Level 1, Unit 4, Section 2 – 2.4 Starting Relays & Devices

2.4.1 — Why Starting Devices Are Needed

All capacitor-start and split-phase motors require a device that performs one critical job: connect the start winding (and start capacitor) at the moment power is applied, then disconnect it once the motor reaches running speed. Failure to connect at start means the motor cannot develop enough torque to accelerate. Failure to disconnect during run means the start winding overheats and burns out within seconds.

The four devices covered in this lesson each detect a different signal to know when the motor has reached the cutout speed of approximately 75% of synchronous speed:

Device	Sensing Signal	Contact at Rest	Coil / Element Connection	Location
Current Relay	Main winding current (drops as motor accelerates)	Normally Open	Series with main (run) winding	External to motor
Potential Relay	Back-EMF voltage across start winding (rises as motor accelerates)	Normally Closed	Parallel with start winding	External to motor
PTC Device	Self-heating by start winding current (resistance rises with temperature)	N/A — solid-state	Series with start winding	External to motor
Centrifugal Switch	Shaft speed (centrifugal force vs. spring force)	Normally Closed	Series with start winding / capacitor	Internal to motor

🔧

Hermetic compressors always use external relays

Hermetic and semi-hermetic compressors are sealed units — no shaft extends outside the housing, so a centrifugal switch is impossible. Current relays, potential relays, and PTC devices are the only options for these motors. The relay mounts directly on the compressor terminal pins or on a separate relay board mounted nearby.

2.4.2 — Current Relay (Series Relay)

The current relay uses the drop in main winding current as the motor accelerates to detect when the start circuit should open. It is the dominant starting device for fractional-horsepower hermetic refrigeration compressors.

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Current Relay — Series / Start Relay

Senses main winding current • Normally-open contacts • Gravity-sensitive

¼–1 HP Hermetic Compressors

At Start-Up (t = 0)

L1 → Series Coil → Run Winding

High inrush current (LRA) flows through the main winding and through the relay’s series coil. The strong magnetic field created by LRA pulls the plunger upward against gravity, closing the normally-open contacts.

Start winding + capacitor now connected through closed contacts
Two-phase starting effect produced
Motor begins to accelerate

Running (~75% Speed Reached)

L1 → Series Coil → Run Winding

As the motor accelerates, current in the run winding drops toward FLA. The weakened coil field can no longer support the plunger against gravity. The plunger drops open, disconnecting the start winding.

Current at dropout: ~50–75% of LRA
Motor continues on run winding only
Relay remains open (plunger resting down) until next start

Operating Principle Current-operated (LRA pulls in, FLA drops out)

Contacts at Rest Normally Open (NO)

Coil Connection Series with main (run) winding

Orientation Gravity-critical — must mount in specified position

Typical HP Range ¼–1 HP (185–750 W)

⚠️

Current relay sizing is motor-specific — do not substitute

The relay must be matched to the motor’s LRA (pull-in) and FLA (drop-out) values. An oversized relay (pickup current too high) may never close on the available LRA — the motor hums but cannot start. An undersized relay (dropout current too high) may not open at run speed, leaving the start winding energised until it burns out. Always cross-reference the replacement relay part number to the compressor model, not just to relay dimensions.

Current Relay — Failure Modes

🔴 Contacts Welded Closed

Start winding permanently energised

Start winding overheats within seconds; motor trips on overload or thermal protector. On the next start attempt the motor may hum and draw LRA continuously. Detect with a resistance or continuity check across relay contacts with power removed.

🟡 Coil Open / Contacts Fail to Close

Start winding never energised

Motor hums at LRA but shaft does not rotate. Trips breaker or overload quickly. Incorrect mounting orientation (relay upside-down) produces the same symptom — always check orientation first before condemning the relay.

2.4.3 — Potential Relay (Voltage / Back-EMF Relay)

The potential relay uses the rise in back-EMF voltage across the start winding as the motor accelerates. Because voltage is the sensing signal, this relay is orientation-independent and is the preferred choice for larger compressor motors where current relays are less reliable.

⚡

Potential Relay — Voltage / Back-EMF Relay

Senses start winding back-EMF • Normally-closed contacts • Orientation-free

½ HP and above Commercial Compressors

At Start-Up (Rotor Stationary)

NC Contacts → Start Cap → Start Winding

At rest, the coil is de-energised (no back-EMF) so contacts remain in their normal position: closed. Start winding and capacitor are immediately connected when power is applied — no waiting for current to build as with a current relay.

Coil connected in parallel with start winding
At standstill, back-EMF ≈ 0 V — coil sees only resistive voltage drop
Contacts remain closed; start circuit active

Running (~75% Speed Reached)

Contacts Open → Cap Isolated ℸ Start Winding

As the rotor accelerates, the rotating magnetic field induces an increasing back-EMF in the start winding. At ~75% synchronous speed, the back-EMF reaches the relay’s pickup voltage (typically 200–450 V). The coil energises and opens the contacts, disconnecting the start circuit.

Coil remains energised as long as motor runs (back-EMF sustained)
Contacts re-close when motor stops and back-EMF collapses
Ready for next start cycle automatically

Operating Principle Back-EMF voltage (rises with speed)

Contacts at Rest Normally Closed (NC)

Coil Connection Parallel with start winding

Orientation Not critical — mounts in any position

Typical HP Range ½–5 HP (370 W–3.7 kW) and above

🔢

Pickup and Dropout Voltage — Why They Matter

Every potential relay is rated with two key voltages:

Pickup voltage — the minimum back-EMF required to energise the coil and open the contacts. If the relay’s pickup voltage is too low, the contacts open before the motor reaches sufficient speed, cutting off the start circuit prematurely. The motor may stall, fail to reach full speed, or repeatedly try to restart.

Dropout voltage — the back-EMF at which the coil de-energises and the contacts re-close (when the motor stops). If the relay’s pickup voltage is too high, the coil never energises during a normal start — the start capacitor remains in the circuit during run, overheating and failing within seconds.

Always match the relay’s rated pickup voltage to the motor manufacturer’s specification. A relay labelled with a pickup range of 200–240 V cannot be substituted on a motor requiring 350–420 V pickup.

Potential Relay — Failure Modes

🔴 Contacts Welded Open (Coil Fails Closed)

Start circuit never connects

Motor hums but cannot start — same symptom as failed-open current relay. Check with ohmmeter: normally-closed contacts should read near 0 Ω with relay de-energised. If contacts read open at rest, relay is defective.

🟡 Contacts Stuck Closed (Coil Fails to Energise)

Start capacitor remains in circuit during run

Start capacitor overheats and fails (often explosively). Motor draws high current and trips overload. Detect by checking whether contacts open when a voltage equal to motor back-EMF is applied to the coil terminals during bench testing.

2.4.4 — PTC Start Device (Positive Temperature Coefficient)

The PTC device is a solid-state thermistor that replaces an electromechanical relay for small hermetic motors. It has no moving parts and no coil — it simply changes resistance with temperature. Resistance is low when cold (allows start current) and very high when hot (blocks start current).

🌡️

PTC Thermistor Start Device

Self-heating solid-state device • No moving parts • Requires cool-down between starts

Under ½ HP Residential Refrigeration

At Start-Up (PTC Cold)

Run Winding ‖ PTC: ~20Ω → Start Winding

When the PTC is at room temperature, its resistance is very low (typically 10–50 Ω). Current flows freely through the start winding, creating the starting torque. Simultaneously, the current heats the PTC.

Start winding energised through low-resistance PTC
Motor begins accelerating
PTC temperature rising rapidly

Running (PTC Hot, 0.5–3 seconds later)

Run Winding ‖ PTC: ~15kΩ ≈ Start Winding

As the PTC heats up, its resistance increases sharply through the Curie point, rising to 10 000–50 000 Ω. At this impedance, start winding current drops to negligible levels — the circuit is effectively open without any mechanical switching.

Start winding essentially de-energised
PTC remains hot while motor runs; stays in high-resistance state
Must cool 2–5 minutes before next start

Operating Principle Self-heating resistance change (solid-state)

Moving Parts None

Circuit Connection Series with start winding

Orientation Not critical

Typical HP Range Under ½ HP (370 W)

PTC Resistance vs. Temperature

Cold State (Ambient Temperature)

10–50 Ω

Low resistance. Allows full start winding current to flow. Typical at room temperature before start-up, or after a full cool-down cycle.

Hot State (Running Temperature)

10 000–50 000 Ω

Very high resistance. Reduces start winding current to near zero, effectively disconnecting the start circuit without mechanical switching.

PTC Operating Cycle

❄️ PTC Cold Low R
Ready to start

→

⚡ Start (0–3 s) Current heats PTC;
R rises rapidly

→

🔥 Running High R sustained;
start wdg blocked

→

🕐 Cool-Down (2–5 min) R returns to low;
ready for next start

🚫

PTC devices enforce a mandatory restart delay

Because the PTC must cool from its operating temperature before resistance drops back to starting levels, a motor equipped with a PTC cannot be immediately restarted after stopping. Attempting a restart before the 2–5 minute cool-down period results in a start attempt with a high-resistance PTC — the motor hums but cannot start. This built-in delay also provides short-cycle protection that helps protect the compressor from damage due to liquid refrigerant flood-back during rapid cycling.

PTC Device — Failure Modes

🔴 PTC Fails Shorted (Resistance Stays Low)

Start winding permanently energised during run

Start winding overheats. Motor runs but draws excessive current. Diagnose by measuring PTC resistance when the motor has been running for at least 30 seconds — a good PTC reads very high resistance; a failed short reads near zero.

🟡 PTC Fails Open (Resistance Always High)

Start circuit never connects

Motor hums but will not start. Diagnose by measuring PTC resistance when cold — a good PTC reads 10–50 Ω; a failed open reads very high or infinite resistance. Also check: has the PTC had enough time to cool before testing?

2.4.5 — Centrifugal Switch

The centrifugal switch is a mechanical device mounted inside the motor on the rotor shaft. It is the most direct method of detecting rotor speed: at ~75% of synchronous speed, centrifugal force on the rotating weights overcomes spring force and physically opens the contact.

At Rest — Contacts Closed 🔘 Spring holds weights inward Spring pressure keeps the flyweights pulled toward the shaft centre. The actuating collar is pushed against the switch contact, holding it closed. Start winding is connected and ready for the next start.

Running — Contacts Open 💫 Centrifugal force throws weights outward At ~75% synchronous speed, centrifugal force exceeds spring force. Weights fly outward, pulling the actuating collar away from the switch contact and opening the start circuit. Contacts stay open while the motor runs.

🔩

Centrifugal Switch

Speed-operated mechanical switch • Internal to motor • Requires disassembly for service

Split-Phase & CS Motors Open Frame Motors

Operating Principle Centrifugal force vs. spring force at ~75% Ns

Contacts at Rest Normally Closed (NC)

Connection Series with start winding & capacitor

Location Internal to motor — requires disassembly

Orientation Not critical (mechanical, not gravity-dependent)

🔧

Inspection and Maintenance

Centrifugal switches are subject to mechanical wear from the repeated opening and closing cycles over the motor’s life. During any motor rebuild or overhaul, the centrifugal switch should be inspected and tested:

Contact condition: Check for burning, pitting, and carbon deposits. Lightly pitted contacts can be cleaned with fine (400-grit) emery cloth — never a file, which leaves grooves that accelerate further arcing. Heavily burned contacts require replacement of the switch assembly.

Mechanical operation: With the motor de-energised and the rotor held stationary, the contacts should be closed. Manually rotate the shaft and verify the contacts open smoothly when the weights extend and close cleanly when released. A sticky or sluggish mechanism indicates worn pivots or a weak return spring.

Gap adjustment: Some designs allow adjustment of the contact gap. Refer to the motor manufacturer’s service data — too large a gap causes high contact resistance and arcing; too small a gap causes premature opening during acceleration before the motor reaches full speed.

Centrifugal Switch — Failure Modes

🔴 Contacts Welded or Stuck Closed

Start winding energised continuously during run

Motor starts normally but immediately overheats and trips. Burned-winding smell is common. The start winding is not rated for continuous duty — failure can occur in under a minute. Caused by contact welding from high inrush arcing or from contamination preventing mechanical opening. Requires motor disassembly to inspect.

🟡 Contacts Burned Open or Fail to Close

Start circuit never connects

Motor hums at LRA but shaft does not rotate. Trips overload or breaker. A common variation: contacts close intermittently, causing the motor to start erratically or start only when the housing is vibrated. Confirm by listening for the characteristic “click” as the switch should open and close with shaft rotation.

👂

The centrifugal switch click — a useful diagnostic

A healthy centrifugal switch produces a faint but audible “click” at two points during normal operation: once when the contacts open as the motor reaches ~75% speed during starting, and again when they close as the motor coasts to a stop after shut-down. If you hear the start click but not the stop click (or vice versa), the mechanical action of the switch is incomplete. No click at all during starting suggests the contacts are already open (failed open); no click during coast-down suggests the contacts never closed back (failed by jamming).

2.4.6 — Device Comparison & Selection Guide

Use the table below when identifying an unknown starting device during service, or when selecting a replacement. The “tells” column lists the quickest field identification clue for each type.

Device	Contact at Rest	Coil / Element Location	Orientation-Sensitive?	Restart Delay?	Typical HP	Quick Field “Tell”
Current Relay	NO	Series with run winding	Yes — gravity	No	¼–1 HP	Heavy series coil; rattles when shaken (plunger)
Potential Relay	NC	Parallel with start winding	No	No	½ HP+	Labelled with pickup voltage (e.g. “330V”); fine coil wire
PTC Device	N/A	Series with start winding	No	Yes — 2–5 min	Under ½ HP	Small disc or cylinder; no moving parts; warm after run
Centrifugal Switch	NC	Internal on rotor shaft	No	No	Any (open frame)	Audible “click” at start and stop; internal — not visible externally

🔁

Current relay vs. potential relay — they cannot substitute for each other

Current relays and potential relays look similar externally but are wired differently and sense opposite quantities. Installing a current relay where a potential relay is required (or vice versa) will result in either the start circuit never activating, or the start capacitor remaining in-circuit during run and failing. Always verify the relay type by checking: (1) normally open vs. normally closed contacts, (2) coil terminals labelled for series vs. parallel connection, and (3) the rated voltage or current on the label.

📋

Field identification checklist

1. Is the device inside or outside the motor? — Inside = centrifugal switch.
2. Does it have a visible coil winding? — Yes = current or potential relay.
3. Does it rattle (plunger inside)? — Yes = current relay. No rattle but has coil = potential relay.
4. Is it a small solid disc or ceramic cylinder with no windings? — PTC device.
5. Is it warm after the motor has run? — Confirms PTC device.

2.4 — Starting Relays & Devices

2.4.1 — Why Starting Devices Are Needed

2.4.2 — Current Relay (Series Relay)

Current Relay — Series / Start Relay

Current Relay — Failure Modes

2.4.3 — Potential Relay (Voltage / Back-EMF Relay)

Potential Relay — Voltage / Back-EMF Relay

Pickup and Dropout Voltage — Why They Matter

Potential Relay — Failure Modes

2.4.4 — PTC Start Device (Positive Temperature Coefficient)

PTC Thermistor Start Device

PTC Resistance vs. Temperature

PTC Operating Cycle

PTC Device — Failure Modes

2.4.5 — Centrifugal Switch

Centrifugal Switch

Inspection and Maintenance

Centrifugal Switch — Failure Modes

2.4.6 — Device Comparison & Selection Guide