Video Companion: How to Test a Capacitor

Capacitors are critical components in HVAC systems, essential for the efficient operation of motors in air conditioners, furnaces, and heat pumps. A malfunctioning capacitor can lead to system failure, reduced efficiency, and increased energy consumption. This guide provides HVAC professionals with a comprehensive, technical overview of capacitor testing procedures, emphasizing safety, proper tool usage, and accurate diagnostic techniques.

Understanding HVAC Capacitors

HVAC systems primarily utilize two types of capacitors: run capacitors and start capacitors. Both are designed to store and release electrical energy, but they serve distinct functions.

Run Capacitors

Run capacitors provide continuous phase-shifted current to the motor's start winding, ensuring smooth and efficient operation once the motor is running. They contribute to maintaining the motor's design efficiency, correct rotational direction, appropriate torque, and a near-unity power factor. Failure of a run capacitor can cause a motor to not start (especially high-torque compressors), run backward, operate slowly, or draw excessive amperage, leading to overheating [1].

Start Capacitors

Start capacitors provide an initial surge of energy to overcome the motor's inertia and initiate rotation. They are typically used for a very short duration during startup and are then disengaged from the circuit by a centrifugal switch or potential relay. A faulty start capacitor will prevent the motor from starting, often resulting in a humming sound without any rotational movement.

Common Causes of Capacitor Failure

Capacitors can fail due to several factors, including [1]:

• Poor Manufacturing Quality: Inherent defects from the production process.
• Overtemperature: Prolonged exposure to high temperatures, which degrades the dielectric material.
• Over-voltage: Exposure to voltage levels exceeding the capacitor's rated capacity.

Safety First: Essential Precautions

Working with capacitors involves handling stored electrical energy, which can be hazardous. Adhering to strict safety protocols is paramount to prevent electrical shock and injury [2].

Personal Protective Equipment (PPE)

Always wear appropriate PPE, including:

• Safety Glasses: To protect eyes from potential electrical arcs or debris.
• Insulated Gloves: To provide a barrier against electrical current.

Power Disconnection and Verification

Before any inspection or testing, ensure the HVAC unit is completely de-energized:

1. Turn Off Power: Disconnect the main power supply to the unit at the service disconnect or circuit breaker [2].
2. Verify Zero Voltage: Use a multimeter, pre-tested on a known live circuit, to confirm the absence of voltage across all terminals (L1 to L2, L1 to ground, and L2 to ground) [1].

Capacitor Discharge Procedure

Capacitors can retain a significant electrical charge even after power is disconnected. It is crucial to discharge them safely before handling [1] [2].

1. High-Resistance Resistor Method: The safest method involves using a high-resistance resistor (e.g., 15,000 to 20,000 ohms, 2-watt) with insulated leads to bridge the capacitor terminals (HERM and Fan terminals across to C, or across a single capacitor). This allows for a controlled discharge [1].
2. Insulated Screwdriver Method (Controversial): While common among some technicians, using an insulated screwdriver to short the terminals is controversial due to the potential for arcing and damage to the capacitor or screwdriver. If this method is used, ensure the screwdriver is properly insulated and held securely [1].

Tools for Capacitor Testing

Accurate capacitor testing requires specialized tools:

• Multimeter with Capacitance Function: Essential for measuring capacitance in microfarads (µF). Ensure the multimeter is calibrated and capable of reading the expected capacitance range [2].
• Insulated Screwdriver: For safely discharging capacitors (if using the controversial method) and removing access panels.
• Needle-Nose Pliers: For disconnecting and reconnecting wires to capacitor terminals.
• Amp Clamp (for under-load testing): Used to measure amperage on motor windings during under-load tests [1].

Step-by-Step Capacitor Testing Procedures

Capacitors can be tested either under load (with the system running) or via a bench test (with the capacitor removed). The choice depends on the system's operational status and the technician's preference [1].

Visual Inspection

Before electrical testing, always perform a visual inspection [1]:

• Physical Damage: Look for signs of physical damage such as bulging tops, oil leakage, or corrosion. Any such indication warrants immediate replacement.
• Rating Verification: Note the microfarad (MFD or µF) and voltage ratings. A replacement capacitor must have the same MFD rating. A higher voltage rating is acceptable, but never lower [1].

Bench Test (Off-Load Testing)

This method is suitable when the system is not running or for verifying new capacitors before installation [1] [2].

1. Isolate and Discharge: Ensure the capacitor is fully discharged and all wires are disconnected from its terminals.
2. Multimeter Setup: Set the multimeter to the capacitance (µF) setting.
3. Connect Probes: Connect the multimeter probes to the capacitor terminals. Ensure a solid connection to the metal spades without touching the probes directly [1].
4. Read and Interpret: Observe the reading on the multimeter. Compare it to the capacitor's rated capacitance. A reading within ±6% of the rated value is generally acceptable. A deviation greater than 10% (either low or high) indicates a faulty capacitor that requires replacement [1] [2].

Under-Load Test (In-Circuit Testing)

This method is performed with the system running and provides real-world performance data. It is not recommended for blower capacitors due to the risk of entanglement with spinning components [1].

1. PPE and Safety: Wear proper PPE and ensure a safe working environment.
2. Measure Start Winding Current: Use an amp clamp to measure the amperage on the start wire, ensuring the wire is centered within the clamp to avoid interference. Multiply this reading by 2652 [1].
3. Measure Voltage Across Capacitor: Measure the voltage across the capacitor terminals [1].
4. Calculate Capacitance: Divide the result from step 2 (amperage x 2652) by the voltage measured in step 3. This will give the capacitance in MFD [1].
5. Evaluate Results: If the calculated MFD is more than 10% below the rated value, replacement is suggested. Readings significantly above the rating often indicate measurement error [1].

Replacing a Faulty Capacitor

If a capacitor is determined to be faulty, it must be replaced with a new one of the exact same microfarad (MFD or µF) rating. A higher voltage rating is permissible, but never a lower one [1] [2].

1. Document Connections: Before disconnecting, take a picture or make a diagram of the wire connections to the capacitor terminals (Common, HERM, FAN) [1] [2].
2. Remove Old Capacitor: Disconnect all wires and remove any straps holding the capacitor in place [1].
3. Bench Test New Capacitor: It is good practice to bench test the new capacitor before installation to confirm its integrity [1].
4. Install New Capacitor: Mount the new capacitor upright with terminals facing up. If the new capacitor has different dimensions, adjust or create a new mounting strap using self-tapping screws, ensuring no coils are punctured [1] [2].
5. Reconnect Wires: Reattach the wires to their corresponding terminals, ensuring snug connections. Use needle-nose pliers to tighten spade connectors if necessary [1] [2].
6. System Test: Restore power to the unit. Verify that the compressor and fan are running at proper amperage. Consider using a power meter to check the motor's power factor, aiming for near unity [1].
7. Final Assembly: Clean the work area and reattach all service panels [1].

Internal Links

• HVACProSales.com/air-conditioners
• HVACProSales.com/furnaces
• HVACProSales.com/heat-pumps
• HVACProSales.com/multimeters
• HVACProSales.com/safety-equipment