3.2 Motor Starting Components & Capacitors

3.2 Motor Starting Components & Capacitors

1. Capacitors: Physical Construction, Dielectric Science & Operational Physics

Capacitors are electrical energy storage devices consisting of two conductive plates separated by an insulating material known as a dielectric. In single-phase HVACR circuits, capacitors create artificial phase displacement: when an alternating voltage is applied, current flows into the capacitor plates, leading the applied voltage by up to 90 electrical degrees ($I$ leads $E$). This phase advance creates the secondary magnetic flux required to start or efficiently operate single-phase induction motors.

Capacitance is measured in microfarads ($\mu\text{F}$ or MFD), representing one-millionth ($10^{-6}$) of a farad.

Start Capacitors (Electrolytic Construction)

• Dielectric & Internal Assembly: Start capacitors provide high capacitance in a compact footprint (ranging from 50 $\mu\text{F}$ to over 600 $\mu\text{F}$). They are constructed of two etched aluminum foil ribbons wound tightly together, separated by porous kraft paper saturated with a conductive glycol- or borax-based liquid/paste electrolyte. The chemical anodization of the aluminum creates an ultra-thin aluminum oxide dielectric film.
• Enclosure & Safety Vent: Because the liquid electrolyte boils when energized, start capacitors are housed in non-conductive black phenolic or molded plastic shells. The top disc incorporates a soft rubber safety vent plug. If internal pressure builds up due to prolonged energization, the rubber vent ruptures to release expanding gas safely, preventing explosive casing fragmentation.
• Duty Cycle Limitation: Start capacitors are intermittent-duty devices. Their permitted energization time and starts per hour come from the component and motor-start design; failure of the opening device can overheat and rupture the capacitor.
• Discharge or relay resistor: Some start-capacitor and potential-relay circuits use a resistor across the capacitor. Its presence, resistance, wattage, and connection come from the start kit or equipment design; it is not a universal field-added component.
  • Function: When the starting relay opens, the start capacitor can retain a DC charge of 300 to 450 volts. The bleed resistor discharges this trapped potential within seconds.
  • Why Critical: Without a bleed resistor, when the compressor cycles on again, the contacts close while the capacitor holds voltage that may be 180 degrees out of phase with line power. The resulting massive voltage differential causes extreme instantaneous current spikes, arcing, contact erosion, and contact welding on the starting relay.

Run Capacitors (Metallized Polypropylene & Oil Construction)

• Dielectric & Internal Assembly: Run capacitors are designed for 100% continuous duty and range from 1.5 $\mu\text{F}$ to 80 $\mu\text{F}$. They consist of ultra-thin polypropylene plastic film with a microscopic vacuum-deposited aluminum or zinc-aluminum alloy metallization on one side. The wound rolls are placed inside a stamped aluminum canister filled with refined, non-PCB dielectric oil (such as epoxidized soybean oil or synthetic hydrocarbon oil). The oil dissipates heat away from the windings to the aluminum shell and quenches internal electrical micro-arcs.
• Internal Pressure-Sensitive Interrupter: Run capacitors incorporate a mechanical safety device. The top terminal cover is curved/dished downward under tension, connected to the internal windings via thin, break-away copper wires. Under internal electrical breakdown, sustained arcing vaporizes dielectric oil, producing hydrogen and hydrocarbon gases. As internal pressure rises, the top cover bows convexly upward. This mechanical expansion snaps the internal break-away connections, permanently isolating the capacitor from line voltage before the metal canister can rupture.
  • Field Diagnosis: A run capacitor with a bulged, crowned, or dome-shaped top terminal cover has tripped its internal interrupter; it must be replaced immediately.
• Dual Run Capacitors: Residential central air conditioners and heat pumps commonly combine two independent capacitors in a single aluminum housing:
  • C (Common): Connected to incoming line power (contactors T1/T2).
  • HERM (Hermetic Compressor): High-capacitance section (e.g., 35 to 70 $\mu\text{F}$) wired to compressor auxiliary/start winding.
  • FAN: Low-capacitance section (e.g., 3 to 10 $\mu\text{F}$) wired to outdoor condenser fan motor auxiliary/start winding.
  • Testing Rule: Always measure capacitance between C and HERM, and between C and FAN. Never measure between HERM and FAN.

Voltage Rating Standards & Vector Addition Physics

Run capacitors are standardly rated at 370 VAC or 440 VAC.

• Voltage Selection Rule: The working voltage rating on a replacement capacitor must always equal or exceed the original rating. A 440 VAC capacitor can safely replace a 370 VAC unit. A 370 VAC capacitor must never be used to replace a 440 VAC unit.
• Vector Sum Voltage Across Run Capacitors: Technicians often wonder why a 440 VAC capacitor is specified on a 240 VAC system. When an induction motor operates, the rotor spinning within the stator winding acts as an electrical generator, producing counter-electromotive force (back-EMF). This generated voltage across the start winding combines vectorially with the incoming line voltage. Under normal operating load, the voltage measured directly across the terminals of a run capacitor often reaches 330 VAC to 390 VAC on a 240 VAC single-phase circuit! A 370 VAC capacitor in this environment operates near dielectric breakdown; installing a 440 VAC unit provides critical safety margin against voltage surges.

Capacitance Calculations & In-Service Testing Formulas

When combining capacitors in the field to achieve a required rating:

1. Parallel Banking: Capacitances add directly; voltage rating is limited to the lowest rated capacitor in the bank: $C_{\text{total}} = C_1 + C_2 + C_3$ (Example: Connecting a 20 $\mu\text{F}$ / 440V capacitor in parallel with a 15 $\mu\text{F}$ / 440V capacitor yields a 35 $\mu\text{F}$ / 440V bank).
2. Series Banking: Capacitance decreases; voltage ratings add: $\frac{1}{C_{\text{total}}} = \frac{1}{C_1} + \frac{1}{C_2} \quad \text{or} \quad C_{\text{total}} = \frac{C_1 \times C_2}{C_1 + C_2}$ (Example: Two identical 100 $\mu\text{F}$ / 370V capacitors in series yield a 50 $\mu\text{F}$ / 740V assembly).

In-Service Capacitance Verification Formula:

To test a run capacitor while the system is running under true operating load: $C (\mu\text{F}) = \frac{\text{Amps (through auxiliary lead)} \times 2650}{\text{Volts (across capacitor terminals)}}$ Where $2650 = \frac{10^6}{2 \times \pi \times 60\text{ Hz}}$. If the calculated value deviates by more than ±5% to ±6% from the stamped nameplate rating, replace the capacitor.

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2. Motor Starting Relays: Potential, Current & Solid-State PTC

Hermetic compressors enclose both the motor and pump mechanism inside a welded steel shell, precluding the use of mechanical centrifugal switches. System designers employ external starting relays to disengage the start winding and start capacitor.

Potential Relays (Counter-EMF Relays)

Potential starting relays are the industry standard for single-phase commercial refrigeration and residential CSCR compressors ranging from 1 to 5+ horsepower.

• Physical Operating Principle: As the compressor rotor accelerates, the auxiliary start winding generates a counter-electromotive force (back-EMF) that increases linearly with rotor speed. The potential relay senses this back-EMF voltage to trigger contact opening.
• Relay Construction & Terminal Identification:
  • The relay incorporates a high-impedance, fine-wire electromagnetic coil (typically 5,000 $\Omega$ to 10,000 $\Omega$ resistance) connected between Terminal 2 and Terminal 5.
  • A set of heavy-duty Normally Closed (NC) contacts is connected between Terminal 1 and Terminal 2.
  • Terminals 4 and 6 are insulated dummy/tie terminals used as mechanical wiring junction points.
• Circuit Operation:
  1. Standstill: Contacts between 1 and 2 are closed. Line power passes through Terminal 1, through the contacts to Terminal 2, feeding high current through the start capacitor into the start winding.
  2. Acceleration: As the motor hits 75% to 80% of synchronous speed, back-EMF induced across the start winding reaches the relay's specified pickup voltage (typically 300 to 420 VAC). The coil energizes, pulling the armature down and snapping contacts 1-2 open. This drops the start capacitor out of the circuit. The run capacitor remains in parallel across the open contacts, keeping the auxiliary winding energized for running efficiency.
  3. Running: While the compressor runs, continuous back-EMF across Terminals 2 and 5 keeps the coil energized and contacts 1-2 held open.
  4. Shutdown or Stall: When power is cut or if the motor stalls under high head pressure, back-EMF collapses below the relay's drop-out voltage (typically 50 to 100 VAC). Spring tension instantly re-closes contacts 1-2, preparing the system for the next start cycle.

Current Relays (Amperage-Sensitive Relays)

Current starting relays are employed on fractional-horsepower refrigeration compressors (typically under 1/2 HP in domestic refrigerators, water coolers, and small ice makers) utilizing Split-Phase (RSIR) or CSIR motors.

• Physical Operating Principle: Operates on the dramatic difference between locked-rotor inrush amperage (LRA) and running amperage (FLA) in the compressor's main run winding.
• Relay Construction: Features a low-resistance coil wound with heavy-gauge wire, connected in series with the compressor's main run winding. A set of Normally Open (NO) contacts is connected in series with the start winding.
• Circuit Operation:
  1. Start: At rest, the contacts are held open by gravity. When the thermostat calls, initial locked-rotor inrush current (5 to 6 times normal FLA) surges through the heavy coil.
  2. Pick-up: The intense magnetic field lifts an internal weighted iron plunger, slamming the NO contacts closed. This energizes the start winding (and start capacitor, if equipped), providing starting torque.
  3. Drop-out: As the rotor reaches 75% to 80% of operating speed, run-winding current drops to normal running levels. The weakened magnetic flux can no longer support the weighted plunger. Gravity pulls the plunger down, dropping the contacts open and taking the start winding off-line.
• Mounting: Install a current relay in the orientation marked by its manufacturer. Gravity-sensitive designs can malfunction when misoriented, but the required position comes from the particular relay rather than a universal arrow-up rule.

Solid-State PTC (Positive Temperature Coefficient) Starters

PTC starters are solid-state ceramic thermistors commonly found in domestic refrigeration and light commercial air conditioning systems as cost-effective starting devices.

• Operating Principle: A round ceramic pill (doped barium titanate) is wired in series with the start winding, or in parallel with the run capacitor. At room temperature (cold), the ceramic pellet exhibits very low electrical resistance (3 $\Omega$ to 25 $\Omega$), allowing high starting current to flow into the start winding.
• Curie Point Transition: As current flows through the disc, internal $I^2R$ power dissipation rapidly heats the ceramic material. Within 0.1 to 0.5 seconds, the disc passes its Curie transition temperature (~120°C / 248°F), causing its resistance to skyrocket exponentially to 10,000 $\Omega$ to 50,000+ $\Omega$. This chokes off start winding current to a harmless trickle (a few milliamps), effectively removing it from the operational circuit.
• Field Limitation / Diagnostic Trap: A PTC starter requires 2 to 3 minutes of cool-down time after compressor shutdown to return to low resistance. If a system experiences a momentary power interruption or rapid thermostat cycling, the PTC remains hot and highly resistive. The compressor attempts to start without start-winding current, drawing locked-rotor amps and tripping its internal thermal overload protector.

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3. Starting Device Comparison & Diagnostic Matrix