Capacitors store energy in an electric field and create the phase shifts that
single-phase motors need to start and run. This lesson covers capacitor principles,
voltage ratings, selection, testing, and safe handling for HVAC/R service.
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3.1.1 — Capacitor Fundamentals
Capacitors are electrical devices that store energy in an electric field between
two conductors separated by an insulating material (dielectric). In motor
applications, capacitors serve critical functions in starting and running
single-phase motors, improving power factor, and enhancing motor performance.
Understanding capacitor types, ratings, and applications is essential for motor
installation, service, and troubleshooting.
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Basic Capacitor Principles
A capacitor consists of two conductive plates separated by a dielectric material
such as paper, plastic film, ceramic, or electrolyte. When AC voltage is applied,
the capacitor repeatedly charges and discharges, with current leading
voltage by 90 electrical degrees. This phase-shift characteristic makes
capacitors valuable for creating rotating magnetic fields in single-phase motors.
Capacitance, measured in microfarads (µF), indicates the
capacitor’s ability to store electrical charge. Larger capacitance values
store more charge and allow higher currents to flow. Motor capacitors typically
range from 2 µF to 1 200 µF
depending on motor size and type.
Voltage Ratings
Voltage rating indicates the maximum continuous voltage the capacitor can withstand.
Exceeding this rating causes dielectric breakdown and capacitor failure. Motor
capacitors must be rated for at least the motor operating voltage, with margins for
voltage transients and supply variations.
Common Voltage Rating
Typical Application
125 VAC
115 V start capacitors
165 VAC
115 V start capacitors — higher margin
250 VAC
208–230 V start capacitors
330 VAC
208–230 V start capacitors — higher margin
370 VAC
208–230 V run capacitors — standard
440 VAC
208–230 V run capacitors — preferred
660 VAC
460 V systems with VFD voltage spikes
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Higher voltage rating is always acceptable
A capacitor rated higher than the minimum required provides additional safety
margin and may extend service life. It will be physically larger and more
expensive. Never use a capacitor with a voltage rating below the
application voltage.
3.1.2 — Capacitor Selection and Sizing
Capacitors must be matched to the motor’s requirements. Always refer to the
motor nameplate or manufacturer specifications for correct capacitor values.
Using incorrect capacitance affects motor performance in predictable ways.
Start Capacitor — Sizing Effects
⬇️ Start Capacitor Too Small
Reduced starting torque
Extended starting time
May fail to start or reach full speed
Excessive starting current in start winding
⬆️ Start Capacitor Too Large
Excessive current in the start winding
Potential start winding or relay damage
May not improve starting torque significantly
Run Capacitor — Sizing Effects
⬇️ Run Capacitor Too Small
Reduced running efficiency and power factor
Increased operating current
Reduced torque output
Motor overheating
⬆️ Run Capacitor Too Large
Excessive current in auxiliary winding
Potential auxiliary winding damage
May reverse phase relationship
Reduced motor life
📐
Replacement tolerance
Replacement capacitors should match the original capacitance rating within
±6% for run capacitors and ±10%
for start capacitors. Exact replacement is always preferred when available.
Voltage Rating Selection
Voltage rating must meet or exceed the motor operating voltage with adequate safety
margin. Common practices for Canadian HVAC/R systems:
For 115 V motors: minimum 125 VAC rating (165 VAC or 250 VAC preferred)
For 208–230 V motors: minimum 250 VAC rating (330 VAC or 370 VAC preferred)
For 460 V three-phase systems with VFDs: 660 VAC rating may be required due to voltage spikes
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Never substitute below minimum voltage rating
A capacitor with a voltage rating below the motor operating voltage will fail
from dielectric breakdown. Higher voltage ratings are always acceptable —
lower ratings are never acceptable, regardless of µF match.
3.1.3 — Capacitor Testing and Troubleshooting
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Discharge capacitors before testing or handling
Even with power disconnected, capacitors can retain dangerous charge for
extended periods. Use a 20 000-ohm 5-watt resistor
across the terminals for 30–60 seconds to safely discharge. Verify
discharge with a voltmeter before touching terminals. Treat all capacitors
as potentially charged.
Visual Inspection
Many capacitor failures can be identified visually before any electrical testing is performed:
👁️
Visual Failure Indicators — Replace Immediately if Present
Swollen or bulging case — internal pressure from overheating or end-of-life
Oil leakage — case rupture or seal failure (run capacitors)
Burned or discolored terminals — overheating at connection points
Cracked or damaged case — structural failure
Corrosion on terminals or mounting bracket — age or environmental exposure
Capacitance Testing
Use a digital capacitance meter or multimeter with capacitance function. Disconnect at
least one lead before testing. Compare the measured value to the nameplate rating:
Measured Result
Interpretation
Action
Within ±6–10% of nameplate
Acceptable
No action required
More than 10% below nameplate
Weakened
Should be replaced
More than 20% below nameplate
Failed
Must be replaced immediately
Zero reading
Open Circuit
Replace immediately
Resistance Testing
Use an analog ohmmeter or digital multimeter in resistance mode with the capacitor fully
discharged and at least one lead disconnected:
✓ Good Capacitor
Initially shows low resistance
Gradually increases toward infinity as the meter battery charges the capacitor
Reversing leads repeats the charging effect
✗ Shorted Capacitor
Shows zero or very low resistance continuously
Reading does not increase over time
✗ Open Capacitor
Shows infinite resistance immediately
No charging effect when leads are reversed
⚠ Leaking Capacitor
Shows some finite resistance — not approaching infinity
Indicates internal breakdown
Replace even if capacitance appears within range
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Resistance testing cannot detect a weakened capacitor
Resistance testing identifies obvious open or shorted failures quickly without
specialized equipment. However, it cannot detect reduced capacitance — the
most common run capacitor failure mode. Always use capacitance measurement when
available and when diagnosing motor performance complaints.
3.1.4 — Capacitor Installation and Safety
🔧
Mounting and Connection Requirements
Mount capacitors securely using the provided bracket or mounting strap
Position away from hot surfaces such as compressor domes and discharge lines
Ensure adequate ventilation for heat dissipation
Protect from physical damage, moisture, and contaminants
Use appropriate wire sizes for capacitor current (typically 14 or 16 AWG for run capacitors)
Use spade or ring terminals appropriate for the capacitor terminal type — make connections secure
Route wiring to avoid sharp edges, hot surfaces, and moving parts
Use terminal covers or shrouds where provided to prevent accidental contact
Safety Hazards
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Electrical Shock Hazard
Capacitors store electrical charge and can deliver dangerous shocks even when
disconnected from power. Always discharge capacitors before service, testing, or
handling. Verify discharge with a voltmeter. Treat all capacitors as
potentially charged until you have verified otherwise.
💥
Explosion Hazard
Failed or overheated capacitors can rupture violently, releasing hot oil and
metal fragments. Never bypass capacitor protection devices. Replace any capacitor
showing signs of swelling, leakage, or damage. Do not over-torque terminal
connections — mechanical stress can damage the case and cause rupture.
♻️
Environmental Disposal
Capacitors may contain PCBs (polychlorinated biphenyls) or other hazardous
materials, particularly older units. Check local regulations for proper disposal
procedures. Many jurisdictions require capacitors to be recycled or disposed of
as hazardous waste. Do not dispose of capacitors in regular
trash.
🛡️
Ratings must not be compromised
Always use capacitors with voltage ratings equal to or greater than the
application voltage. Never substitute with lower voltage ratings. Ensure
capacitance values are appropriate for the motor and application. When in
doubt, reference the original equipment manufacturer specifications or the
motor nameplate data.