Film capacitor production process

The performance of capacitor standards from batch-by-batch inspection to periodic test are all four major parameters, namely capacitance, loss tangent value (hereinafter referred to as loss), insulation resistance and withstand voltage. The manufacturing process of capacitors is also closely focused on ensuring that the four parameters meet the requirements.

 

Film capacitor structure:

 

1‐Nut;

2‐Bottom cover;

3‐Al case;

4‐Label;

5‐Element;

6‐Conductor plate;

7-Core;

8‐Insolated plasTIc cover;

9‐Electrode;

10‐PlasTIc cover;

 

The influence of the manufacturing process of the metallized film capacitor on the four parameters:

Process factors that lead to deviations in capacitance during the manufacturing process:

The capacitance of the wound capacitor C = 0.177ε s / d

ε is the dielectric constant of the medium;

s is the effective area of the plate;

d is the thickness of the medium;

The capacitance is proportional to ε and s, and inversely proportional to d.

 

Although precise regulations have been made, these three parameters will change during the process, resulting in capacity deviations. The focus of manufacturing is to reduce these deviations and improve the capacity hit rate.

 

a. Winding process

There are errors in the width, thickness or margins of the film;

The change of the tension of the film from the large circle to the small circle, the tension error of each winding machine;

The pressure of the roller is too small. Deviation during winding process, wrong edge error;

When the air humidity is high, the core capacity is too large;

 

b. Hot pressing process

The thickness error of the core is not uniformly stressed, resulting in uneven core tightness and capacity dispersion;

The hot pressing plate is uneven;

Temperature error

 

c. Heat treatment time or temperature error

 

d. Impregnation

Vacuum degree error;

Time error

Error of curing temperature;

 

Process introduction of film capacitors:

 

Winding technology:

In the traditional production process, metallized plastic film capacitors are made by winding a pair of films on a plastic core rod and then wrapping the capacitor with an insulating sleeve.

 

 

Terminology:

1. Nominal capacity CN: The designated capacity value usually marked on the capacitor.

2. Rated voltage UN, and ripple voltage Ur (as shown below).

 

 

Rated DC voltage UNDC: The highest operating peak voltage that can be operated continuously in either polarity of the non-repetitive waveform used in the design of capacitors.

 

3. Aperiodic surge voltage Us: The peak voltage caused by switching or any other disturbance in the system. This voltage is only allowed to appear a limited number of times and its duration is shorter than the basic period.

 

4. Insulation resistance Rins: The insulation resistance is the ratio of the DC voltage applied after the capacitor is charged for one minute to the value of the leakage current flowing through the capacitor.

 

5. Insulation voltage Ui: The root mean square value of the sine wave voltage designed for the insulation between the capacitor terminal and the shell or the ground.

 

6. Maximum peak current I: the maximum peak current that can be repeated during continuous operation.

 

7. Maximum current Imax: the root mean square value of the maximum current during continuous operation.

 

8. Maximum surge current Is: The non-repeatable peak current caused by switching or any other disturbances in the system. This current is only allowed to appear a limited number of times and its duration is shorter than the basic period.

 

9. Series resistance Rs: Under specified operating conditions, the effective ohmic resistance of the conductor of the capacitor.

 

10. Equivalent series resistance Resr: an effective resistance, when it is connected in series with an ideal capacitor with the same capacitance value of the capacitor under discussion, under the specified operating conditions, the power loss of the resistor will be equal to the dissipation in the capacitor Active power.

 

11. Capacitor loss tangent tan δ: The ratio between the equivalent series resistance of the capacitor and its capacitive reactance under the specified sinusoidal AC voltage, frequency and temperature.

 

12. Capacitor loss: the active power consumed in the capacitor.

 

13. Maximum power loss Pmax: The maximum power loss that the capacitor can operate at the highest case temperature.

 

14. Shell temperature rise Δθcase: the difference between the temperature of the hottest spot of the shell and the temperature of the cooling air.

 

15. Conditions of steady state: the thermal balance reached by the capacitor under constant output and constant cooling air temperature.

 

16. Hot spot temperature θhs: The temperature of the area where the highest temperature appears in the capacitor.

 

17. Thermal resistance Rth: Thermal resistance is expressed by the degree that the hot spot temperature of the capacitor rises from the ambient temperature per watt of heat loss.

 

Process points to improve the capacity hit rate:

1. To accurately determine the central value of the winding capacity (also called the correction value), the change rate of the process capacity of hot (cold) pressing, heat treatment and encapsulation must be included in the core capacity correction value. Different models, different specifications and even different winding machines have different correction values;

2. The capacity and height of the core are regularly measured during the winding process to control the discreteness of the capacitance;

3. After sampling the core capacity after flattening and shaping, if the deviation is found, adjust the winding center value in time.

4. Tracking the distribution status of finished product capacity, and timely feedback when found out of tolerance to adjust the capacity correction value;

 

The law of volume change in finished product storage and corresponding measures:

 

If the product is tested for a short period of time after being encapsulated, the capacity of the product will change after being stored for a period of time, causing the capacity to be out of tolerance. Polyester capacitors are more obvious and generally shift in the positive direction.

There are several solutions:

1. After encapsulation, the product undergoes another heat treatment.

2. After encapsulation, store it naturally for one week before testing.

3. Control the deviation value during the test according to the change law, such as the polyester capacitor is clamped in the positive deviation, and the negative deviation is relaxed.

 

Process flow chart of film capacitor production: