A fan capacitor is a small electrical component that works alongside the motor in ceiling fans, table fans, floor fans, and other AC-powered fan types. The capacitor plays an important role in starting the fan’s induction motor and regulating its speed.
In this article, we will cover the following topics:
- What is a capacitor and how does it work?
- Capacitor function in fan motors
- Types of fan capacitors
- Capacitor specifications
- Capacitor testing and troubleshooting
- Safety considerations
- Capacitor upgrades and improvements
Understanding fan capacitor operation and characteristics helps ensure proper fan performance, trouble-free operation, and informed repairs when needed.
What is a Capacitor?
A capacitor is a passive electronic component that can store electrical energy in the form of an electrostatic field. It consists of two conducting plates separated by an insulating dielectric material.
When voltage is applied across the capacitor’s terminals, opposite electric charges accumulate on the two plates. The capacitor stores the charge rather than allowing significant current to flow.
The amount of charge stored is proportional to the voltage applied, and depends on the capacitor’s capacitance value. Capacitance (C) is a measure of the capacitor’s charge storage capacity, specified in units of Farads (F). Larger capacitance allows more charge to be stored at a given voltage.
Capacitors exhibit the following key properties:
- Store and release electrical energy
- Block direct current (DC) while allowing alternating current (AC) to pass
- Smooth out voltage fluctuations in power circuits
- Help suppress electromagnetic interference (EMI)
- Serve as frequency filters in electronic circuits
These characteristics make capacitors useful in a wide range of electrical and electronic applications.
Capacitor Function in Fan Motors
In ceiling, table and floor fan motors, the capacitor serves two important functions:
1. Motor Starting
At startup, the capacitor provides a phase shift between the start and run windings in the motor. This allows the motor to start spinning from rest and overcome initial inertia.
Without the capacitor, the two windings would be in phase during starting and the motor would not rotate. The capacitor brings the windings out of phase to produce a rotating magnetic field that starts the motor.
2. Speed Control
Once the motor reaches operating speed, the capacitor stays connected in series with the run winding. This allows variable speed control based on the capacitor value.
The capacitor shifts the phase angle between the start and run windings. Varying this phase shift varies the speed of the rotating magnetic field, which changes the motor rpm. Higher capacitance gives lower speeds while lower capacitance provides higher speeds.
Capacitor connections in a fan motor circuit
Thus the capacitor allows proper starting of the motor from rest and then controls the operating speed once the motor is running.
Types of Fan Capacitors
Fan motors use AC electrolytic capacitors that are polarized and have relatively large capacitance values.
Electrolytic capacitors use an insulating liquid electrolyte between their plates to achieve high capacitance in a small size. But this electrolyte requires correct polarity when connecting the capacitor in a circuit.
The main types of electrolytic capacitors used in fans include:
1. Run Capacitor
This remains permanently connected to the motor during starting and running to control its speed.
It is non-polarized, meaning polarity does not matter when connecting to AC power. Run capacitors have capacitance in the range of 3-12 microfarads (μF).
2. Start/Run Capacitor
This combines start and run capacitor functionality into a single unit. It is polarized, so proper orientation must be maintained when fitting on the motor.
Start/run capacitors have higher capacitance, typically in the 30-70 μF range, since they must provide the higher starting torque.
3. Start Capacitor
This is used only during initial motor startup to get it spinning. Once the motor reaches operating speed, the start capacitor is disconnected via a centrifugal switch inside the motor.
Start capacitors have very large capacitance of 200-800 μF to provide high starting torque. They are always polarized electrolytic units.
Dual capacitors that combine a run capacitor and a start capacitor in a single casing are also sometimes used.
Capacitor Specifications
Fan capacitors are rated by the following key parameters:
1. Capacitance
This is the main rating defining the capacitor’s charge storage capacity in microfarads (μF). Typical fan capacitance values range from 3 to 70 μF.
Higher capacitance is needed for starting the motor from rest versus keeping it running.
2. Tolerance
It specifies the acceptable capacitance tolerance, such as ±5% or ±10%. Tighter tolerance provides more accurate capacitance.
3. Voltage
Rated voltage is the maximum DC working voltage that can be applied to the capacitor. Fan capacitors are available in voltage ratings from 125V up to 450V.
Higher voltage units are specified where the line voltage can fluctuate widely.
4. Temperature
This indicates the operating temperature range of the capacitor, usually something like -30°C to +85°C for fan applications.
High temperature plastic film dielectrics extend the range up to 105°C.
5. Polarity
Whether the capacitor is non-polarized or polarized determines proper installation orientation on the motor terminals. Non-polarized units work in either AC direction.
These key ratings must be verified when replacing a faulty capacitor to ensure correct operation and longevity.
Capacitor Testing and Troubleshooting
If a fan is not starting properly or not reaching full speed, the capacitor is often the cause of such symptoms. Some steps for testing capacitors:
- Use a multimeter switched to capacitance measurement mode to measure the capacitance. It should match the original specification printed on the capacitor.
- For polarized capacitors, verify continuity for forward polarity but no continuity in reverse, using a multimeter set to resistance mode.
- Use an ohmmeter to check short circuit and open circuit faults. A short or open reading indicates failure.
- Test capacitor breakdown voltage using a adjustable voltage source gradually increased up to the capacitor’s voltage rating.
- Use a capacitor analyzer or ESR meter to accurately determine capacitance and equivalent series resistance (ESR). High ESR indicates capacitor aging.
- Check for capacitor bulgeging, leaking electrolyte, burned spots or popped vent caps which indicate it needs replacement.
If testing points to a bad capacitor, it should be replaced with an identical or very similar rated unit. This will restore proper starting and running operation in the fan.
Safety Considerations
Working with fan capacitors requires some safety measures:
- Disconnect power supply before accessing capacitors in the fan motor. Wait 5 minutes after powering off to allow voltage discharge before handling capacitors.
- Never short circuit the capacitor terminals with metal objects as this can cause burns or explosion due to rapid discharge. Use an insulated screwdriver for diagnosis.
- Do not puncture or damage the capacitor casing and electrolyte seal which can lead to leaking.
- Verify AC power is off before disconnecting or installing replacement units.
- Use adequate hand and eye protection when testing capacitors which may rupture or leak.
- Ensure polarity is correct when installing polarized capacitors – improper connection can explode the capacitor.
Taking suitable precautions when testing or replacing capacitors prevents possible injury.
Capacitor Upgrades and Improvements
Users may wish to consider upgraded capacitors when repairing older fans or modifying fan performance:
- Longer lifetime units – Specify 105°C rated capacitors for 30,000 hour lifetime or more. This improves reliability where fans operate continuously in hot environments.
- Low ESR capacitors – Lower equivalent series resistance extends the capacitor life and improves efficiency by reducing wasted heat.
- Self-healing types – These contain a dielectric that can repair minor internal faults, preventing failure. Improves reliability.
- Higher torque rating – Larger microfarad rating increases starting torque and allows getting old inefficient fan motors spinning again.
- Noise suppression – Low ESR and self-healing capacitors also reduce audible humming and crackling in aging fan motors.
With proper installation, upgraded modern capacitors enhance the performance and lifetime of existing fan motors.
Conclusion
The key points on fan capacitors covered in this article are:
- Capacitors store electrical charge and help start fan motors and control their speed.
- Fan motors employ electrolytic capacitors for their high capacitance in a compact size.
- Run capacitors keep the motor running while start capacitors provide high starting torque.
- Capacitor ratings like capacitance, voltage, tolerance and temperature range must match the motor design.
- Testing with multimeters and ESR meters can identify faulty capacitors needing replacement.
- Safety steps like de-energizing and discharging capacitors prior to handling them are mandatory.
- Upgraded capacitors can improve reliability, efficiency, torque and noise in older fan motors.
In summary, capacitors are vital components enabling proper functioning of AC fan motors. Understanding their characteristics and following sound maintenance practices ensures safe, reliable and optimal fan operation.
Frequently Asked Questions
Question 1: How often should fan capacitors be replaced?
Electrolytic capacitors age and degrade over years of use. As a guideline, fan capacitors should be proactively replaced every 3-5 years to avoid in-service failures, especially for fans in continuous operation.
Question 2: Can a different microfarad rating be used when replacing a failed capacitor?
It is strongly advised to use the same capacitance rating specified on the original capacitor. Very small variations up to +/- 10% may be acceptable, but significantly changing the capacitance can affect starting and speed performance.
Question 3: How can you determine capacitor polarity when replacing a failed one?
Refer to any product diagram or markings on the motor winding connections. If unavailable, use a multimeter set to diode checking mode to determine which terminal gives a forward drop versus reverse drop – this indicates capacitor polarity.
Question 4: Why may both a start and a run capacitor be used together in a fan motor?
The start capacitor provides the very large capacitance needed to start the motor from rest. Once running, the smaller run capacitor is switched in series with the windings for more efficient speed control during operation.
Question 5: Is it safe to discharge a capacitor by shorting the terminals with a screwdriver?
No, shorting the terminals can cause a violent discharge with arcing that can destroy the capacitor and also potentially cause injury. It is much safer to discharge a capacitor through a resistor of at least 1 kilo-ohm before handling.
