8.3 Capacitors (Start & Run), Potential Relays & Motor Starting Circuits

8.3 Capacitors (Start & Run), Potential Relays & Motor Starting Circuits

Single-phase alternating current supply produces a pulsating, stationary magnetic field across a motor stator rather than a naturally rotating field. Consequently, a single-phase AC induction motor has zero net starting torque at standstill and cannot rotate on its own without auxiliary phase-splitting assistance.

To overcome this physical limitation, single-phase HVAC compressors and fan motors utilize Run Capacitors, Start Capacitors, and Potential Starting Relays. An Arizona HVAC contractor must thoroughly understand capacitor sizing, active under-load testing, and Counter-Electromotive Force (CEMF) relay dynamics to ensure reliable compressor starting under extreme desert head pressures.

1. Capacitor Operating Principles & Phase Displacement

A capacitor consists of two conductive aluminum foil plates separated by an insulating dielectric material (polypropylene film bathed in dielectric oil for run capacitors, or an etched aluminum oxide electrolytic paste for start capacitors).

                      CAPACITOR PHASE SHIFT DYNAMICS

     RUN WINDING CURRENT (I_run)             START WINDING CURRENT (I_start)
     (High Inductance -> Lags Voltage)       (Series Capacitor -> Leads Voltage)
                     │                                       ▲
                     │                                      / 
                     │                                     /  Phase Displacement
                     │                                    /   Angle: up to 90°
                     │                                   /    (Creates Rotating
                     ▼                                  /      Magnetic Field!)
     ──────────────────────────────────────────────────► VOLTAGE (V_line)

The $90^\circ$ Phase Angle Shift

When an alternating voltage is applied across a capacitor, the capacitor continuously charges and discharges. The alternating current through the capacitor leads the applied voltage by $90^\circ$ electrical degrees ($I_C$ leads $V_C$ by $90^\circ$).

1. The motor's Main (Run) Winding has high inductance and low resistance, causing current to lag applied line voltage by approximately $45^\circ\text{ to }60^\circ$.
2. The motor's Auxiliary (Start) Winding is connected in series with a capacitor, causing start winding current to lead line voltage by $30^\circ\text{ to }45^\circ$.
3. The resulting net current phase displacement between the two windings approaches $90^\circ$. This simulated two-phase condition produces a true Rotating Magnetic Field (RMF) across the stator, inducing rotor currents that generate powerful starting torque.

2. Run Capacitors vs. Start Capacitors: Comparative Analysis

Run and start capacitors fulfill completely different engineering roles inside an HVAC electrical circuit and cannot be interchanged.

                      RUN CAPACITOR VS. START CAPACITOR
 ┌───────────────────────────┬─────────────────────────────────────────────────┐
 │ SPECIFICATION / FEATURE   │ RUN CAPACITOR          │ START CAPACITOR        │
 ├───────────────────────────┼────────────────────────┼────────────────────────┤
 │ Operating Duty Cycle      │ Continuous Duty (100%) │ Intermittent (< 3 sec) │
 │ Physical Construction     │ Oil-Filled Metal Can   │ Dry Plastic / Bakelite │
 │ Dielectric Material       │ Metallized Polyprop.   │ Aluminum Oxide Paste   │
 │ Capacitance Range         │ 1.5 μF to 80 μF        │ 50 μF to 600+ μF       │
 │ Voltage Rating (VAC)      │ 370 VAC or 440 VAC     │ 250 VAC to 330 VAC     │
 │ Internal Safety Feature   │ Pressure Interrupter   │ None (Vents/Explodes)  │
 │ Bleed Resistor Fitted?    │ No                     │ Yes (15kΩ - 20kΩ, 2W)  │
 └───────────────────────────┴────────────────────────┴────────────────────────┘

1. Run Capacitors (Continuous Duty) & Dual Round/Oval Cans

• Role: Remains permanently in series with the auxiliary start winding throughout the entire operating cycle of the motor. It optimizes running torque, lowers motor running current (FLA/RLA), and corrects the motor's operating Power Factor to $0.92\text{--}0.96$.
• Dual Capacitor Construction: Most residential outdoor condensing units combine two independent run capacitors into a single three-terminal metal canister:
  • C (Common): Connected to incoming Line 2 power from the contactor.
  • HERM (Hermetic Compressor): Connected to the compressor Start (S) terminal (typically $35\ \mu\text{F}\text{ to }70\ \mu\text{F}$).
  • FAN (Condenser Fan Motor): Connected to the condenser fan Start terminal (typically $5\ \mu\text{F}\text{ to }10\ \mu\text{F}$).
• Internal Pressure Sensitive Interrupter (PSI): If internal electrical arcing vaporizes dielectric oil, internal pressure expands the convex top dome of the metal can, physically tearing internal ribbon connections to disconnect the capacitor safely before the case ruptures.

2. Start Capacitors (Intermittent Duty) & Bleed Resistors

• Role: Provides massive microfarad capacitance ($50\ \mu\text{F}\text{ to }600+\ \mu\text{F}$) for an instant ($< 1\text{ second}$) during compressor startup, boosting starting torque up to $300%\text{ to }400%$ of full-load torque. Essential when starting against un-equalized TXV refrigerant pressures.
• Duty Limit: Designed for a maximum of 20 starts per hour, with each start lasting no longer than 3 seconds. If a defective starting relay sticks closed, a start capacitor will overheat, boil its internal paste, and vent catastrophically within 10 to 15 seconds.
• Bleed Resistor: A $15,000\ \Omega\text{ to }20,000\ \Omega$ ($15\text{k}\text{--}20\text{k}\ \Omega$), 2-Watt carbon resistor must be soldered across the start capacitor terminals. It safely discharges residual DC voltage between cycles, preventing contact arcing and welding across potential relay contacts.

3. Capacitance Testing & The Active Under-Load Formula

Capacitors degrade over time as extreme desert heat dries out dielectric fluid or breaks down internal foil layers. Capacitance must be verified during routine maintenance and troubleshooting.

1. Bench / Passive Capacitance Test

• Disconnect power, discharge capacitor terminals with a 20kΩ resistor, and isolate the terminals.
• Set a digital multimeter to Capacitance ($\mu\text{F}$).
• Manufacturer Tolerance: A capacitor must measure within $\pm 6%\text{ to }\pm 10%$ of its stamped nameplate rating (e.g., a $45\ \mu\text{F} \pm 6%$ capacitor must read between $42.3\ \mu\text{F}$ and $47.7\ \mu\text{F}$). Any reading outside this window requires immediate replacement.

2. Active Under-Load Capacitance Test Formula

Bench testing with a 9V multimeter battery does not reveal capacitor dielectric breakdown occurring under full $240\text{V}$ operating load. The most rigorous diagnostic test measures current and voltage while the motor is actively running:

$\mu\text{F} = \frac{\text{Start Winding Amperes} \times 2652}{\text{Capacitor Measured Voltage (VAC)}}$

Where:

• $\text{Start Winding Amperes}$ = Current measured with a True-RMS clamp meter on the auxiliary start winding wire.
• $\text{Capacitor Measured Voltage}$ = Voltage measured directly across the capacitor terminals (C to HERM or C to FAN) while operating.
• $2652$ = Constant derived from standard $60\text{ Hz}$ AC line frequency: $X_C = \frac{V_C}{I_{\text{start}}} = \frac{1}{2\pi f C} \implies C = \frac{I_{\text{start}}}{2\pi (60) V_C} = \frac{I_{\text{start}}}{376.99 \times V_C} \times 10^6 = \frac{I_{\text{start}} \times 2652}{V_C}$

Worked Example: Under-Load Capacitance Calculation

Problem: A technician diagnoses a running 4-ton compressor with a stamped $50\ \mu\text{F}$ run capacitor. While the compressor runs under full cooling load:

• Measured Start Winding Current ($I_{\text{start}}$) = $6.2\text{ Amperes}$
• Measured Voltage Across Capacitor Terminals ($V_C$) = $360\text{ VAC}$

Calculate actual running microfarads and determine if the capacitor meets $\pm 6%$ tolerance.

$\text{Actual } \mu\text{F} = \frac{6.2\text{ A} \times 2652}{360\text{ V}} = \frac{16,442.4}{360} = \mathbf{45.67\ \mu\text{F}}$

• Tolerance Check:
  $\text{Deviation} = \left(\frac{45.67 - 50.0}{50.0}\right) \times 100 = \frac{-4.33}{50.0} \times 100 = -8.66\%$
• Diagnostic Conclusion: The capacitor has drifted $-8.66%$, exceeding the allowable $\pm 6%$ manufacturer tolerance. The capacitor is degraded and must be replaced to prevent compressor motor overheating.

4. Potential Starting Relays: Counter-EMF Operating Mechanics

The Potential Starting Relay (PSR) is the industry-standard starting device for high-efficiency single-phase air conditioning and heat pump compressors equipped with thermal expansion valves (TXVs).

                      POTENTIAL RELAY OPERATING CYCLE

   1. STANDSTILL (0% RPM)       2. ACCELERATION (0-75% RPM)   3. RUNNING (75-100% RPM)
   ┌──────────────────────┐     ┌──────────────────────┐      ┌──────────────────────┐
   │ CEMF = 0 Volts       │     │ CEMF Rising Across S │      │ CEMF Reaches Pickup  │
   │ Contacts 1-2 CLOSED  │ ──► │ Start Cap in Parallel│ ──►  │ (300V - 450V)        │
   │ Full Inrush Torque   │     │ High Acceleration    │      │ Coil Energizes       │
   │ Applied (300-400%)   │     │ Torque Delivered     │      │ Contacts 1-2 SNAP OPEN│
   └──────────────────────┘     └──────────────────────┘      └──────────────────────┘

Counter-Electromotive Force (CEMF / Back-EMF)

As the compressor rotor spins inside the stator, the auxiliary start winding behaves like an electrical generator. The spinning magnetic field cuts across the fine start winding turns, inducing a Counter-Electromotive Force (CEMF / Back-EMF) voltage across the start winding.

• CEMF is directly proportional to rotor rotational speed ($RPM$).
• Because the start winding has significantly more wire turns than the run winding, generated CEMF voltage exceeds line voltage, frequently reaching $300\text{ VAC to }450+\text{ VAC}$ on a nominal $240\text{V}$ system at full operational speed!

Potential Relay Terminal Numbering & Internal Connections

Industry-standard potential relays utilize a universal three-terminal layout (1, 2, and 5):

• Terminal 1: One side of the heavy-duty Normally Closed (NC) contacts; wired in series with the start capacitor.
• Terminal 2: The junction of the NC contacts and the high-resistance continuous voltage-sensing coil; wired directly to the compressor Start (S) terminal.
• Terminal 5: The opposite end of the high-resistance sensing coil; wired to incoming Line 2 power (Compressor R / Run terminal).

Critical Operating Voltage Ratings

1. Pick-Up Voltage ($V_{\text{pickup}}$): The precise CEMF voltage required across the sensing coil (Terminals 2–5) to generate sufficient electromagnetic force to overcome spring tension and snap contacts 1–2 OPEN, taking the start capacitor out of the circuit. Occurs when the motor reaches $75%\text{ to }80%$ of synchronous speed (within $\approx 0.5\text{ seconds}$).
2. Hold-In Voltage: The minimum CEMF voltage required to keep the coil energized and contacts 1–2 held open while the motor runs. Always lower than pick-up voltage.
3. Drop-Out Voltage: When the compressor shuts off and the rotor decelerates, CEMF collapses. When CEMF drops below the drop-out rating, internal spring tension drops the armature, re-closing contacts 1–2 so the start capacitor is ready for the next starting cycle.

[!CAUTION] Diagnostic Trap: Potential Relay vs. Current Relay: Potential relays have Normally Closed (NC) contacts and a high-resistance (high-turn) continuous voltage coil that senses CEMF across terminals 2 and 5. Current starting relays (used on small fractional HP domestic refrigeration) have Normally Open (NO) contacts and a low-resistance (few turns of heavy wire) current coil wired in series with the main run winding that senses high starting inrush amperage. Never confuse these devices—installing a potential relay incorrectly will burn out start capacitors or prevent motor starting.