Why Low Dissipation Factor Matters in Motor Run Capacitors
Objective
This blog helps you understand how a low dissipation factor affects motor run capacitor performance, reduces internal electrical losses and supports reliable operation. We will also explain the role of polypropylene film, insulation resistance and capacitor quality when selecting the right product.
Key Takeaways
Generally, with a reduction in dissipation factor, internal electrical losses reduce.
With a reduction in dielectric losses, the amount of heat generated in a capacitor reduces.
Polypropylene film aids in the construction of low-loss capacitors.
Insulation resistance helps in controlling leakage of current in undesirable paths.
Correct voltage, capacitance and temperature ratings help in reliable operation of the capacitor.
Always check the technical specifications and applicable safety standards before purchasing a product.
Introduction
Motors may rely on several components to operate, but have you thought about the role of a small capacitor in a motor? Have you wondered why some motor capacitors experience more internal heating? One important factor is the dissipation factor.
A low dissipation factor capacitor has lower internal electrical losses under comparable operating conditions. This matters in applications such as water pumps, submersible pumps and heavy-duty fans, where capacitors may operate for extended periods.
Did you know? In May 2025, the Bureau of Indian Standards (BIS) released revised Product Manuals for IS 2993 (Part 1 & 2). These manuals highlight the performance, test, and safety requirements of AC Motor Capacitors.
This shows the importance of checking product specifications and applicable safety standards while selecting a capacitor.
Table of Contents
What Is Dissipation Factor in a Motor Run Capacitor?
Why Does Low Dissipation Factor Matter?
How Does Polypropylene Film Reduce Capacitor Losses?
Why Is Insulation Resistance Important?
Where Are Low-Loss Motor Run Capacitors Used?
How to Choose the Right Motor Run Capacitor?
What Is Dissipation Factor in a Motor Run Capacitor?
The dissipation factor (DF) measures the electrical energy lost inside a capacitor compared with its capacitive behaviour. In simple terms, it tells us how much energy is converted into heat instead of being stored and released through the capacitor's normal operation.
An ideal capacitor would have no internal losses. However, real capacitors contain dielectric materials and other components that cause some energy loss.
Understanding dielectric losses
A capacitor contains two conductive electrodes separated by an insulating material called a dielectric. When AC voltage is applied, the dielectric experiences electrical stress and some energy is lost as heat.
The amount of loss depends on factors such as:
Dielectric material quality.
Operating frequency.
Temperature.
Capacitor construction.
Electrical operating conditions.
Relationship between dissipation factor and ESR
Dissipation factor is also related to Equivalent Series Resistance (ESR).
For a simplified series equivalent circuit:
DF = ESR / Xc
Here, ESR represents equivalent series resistance, while Xc represents capacitive reactance.
A lower DF generally indicates lower electrical losses when the measurement conditions are comparable.
Why Does Low Dissipation Factor Matter in Motor Run Capacitors?
A motor run capacitor must support the motor circuit during normal operation. Its electrical properties matter because the capacitor can remain in service for long periods.
Here are four reasons why a low dissipation factor capacitor is worth considering.
1. Helps reduce internal heat generation
Ideally, the electrical energy lost in a capacitor is 0. However, due to the nature of the dielectric materials used in a capacitor, heat is produced.
For equipment that will be used to run motors for extended periods of time, the motors used should also produce as little heat as possible, so using low-loss capacitors is also beneficial.
2. Supports consistent electrical performance
The capacitor must maintain the required capacitance and operate within its rated voltage and temperature limits.
Lower dielectric losses support better electrical performance, although the correct capacitance and voltage ratings remain equally important.
3. Helps manage thermal stress
Excessive internal heating can place additional stress on dielectric materials, insulation and other capacitor components.
A capacitor with suitable low-loss properties can help limit this source of thermal stress. However, its overall service life also depends on construction quality and actual operating conditions.
4. Supports dependable operation
When selecting a motor run capacitor, we should consider more than its price or physical size.
Its dissipation factor, insulation resistance, voltage rating and temperature limits all help us assess whether it suits the intended application.
How Does Polypropylene Film Help Reduce Capacitor Losses?
Polypropylene film is widely used as a dielectric material in motor run capacitors because of its favourable electrical properties.
It separates the conductive electrodes and helps the capacitor store and release electrical energy during operation.
What is SH-MPP film?
SH-MPP stands for Self-Healing Metallised Polypropylene film.
This construction uses a thin metallised layer on polypropylene film. Under certain local electrical faults, the metallisation around the affected area can clear, helping the capacitor continue operating.
The self-healing feature does not mean that every type of electrical fault can be repaired. Its effectiveness depends on the fault and the capacitor's design.
Why does material quality matter?
The dielectric material and manufacturing process influence:
Dissipation factor.
Electrical insulation.
Thermal performance.
Capacitance stability.
Overall capacitor reliability.
Concap lists high-temperature SH-MPP film, polyurethane resin, low dissipation factor and better heat dissipation among the features of its motor run capacitors.
These product details show why we should review the material and construction rather than selecting a capacitor based only on its MFD rating.
Why Is Insulation Resistance Important?
A high insulation resistance capacitor helps limit unwanted leakage current through its insulating system.
Insulation resistance and dissipation factor measure different electrical properties, so we should not treat them as interchangeable.
Parameter | What it tells us |
Dissipation factor | The level of internal electrical losses |
Insulation resistance | How effectively insulation resists leakage current |
Capacitance | The capacitor's ability to store electrical charge |
Rated voltage | The voltage the capacitor is designed to withstand under specified conditions |
Rated voltage The voltage the capacitor is designed to withstand under specified conditions
Both dissipation factor and insulation resistance are useful when assessing capacitor quality.
Where Are Low-Loss Motor Run Capacitors Used?
Motor run capacitors are used in several types of equipment across domestic, commercial and industrial settings.
Water and submersible pumps
Due to the nature of water pumps, which can be continuously loaded, it is important to select a capacitor with the correct electrical and thermal ratings to ensure the motor is not overloaded.
Heavy-duty fans
In order to have adequate air movement, the capacitors used with heavy-duty fans need to have the correct ratings to match the motor requirements.
Agricultural equipment
Irrigation pumps and other motor-driven equipment need suitable capacitors for their intended duty.
The important point is that we should select a capacitor according to the motor manufacturer's requirements rather than assuming one rating suits every application.
How to Choose the Right Motor Run Capacitor
Before purchasing a capacitor, follow this simple checklist.
Check capacitance: Match the required MFD rating with the motor manufacturer's recommendation.
Verify rated voltage: Choose the correct voltage rating for the application.
Review dissipation factor: Ask the manufacturer for the specified value and test conditions.
Check insulation resistance: Review the specified value and measurement method.
Check dielectric material: Look for suitable polypropylene film construction.
Verify product standards: Check applicable BIS certification, markings and IS 2993 requirements.
You should also review the manufacturer's technical datasheet and operating temperature limits.
For example, when comparing two capacitors for a water pump, do not select one simply because it has a lower quoted price. Compare their capacitance, voltage, dielectric properties, insulation resistance and test documentation.
If you are evaluating suppliers, you can also review the experience and product range of a single-phase motor start capacitor manufacturer before deciding which manufacturer can meet your wider capacitor requirements.
Make Capacitor Quality a Priority
All the features of a capacitor influence its performance in one way or the other. Internal losses affect how much heat is generated and how well the capacitor performs electrically. A low dissipation factor capacitor can reduce dielectric losses. Good insulation resistance and polypropylene film provide better operation.
However, no single specification tells the complete story. We should always consider voltage, capacitance, temperature, construction and applicable safety requirements together.
If you need motor run capacitors for pumps, fans or industrial equipment, review the technical specifications carefully before making a purchase.
Looking for motor run capacitors for your applications?
Explore the motor run capacitor range from Concap and review the available product specifications to identify a suitable option for your requirements.
Frequently Asked Questions
1. What is a low dissipation factor capacitor?
A capacitor with low internal electrical losses is referred to as a low dissipation factor capacitor. It reduces energy converted to heat during operation.
2. Why is dissipation factor important in motor run capacitors?
It indicates the level of internal electrical losses. Lower values indicate less energy lost to heat and better performance.
3. How does polypropylene film improve capacitor performance?
Capacitors utilise polypropylene film as a dielectric to improve performance. The quality and construction of the polypropylene film impact the performance of the capacitor.
4. What is the difference between dissipation factor and insulation resistance?
Dissipation factor measures internal energy losses, while insulation resistance measures how effectively the insulating system resists unwanted electrical leakage.
5. How do we choose a reliable motor run capacitor?
Check its capacitance, rated voltage, dissipation factor, insulation resistance, temperature limits, construction and applicable certifications before purchasing.
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