32.1 Belts, Pulleys, Motor Rotation & Seized Compressors

32.1 Belts, Pulleys, Motor Rotation & Seized Compressors

Trade Area F (Maintenance Service) is 14 percent of both Class A and Class B. Chapter 31 recovered refrigerant, repaired compressor internals as a component, and calibrated controls. This chapter is the rest of Trade F: mechanical service. The numbered items that land in 32.1 are aligning pulleys, adjusting V-belt tension, electric actuating valves, electric humidistats, reversing three-phase motors, reversing single-phase motors, and starting seized hermetic compressor motors. Installing belts and sheaves was Chapter 28.2. Wiring rotation as a three-phase install step was Chapter 28.1. Amp-draw and terminal ohms were Chapter 30.1. Here the exam wants the service sequence: a glazed belt, a motor running backward, a solenoid that will not open, a humidistat that will not cut out, and a hermetic that hums and will not start. The 2026 Air Conditioning CBT books for this cluster are Refrigeration & Air Conditioning Technology, 9th Edition (2021), and Air Conditioning and Refrigeration Troubleshooting Handbook, 2nd Edition (2003). OSHA 29 CFR 1926 (July 1, 2025 edition on the reference list) still wants lockout/tagout (LOTO) before a belt guard comes off and before an ohmmeter touches compressor pins. F.S. 489.105 still fences line-voltage work to dedicated HVAC circuits, existing single-phase disconnects, circuit-breaker locks, and low-voltage controls. Class B still services drives and hermetics on legal systems through 25 tons cooling and 500,000 Btu heating; a 30-ton belt-drive air handler is Class A to contract, but the theory of alignment, reverse rotation, and a seized hermetic is on both outlines.

Quick Answer: Align sheaves with a straightedge — shafts parallel, faces in one plane. Tension a V-belt about 1/64 inch of deflection per inch of span at mid-span, or use the manufacturer's gage. Swap any two power leads to reverse three-phase. Reverse single-phase at the start winding, not by swapping L1 and L2 on a permanent split-capacitor compressor. Never bump a grounded hermetic. LOTO, ohm to the shell, then a listed hard-start attempt while watching that amps leave locked-rotor amps (LRA). Electric humidistats open on rise for humidifiers and close on rise for dehumidifiers. Electric actuating valves are solenoids and motorized valves — prove the coil, then the plunger.

Why this is service, not a parts swap

A burned belt, a backward condenser fan, a stuck liquid-line solenoid, and a hermetic that will not break away can all be sold as replace the compressor until you align, tension, reverse, ohm, and amp-check. Trade F scores that discipline. The CILB card is not a live-work permit: guards come off under LOTO; live amp checks use a clamp-on on one insulated conductor with listed covers in place. Jumping a contactor to bump a machine whose winding is already on the shell is how you flash the cabinet.

Aligning pulleys and setting V-belt tension

A belt-drive air handler, exhaust fan, or cooling-tower fan uses a motor sheave (pulley), a driven sheave, and one or more V-belts (or a banded belt). Chapter 28.2 already used the speed formula; service still needs it because a replaced sheave with the wrong pitch diameter changes revolutions per minute (RPM) and, by the fan laws, cubic feet per minute (CFM) and horsepower:

(\text{RPM}{\text{fan}} = \text{RPM}{\text{motor}} \times (D_{\text{motor sheave}} / D_{\text{fan sheave}}))

Worked sheave check. A 1725 RPM motor, 4-inch motor sheave, 8-inch fan sheave: (1725 \times (4/8) = 862.5) RPM. If a helper installs a 6-inch motor sheave on the same 8-inch fan sheave, fan speed becomes (1725 \times (6/8) = 1{,}294) RPM. CFM rises with RPM; horsepower rises with RPM cubed. That one-size-up sheave can cook a motor that was legal at 862 RPM. Variable-pitch sheaves are set at commissioning and locked; they are not a weekly CFM handle.

Alignment is two requirements at once: the shafts must be parallel, and the sheaves must sit in the same plane. A steel straightedge (or a laser) across both faces should kiss both sheaves at four points. Angular misalignment (shafts not parallel) and parallel offset (sheaves not in one plane) both eat the belt sidewall, heat the belt, and thrust the bearings. A belt that climbs out of the groove is an alignment problem, not a needs-more-tension problem.

Tension. The field rule the troubleshooting handbook and the technology text both support is about 1/64 inch of deflection per inch of belt span, measured at mid-span, under the force the belt maker publishes (or firm thumb pressure when the item does not give a gage).

Worked deflection. Motor shaft to fan shaft is 16 inches of span. (16 \times (1/64) = 1/4) inch of deflection at the middle of the belt. Too loose: slip, squeal on start, glaze (shiny belt face), heat, lost CFM, and a motor that runs while the fan does not. Too tight: bearing thrust, bent shafts, and a motor that runs hot at a legal amp draw because the mechanical load is the belt, not the air. Never pry a belt over a sheave with a screwdriver; roll it on by opening the adjustable motor base, square the motor, and lock the bolts. Replace matched sets together — a new belt on a stretched partner carries the load and dies. A guard goes back on before the disconnect is closed; OSHA 1926 still wants the rotating parts covered.

Electric actuating valves and electric humidistats

An electric actuating valve is a valve whose stem is moved by a solenoid coil or by a motorized actuator, not by a pressure bellows. The liquid-line solenoid on a pump-down circuit is the exam's favorite: normally closed (NC), it opens when the coil is energized and closes when the thermostat (or the pump-down relay) drops out, so the compressor can pump the evaporator down to the low-pressure (LP) cutout. Hot-gas solenoids, unloader solenoids (Section 32.2), humidifier water valves, and two-position chilled-water or condenser-water valves are the same family. Modulating motorized valves take a 0–10 V or 4–20 mA signal from a controller; two-position motorized valves are 24-volt or line-voltage open/close.

Service the coil first. After LOTO on line-voltage coils (or with the circuit designed for a live 24-volt measurement), read coil ohms against the stencil and read voltage at the coil on a call. Infinite ohms is an open coil. A coil that is in spec with full voltage and a valve that still will not pass is a stuck plunger — scale, sludge, or a burned seat — not another thermostat. Manual-open stems exist on many water valves for diagnosis; they are not a substitute for a rebuild. Do not hammer the valve body to free it on a charged liquid line.

An electric humidistat is a humidity-actuated switch. Action depends on the load:

• Humidifier control: contacts open on rise of relative humidity (RH) — when the space is wet enough, the humidifier stops.
• Dehumidifier (or a dehumidify call into a thermostat or a dedicated dehumidifier): contacts close on rise of RH — when the space is too wet, the machine runs.

Florida cooling season is a dehumidification climate. A humidistat wired as a humidifier in a Miami closet that never sees 30 percent RH will sit in the wrong state all summer. Sensing location matters: in the return air or in the occupied space, not in a supply dump and not taped to a wet coil. Dust on an element drifts calibration; Chapter 31 already calibrated controls — here you replace or clean the element and prove the switch makes and breaks at the setpoint. Low-voltage humidistat cable is inside both Class A and Class B (F.S. 489.105).

Reversing motors, then starting a seized hermetic

Three-phase reverse: swap any two of the three power leads. That is the exam sentence. Scroll compressors, some pumps, and propeller fans are rotation-sensitive. A condenser fan running backward still moves some air and still makes noise while head pressure climbs. A momentary rotation check on a healthy newly wired motor, with listed covers, is an install/service check (Chapter 28.1). Swapping one lead does nothing useful. Swapping L1 and L2 on a single-phase run-capacitor compressor does not reverse it.

Single-phase reverse: a reversible split-phase or capacitor-start motor reverses when you interchange the start-winding leads relative to the run winding. Many permanent split-capacitor (PSC) blower motors have identified reversing leads in the wiring diagram. Shaded-pole motors are generally not field-reversed. Hermetic PSC compressors are not reversed as a capacity trick.

Starting a seized hermetic is a diagnosis, not a dare. A compressor that hums and will not start is at LRA until you kill the power. Causes include unequalized pressures (short-cycle, thermostatic expansion valve still holding), a flooded start (liquid refrigerant in the crankcase, oil diluted, bearings tight), a failed start capacitor or potential relay, an open start winding, a grounded winding, or a mechanically wiped bearing.

Required sequence:

1. LOTO the dedicated HVAC disconnect and the breaker lock. Verify zero volts.
2. Discharge capacitors through a resistor. Lift the leads.
3. Ohm common–run–start (C–R–S) or T1–T2–T3, and every pin to the shell.
4. Any continuity to the shell is a grounded winding. Recover the refrigerant under EPA Section 608, replace the compressor, and clean the system as Chapter 31 required. Do not bump it. Do not reverse-bump it. Do not hold the contactor in. A grounded hermetic that is forced to run is a live cabinet and a flash hazard.
5. If the windings identify, are ungrounded, and the shell is not sitting on an open internal overload, restore power with listed covers. Confirm pressures have equalized. Confirm start-circuit parts.
6. A listed hard-start kit (start capacitor plus potential relay, or a listed positive-temperature-coefficient device) is a single-phase tool for an ungrounded motor that is tight, not a grounded motor. Watch the clamp-on: amps must leave LRA in a few seconds. If they do not, kill power. Repeated stalls cook the windings you just proved were good.
7. Three-phase reverse bumping to rock a stuck rotor is not the first answer and is never the answer on a grounded machine. If windings are good, pressures are equal, and the rotor is mechanically locked, replace the compressor. Do not sit on LRA hoping it breaks away.

Amp check is the live half of that sequence: clamp one conductor. Compare running amps to rated-load amps (RLA) after a successful start. LRA that never drops is a locked rotor, not a slow start. A crankcase heater that was left open overnight in a Florida garage is a flooded-start story, not a bumping story.

Florida scenario

Bayshore Mechanical, a certified Class B shop in Hillsborough County, is on a 5-ton belt-drive air handler over a finished Tampa living room and a 3-ton single-phase condensing unit on the pad. The air handler squeals; the helper has already pried a new belt on with a screwdriver and tightened it until the motor groans. Straightedge shows the motor sheave 1/4 inch offset. The correct service is LOTO, roll the belt off the adjustable base, align both faces, set 1/64 inch per inch of span, replace the matched belt if it is already glazed, and put the guard back. The condensing unit hums and trips. The helper wants to bump it in reverse. After LOTO, C–R–S identifies, but common to shell is near zero ohms. That compressor is grounded. Recover, replace, do not energize. The same owner has a 30-ton three-phase rooftop on the strip center. Class B still has to know that swapping two leads reverses the supply fan and that a seized 30-ton hermetic follows the same ground-first rule. Class B does not contract that 30-ton system (F.S. 489.105(3)(g)).

Traps: (1) Prying belts on with a screwdriver. (2) Mixing old and new belts. (3) Over-tension as a cure for misalignment. (4) Swapping L1/L2 to reverse a PSC compressor. (5) Bumping a grounded hermetic. (6) Comparing a stalled motor to RLA instead of noticing it is sitting on LRA. (7) Treating a humidistat as a thermostat with the same open-on-rise logic for dehumidifiers. (8) Class B installing the 30-ton drive because we only reversed it.