How to Check a Start Capacitor: A Comprehensive Guide
Start capacitors are essential components in many electrical appliances, particularly those that use single-phase AC motors. These motors, unlike their three-phase counterparts, lack the inherent starting torque needed to get them rotating. That’s where the start capacitor comes in, providing a temporary boost of power to initiate the motor’s rotation. However, like any electrical component, start capacitors can fail over time. A failed start capacitor can cause a motor to hum, fail to start, or even overheat and burn out. Therefore, knowing how to check a start capacitor is crucial for both troubleshooting and preventative maintenance. This comprehensive guide will walk you through the process, providing detailed steps and essential safety information.
## What is a Start Capacitor and How Does it Work?
Before diving into the testing procedure, it’s essential to understand the role of a start capacitor in a motor circuit.
* **Function:** A start capacitor provides a brief, high-current surge to the motor’s starting winding, creating a phase shift that initiates rotation. This surge helps the motor overcome its initial inertia and begin spinning.
* **Operation:** Unlike run capacitors, which remain in the circuit continuously, start capacitors are designed for intermittent use. Once the motor reaches a certain speed (typically around 75% of its rated speed), a centrifugal switch or a potential relay disconnects the start capacitor from the circuit. This prevents the capacitor from overheating and failing due to prolonged exposure to high voltage.
* **Construction:** Start capacitors are typically electrolytic capacitors, known for their high capacitance in a relatively small package. However, electrolytic capacitors have a limited lifespan and are more susceptible to failure than other types of capacitors.
## Identifying a Start Capacitor
Start capacitors are usually cylindrical and housed in a plastic or metal case. They are typically labeled with the following information:
* **Capacitance:** Measured in microfarads (µF or mfd). This value indicates the capacitor’s ability to store electrical charge.
* **Voltage:** Measured in volts (V). This is the maximum voltage the capacitor can safely withstand.
* **Operating Temperature:** Indicates the acceptable temperature range for operation.
* **Terminal Type:** May be quick-connect terminals or screw terminals.
Start capacitors are typically found in appliances like:
* Air conditioners
* Refrigerators
* Washing machines
* Dryers
* Furnaces
* Pumps
* Power tools (e.g., drill presses, table saws)
## Safety Precautions
Working with electrical components can be dangerous if proper safety precautions are not followed. Before attempting to check a start capacitor, ensure you understand and adhere to the following safety guidelines:
* **Disconnect Power:** Always disconnect the appliance from its power source before working on it. This is the most crucial safety step to prevent electric shock. Unplug the appliance or turn off the circuit breaker supplying power to it.
* **Discharge the Capacitor:** Start capacitors store electrical energy even after the power is disconnected. This stored energy can deliver a painful and potentially dangerous shock. **Always discharge the capacitor before handling it.**
* **Use Insulated Tools:** Use tools with insulated handles to minimize the risk of electric shock.
* **Wear Safety Glasses:** Wear safety glasses to protect your eyes from debris or accidental sparks.
* **Work in a Dry Environment:** Avoid working in damp or wet conditions, as water increases the risk of electric shock.
* **Proper Ventilation:** If you’re working indoors, ensure adequate ventilation to prevent the buildup of fumes from cleaning agents or other materials.
* **Consult a Qualified Technician:** If you’re uncomfortable working with electrical components or unsure about any part of the process, consult a qualified electrician or appliance repair technician.
## Tools and Materials Needed
Before you begin, gather the following tools and materials:
* **Screwdriver:** To access the capacitor.
* **Multimeter:** A multimeter is essential for measuring capacitance and voltage. It should be capable of measuring capacitance in microfarads (µF) and voltage in AC and DC volts.
* **Alligator Clips (optional):** Can be helpful for connecting the multimeter to the capacitor terminals.
* **Safety Glasses:** To protect your eyes.
* **Insulated Gloves (optional):** For added protection against electric shock.
* **Discharge Tool (optional):** A resistor with insulated leads can be used to safely discharge the capacitor. A screwdriver with an insulated handle *can* be used, but is a less safe and less controlled method.
* **Replacement Capacitor (if needed):** If the capacitor is found to be faulty, you’ll need a replacement with the same capacitance and voltage rating.
* **Camera or Phone:** Take pictures of the wiring before disconnecting anything to ensure proper reassembly.
## Step-by-Step Guide to Checking a Start Capacitor
Here’s a detailed step-by-step guide to checking a start capacitor:
**Step 1: Disconnect Power**
The very first step, and the most important, is to disconnect the appliance from its power source. Unplug the appliance from the wall outlet or turn off the circuit breaker that supplies power to it. Double-check that the power is off by using a voltage tester on the appliance’s power cord.
**Step 2: Access the Capacitor**
The location of the start capacitor varies depending on the appliance. In most cases, it’s located near the motor. You may need to remove access panels or covers to reach it. Refer to the appliance’s service manual or online resources for specific instructions on how to access the capacitor.
* **Air Conditioners:** Typically found in the control panel on the outdoor unit.
* **Refrigerators:** Usually located in the compressor compartment at the back of the refrigerator.
* **Washing Machines:** Often found behind the control panel or near the motor.
* **Dryers:** Typically located behind the rear access panel.
Use a screwdriver to remove any screws or fasteners holding the access panels in place. Carefully remove the panels and set them aside.
**Step 3: Identify the Capacitor**
Once you’ve accessed the components, locate the start capacitor. It’s usually a cylindrical component with terminals on top. Verify that it matches the description provided earlier (capacitance, voltage, etc.).
**Step 4: Discharge the Capacitor (Crucial!)**
This is a critical safety step. Never skip it. Capacitors store electrical energy, and even after the appliance is disconnected from power, the capacitor can still hold a dangerous charge.
**Method 1: Using a Resistor (Recommended)**
1. Use a resistor between 5000 and 20000 ohms with a wattage rating of at least 5 watts. The higher the wattage rating, the safer the resistor. The higher the resistance value, the slower the discharge, but also the safer it is for the capacitor.
2. Connect insulated leads to each end of the resistor.
3. Hold the insulated leads and touch one lead to each terminal of the capacitor. Maintain contact for several seconds (at least 15-30 seconds) to allow the capacitor to discharge fully.
4. Remove the resistor and, using your multimeter set to DC voltage, test the voltage across the capacitor terminals to confirm that it has been discharged to a safe level (ideally below 5V). If there is still significant voltage, repeat the discharging process.
**Method 2: Using a Screwdriver (Less Safe)**
* **Warning:** This method is less safe and should only be used as a last resort if you don’t have a resistor. Use extreme caution.
1. Wear safety glasses and insulated gloves.
2. Use a screwdriver with a well-insulated handle.
3. Carefully touch the metal shaft of the screwdriver to both terminals of the capacitor simultaneously. This will create a short circuit, discharging the capacitor.
4. You may see a spark when the screwdriver touches the terminals. This is normal, but be prepared for it.
5. After shorting the terminals, use your multimeter set to DC voltage to test the voltage across the capacitor terminals to confirm that it has been discharged to a safe level (ideally below 5V). If there is still significant voltage, repeat the discharging process.
**Step 5: Disconnect the Capacitor**
Once you’ve confirmed that the capacitor is discharged, you can safely disconnect it from the circuit. Before disconnecting the wires, take a picture or draw a diagram of how the wires are connected to the capacitor terminals. This will ensure that you reconnect them correctly later.
Use pliers or your fingers to carefully disconnect the wires from the capacitor terminals. If the wires are held in place by screws, use a screwdriver to loosen the screws and remove the wires.
**Step 6: Inspect the Capacitor**
Visually inspect the capacitor for any signs of damage, such as:
* **Bulging or Swelling:** This indicates internal pressure and is a sign of a failed capacitor.
* **Cracks or Leaks:** Cracks in the casing or leaking fluid indicate that the capacitor is damaged.
* **Burn Marks:** Burn marks suggest that the capacitor has overheated.
If you observe any of these signs, the capacitor is likely faulty and needs to be replaced.
**Step 7: Test the Capacitor with a Multimeter**
Use a multimeter to test the capacitor’s capacitance and continuity. Here’s how:
**A. Testing Capacitance:**
1. Set your multimeter to the capacitance (µF) setting. The symbol for capacitance is typically “F” or “µF.”
2. If your multimeter has a range selection, choose the appropriate range for the capacitor’s capacitance value. For example, if the capacitor is rated at 20 µF, select a range that includes 20 µF.
3. Connect the multimeter’s probes to the capacitor’s terminals. Polarity doesn’t matter for most start capacitors.
4. Wait for the multimeter to stabilize and display a reading. This may take a few seconds.
5. Compare the reading to the capacitor’s rated capacitance value. A healthy capacitor should have a capacitance value within 10% of its rated value. For example, a 20 µF capacitor should read between 18 µF and 22 µF.
6. If the reading is significantly outside of this range, the capacitor is likely faulty and needs to be replaced. A reading of zero indicates a shorted capacitor. A very low reading indicates a capacitor with low capacitance and poor starting ability.
**B. Testing Continuity (Optional but Recommended):**
1. Set your multimeter to the continuity setting. This setting is usually indicated by a diode symbol or a speaker symbol.
2. Connect the multimeter’s probes to the capacitor’s terminals.
3. A healthy capacitor should initially show a short circuit (a beep or a low resistance reading) as it charges up. Then, the resistance should gradually increase to infinity (an open circuit). This indicates that the capacitor is able to store a charge.
4. If the multimeter shows a continuous short circuit (a constant beep or a very low resistance reading), the capacitor is shorted and needs to be replaced. If the multimeter shows an open circuit immediately and never beeps, the capacitor is open and needs to be replaced.
**Step 8: Replace the Capacitor (If Necessary)**
If the capacitor fails either the visual inspection or the multimeter tests, it needs to be replaced. When replacing the capacitor, make sure to use a replacement capacitor with the same capacitance and voltage rating as the original. Using a capacitor with a different rating can damage the motor or other components.
1. Obtain a replacement capacitor with the correct specifications. Double-check the capacitance (µF) and voltage (V) ratings.
2. Connect the wires to the new capacitor terminals, matching the original wiring configuration (refer to the picture or diagram you took earlier).
3. If the capacitor is mounted in a bracket or clamp, secure the new capacitor in place.
4. Double-check all connections to ensure they are secure and properly insulated.
**Step 9: Reassemble the Appliance**
Once you’ve replaced the capacitor, reassemble the appliance in the reverse order of disassembly.
1. Replace any access panels or covers that you removed earlier.
2. Secure the panels with screws or fasteners.
3. Double-check that all wires and components are properly positioned and not pinched or damaged.
**Step 10: Restore Power and Test**
1. Restore power to the appliance by plugging it back into the wall outlet or turning on the circuit breaker.
2. Turn on the appliance and observe its operation. The motor should start smoothly and run without any unusual noises or vibrations.
3. If the motor still fails to start or runs poorly, there may be other problems with the appliance that require further troubleshooting.
## Troubleshooting Tips
* **Motor Hums But Doesn’t Start:** This is a classic symptom of a failed start capacitor. The motor is receiving power, but it doesn’t have enough starting torque to overcome its inertia.
* **Motor Starts Slowly or Weakly:** This could also be caused by a weak or failing start capacitor. The capacitor is providing some starting torque, but not enough to start the motor quickly and efficiently.
* **Motor Overheats:** A failing start capacitor can cause the motor to work harder, leading to overheating. However, overheating can also be caused by other motor issues, so rule out other problems as well.
* **Capacitor Shows Signs of Physical Damage:** As mentioned earlier, bulging, cracks, leaks, or burn marks are all signs of a faulty capacitor.
* **Multimeter Readings are Outside of Tolerance:** If the capacitor’s capacitance is significantly outside of its rated value (e.g., more than 10% off), it’s likely faulty.
## When to Call a Professional
While checking and replacing a start capacitor can be a relatively straightforward task, there are situations where it’s best to call a qualified electrician or appliance repair technician:
* **You’re Uncomfortable Working with Electrical Components:** If you’re not confident in your ability to safely work with electrical components, it’s best to leave the job to a professional.
* **You’re Unsure About the Problem:** If you’ve checked the start capacitor and the motor still isn’t working correctly, there may be other underlying issues that require professional diagnosis.
* **The Appliance is Under Warranty:** If the appliance is still under warranty, attempting to repair it yourself may void the warranty. It’s best to contact the manufacturer or an authorized service center.
* **You Suspect a More Complex Electrical Problem:** If you suspect a more complex electrical problem, such as a wiring issue or a faulty motor winding, it’s best to consult a qualified electrician.
* **The Capacitor is Difficult to Access:** Some appliances have capacitors located in difficult-to-reach areas, requiring specialized tools or expertise to access.
## Conclusion
Checking a start capacitor is a valuable skill for anyone who wants to maintain their appliances and avoid costly repairs. By following the steps outlined in this guide, you can safely and effectively test a start capacitor and determine if it needs to be replaced. Remember to always prioritize safety and disconnect power before working on any electrical appliance. If you’re unsure about any part of the process, don’t hesitate to consult a qualified electrician or appliance repair technician. Regularly checking the start capacitors in your appliances can help prevent motor failures and extend the lifespan of your equipment. Proper maintenance not only saves you money in the long run but also ensures the safe and efficient operation of your appliances. By understanding the function, testing procedures, and safety precautions associated with start capacitors, you can empower yourself to handle common appliance repairs and keep your home running smoothly.