Motor Start Capacitor and Motor Run Capacitor. What do they do?

Permanently decoupled capacitor type AC motors (also known as motor start capacitors and motor run capacitors of AC motors) require capacitors for proper operation. Enjoy your coffee with an explanation of why.

Simple Experiment

To show the importance of capacitors, we can start with a simple experiment. Use a single-phase split permanent capacitor AC motor and connect its leads directly to the single-phase power supply (skipping capacitors). Unless the shaft is rotated by an external force, the motor may not operate under load (this is much easier with a gearless shaft drive). This is because the stator requires at least two phases to generate a rotating magnetic field. This is why we need capacitors specifically motor start capacitors in this case.

What does a capacitor do?

Originally called a capacitor, a capacitor is a passive electronic component that has at least two conductors (two plates) separated by an insulator (insulator). The conductor can be metal, aluminum foil, or a thin film from the disc. The insulator can be glass, ceramic, plastic, air or paper. Capacitors, when connected to a voltage source, store the charge between the conductors in the form of an electrostatic field.

Compared to batteries, batteries use chemicals to store electrical charges and slowly discharge them through circuits. This may take years. Capacitors release their energy faster within seconds. An example of a common application is the flash from the camera.

Note: Never touch the terminals of the capacitor as it carries an electric charge. If you need it for any reason, make sure that the charge is completely discharged.

Difference Between Motor Start Capacitors and Motor Run Capacitors?

Capacitors are intended to create a multi-phase power supply from a single-phase power supply. Using a multi-phase power supply, the motor can:

1. Set the direction of rotation (Motor Start Capacitors).

2. Provides engine starting torque and increases torque during operation (Motor Run Capacitors).

Estimated delivery time

The rated conduction time is the minimum design life of the capacitor when operating at rated load, rated voltage, rated temperature, and rated frequency. Standard life 40,000 hours. Capacitors that break at the end of their life can smoke or catch fire. It is recommended to replace the capacitors after the specified connection time to avoid possible problems.

Capacitor safety features

Some capacitors are equipped with a safety feature that allows them to be safely and completely removed from the circuit to prevent smoke and fire in the event of a dielectric failure. 

How are capacitors evaluated and why is this important?

Capacitor ratings include capacitance, operating voltage, margin of error, leakage current, operating temperature, and equivalent series resistance. For motor matching purposes, the two most important specifications are capacitance and operating voltage. The rated voltage is usually about twice the value of the motor's rated input voltage in volts (in fact, there is a formula for determining the motor's capacitance, but save it for later use). For compact AC motors, the unit of measurement for capacitance is "microfarad" or μF. These specifications can be found on both the motor label and the capacitor label.

The use of capacitors of different capacities can lead to increased engine vibration, heat generation, power consumption, torque fluctuations and unstable operation. If the capacitance is too large, the motor torque will increase, but overheating and excessive vibration may occur. If the capacitance is too small, the torque will decrease. Using a capacitor that exceeds the rated voltage may cause damage or fire.

What if I used a different capacitor?

To ensure that your engine is operating at maximum efficiency, always use the dedicated capacitors that come with your engine. The dedicated capacitor produces 90 electrical phase transitions from the auxiliary (capacitor) phase to the main phase. Using the wrong capacitor can distort the capacitor by more than 90 degrees, and the resulting inefficiency can cause the motor to overheat and cause inconsistent torque or speed performance. Custom capacitors are sized to allow the motor to generate the ideal torque/speed curve. Pay attention to the "Rated Speed" and "Rated Torque". This operating point (where these two shapes intersect on a curve) is where the highest efficiency occurs. All motors are designed for rated loads. This is why it is not the best way to increase the size of AC motors.

Fluctuations in capacitor capacitance affect both rated speed and rated torque as the operating point moves away from maximum efficiency. If you use two identical motors with very different capacitors, the results will be completely different.

If you lose maximum efficiency, the engine will overheat. Excess heat can degrade bearing grease and reduce engine life. However, it is good to know that when the winding temperature reaches 130 degrees Fahrenheit, the thermal protection circuit inside the motor becomes active and the motor stops until it cools down.

How do I connect a capacitor?

For 3-wire AC motors, connect the red and white leads to the opposite terminals of the capacitor. Connect the black wire to the N (neutral) side of the power supply. For one-way operation, simply connect the L (direct) side of the power supply to the terminal closest to the red wire (clockwise) or white wire (counterclockwise) to start the rotation. 

Tip: The two closest stations are inland. For bidirectional operation, use a single-pole double throw switch (SPDT) between the direct line and the directional switching capacitor terminals.

However, to switch the direction of the induction motor, you must wait until the motor stops completely. You can change direction instantly using a reverse drive.

for more info on different types of capacitors visit Amber Capacitors.