[BLOG] Improvement of electrical characteristics and mechanical aging mechanism of the motor capacitors in plastic boxes

Wednesday, 13 March 2019

Motor run and motor start capacitors are designed for obtaining an auxiliary phase of single-phase asynchronous motors and in PCB-s for industrial electronics, where capacitors are subjected with lower impulse loads (DU/Dt):

Typical applications: 

• Pumps
• Circular saws
• Compressors
• Fans

Figure 1: Design

Construction of capacitors is made of metalized polypropylene film with self-healing features. Capacitors are designed for a rated voltage of 250 VAC to 460 VAC and for different lifetime classes (class A, B, C, D). Well-known Paschen minimum voltage for initial partial discharges starts at around 300 VAC. The highest density of electrical field in the capacitor is at the metallization edges, that causes partial discharges at lover voltages (200 – 300 VAC). After the winding and thermal treating, the capacitors usually have different size of gaps on both ends coated with metal-sprayed zinc layer. 

When the capacitor is charged with a voltage higher than 300 VAC, the injuries of dielectric could lead to partial discharges. Those damages lids to dielectric breakdown, following by self-healing process which consequently vaporizing metalized layer, clean and isolate it around breakdown spot. Each of those isolated breakdown spots lead to a small reduction of electrode surface and capacitance.

 
Figure 2: Self-healing

Therefore, at the dry capacitors, corona starts in the air gaps at the metallization edges and evaporates the metallization from outside towards the inside of the metalized film. 

Capacitors, which are designed for higher Lifetime, class (cl. A) number of clearings (self-healing process) slowly increases because of partial discharge damages and meanwhile gas creation, which increases air gaps. Therefore, at the dry capacitors, corona starts in the air gaps on the metallization edges and evaporates the metallization from outside towards the inside of the capacitor. 

Figure 3: Clearing and corrosion

Capacitor exposing for a long period on humidity and temperature increases mentioned damages, because of reducing dielectric rigidity of the gas (gas capability to withstand ionization).

Capacitance loses occur mainly because of decreasing electrode effective surface at the following mechanisms: electrode corrosion and contact lost between the metalized layer and metal spraying surface. 

To improve electrical properties and aging mechanisms partial impregnation of capacitor windings is highly important. Partial impregnation penetrates to outer turns and electrode margins, hence significant reduce damages on metallization edges and electrodes, reduces size and frequency of dielectric damages because of partial discharges and protect the outer surface of windings.
 
Mentioned conclusions have been proved in laboratory and information from the market.

CONCLUSION

Partial impregnation is very important because it fills air gaps between side parts layers (turns) of winding element and electrode margins, which prevent damages because of partial discharges and subsequent loss of capacitance caused by the frequency of clearing i.e. self-healing process.
Very important aging mechanisms are dielectric short circuit, voltage, time, temperature and moisture.

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