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newSchoolWorks.com › VoTechWorks › Building & Construction › HVAC/R › Electrical Controls for HVAC/R Systems › Lesson 6
BLD-HVAC07-03 · Lesson 6 · 13.5 clock hours · lab

Contactors, Relays, and Motor Starters

Objective. Students will wire, energize, and verify correct operation of a contactor and a motor-start relay, measure coil and contact resistance, and explain how each component functions in a system circuit.

Contactors, Relays, and Motor Starters

What These Devices Do

Contactors, relays, and motor-start relays are electromechanical switching devices. A low-voltage control signal energizes a coil, which creates a magnetic field that pulls an armature, opening or closing a set of contacts in a higher-voltage circuit. This allows a thermostat or control board operating at 24 VAC to switch compressors, condenser fans, and other loads running at 120, 208, 240, or 480 VAC.

Contactors

A contactor is a heavy-duty switching device used to connect and disconnect power to loads that draw significant current — compressors and large fan motors are the most common applications in HVAC/R. Contactors have:

Contactor contacts are normally open (NO) — they close when the coil is energized. A small amount of voltage drop across closed contacts is normal; excessive drop (typically above the manufacturer's limit, often around 50 mV at rated current) signals pitted or burned contacts that need replacement.

Relays

A relay is functionally identical to a contactor but rated for lighter loads. Control relays in HVAC systems switch low-amperage circuits — fan relays, time-delay relays, and sequencer relays are common examples. Relays can have normally open (NO) contacts that close on coil energization, normally closed (NC) contacts that open on coil energization, or combinations of both on the same device. Reading the schematic symbol carefully before wiring prevents miswired control circuits.

Potential-Type Motor-Start Relays

Hermetic compressor motors often need a start capacitor and a start winding to develop enough torque to overcome system pressure at startup. Once the motor reaches approximately 75–80 percent of its rated speed, the start capacitor must be switched out of the circuit; leaving it in causes overheating and capacitor failure.

A potential-type (voltage-type) start relay accomplishes this automatically:

1. At rest, the relay coil contacts (which are normally closed) keep the start capacitor in the circuit.

2. As the motor accelerates, back-EMF (counter-electromotive force) across the start winding rises.

3. When back-EMF reaches the relay's pick-up voltage, the coil energizes and the NC contacts open, dropping the start capacitor out of the circuit.

4. The motor then runs on the run capacitor and run winding only.

During a bench test you will observe this as a rapid drop in line current — starting current is several times higher than running current. Documenting both values and comparing them to the data sheet confirms the relay is functioning correctly.

Safety Rules for This Lab

Lockout/Tagout (LOTO): Before any wiring change, de-energize the training board, apply your personal lockout device, and verify with a meter that voltage is absent — test the meter on a known source first (live-dead-live procedure). Never assume power is off because a switch is open.

PPE: Wear safety glasses throughout. When energizing circuits, stand to the side of the panel — not directly in front. Keep loose clothing and jewelry away from terminals.

Capacitor discharge: Start and run capacitors store a charge even after power is removed. Before handling any capacitor, discharge it through a 20,000-ohm, 5-watt resistor or an approved discharge tool. Never short capacitor terminals with a screwdriver.

Arc hoods: Reinstall arc hoods on contactors before energizing. Do not defeat them.

Meter leads: Use insulated, CAT III–rated test leads with no cracked insulation. Confirm meter function category is appropriate for the voltage being measured.

Reading a Wiring Diagram

OEM wiring diagrams show the coil terminals (A1/A2), contact sets (labeled by type — NO or NC — and numbered), and the load circuit. Always follow the manufacturer's diagram for the specific device. Substituting a diagram from a different model can result in incorrect wiring because coil voltage ratings, contact arrangements, and terminal numbering vary.

Measuring Contact Condition

With the contactor coil de-energized and locked out, set your DMM to the resistance (ohms) function. Measure coil resistance between A1 and A2 — a reading of open circuit (OL) indicates a burned coil; a reading far below specification indicates shorted turns. With the coil energized and the circuit under load, measure AC millivolts across each closed contact set (L1 to T1, etc.). High voltage drop relative to the manufacturer's specification means high contact resistance from pitting, burning, or spring fatigue.

How This Relates to Your Credentials

EPA 608 questions cover refrigerant system components, and contactors control compressor operation directly. NATE, HVAC Excellence, and NCCER HVAC exams all include electrical diagnosis questions requiring you to interpret wiring diagrams, identify relay types, and predict circuit behavior — skills practiced in this lab.

Key terms9

ContactorA heavy-duty electromechanical switch with a low-voltage coil that opens or closes high-current contacts to control compressors and large motors.
Normally Open (NO) contactA contact set that is open (no continuity) when the coil is de-energized and closes when the coil is energized.
Normally Closed (NC) contactA contact set that is closed (continuity present) when the coil is de-energized and opens when the coil is energized.
Arc hood (arc chute)A molded insulating barrier on a contactor that contains and extinguishes the electrical arc produced when contacts open under load.
Potential-type start relayA voltage-sensing relay used with hermetic compressors that opens its normally closed contacts — dropping out the start capacitor — when back-EMF rises to the relay's pick-up voltage as the motor accelerates.
Back-EMF (counter-electromotive force)The voltage generated by a spinning motor winding that opposes the supply voltage; it rises proportionally with motor speed and is used by a potential relay to sense when the motor is near full speed.
Contact voltage dropThe small AC voltage measured across a closed contact set when current is flowing; excessive drop indicates high contact resistance from pitting or wear.
Coil resistanceThe DC resistance of a relay or contactor coil winding, measured in ohms; an open or shorted reading indicates coil failure.
Lockout/Tagout (LOTO)An OSHA-required procedure in which a technician de-energizes equipment, applies a personal lock to the energy-isolation point, and verifies absence of voltage before performing any work on electrical components.

Worked examples4

1. A contactor coil is rated 24 VAC. You measure 18.5 Ω between terminals A1 and A2 on your DMM (Ω setting). Is this reading useful, and what would an OL reading mean?
  1. Step 1 — Confirm the circuit is de-energized and locked out before placing the meter in resistance mode.
  2. Step 2 — Connect DMM leads to A1 and A2. The meter sources a small DC test current and displays resistance.
  3. Step 3 — A finite reading (18.5 Ω in this case) confirms the coil winding is continuous — wire is intact. Compare to the OEM specification if published.
  4. Step 4 — An OL (overload / open-loop) reading means the coil winding is broken (open circuit). The coil will not energize, and the contactor will not pull in.
Answer: 18.5 Ω indicates a continuous coil winding. An OL reading indicates an open (broken) coil that must be replaced before the contactor will operate.
2. With a contactor energized and carrying load, you measure 68 mV AC across the L1-to-T1 contact set using the millivolt AC range. The manufacturer specifies a maximum allowable contact voltage drop of 50 mV at rated current. What does this tell you?
  1. Step 1 — Note the measured value: 68 mV.
  2. Step 2 — Note the manufacturer's limit: 50 mV.
  3. Step 3 — Compare: 68 mV > 50 mV, so the contact drop exceeds the specification.
  4. Step 4 — Excessive voltage drop across a closed contact indicates high contact resistance caused by pitting, burning, or spring fatigue. The contact set (or whole contactor) should be replaced.
Answer: The L1-T1 contact is out of specification. At 68 mV versus a 50 mV limit, the contacts show excessive resistance and must be replaced to prevent overheating and compressor damage.
3. A clamp ammeter placed on the common (C) lead of a hermetic compressor simulator reads 28 A during the first two seconds of startup, then drops to 7 A during steady-state run. What is the ratio of starting current to running current, and what event caused the current to drop?
  1. Step 1 — Starting current = 28 A; running current = 7 A.
  2. Step 2 — Ratio = 28 A ÷ 7 A = 4.0.
  3. Step 3 — Identify the cause: as the motor accelerated to approximately 75–80% of rated speed, back-EMF across the start winding rose to the potential relay's pick-up voltage.
  4. Step 4 — The potential relay coil energized, opening its NC contacts and removing the start capacitor from the circuit. With the start winding/capacitor branch disconnected, current dropped to the lower running value.
Answer: The starting-to-running current ratio is 4.0 (28 A to 7 A). The current drop was caused by the potential-type start relay opening its contacts and dropping the start capacitor out of the circuit once the motor reached sufficient speed.
4. A 24 VAC contactor coil has a measured resistance of 22 Ω. Using Ohm's law, estimate the steady-state coil current draw.
  1. Step 1 — Ohm's law for AC (approximate, ignoring inductive reactance for a quick estimate): I = V ÷ R.
  2. Step 2 — I = 24 V ÷ 22 Ω.
  3. Step 3 — I = 1.09 A (approximately 1.1 A).
  4. Step 4 — Note: actual coil current will be lower because the coil is inductive (it has reactance as well as resistance), so this is a maximum estimate. The calculation is useful for quick sanity-checking that a control transformer has adequate capacity.
Answer: Approximately 1.1 A maximum estimated coil current. Actual current will be less due to inductive reactance. This confirms the 24 VAC control transformer must supply at least this much current to pull in the contactor.

The math in this lesson2

Learners read contact voltage drop in millivolts and compare it to the manufacturer maximum.
N-Q.1A-CED.4
Brush up in the math halls at newSchoolWorks.com:
In class: Disassembly and inspection of used contactors and relays: students identify coil terminals, normally-open and normally-closed contacts, arc hoods, and wear indicators, recording observations in the lab log. · Students wire a contactor into a training-board circuit, apply 24 VAC to the coil, verify line-side and load-side voltages, and measure voltage drop across closed contacts to confirm contact condition. · Potential-type motor-start relay bench test: students wire a start relay for a hermetic compressor simulator, observe the start capacitor drop-out sequence with an ammeter, and compare starting current to run current on a data sheet.
Materials: Single-phase and three-phase contactors, 24 VAC coil relays, potential-type motor-start relays, start and run capacitors, hermetic compressor training simulator or equivalent motor, training boards, digital multimeters, clamp ammeters, lab log forms, OEM wiring diagrams
Check: Contactor circuit wired correctly without reversed coil polarity, load-side voltage verified, and contact voltage drop within published specification; relay bench test amperage trace documented with pass/fail notation = pass.

Practice check8

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

Electrical — NCCER HVAC / HVAC Excellence Electrical Systems
1. A contactor coil is de-energized. What is the normal state of its main power contacts?
Safety — NATE Core / NCCER HVAC
2. Why must arc hoods be reinstalled on a contactor before it is energized?
Electrical — NATE Core / HVAC Excellence Electrical Systems
3. A potential-type motor-start relay has normally closed (NC) contacts connected in series with the start capacitor. What causes those contacts to open during compressor startup?
Electrical Diagnosis — NATE Core / NCCER HVAC
4. You measure 95 mV AC across the L2-to-T2 contacts of an energized contactor. The OEM limit is 50 mV. Which action is correct?
Safety — OSHA LOTO / NATE Core
5. Before measuring coil resistance on a contactor that was just powered down, a technician must FIRST:
Electrical — NATE Core / HVAC Excellence
6. A clamp ammeter on a compressor common lead shows 32 A for three seconds then drops to 8 A. What is the most likely explanation?
Blueprint Reading — NCCER HVAC / HVAC Excellence
7. On a wiring diagram, a relay contact symbol is shown with a diagonal line through it. This indicates the contact is:
Safety — EPA 608 / NATE Core / NCCER HVAC
8. A start capacitor must be discharged before handling. Which tool or method is correct?
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