13.1 Single-Phase Electric Motors & Capacitors

13.1 Single-Phase Electric Motors & Capacitors

Electric motors are the primary electro-mechanical workhorses of modern heating, ventilation, air conditioning, and refrigeration (HVAC/R) systems. They convert electrical energy into rotational mechanical energy to drive hermetic compressor mechanisms, indoor air-handling blowers, outdoor condenser fans, and combustion draft inducers. To excel on the NATE Core exam and diagnose complex field failures, technicians must master the electromagnetic operating principles of induction motors, the physics of phase splitting, and the diagnostic protocols for motor windings, capacitors, and nameplate data.

---

Motor Fundamentals: Stators, Rotors & Electromagnetic Induction

All standard single-phase HVAC/R fan and compressor motors operate on the principle of electromagnetic induction, first formulated by Michael Faraday and Heinrich Lenz. An induction motor contains two primary physical assemblies:

1. The Stator: The stationary outer frame of the motor containing a laminated core of high-permeability electrical silicon steel. Insulated copper magnet wire is wound into slots inside the stator core to create concentrated electromagnetic pole pairs.
2. The Rotor: The rotating inner cylindrical member mounted on heavy-duty bronze sleeve bearings or sealed ball bearings. Nearly all HVAC induction motors utilize a squirrel-cage rotor. Rather than wire windings, the squirrel-cage rotor consists of longitudinal conductive aluminum or copper bars cast into laminated iron slots and solidly short-circuited at both ends by heavy conductive end-rings.

Squirrel-Cage Induction Motor Construction:
=========================================================================
       +-------------------------------------------------------+
       |                    STATOR HOUSING                     |
       |   +-----------------------------------------------+   |
       |   |      Laminated Silicon Steel Stator Slots     |   |
       |   |       [ Main Run & Auxiliary Start Coils ]    |   |
       |   +-----------------------------------------------+   |
       |                     Air Gap (~0.015 to 0.030 in.)     |
       |        +-------------------------------------+        |
       |        |   Conductive Rotor End-Ring         |        |
       |  SHAFT |   |===|===|===|===|===|===|===|===| | SHAFT |
       | =======|===| Skewed Aluminum Rotor Bars    |===|=======|
       |        |   |===|===|===|===|===|===|===|===| |        |
       |        |   Conductive Rotor End-Ring         |        |
       |        +-------------------------------------+        |
       +-------------------------------------------------------+
=========================================================================

When alternating current (AC) flows through the stator windings, it establishes an alternating magnetic flux. This flux sweeps across the small air gap separating the stator and rotor, cutting across the conductive bars of the squirrel-cage rotor. According to Faraday's Law, an electromotive force (voltage) is induced across the rotor bars. Because the bars are shorted together at each end by the conductive end rings, massive circulating currents flow through the rotor bars.

According to Lenz's Law, this induced rotor current establishes its own secondary magnetic field that directly opposes the changing stator magnetic field. The interaction between the stator magnetic poles and the induced rotor magnetic poles produces a mechanical tangential force—rotational torque—that forces the rotor shaft to turn.

---

Synchronous Speed & Rotor Slip

The speed at which the stator magnetic field rotates is known as the synchronous speed ($N_s$). Synchronous speed is determined strictly by two variables: the frequency of the alternating current power supply ($f$, in Hertz) and the number of physical electromagnetic stator poles ($P$) wound into the motor.

Synchronous Speed Formula

$N_s = \frac{120 \times f}{P}$

Where:

• $N_s$ = Synchronous speed of the rotating magnetic field in Revolutions Per Minute (RPM)
• $f$ = Electrical line frequency in cycles per second ($60\text{ Hz}$ standard in North America)
• $P$ = Total number of electromagnetic poles per phase (must be an even integer: 2, 4, 6, 8, etc.)
• $120$ = Mathematical constant ($60\text{ seconds/minute} \times 2\text{ poles/pole-pair}$)

The Physics of Rotor Slip

An induction motor can never rotate at synchronous speed while operating under mechanical load. If the rotor shaft were to spin at the exact synchronous speed of the stator magnetic field, the rotor bars would travel at the exact same velocity as the rotating magnetic flux. The relative velocity between the rotor bars and the stator magnetic field would equal zero.

With zero relative motion, no magnetic lines of flux would be cut by the rotor bars, zero electromotive force would be induced, zero rotor current would flow, and the motor would generate zero electromagnetic torque. The rotor would instantly decelerate due to mechanical friction and external load.

Therefore, the rotor must always "lag behind" or "slip" relative to the rotating stator field. The difference between the synchronous stator speed ($N_s$) and the actual mechanical rotor shaft speed ($N_r$) is termed rotor slip:

$\text{Rotor Slip (RPM)} = N_s - N_r$ $\text{Percent Slip (\%)} = \frac{N_s - N_r}{N_s} \times 100\%$

Under normal full-load operating conditions, standard single-phase induction motors exhibit $3%$ to $5%$ slip. If the mechanical load on the motor increases (such as an air handler operating against higher duct resistance or a compressor operating against higher head pressure), the rotor slows down slightly. This increases the slip percentage, causing the rotor bars to cut magnetic flux lines at a higher frequency. This in turn induces higher rotor currents, draws higher stator amperage, and generates the additional torque required to match the higher load.

Worked Problem: Rotor Slip Calculation
A direct-drive indoor blower motor operates on a 60 Hz single-phase power supply.
The stator is wound with 4 electromagnetic poles, and a technician measures an actual
shaft speed of 1,728 RPM using an optical digital tachometer.

Step 1: Calculate Synchronous Speed (Ns):
Ns = (120 * f) / P = (120 * 60) / 4 = 7,200 / 4 = 1,800 RPM

Step 2: Calculate Rotor Slip in RPM:
Slip = Ns - Nr = 1,800 RPM - 1,728 RPM = 72 RPM

Step 3: Calculate Percent Rotor Slip:
Percent Slip = (Slip / Ns) * 100% = (72 / 1,800) * 100% = 0.04 * 100% = 4.0%
Conclusion: The motor is operating normally within the standard 3% to 5% full-load slip window.

---

The Phase Split Requirement in Single-Phase Motors

Single-phase alternating current consists of a single sinusoidal voltage wave. When single-phase AC is supplied to a single stator winding, it creates a magnetic field that pulsates in intensity and alternates in polarity along a stationary physical axis. It does not rotate.

Pulsating Field Vector (Stationary Axis) vs. Rotating Field Vector:
=========================================================================
Single-Phase (Pulsating Only):           Two-Phase Split (True Revolving):
          [ NORTH ]                                  12:00 (Peak L1)
              ^                                            ^
              |                                            |
        +-----+-----+                                \     |     /
        |   ROTOR   |                                 \    |    /
        +-----+-----+                         9:00 <---+---+---> 3:00 (Peak L2)
              |                                       /    |    \
              v                                      /     |     \
          [ SOUTH ]                                        v
(Net torque on stationary rotor = 0)                     6:00
=========================================================================

Mathematically and physically, a pulsating magnetic field can be resolved into two identical magnetic fields of equal strength rotating in opposite directions at synchronous speed. When the rotor is stationary (at rest), the forward rotating magnetic field induces a forward torque that is exactly equal and opposite to the backward torque induced by the reverse rotating field: $T_{\text{forward}} + T_{\text{reverse}} = 0$

Because the net starting torque is exactly zero, a single-phase motor connected to a single winding will merely sit still, hum loudly, draw locked-rotor current, and overheat rapidly until its thermal overload trips. However, if the rotor is manually spun in either direction by hand, the relative slip between the rotor and the field rotating in that direction decreases, the opposing field's influence diminishes, and the motor will accelerate up to running speed in whichever direction it was pushed.

To make a single-phase motor self-starting, a secondary auxiliary winding—the start winding—must be added to the stator. This process is called phase splitting:

1. Spatial Displacement: The start winding is positioned in the stator slots physically displaced by $90\text{ electrical degrees}$ from the primary run winding.
2. Electrical Phase Displacement: An electrical phase shift must be established between the current flowing through the run winding and the current flowing through the start winding. When the currents in the two windings are out of phase with each other (ideally approaching $90^\circ$), their respective magnetic fields peak at different times. The vector sum of these two spatially and temporally displaced magnetic fields creates a continuous, true rotating magnetic field that pulls the rotor into motion.

---

Single-Phase Motor Classifications

Single-phase motors are categorized by how they achieve the initial phase split and whether auxiliary components remain active during normal running operation.

Phase-Splitting Vector Diagrams Across Motor Types:
=========================================================================
Split-Phase Motor:             PSC Motor:                  CSCR Motor:
(Inductive Winding Split)      (Run Capacitor Shift)       (Dual Capacitor Shift)

      V_line                         V_line                      V_line
        |                              |                           |
        |-- I_run (Lags ~70 deg)       |-- I_run (Lags ~45 deg)    |-- I_run (Lags ~45 deg)
        |                              |                           |
        \                              |                           |--- I_start (Leads ~40 deg)
         \ I_start (Lags ~35 deg)      |--- I_start (Leads ~35 deg)    (Max starting phase shift
        (Small phase shift ~35 deg;   (Continuous phase shift;          approaching ~85 deg;
         moderate starting torque)     smooth run, fair start)          massive starting torque)
=========================================================================

1. Shaded-Pole Motors

• Operating Principle: A shaded-pole motor uses salient (projecting) laminated stator poles. A small portion of each pole face is physically notched, and a heavy, uninsulated solid copper ring or loop—called a shading coil or shading ring—is wrapped around that notched section.
• Electromagnetic Action: As alternating current in the main stator coil increases, magnetic flux expands through the pole face. The expanding flux cuts the copper shading ring, inducing a heavy circulating current within it. According to Lenz's Law, this induced current creates a local magnetic counter-flux that delays the buildup of magnetic flux in the shaded portion of the pole. When the main flux reaches its peak and begins to collapse, the shading ring's induced current reverses, maintaining magnetic flux in the shaded portion after the unshaded portion has collapsed. This creates a sweeping magnetic wave that moves across the pole face from the unshaded section toward the shaded section.
• Performance Characteristics:
  • Starting Torque: Extremely low ($50%$ to $75%$ of full-load torque).
  • Operating Efficiency: Lowest of all types ($30%$ to $35%$). High copper and eddy-current losses in the shading rings generate substantial internal heat.
  • Running Characteristics: Fixed speed, high slip ($8%$ to $12%$).
• Field Applications: Sub-fractional and fractional horsepower applications ($1/20\text{ HP}$ to $1/6\text{ HP}$), including furnace draft inducers, small bathroom exhaust fans, domestic refrigerator evaporator fans, and small condensate removal pumps.
• Field Diagnostic Notes: Shaded-pole motors have high internal inductive impedance. If mechanically seized or locked by debris, they draw relatively low locked-rotor current and can remain stalled for hours without catching fire or burning insulation. They are non-reversible electrically; to change rotation, the entire stator core must be physically disassembled and flipped end-for-end relative to the rotor shaft.

2. Split-Phase Motors (Induction-Start Induction-Run - ISIR)

• Operating Principle: Employs two distinct stator windings: a run winding wound with thick, heavy-gauge magnet wire placed deep in the stator slots (giving it high inductive reactance and low resistance) and a start winding wound with much thinner wire placed near the stator surface (giving it high resistance and low inductive reactance).
• Electromagnetic Action: Because the run winding is highly inductive, its current lags applied line voltage by approximately $70^\circ$ to $80^\circ$. The start winding, being more resistive, has current that lags line voltage by only $30^\circ$ to $40^\circ$. This difference creates a phase displacement of approximately $30^\circ$ to $45^\circ$ between the two currents, producing sufficient rotating magnetic flux to accelerate the rotor.
• Disconnect Mechanism: Once the rotor reaches approximately $75%$ of synchronous speed, an internal mechanical centrifugal switch mounted on the rotor shaft snaps open due to centrifugal force. This disconnects the thin start winding from power. If the start winding remained energized, its high resistance would quickly cause it to burn out. The motor continues running solely on the heavy run winding as an induction-run machine.
• Performance Characteristics:
  • Starting Torque: Moderate ($100%$ to $150%$ of full-load torque).
  • Operating Efficiency: Moderate ($50%$ to $65%$).
  • Starting Current: High inrush current ($6\times$ to $8\times$ full-load amps).
• Field Applications: Belt-driven residential furnace blowers (older installations), direct-drive oil burner fuel pumps, and workshop machinery.
• Field Diagnostic Notes: Centrifugal switch contacts frequently pit, oxidize, or foul with lint. If the switch fails to close when the motor stops, the motor will hum and trip on overload during the next start attempt. If the switch sticks closed, the start winding will smoke and burn open within 10 to 30 seconds of operation.

3. Permanent Split Capacitor (PSC) Motors

• Operating Principle: The PSC motor contains both a run winding and a start winding, but incorporates a run capacitor wired permanently in series with the auxiliary start winding. There is no centrifugal switch, no potential relay, and no starting switch.
• Electromagnetic Action: A capacitor causes current to lead voltage ($I$ leads $E$). The series run capacitor overcomes the start winding's natural inductance, causing the current in the start winding to lead the current in the run winding by up to $80^\circ$. Because the capacitor remains in the circuit continuously during operation, the motor runs as a balanced two-phase motor.
• Performance Characteristics:
  • Starting Torque: Low to moderate ($50%$ to $100%$ of full-load torque). Because the run capacitor has a relatively low capacitance value ($3$ to $15\ \mu\text{F}$ for fan motors; $25$ to $60\ \mu\text{F}$ for compressors), starting torque is limited.
  • Operating Efficiency: Good ($60%$ to $70%$). The continuous capacitor improves the motor's running power factor close to unity ($0.95\text{--}0.98$), lowering running amperage and operating temperature.
• Field Applications: The standard workhorse for residential direct-drive furnace and air-handler blowers, draft inducers, outdoor condenser fan motors, and small hermetic compressors with equalizing expansion valves (capillary tube or fixed orifice).
• Field Diagnostic Notes: A weakened or open run capacitor causes a PSC motor to hum, fail to start under load, or run hot and trip its internal thermal overload. If a run capacitor drops more than $10%$ below its rated microfarad ($\mu\text{F}$) value, motor operating temperature climbs significantly, accelerating winding insulation failure.

4. Capacitor-Start Induction-Run (CSIR) Motors

• Operating Principle: Utilizes a high-capacitance, non-continuous start capacitor ($100$ to $400+\ \mu\text{F}$) wired in series with the start winding. The start capacitor is disconnected from the circuit at approximately $75%$ of synchronous speed by a mechanical centrifugal switch (open motors) or an electromagnetic potential/current relay (hermetic compressors).
• Electromagnetic Action: The high capacitance creates an ideal $90^\circ$ phase angle shift between start and run winding currents, generating immense starting torque. Once up to speed, the start winding and start capacitor are de-energized, and the motor runs strictly on its main winding.
• Performance Characteristics:
  • Starting Torque: High ($300%$ to $400%$ of full-load torque).
  • Operating Efficiency: Moderate ($60%$ to $70%$).
• Field Applications: Commercial refrigeration condensing units, walk-in coolers, heavy-duty belt-driven exhaust fans, and positive displacement pumps.

5. Capacitor-Start Capacitor-Run (CSCR) Motors

• Operating Principle: Combines the immense starting torque of a start capacitor with the high running efficiency and quiet operation of a continuous run capacitor. It features both capacitors wired in parallel with each other, and that entire parallel capacitor bank is wired in series with the auxiliary start winding.
• Electromagnetic Action:
  1. During Starting: Both the start capacitor (e.g., $189\text{--}227\ \mu\text{F}$) and run capacitor (e.g., $45\ \mu\text{F}$) are in the circuit in parallel, providing combined capacitance ($C_{\text{total}} = C_1 + C_2 = 250+\ \mu\text{F}$) to produce maximum phase displacement and extreme starting torque.
  2. At $75%$ Speed: A potential starting relay senses the rising back-EMF generated across the start winding. Its normally closed (NC) contacts open, disconnecting the start capacitor and its bleed resistor from the circuit.
  3. During Running: The run capacitor remains continuously in series with the start winding, optimizing running power factor, lowering operating current, and maximizing motor efficiency.
• Performance Characteristics:
  • Starting Torque: Highest of all single-phase motors ($350%$ to $450%$ of full-load torque).
  • Operating Efficiency: Highest among single-phase designs ($75%$ to $85%$).
• Field Applications: Commercial refrigeration compressors, residential central air conditioners, and heat pumps utilizing non-bleed thermostatic expansion valves (TXVs) that maintain high unequalized differential pressures across the compressor upon shutdown.

Single-Phase Motor Typology Comparison Table:
+----------------------+--------------------+----------------+----------------------+-----------------------+
| Motor Type           | Starting Torque    | Running Eff.   | Starting Components  | Running Components    |
+----------------------+--------------------+----------------+----------------------+-----------------------+
| Shaded-Pole          | 50% - 75%          | 30% - 35%      | Copper shading ring  | Main stator coil only |
| Split-Phase (ISIR)   | 100% - 150%        | 50% - 65%      | Start wdg + Cent sw  | Main run winding only |
| PSC                  | 50% - 100%         | 60% - 70%      | Start wdg + Run cap  | Start wdg + Run cap   |
| CSIR                 | 300% - 400%        | 60% - 70%      | Start cap + Relay/sw | Main run winding only |
| CSCR                 | 350% - 450%        | 75% - 85%      | Start/Run cap+Relay  | Start wdg + Run cap   |
+----------------------+--------------------+----------------+----------------------+-----------------------+

---

Tapped Multi-Speed PSC Blower Motors

Direct-drive residential furnace blowers and air handlers frequently use tapped multi-speed PSC motors to provide different airflow rates (CFM) for different operating modes (e.g., High speed for cooling, Medium-Low for gas heating, and Low for continuous fan circulation).

Tapped Multi-Speed PSC Motor Internal Schematic:
=========================================================================
        +---[ L1 Power Input - Switched by Fan Relay / Control Board ]
        |     | (High Speed Tap)   -> Directly to Run Winding End
        |     | (Med-High Tap)     -> Series Inductive Speed Coil 1
        |     | (Med-Low Tap)      -> Series Inductive Speed Coil 2
        |     | (Low Speed Tap)    -> Series Inductive Speed Coil 3
        |     v
        |   +-------------------------------------------------------+
        |   |   [Speed Coil 3]--[Speed Coil 2]--[Speed Coil 1]      |
        |   |         |               |               |             |
        |   |         +---------------+---------------+             |
        |   |                         v                             |
        |   |               [ MAIN RUN WINDING ]                    |
        |   +-------------------------+-----------------------------+
        |                             |                             
        |                             v                             
        |                     (C) COMMON TERMINAL                   
        |                             |                             
        +-----------------------------)---------[ L2 / Neutral ]    
                                      |                             
                 +---[ RUN CAPACITOR ]+                             
                 |                    |                             
                 v                    |                             
        [ START WINDING ]             |                             
                 |                    |                             
                 +--------------------+                             
=========================================================================

How Speed Selection Actually Works

Many technicians mistakenly believe that a multi-speed PSC motor changes speed by switching between different numbers of stator poles. This is incorrect. A 4-pole multi-speed motor remains a 4-pole motor on all speeds, with a constant synchronous speed of $1,800\text{ RPM}$.

Instead, speed control is achieved by modulating inductive reactance and rotor slip under aerodynamic load:

1. High Speed (Black Lead): Line voltage ($120\text{ VAC}$ or $240\text{ VAC}$) is applied directly across the main run winding. The winding receives full line voltage, draws maximum current, develops maximum electromagnetic flux, and produces maximum motor torque. Operating against the resistance of the blower wheel, rotor slip is minimized, resulting in maximum shaft speed (typically $1,050\text{--}1,075\text{ RPM}$ on a 6-pole motor, or $1,650\text{--}1,700\text{ RPM}$ on a 4-pole motor).
2. Lower Speeds (Blue, Yellow, Red Leads): Selecting a lower speed tap routes line voltage through additional internal speed coils wired in series with the main run winding. Adding these series inductive coils increases the total inductive reactance ($X_L = 2\pi f L$) and impedance of the stator circuit.
3. Slip Increase: The increased impedance reduces current flow through the main run winding, weakening the stator's magnetic field and diminishing the motor's developed torque. Because the blower wheel's mechanical air resistance remains high, the weakened motor cannot maintain high speed. The motor is forced to operate at a higher slip percentage, causing shaft RPM to drop.

[!IMPORTANT] The Aerodynamic Load Rule: Because multi-speed PSC speed control relies entirely on torque reduction and increased rotor slip, a multi-speed PSC motor will only change speeds when mechanically loaded by a blower wheel moving air. If a technician tests a tapped PSC motor on a workbench with no fan wheel attached, the motor will operate at virtually synchronous speed (~1,780 RPM) on all speed taps because with zero mechanical load, near-zero torque is required to spin the bare shaft.

Industry-Standard PSC Motor Lead Color Coding

While technicians must always verify the motor wiring diagram printed on the motor housing, North American manufacturers adhere to a nearly universal color code for direct-drive blower motors:

• Common: White ($120\text{ V}$ neutral) or Yellow ($240\text{ V}$ line)
• High Speed: Black (typically connected to cooling relay terminal COOL or Y)
• Medium-High Speed: Blue (used for high-static heating or electric strip heat)
• Medium-Low Speed: Yellow or Orange (standard residential gas furnace heating HEAT)
• Low Speed: Red (used for continuous air circulation FAN ON or low-fire heating)
• Capacitor Leads: Two Brown wires (one may have a white stripe; connect directly across the run capacitor)

---

Hermetic Motor Terminal Identification & Resistance Testing

Hermetic and semi-hermetic refrigeration compressors house both the electric motor stator and the mechanical compression cylinder/scroll inside a welded steel pressure vessel or cast-iron body. The internal motor windings terminate at three electrical pin terminals that penetrate the shell through fused ceramic-glass hermetic seals. These pins are designated:

• C: Common (the shared electrical junction between the run and start windings)
• R: Run (the opposite terminal of the main run winding)
• S: Start (the opposite terminal of the auxiliary start winding)

Hermetic Terminal Internal Circuit & Resistance Triangle:
=========================================================================
                      [ C ] COMMON
                     /            \
                    /              \
                   /                \
          (R_CR)  /                  \  (R_CS)
         Low     /                    \ High
     Resistance /                      \ Resistance
               /                        \
              /                          \
             /                            \
         [ R ] ========================== [ S ]
          RUN           (R_RS)             START
                   Highest Resistance
              (Sum: R_RS = R_CR + R_CS)
=========================================================================

The Governing Resistance Relationships

The physics of the two windings dictates exact resistance relationships:

1. Run Winding ($R_{\text{CR}}$): Wound with heavy-gauge magnet wire with fewer turns to provide low resistance and high inductive current capacity under continuous operation. Therefore, the resistance between Common and Run ($R_{\text{CR}}$) is the lowest resistance reading.
2. Start Winding ($R_{\text{CS}}$): Wound with thinner-gauge wire with significantly more turns to generate high starting flux and phase displacement. Therefore, the resistance between Common and Start ($R_{\text{CS}}$) is an intermediate (higher) resistance reading.
3. Run-to-Start ($R_{\text{RS}}$): When measuring between Run and Start, the ohmmeter measures current flowing through the Run winding to Common, and then through the Start winding. The two windings are in series across these two pins. Therefore, the resistance between Run and Start ($R_{\text{RS}}$) is the highest resistance reading, and must equal the mathematical sum of the individual windings:

$R_{\text{RS}} = R_{\text{CR}} + R_{\text{CS}}$ $R_{\text{CR}} < R_{\text{CS}} < R_{\text{RS}}$

Step-by-Step Terminal Identification Procedure

When terminal labels on a compressor have corroded, melted, or washed away, a technician uses a digital multimeter set to the lowest resistance ($\Omega$) scale to identify every terminal with $100%$ accuracy:

1. Isolate Power: Lock out and tag out the equipment disconnect switch. Discharge all capacitors with a $20\text{ k}\Omega$, $5\text{ W}$ resistor. Remove all three field push-on terminal wires from the compressor pins.
2. Measure and Record All Three Combinations:
   • Terminal Pair 1-2
   • Terminal Pair 2-3
   • Terminal Pair 1-3
3. Identify Common (C): Find the pair of pins that produces the highest resistance reading. The remaining third terminal—the one not involved in this highest reading—is unconditionally Common (C).
4. Identify Run (R) and Start (S): Place one meter lead on the newly identified Common terminal:
   • Measure from Common to one of the remaining pins: the pin that yields the lower resistance is Run (R).
   • Measure from Common to the other remaining pin: the pin that yields the higher resistance is Start (S).
5. Verify the Series Sum: Verify that $R_{\text{CR}} + R_{\text{CS}} = R_{\text{RS}}$ within the calibration tolerance of the meter (typically $\pm 0.1\text{--}0.2\ \Omega$).

Worked Field Problem: Compressor Terminal Identification
A technician inspects an unmarked 3-ton R-410A hermetic compressor.
With leads disconnected, the technician records the following resistance values:
- Between Terminal A and Terminal B: 4.8 Ohms
- Between Terminal B and Terminal C: 1.4 Ohms
- Between Terminal A and Terminal C: 3.4 Ohms

Step 1: Identify the highest resistance reading:
The highest reading is between Terminal A and Terminal B (4.8 Ohms).
Therefore, the opposite terminal, Terminal C, is COMMON (C).

Step 2: Identify Run and Start from Common (C):
- From Common (C) to Terminal B = 1.4 Ohms (Lowest reading from Common).
  Therefore, Terminal B is RUN (R).
- From Common (C) to Terminal A = 3.4 Ohms (Higher reading from Common).
  Therefore, Terminal A is START (S).

Step 3: Verify the mathematical sum:
R_CR + R_CS = 1.4 Ohms + 3.4 Ohms = 4.8 Ohms.
This exactly matches the measured R_RS (4.8 Ohms).
Conclusion: Pin A = Start, Pin B = Run, Pin C = Common.

Diagnosing Winding Faults with an Ohmmeter

• Open Winding: A reading of infinite resistance (displayed as O.L. on digital meters) between Common and Run or Common and Start indicates a severed or burned-open winding (or an open internal thermal overload if measuring from Common).
• Shorted Winding Turns: If $R_{\text{CR}} + R_{\text{CS}}$ does not equal $R_{\text{RS}}$ (for example, if $R_{\text{CR}} = 1.4\ \Omega$, $R_{\text{CS}} = 3.4\ \Omega$, but $R_{\text{RS}}$ reads only $2.2\ \Omega$), turns of magnet wire have melted together and short-circuited. The motor will draw excessive current and trip on startup.
• Grounded Winding: Set the meter to the highest resistance scale (or use a $500\text{ V}$ or $1,000\text{ V}$ megohmmeter). Measure between each terminal pin and clean, bare copper suction tubing or unpainted compressor metal. Any reading less than infinite resistance (or $<100\text{ M}\Omega$ on a megohmmeter) indicates compromised insulation and a grounded winding, requiring immediate compressor replacement.

---

Motor Nameplate Data & Electrical Ratings

The National Electrical Code (NEC) and NEMA (National Electrical Manufacturers Association) mandate that every motor display a durable stamped metal nameplate. Technicians must be able to interpret every parameter:

Standard NEMA Motor Nameplate Representation:
=========================================================================
 MODEL: HF-56-1725       VOLTS: 208-230 VAC      PHASE: 1
 HP: 1/2                 AMPS: 4.2 / 3.8         HZ: 60
 RPM: 1725               FRAME: 48Y              INS. CLASS: B
 CAP: 7.5 MFD / 370V     SF: 1.15                DUTY: CONT.
 LRA: 21.0               CODE: J                 THERMALLY PROTECTED
=========================================================================

Key Electrical Ratings Defined

1. Full Load Amps (FLA): The continuous operating current drawn by the motor when delivering its rated mechanical horsepower at rated voltage and frequency. FLA is used to size branch circuit conductors, overload heaters, and disconnects for open motors.
2. Locked Rotor Amps (LRA): The massive inrush current drawn by the motor at the exact instant of startup when the rotor is stationary ($0\text{ RPM}$) and slip is $100%$. LRA is typically $5\times$ to $6\times$ the FLA rating. LRA is used to verify power supply adequacy and size instantaneous circuit breakers.
3. Rated Load Amps (RLA): Applied specifically to hermetic refrigeration compressors under NEC Article 440. Because a hermetic motor is cooled by cold suction refrigerant vapor rather than ambient air, it can handle higher electrical loading than an open motor of equivalent physical size. RLA is a calculated value ($RLA = \text{Maximum Continuous Current} / 1.56$, or established during UL calorimeter testing). Sizing of contactors and wire feeds for compressors is based on RLA.
4. Service Factor (SF): A multiplier indicating the continuous overload capacity a motor can sustain without exceeding safe thermal limits, provided nominal voltage and ventilation are maintained. A motor with a $1.0\text{ HP}$ rating and a $1.15\text{ SF}$ can deliver $1.15\text{ HP}$ ($1.0 \times 1.15$) continuously without burning its insulation.
5. NEMA Frame Sizes: Dictates standard physical dimensions, including shaft diameter, shaft height, mounting bolt hole patterns, and motor shell diameter:
   • Frame 48: $5.6\text{-inch}$ outer diameter body with a $1/2\text{-inch}$ shaft (standard residential blower and condenser fan motor).
   • Frame 56: Heavy-duty industrial frame with a $5/8\text{-inch}$ shaft and rigid mounting base.
6. Insulation Class: Designates the maximum internal operating temperature the motor's magnet wire enamel, slot liners, and varnishes can withstand for a standard 20,000-hour design life:
   • Class A: $105^\circ\text{C}$ ($221^\circ\text{F}$)
   • Class B: $130^\circ\text{C}$ ($266^\circ\text{F}$) — standard for HVAC blower motors
   • Class F: $155^\circ\text{C}$ ($311^\circ\text{F}$) — standard for commercial motors and hermetic compressors
   • Class H: $180^\circ\text{C}$ ($356^\circ\text{F}$) — high-temperature severe-duty motors