: L1, L2, L3 to the contactor line terminals (top); T1, T2, T3 from the contactor load terminals (bottom) to the motor leads.

3. Wire the control transformer primary to the designated supply taps; wire the secondary (X1, X2) to the control circuit rails.

4. Install the Stop button (NC), Start button (NO), and holding contact per the ladder diagram.

5. Connect the overload relay's NC output contact in series in the control circuit.

6. Set the overload relay dial to the motor's nameplate FLA.

7. Perform a visual check: all terminations tight, no bare conductors touching, no crossed phase conductors.

8. Call the instructor for the mandatory safety inspection before energizing.

Functional Test Procedure

After inspector approval, the instructor energizes the circuit. Students then:

Key terms10

ContactorAn electromagnetically operated switch that opens and closes main power contacts to start and stop a motor when its coil is energized or de-energized.
Magnetic StarterA combination device consisting of a contactor plus a thermal overload relay, used to control and protect a three-phase motor.
Thermal Overload RelayA protective device wired in series with motor leads that uses a bimetal element to trip and open the control circuit when motor current exceeds the set value for a sustained period.
Three-Wire ControlA control scheme using momentary Start and Stop push buttons plus a holding (memory) contact that maintains the coil circuit after the Start button is released, providing low-voltage protection.
Holding ContactAn auxiliary contact on the contactor, wired in parallel with the Start button, that seals in the coil circuit after the Start button is released.
Full-Load Amperage (FLA)The current drawn by a motor when operating at its rated horsepower and voltage, listed on the motor nameplate; used to set the overload relay.
NEMA SizeA standardized rating system from the National Electrical Manufacturers Association that classifies magnetic starters by the maximum horsepower and voltage they can control.
Trip ClassA designation (commonly Class 10 or Class 20) indicating how many seconds an overload relay takes to trip at a specified multiple of FLA, matched to the motor's acceleration characteristics.
Ladder DiagramA schematic drawing that represents the control circuit as horizontal rungs between two vertical power rails, showing the logic sequence of contacts and coils.
Low-Voltage ProtectionA feature of three-wire control that prevents automatic motor restart after a power interruption because the holding contact opens when the contactor drops out.

Worked examples4

1. A three-phase motor nameplate reads: 1 HP, 208 V, FLA = 4.6 A. What value should you set on the overload relay dial?
  1. Locate the FLA on the motor nameplate: 4.6 A.
  2. The overload relay dial should be set to match the motor FLA. Set the dial to 4.6 A (or the nearest marked increment at or just above 4.6 A, per the relay manufacturer's instruction).
  3. Do not set higher than the nameplate FLA without consulting the relay manufacturer's guidelines, as this reduces overload protection.
Answer: Set the overload relay to 4.6 A.
2. With a clamp ammeter you measure the following three phase currents on a running motor: Phase A = 4.4 A, Phase B = 4.5 A, Phase C = 4.3 A. What is the average running current, and how does it compare to the 4.6 A FLA?
  1. Add all three phase readings: 4.4 + 4.5 + 4.3 = 13.2 A.
  2. Divide by 3 to find the average: 13.2 ÷ 3 = 4.4 A.
  3. Compare to FLA: 4.4 A average versus 4.6 A FLA.
  4. The motor is running at approximately 96 % of FLA, which is normal under light or no-load conditions in a training environment.
Answer: Average running current is 4.4 A, which is slightly below the 4.6 A FLA — normal for a lightly loaded motor.
3. The overload relay is set to 4.6 A and has a Class 20 designation. A technician checks whether the relay provides protection: at 600 % of the trip setting, the relay should trip within 20 seconds. What current level represents 600 % of the 4.6 A setting?
  1. Convert 600 % to a decimal multiplier: 600 ÷ 100 = 6.0.
  2. Multiply the trip setting by the multiplier: 4.6 A × 6.0 = 27.6 A.
  3. A Class 20 relay must trip within 20 seconds when the motor draws 27.6 A (600 % of 4.6 A).
Answer: 600 % of 4.6 A = 27.6 A. The Class 20 relay must trip within 20 seconds at that current level.
4. During the functional test you measure 118 V across the control transformer secondary (X1 to X2) and 0 V at the coil terminals while the Start button is held in. What is the voltage drop across the Stop button, and what does this indicate?
  1. If 118 V is available at the transformer secondary but 0 V appears at the coil, nearly all 118 V is being dropped somewhere in the series control circuit.
  2. Measure voltage across the Stop button: if the meter reads approximately 118 V across a normally closed Stop button, the Stop button contact is open (failed or wired incorrectly).
  3. A properly closed NC Stop contact should read 0 V across it (no voltage drop across a closed contact). A reading near supply voltage means that contact is open.
  4. Conclusion: the Stop button circuit is open — check that the NC terminal pair is used and the button is wired correctly.
Answer: Approximately 118 V drop across the Stop button indicates its normally closed contact is open; the button is likely wired to the wrong terminals or is faulty.

The math in this lesson2

Learners average three phase currents, compare to overload setting, and compute 600% trip current.
7.RP.3S-ID.2
Brush up in the math halls at newSchoolWorks.com:
In class: Hands-on wiring: students mount a NEMA-size magnetic starter in a training enclosure, terminate three-phase power conductors to the contactor line terminals and load terminals to the motor leads per the schematic. · Control circuit wiring: students wire a 120 V control transformer, start-stop push-button station, and holding contact on the contactor, following the three-wire ladder diagram. · Functional test: instructor energizes the circuit; students verify motor starts and runs, then manually trip the overload relay, observe and reset, and record motor FLA with a clamp meter versus overload relay trip setting.
Materials: NEMA size-1 magnetic starter with integral overload relay; 120 V control transformer; start-stop push-button station; three-phase training motor (fractional or 1 HP); training panel board; insulated screwdrivers; wire strippers; 14 AWG THHN wire; clamp ammeter; multimeter; ladder diagram handout
Check: Instructor inspects wiring for correct terminal connections, conductor routing, and secure terminations before energizing (100% safety check); functional test: motor starts and stops on command and overload trips within one attempt; any wiring fault must be corrected and retested.

Practice check8

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

NCCER Electrical — Motors and Controls
1. What is the primary function of the holding contact in a three-wire motor control circuit?
NCCER Electrical — Motors and Controls
2. A thermal overload relay in a magnetic starter is wired in series with the motor leads. When it trips, what happens in the control circuit?
OSHA 10 — Electrical Safety / Lockout-Tagout
3. Before energizing a training panel for a wiring lab, which safety step is required?
NCCER Electrical — Motors and Controls
4. On a three-phase magnetic starter, the line terminals are labeled L1, L2, and L3. Where do the conductors from those terminals terminate?
NCCER Electrical — Motors and Controls
5. A motor nameplate lists FLA = 6.8 A. You are setting a Class 20 thermal overload relay for this motor. What is the correct initial dial setting?
NCCER Electrical — Motors and Controls
6. During a functional test you press Start, but the motor does not run and the contactor does not pull in. You measure 120 V across the coil terminals. What is the most likely cause?
NCCER Electrical — Motors and Controls
7. Which of the following best describes low-voltage protection as provided by three-wire control?
NCCER Electrical — Motors and Controls
8. A student measures the following phase currents on a running three-phase motor: 5.1 A, 5.0 A, and 5.2 A. The motor nameplate FLA is 5.0 A and the overload relay is set to 5.0 A. Which conclusion is most accurate?
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