30.2 Testing Relays, Capacitors, Starters, Transformers & Defrost Stats

30.2 Testing Relays, Capacitors, Starters, Transformers & Defrost Stats

The rest of the electrical Trade E cluster is component tests: current relays, capacitors, defrost thermostats (defrost stats), high-voltage transformers, high-voltage relays, low-voltage relays, magnetic starters, potential relays, and the motor terminals already covered in 30.1. Chapter 27.3 installed transformers, capacitors, contactors, and starting relays. Chapter 27.4 installed defrost hardware, solenoid coils, and magnetic starters. This section is whether the part that is already in the machine is still the part. The CBT spine is still Refrigeration & Air Conditioning Technology, 9th Edition (2021), and the Troubleshooting Handbook, 2nd Edition (2003). Lockout/tagout before a cover comes off for ohms or microfarads. Live coil-voltage and clamp-on tests stay on a rated meter with the listing’s cover rules.

Quick Answer: Capacitor: measure µF, keep about ±10 percent of nameplate, never short the terminals. Current relay: low-ohm coil, NO contacts, series with run. Potential relay: high-ohm coil, NC contacts, across start, opens on back-EMF. Transformers: 24 V under load; HV packs are kilovolts. Starters: coil plus FLA overloads on each phase. Defrost stats: closed cold, open when the coil is clear.

Capacitors — microfarads, not a spark

A run capacitor is continuous-duty and stays in the circuit. A start capacitor is intermittent-duty and must be dropped out by a start relay after a second or two. Dual run cans are labeled HERM, FAN, and C. Chapter 27.3 installed them. Trade E tests them.

Never test a capacitor by shorting the terminals. Shorting a charged can dumps the energy in a spark, can explode the can, can weld a screwdriver, and tells you nothing about microfarads. LOTO, then discharge through a resistor (a 20,000-ohm, 2-watt resistor is the usual field picture — use what the instrument maker lists). Then measure with a capacitor tester or a meter that reads µF. Typical field band versus nameplate is about ±10 percent (some listings print ±6 percent; ±10 percent is the common exam band). A 45 µF HERM section is in band at 40.5–49.5 µF. 35 µF is failed — high compressor amps, hot run winding, hard start. A 5 µF FAN section is in band at 4.5–5.5 µF.

Worked dual-can test. Nameplate 45/5 µF, 370/440 V. HERM measures 47 µF (in band). FAN measures 3.1 µF (out of band). Replace the can — both sections share a case, and a failed FAN section is a failed condenser-fan capacitor even if HERM still looks pretty. Match VAC at or above the old can. An open capacitor reads 0 µF (or OL) and the motor hums. A shorted capacitor shows near 0 Ω between terminals and will trip an overload the instant you close the contactor. Bulged cans and oil on the top are failed even if µF still limps inside the band.

Current relays versus potential relays

A current (series) starting relay sits in series with the run winding. High start current closes a normally-open contact that puts the start winding (and often a start capacitor) in the circuit. As run current falls, the relay drops out. The coil is a few turns of heavy wire: very low ohms, often a fraction of an ohm to a couple of ohms. Contacts at rest are open. A welded-closed current relay holds the start winding in and cooks it. An open coil never closes the contacts — the motor hums and does not start. Do not read “almost 0 Ω on the coil” as a short; that is the coil’s normal picture.

A potential (voltage) starting relay sits across the start winding. As the motor approaches speed, back-EMF rises and the coil opens its normally-closed contact, dropping the start capacitor. The coil is many turns of fine wire: thousands of ohms (commonly in a 3,000–18,000 Ω neighborhood — use the relay’s chart). Contacts at rest are closed. An open coil never picks up, so the start capacitor stays in the run circuit and dies. Contacts that will not open do the same. Contacts that will not close at rest mean no start capacitor, so the motor does not start.

Do not interchange them. A current relay on a potential-relay motor (or the reverse) either leaves the start winding energized or never starts it. Hard-start kits are listed potential relay plus start capacitor accessories; they are not a substitute for a failed run capacitor and not a substitute for a low-voltage problem.

Transformers, HV packs, and HV versus LV relays

A 24-volt Class 2 control transformer is tested two ways. Live: secondary voltage under load (contactor pulled in, damper motors sitting on R). A no-load 27 V that collapses to 18 V when the coil pulls in is an undersized or failed transformer, or a shorted 24-volt cable — not a “weak contactor.” De-energized (LOTO): primary and secondary ohms. An open winding is infinite. A shorted winding is near zero and will blow the primary fuse or the breaker. Primary taps must still match site voltage (208 versus 240) or every coil on the machine is a voltage-drop story.

High-voltage transformers on this outline are ignition transformers and electronic-air-cleaner power packs (and similar ionizing supplies). Secondaries are thousands of volts. You do not put a 24-volt meter on that secondary. Test the primary ohms and voltage, the door interlock (HV must die when the access door opens — Chapter 27.4), and the listing’s HV probe if you have one. Do not megger through a solid-state pack. Wet EAC cells with the power on are a shock and a warped-plate complaint, not a test.

High-voltage relays switch those HV or line-voltage loads (EAC, ignition, some defrost heaters). Low-voltage relays are the 24-volt fan, reversing-valve, and accessory relays. Coil voltage must match. Coil ohms: LV coils are typically tens of ohms; an open coil is infinite and the relay never pulls in. Contacts: check continuity in the rest state and change of state when you apply the rated coil voltage on a bench, or when the thermostat actually calls. Pitted or welded contacts that “work with a tap” are failed contacts. A 24-volt coil landed on 120 V is a one-time event, not a stronger relay.

Magnetic starters and defrost stats

A magnetic starter is a contactor plus overload protection. Trade E tests the coil (open, shorted, wrong voltage), the power contacts (pitted, high resistance, one pole open — a single-phasing machine of your own making), and the overloads. Overload heaters or electronic FLA dials are selected from motor FLA, all three phases on three-phase. Heaters one size large “to stop nuisance trips” are how a single-phased semi-hermetic burns. An overload that will not reset, or that trips with balanced current below FLA, is a failed overload or a real mechanical overload — prove amps before you upsize the heaters. The starter coil still sits downstream of the safeties. Jumping L1 to T1 around the starter is not a test; it is a bypass.

A defrost thermostat (termination stat) on electric defrost is closed on a cold coil so heaters can run when the clock or demand board calls defrost, and it opens when the coil is clear — commonly in a 50–70°F coil-temperature band (use the listing). Fail-safe time on the clock still ends defrost if the stat never opens. Bench test after LOTO: ice or a freeze spray should close a termination stat that is open at room temperature; heat should open it. Continuity that never changes is a stuck stat. Paralleling the termination stat so heaters “always finish” is how you bake a walk-in (Chapter 27.4). Heat-pump outdoor defrost sensors are usually thermistors on a board — compare ohms to the manufacturer’s temperature chart; they are not a 24-volt coil.

Florida HVAC scenario

A certified Class B shop in Miami-Dade is on a walk-in freezer (legal under the 25-ton cap) and a 3-ton heat pump next door. On the freezer the helper “tests” the dual run capacitor by shorting it, then jumpers the defrost termination because heaters “were not finishing.” On the heat pump the helper swaps a current relay onto a potential-relay compressor, reads the potential-relay coil as “open” because a low-ohm range showed OL on a 10 kΩ coil, and upsizes the magnetic-starter heaters on a legal 15-ton rack because one leg was hot. Four independent Trade E failures: capacitor spark instead of µF, termination bypassed, relay types swapped and mis-read, overloads enlarged instead of finding the lost phase. The 15-ton rack is Class B work. The hospital’s 30-ton sister rack is Class A; Class B still has to test a 30-ton starter’s FLA heaters and a 30-ton potential relay on the exam.

Traps: (1) Shorting capacitors. (2) Treating a current-relay coil’s near-zero ohms as a short. (3) Treating a potential-relay coil’s thousands of ohms as open on a low range. (4) Swapping relay types. (5) Meggering an EAC pack. (6) Oversized starter heaters. (7) Jumping defrost termination. (8) Ohm tests without LOTO.