28.2 Motor Types, Protection, Controllers, Belts & Pulleys

28.2 Motor Types, Protection, Controllers, Belts & Pulleys

The next Trade D cluster is the rotating machinery on the HVAC unit: capacitor-start motors, shaded-pole motors, split-phase motors, fan blades, fan belts, and drive pulleys. Chapter 18.2 sized fan motors from CFM. Chapter 24.2 installed the fan and blower as an air-side component. Chapter 27.4 hung magnetic starters and hermetic overloads. Chapter 32 will service seized compressors and slipping belts. This section is selecting and installing the motor type, the protector, the controller, and the belt-and-pulley drive so the exam’s numbered items are not a pile of catalog names. Refrigeration & Air Conditioning Technology, 9th Edition (2021) is the technology book. F.S. 489.105 still fences three-phase power (Section 28.1) and Class B at 25 tons / 500,000 Btu.

Quick Answer: Split-phase = start winding + centrifugal switch, modest torque. Capacitor-start = start capacitor + switch, high torque. Shaded-pole = shading ring, tiny fans only. PSC keeps a run capacitor in. Protect from FLA, control with a contactor or starter, set belts by ratio and alignment, and match blade rotation to the housing.

Split-phase, capacitor-start, and shaded-pole — what the outline names

A single-phase motor does not create a rotating field by itself. It needs a start device that shifts current in a second winding (or a shading coil) so the rotor sees a rotating field, then that start device must drop out (except on PSC and shaded-pole, which stay in).

A split-phase motor has a run winding (heavier wire, lower resistance) and a start winding (finer wire, higher resistance) displaced about 90 electrical degrees. A centrifugal switch (or electronic relay) opens the start winding at roughly 75 percent of synchronous speed. Starting torque is modest — commonly around 150 percent of full-load torque. HVAC uses: some older belt-drive blowers, small pumps, oil burners. They are not the first choice for a hard-starting hermetic. If the centrifugal switch welds closed, the start winding stays in and cooks. If it never closes, the motor hums and draws LRA until the overload opens.

A capacitor-start (CS) motor is a split-phase machine with a start capacitor in series with the start winding. The capacitor increases the phase shift and starting torque — commonly 300 to 400 percent of full-load torque — so the motor can start a compressor, a loaded pump, or a high-static blower. The same centrifugal switch (or a potential relay on hermetics) drops the start capacitor and start winding out after a second or two. Never leave that start capacitor in the run circuit: it is intermittent-duty and will swell, leak, and take the start winding with it. Match µF and VAC to the listing (Chapter 27.3).

A shaded-pole motor has only one main winding. A copper shading ring (a shorted turn) on a portion of each pole delays the flux in that pole tip and produces a weak rotating field. Starting torque is low (often 25 to 50 percent of full-load). Uses: small propeller fans, bathroom-style exhausts, timer motors, some condenser-fan replacements on fractional-horsepower appliances. They are cheap, they run hot, they are not reversible by swapping leads in any useful way unless the listing provides a reverse connection, and they are not a 3-horsepower blower motor. Putting a shaded-pole in a high-static air handler is a no-start, not a “simple swap.”

Two cousins the exam will still show you because they are what you actually hang:

• Permanent-split capacitor (PSC): a run capacitor stays in series with the start (auxiliary) winding the entire run. Moderate starting torque, quiet, the usual condenser fan, direct-drive blower, and many hermetic compressors. No centrifugal switch to fail. A failed run capacitor is high amps and a hot winding (Chapter 27.3).
• Capacitor-start, capacitor-run (CSCR): start capacitor plus run capacitor. High start torque and better run efficiency. The start capacitor still must drop out.

Three-phase motors (induction, often squirrel-cage) do not need start capacitors or shading rings; the three legs already produce a rotating field. They still need overloads, a controller, and the rotation check from 28.1.

Motor protection and controllers

Protection keeps the winding from cooking. Integral protectors (Klixon on the shell, buried hermetic overload) open on temperature/current and may take minutes to reset — Section 27.4. External overloads in a magnetic starter are selected from motor FLA (or compressor RLA per the starter table) with sensing in each phase on three-phase. Electronic overload modules dial the same FLA. Do not size heaters from breaker size or from “the next size up so it stops nuisance tripping.” Single-phasing will then burn two windings while the third sits idle.

Duty must match the listing: continuous, intermittent, air-over. A condenser-fan motor stamped air-over that you bury in a doghouse with no through-air will take out the protector, then the winding. Nameplate ambient (often 40°C) is not a Florida-roof exemption; it is the test condition. Use a motor rated for the outdoor or high-static duty you actually have.

A motor controller is the listed means of starting and stopping: a contactor (across-the-line, the usual HVAC compressor switch), a magnetic starter (contactor plus overloads, often with HOA), part-winding or star-delta on large machines, a soft start, or a VFD. VFD output is not a place to hang a standard phase monitor on the motor leads without reading the listing; the monitor belongs on line side unless the drive documentation says otherwise. Controller coil voltage must match the circuit (24, 120, or 240 V). Line-side power to that controller still sits inside the 489.105 fence: HVAC lands what the dedicated-circuit / single-phase-disconnect grants allow; a new 460-volt starter feeder is an electrician.

Fan blades, belts, and drive pulleys

A fan blade (propeller) or blower wheel (forward-curve squirrel cage, backward-inclined, airfoil) must match rotation and housing. An arrow on the housing and on the motor is not decoration. A propeller on a condenser is usually draw-through the coil; install it so air leaves toward the discharge guard. A forward-curve wheel run backward still moves some air and sounds “almost right” while CFM collapses. Balance and set-screws (or a clamp hub) on a clean shaft: a missing blade, a bent blade, or a wheel that has walked on the shaft takes bearings and then the motor. Never “trim” a blade with tin snips to clear a housing; replace the listed wheel. Guard it. OSHA 1926 still wants the moving parts covered (Chapter 33).

Belt-drive air handlers and exhaust fans use a motor sheave (pulley), a fan sheave, and one or more V-belts (or a banded belt). Speed formula:

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

Worked pulley. A 1725 RPM motor, 4-inch motor sheave, 8-inch fan sheave:

(1725 \times (4/8) = 862.5\ \text{RPM})

Need 1,000 RPM on the same 1725 RPM motor? (D_m / D_f = 1000/1725 \approx 0.58). A 5-inch motor sheave on an 8.6-inch fan sheave (or the closest listed pair) is the direction — larger motor sheave or smaller fan sheave raises fan RPM. Fan laws from Chapter 16.3 still apply: CFM tracks RPM, horsepower tracks RPM³. A “one-size-up” motor sheave that looks harmless can overload the motor and the breaker.

Alignment: both pulleys in the same plane, shafts parallel. A straightedge across both faces should kiss both sheaves. Angular misalignment eats belts and bearings. Tension: a common field rule is about 1/64 inch of deflection per inch of span under moderate thumb pressure at mid-span, or the manufacturer’s belt-tension gage. Too loose = slip, glaze, heat, lost CFM. Too tight = bearing thrust and a cooked motor. Replace belts as a matched set. Do not pry a belt over the sheave with a screwdriver; roll it on with the motor base adjusted. Set the motor on the adjustable base, square it, lock the bolts, and leave belt-length adjustment — not a twist in the belt — for take-up. Chapter 32 returns to service of glazed belts and seized compressors; the install rule is right ratio, aligned, tensioned, guarded.

Variable-pitch motor sheaves are set at commissioning to hit scheduled CFM, then locked. They are not a weekly “more air” handle for a salesperson. Direct-drive ECM blowers do not get a pulley lecture; they get the correct motor and wheel pair from the listing.

Florida HVAC scenario

A certified Class B shop in Lee County replaces a 5-ton belt-drive air handler (legal tons). The old motor is a split-phase 1725 RPM; the new listing wants a PSC with a run capacitor. The helper hangs a shaded-pole “because it was on the truck and it is a fan motor,” then throws a capacitor-start farm-motor on the rails when the shaded-pole will not start the wheel. Two wrong types. The listing PSC goes in with the run capacitor, overloads (or integral protector) matching FLA, and the belt drive is set 4-inch motor / 8-inch fan on 1725 RPM → 863 RPM. TAB needed 1,000 RPM; the helper opens the variable sheave the wrong way, overspeeds the wheel, and trips the starter. Alignment is off a half-belt width, so the new belts glaze in a week. Independently, the same owner points at a 30-ton belt-drive penthouse. Class B still has to compute the pulley ratio and name a capacitor-start versus shaded-pole on the exam; Class B does not contract that 30-ton system. The 30-ton three-phase starter feeder is still an electrician (Section 28.1).

Traps: (1) Shaded-pole as a blower motor. (2) Start capacitor left in run. (3) Oversized overload heaters. (4) Raising fan RPM with a bigger motor sheave without checking horsepower cubed. (5) Misaligned or overtight belts. (6) Backward blower wheel. (7) Class B on a 30-ton belt-drive AHU because the pulley is small.