What Does a Capacitor Do?
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For permanent-split capacitor (PSC) type AC motors—commonly referred to as capacitor start and run AC motors—a capacitor is essential for proper functioning. Let’s take a moment to understand why this is so.
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A Simple Experiment...
To illustrate the importance of a capacitor, consider this simple demonstration. Take a single-phase PSC motor and connect its leads directly to a single-phase power supply without including the capacitor. You’ll find that the motor probably won’t run unless you manually rotate the shaft using an external force (this is easier with an ungeared motor with a round shaft). This happens because generating a rotating magnetic field in the stator requires at least two phases, which is where the capacitor steps in.
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What Does a Capacitor Do?
Originally called a “condenser,†a capacitor is a passive electronic component consisting of at least two conductors (plates) separated by an insulating material (dielectric). These conductors can be made of thin metal films, aluminum foil, or disks, while the insulator might be glass, ceramic, plastic film, air, or paper. When connected to a voltage source, a capacitor stores electrical charge in the form of an electrostatic field between its plates.
In contrast to a battery, which stores charge chemically and releases it slowly over time (potentially taking years), a capacitor discharges its energy much faster—typically within seconds or less. A classic example of this is the flash from a camera.
**Caution:** Since a capacitor holds an electrical charge, avoid touching its terminals. If you must handle it, ensure the charge is fully discharged first.
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Why Are Capacitors Essential for Motors?
The primary role of a capacitor in a motor is to create a poly-phase power supply from a single-phase one. This allows the motor to:
1. Determine the direction of rotation.
2. Provide starting torque and increase torque during operation.
Oriental Motor’s AC motors are all permanent-split capacitor types (capacitor start and run). These motors include a main winding and a secondary auxiliary winding. The capacitor is connected in series with the auxiliary winding, causing the current in the auxiliary winding to lag behind the current in the main winding by 90 electrical degrees (a quarter of the full cycle). This effectively generates a poly-phase power supply from a single-phase one.
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**Without a Capacitor vs. With a Capacitor**
| Without a Capacitor | With a Capacitor |
|---------------------|------------------|
|  |  |
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What Type of Capacitor Does Oriental Motor Use?
Oriental Motor utilizes vapor-deposited electrode capacitors certified by UL. This type of capacitor employs metallized paper or plastic film as its core element. It’s also known as a "self-healing (SH) capacitor." While earlier capacitors often used paper elements, plastic film capacitors have become more popular in recent years due to their compact design.
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Rated Conduction Time
The rated conduction time represents the minimum lifespan of the capacitor under rated load, voltage, temperature, and frequency conditions. The standard expected life span is 40,000 hours. If a capacitor fails at the end of its life, it may smoke or even catch fire. To avoid such issues, it’s recommended to replace the capacitor after reaching the rated conduction time.
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Safety Features of Capacitors
Some capacitors come equipped with safety features to safely remove them from circuits in case of a dielectric breakdown, preventing smoke or fire hazards. Oriental Motor products use capacitors with UL-recognized safety features that meet the UL 810 requirement of a 10,000 A fault current test.
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How Are Capacitors Rated, and Why Is This Important?
Capacitors are rated based on several factors, including capacitance, working voltage, tolerance, leakage current, working temperature, and equivalent series resistance. For motor applications, the two most critical specifications are capacitance and working voltage. Typically, the voltage rating is about double the motor’s rated input voltage in volts (though there’s a specific formula to calculate the exact capacitance for a motor). For compact AC motors, capacitance is measured in microfarads (μF), and these specifications appear on both the motor label and the capacitor label.
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Motor Label with Recommended Capacitor | Capacitor Label
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Using a capacitor with incorrect capacitance values can lead to increased motor vibration, heat generation, power consumption, torque variation, and unstable operation. Too high a capacitance can boost torque but may cause overheating and excessive vibration, whereas too low a capacitance reduces torque. Using a capacitor with a voltage rating exceeding the motor’s rated voltage may result in damage, potentially causing the capacitor to smoke or ignite.
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Do I Need to Size the Right Capacitor for Oriental Motor’s AC Motors?
No. Each single-phase AC motor from Oriental Motor comes with a dedicated capacitor specifically matched to the motor for optimal efficiency and performance. There’s no need for capacitor sizing.
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What Happens If I Use a Different Capacitor?
To ensure the motor operates at peak efficiency, always use the dedicated capacitor provided with the motor. The dedicated capacitor creates a precise 90-degree phase shift between the auxiliary (capacitor) phase and the main phase. Using the wrong capacitor can disrupt this phase shift, leading to inefficiencies, overheating, inconsistent torque, or fluctuating speeds.
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The dedicated capacitor ensures the motor achieves its ideal torque-speed curve. Notice the "Rated Speed" and "Rated Torque." The operating point where these intersect is where the motor operates most efficiently. Every motor is designed for a specific rated load, so simply oversizing the motor isn’t the best approach.
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AC Motor Speed-Torque Curve | Effect of Capacitance on AC Motor Speed-Torque Curve
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A change in the capacitor’s capacitance affects the motor’s rated speed and torque, shifting the operating point away from its optimal efficiency. Even two identical motors wired with different capacitors will yield drastically different results. Once efficiency drops, heat generation rises, potentially degrading bearing grease and reducing the motor’s long-term lifespan. However, if the winding temperature reaches 130°F, the motor’s thermal protection circuit will activate, shutting off the motor until it cools down.
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How Do I Wire a Capacitor?
For a three-wire AC motor, connect the red and white lead wires to opposite capacitor terminals, and the black lead wire to the neutral (N) side of the power supply. For unidirectional operation, connect the live (L) side of the power supply to the terminal closest to either the red lead wire (for clockwise rotation) or the white lead wire (for counterclockwise rotation). Hint: The two closest terminals are internally joined. For bidirectional operation, use a single-pole double-throw (SPDT) switch between the live wire and the capacitor terminals to switch directions.
However, reversing the direction of an induction motor requires waiting for the motor to stop completely. Reversible motors allow instantaneous direction changes.
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Now that you understand the importance of capacitors, don’t misplace them. If you do, refer to the motor label to identify the correct capacitor. Stay tuned for more troubleshooting tips.
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**Related Resources:**
- [Features and Types of Constant Speed Motors](http://bsg-i.nbxc.com/blog/46f36e7cbb3e77757761b7b4bdaced17.png)
- [Show & Tell: AC Induction Motors](http://bsg-i.nbxc.com/blog/1beaf45830cb7facd2b7caf90e6b6d27.png)
- [Show & Tell: AC Reversible Motors & AC Electromagnetic Brake Motors](http://bsg-i.nbxc.com/blog/68a0663b408849973e5f37c9d823e4a4.png)
- [How-To Videos: Wiring Different AC Motors](http://bsg-i.nbxc.com/blog/520546fed3b10b1c82c63892c1ef079a.png)
- [White Papers: AC Motor Technology](http://bsg-i.nbxc.com/blog/d78dc7e2c914ff92f5aac2299cdd130b.png)
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I hope this helps clarify the role of capacitors in AC motors!