3.4 Motor Testing, Insulation Resistance, Bearings, and Belt Drives

3.4 Motor Testing, Insulation Resistance, Bearings, and Belt Drives

The Electrical sheet of the Competency and Task List names a specific set of test instruments — ohmmeter, multimeter, ammeter, voltmeter, wattmeter, hermetic compressor analyzer, relay tester, megger meter, capacitor analyzer — and requires the technician to demonstrate "the purpose of checking the resistance of motor windings," to define and measure locked rotor amps and full load amps, and to clean, evaluate, and install shaded-pole, split-phase, PSC, CSR, and ECM motors. This section is the diagnostic companion to Section 3.2.

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1. The Three-Test Motor Evaluation

Every motor failure resolves into one of three conditions, and there is one test for each.

A standard digital multimeter on the ohms range applies only a few volts and will report a grounded compressor as "good" because a carbonized path only conducts under high potential. This is exactly why the task list names the megger separately.

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2. Insulation Resistance Testing (Megohmmeter)

A megohmmeter ("megger") applies a high DC test voltage — typically 500 V or 1,000 V for HVACR motors — between the windings and the motor frame and measures the leakage in megohms.

Procedure

1. De-energize and lock out. Confirm zero voltage.
2. Discharge and disconnect run and start capacitors, and disconnect the motor leads from the contactor, board, and any electronics.
3. Connect the negative/earth lead to a bare, unpainted point on the compressor shell or motor frame, and the positive lead to a motor terminal.
4. Apply the test voltage for 60 seconds and read.

Interpretation for a hermetic compressor

The classic rule of thumb for open motors is 1 MΩ per 1,000 V of rating plus 1 MΩ, so a 240 V motor should read at least about 1.24 MΩ. Hermetic compressors are held to the much higher standard above because refrigerant and oil are excellent insulators until they are contaminated.

Never megger an ECM, a variable-frequency drive, an inverter-driven compressor, a control board, or any motor whose electronics cannot be isolated. The test voltage will destroy the semiconductors. On an ECM, disconnect the module and test the stator only, or follow the manufacturer's resistance-check procedure.

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3. Winding Resistance and Terminal Identification

A single-phase hermetic compressor has three terminals: C (common), S (start/auxiliary), R (run/main). The identity that governs them is: $R_{C-S} + R_{C-R} = R_{S-R}$ The start winding has the highest resistance (more turns of finer wire), the run winding the lowest, and the reading between S and R is the sum.

Identifying unmarked terminals: measure all three pairs. The highest reading is between S and R, so the terminal not involved in that reading is common. Of the two remaining readings from common, the larger is C-S and the smaller is C-R.

Interpreting failures

• Any pair reading OL → open winding. If C-S is open, a PSC motor will hum and not start.
• $R_{C-S} + R_{C-R} \neq R_{S-R}$ (typically the sum reads high relative to S-R) → shorted turns inside a winding.
• Any terminal reading low resistance to the shell → grounded. Confirm with the megger.
• Three-phase motors should show equal resistance across all three pairs; more than about 5% deviation indicates a developing short.

A hermetic compressor analyzer combines these tests with an applied-voltage start attempt, and a relay tester verifies that a potential or current start relay picks up and drops out at the correct voltage or current.

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4. FLA, RLA, LRA, and What the Nameplate Means

RLA is not a target. A compressor's actual running current is usually well below nameplate RLA; a healthy 3-ton scroll rated at 16.7 RLA may draw 11–13 A on a mild day. Running current climbs with load, so measure and compare against manufacturer performance data rather than against RLA.

LRA is normally four to eight times FLA. Measuring it requires a clamp meter with inrush (MIN/MAX peak-hold) capability. Diagnostic uses:

• Compressor draws LRA and will not start → locked rotor (mechanically seized), or a failed start component. Confirm the run capacitor and the start relay before condemning.
• Compressor draws LRA, trips on internal overload, restarts after cooling → classic locked rotor or severe undervoltage.
• A hard-start kit (start capacitor plus potential relay) increases starting torque and shortens the LRA period; it is a legitimate repair for a marginal compressor and a required accessory on some long-line-set installations.

Wattmeter checks are the cleanest way to prove a compressor is doing work: compare measured watts against the manufacturer's performance table at the observed suction and discharge saturation temperatures. A compressor drawing low amps and low watts with high suction pressure is not pumping.

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5. Bearings, Lubrication, and Motor Mounts

• Sleeve bearings are common on residential direct-drive blowers. Many have oil ports; use a light SAE 10 or 20 non-detergent electric-motor oil, a few drops only. Never use penetrating oil or automotive motor oil — detergents strip the wick and the oil breaks down at motor temperature.
• Ball bearings carry belt-drive side loads. Sealed bearings are lubricated for life; re-greasable bearings take a small amount of the manufacturer's specified grease with the relief plug removed and the motor running, so old grease is purged rather than blown past the seal.
• Failure symptoms: a growl or rumble that changes with speed is a bearing; a squeal on start is usually a belt; a hum with no rotation is an electrical or seized condition. Excess end play or shaft lift is a worn sleeve bearing.
• Motor mounts on residential blowers are typically resilient rubber-ring mounts in a band or cradle, chosen to isolate vibration. Hardened or cracked mounts transmit noise into the ductwork and can allow the rotor to rub. Belly-band, cradle, and rigid-base mounts are the three arrangements the task list expects you to identify.

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6. Belt-Drive Blowers

The Electric Heat and Gas Heat sheets both require adjusting airflow on a belt-driven blower assembly.

• Alignment first. Lay a straightedge across both sheave faces. Angular or parallel misalignment destroys belts and bearings faster than any other single cause. Correct alignment before tensioning.
• Tension by deflection. Press at the midpoint of the span with moderate force; correct deflection is about 1/64 inch per inch of span — roughly 1/2 inch on a 32-inch span. Over-tensioning overloads motor and blower bearings; under-tensioning causes slip, glazing, squeal, and lost airflow.
• Adjust airflow with the motor sheave. Loosen the setscrew and turn the adjustable half: closing it (fewer turns out) raises the effective pitch diameter and increases blower RPM and CFM; opening it lowers RPM and CFM. Never adjust more than one full turn without rechecking tension and motor amperage.
• Verify with amperage. Increasing airflow increases the blower motor's load. After any sheave change, measure motor current and confirm it remains at or below nameplate FLA, and re-measure external static pressure and temperature rise.
• Replace belts in matched sets on multi-belt drives, and never pry a belt over a sheave — back off the motor base instead.