Level 1, Unit 4, Section 3 – 3.2 Types of Capacitors

Unit 4 — Electrical Fundamentals

Section 3 — Working With Motors

3.2 — Types of Capacitors

Single-phase motors use two distinct capacitor types — start and run — each engineered for a specific duty cycle and function. This lesson covers construction, characteristics, applications, and failure modes for start, run, and dual capacitors.

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3.2.1 — Start Capacitors (Electrolytic)

Start capacitors are high-capacitance electrolytic capacitors designed for intermittent duty during motor starting. They provide the large phase shift and high current necessary for maximum starting torque but can operate for only short periods — typically 1 to 3 seconds — before overheating and failing.

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Start Capacitor — Electrolytic

Intermittent Duty Only

Construction

Aluminum foil electrodes separated by paper soaked in electrolyte solution, wound tightly and sealed in a metal or plastic case. This construction provides high capacitance in a compact size but limits duty cycle and lifespan. Cases are typically cylindrical or oval, brown or black.

Applications

Used in capacitor-start (CS) motors and capacitor-start capacitor-run (CSR) motors. Always disconnected by a relay or centrifugal switch after the motor reaches approximately 75% of full speed. Continuous operation destroys start capacitors rapidly.

Key Characteristics

Capacitance range: 50 to 1 200 µF typically
Voltage ratings: 110–125 VAC, 165 VAC, 220–250 VAC, 330 VAC
Intermittent duty only — 1 to 3 seconds per start
Typical lifespan: 10 000 to 20 000 start cycles
Lower cost compared to run capacitors

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Intermittent duty only

If the starting switch fails and leaves the start capacitor in circuit, it will overheat and fail within seconds — often violently.

Used in: Capacitor-Start (CS) Motors Capacitor-Start Capacitor-Run (CSR) Motors

Failure Modes

🔴 Open Circuit (No Capacitance)

Prevents starting or reduces torque to split-phase levels
Motor hums but will not start, or starts only when given a manual spin
Extended start time or failure to reach full speed

⚡ Shorted (Zero Resistance)

May blow fuses or trip branch-circuit breaker on start
Can damage the starting relay or centrifugal switch contacts
Inspect starting device immediately when a start cap is found shorted

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Weakened capacitor (reduced capacitance)

A start capacitor that has lost capacitance but has not failed completely will provide inadequate starting torque — causing extended start times, failure to reach full speed, or overheating of the start winding. Capacitance measurement is the only reliable way to catch this failure mode early.

3.2.2 — Run Capacitors (Oil-Filled)

Run capacitors are designed for continuous duty and remain in the circuit during motor operation. They improve motor efficiency, power factor, and performance by maintaining an optimal phase relationship between main and auxiliary winding currents.

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Run Capacitor — Oil-Filled

Continuous Duty

Construction

Metalized plastic film (typically polypropylene) as the dielectric, sealed in metal cases filled with insulating oil for heat dissipation. The self-healing construction automatically repairs minor dielectric breakdowns, contributing to long service life under continuous load. Cases are typically cylindrical metal, silver or gray.

Applications

Used in PSC motors, capacitor-run motors, and CSR motors. Always connected during motor operation to improve running efficiency, torque, and power factor.

Key Characteristics

Capacitance range: 2 to 80 µF typically
Voltage ratings: 370 VAC and 440 VAC most common
Continuous duty rated
Typical lifespan: 60 000+ operating hours
Self-healing construction
Higher cost than start capacitors

Used in: PSC Motors CSR Motors Condenser Fan Motors Indoor Fan Motors

Failure Modes

Run capacitors typically fail gradually by losing capacitance as the dielectric degrades. Unlike start capacitors that fail abruptly, run capacitor degradation causes a slow decline in motor performance that worsens over time.

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Effects of a Weakened Run Capacitor

Increased motor current draw
Reduced running efficiency
Lower power factor
Motor overheating and reduced insulation life
Eventual motor failure if not corrected

Complete failure (open circuit) significantly reduces motor torque and efficiency, and may prevent starting on difficult loads.

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Diagnosing a failed PSC run capacitor in the field

A motor that runs but draws high current, runs hot, or has reduced airflow is a classic run-capacitor symptom. Capacitance measurement is the only reliable test — a value more than ±6% from the nameplate rating warrants replacement. Resistance testing alone cannot detect a weakened run capacitor.

3.2.3 — Dual Round Capacitors

Dual capacitors, also called dual run capacitors, contain two separate capacitor sections in a single cylindrical case with three terminals. These are standard in residential and light-commercial air conditioning systems where both the compressor motor and condenser fan motor require run capacitors.

A typical dual capacitor is rated 40/5 µF 440 VAC — providing 40 µF for the compressor and 5 µF for the fan motor. Dual capacitors reduce component count and simplify wiring compared to two separate run capacitors.

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Terminal Identification and Testing

Standard Terminal Labels

C or COM — Common; connected to power supply
HERM — Hermetic compressor connection
FAN — Condenser fan motor connection

Testing Dual Capacitors

Test each section independently between its terminal and COM
Measure HERM-to-COM for the compressor section
Measure FAN-to-COM for the fan section
Compare each reading to the rating on the label

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One failed section means full capacitor replacement

If only the fan section or compressor section fails, the entire dual capacitor must be replaced. Individual sections cannot be serviced in the field. Always replace with a unit that matches both µF ratings and the voltage rating.