Motor Capacitor Size Calculator

The motor capacitor size calculator allows you to determine the specific amount of capacitance needed to store a defined amount of electrical energy at a specified voltage.

The appropriate choice of capacitor size is critical in designing or analyzing electrical circuits. A capacitor with insufficient capacitance will fail to meet your application’s energy storage needs, while oversized capacitors increase your total project costs and typically require additional physical space, both of which can negatively impact the performance of an electrical circuit.

A proper understanding of how capacitance and energy relate to voltage will enable you to make informed decisions about the physical properties of capacitors for your project.

In this article, we will start with:

• What is a motor capacitor?

Then you will learn about:

• How the capacitor-sizing calculation works;
• The motor capacitor size formula; and
• Plus considerations in making the proper choice of suitable capacitors.

Motor capacitors are components of motors. They store energy for motor start-up and operation. They create phase differences in the windings, generating a rotating magnetic field that produces torque. Motors without an appropriately sized motor capacitor may fail to start or run efficiently, and may overheat when loaded. Motors, found in air conditioning units, ceiling fans, water pumps, compressors, refrigeration units, etc., use motor capacitors.

Motor capacitors can mainly be of two types:

Start Capacitor

Start capacitors are designed to offer a surge of electrical energy to a motor when it starts. This surge of energy allows the torque to overcome the motor’s inertia and quickly accelerate it to speed. Start capacitors are removed from the electrical circuit after the motor has started because their function is only for a brief period.

Run Capacitor

Run capacitors are designed to be part of the motor throughout its operation. Instead of aiding the motor to start, it allows the motor to run more efficiently and smoothly by maintaining the phase shift of the windings. It helps reduce the motor's power consumption and operating temperature.

Omni’s motor capacitor size calculator is designed to determine the capacitor size based on voltage and startup energy. These terms might sound heavy, but the tool is as straightforward as it can get.

1. First, input the voltage of the motor. The default unit is volts (V), but you can change it from the available list.
2. Next, input the startup energy. The default unit is joules (J); you can change it as well.
3. That’s it. Watch the magic happen as the tool instantly determines the capacitance, measured in microfarads (μF).
4. You can use the tool in any order as well. Just enter any two available values and get the results.

Since we are on the topic of capacitance, we have two simple yet interesting tools that are worth checking out. The capacitors in series calculator and the parallel capacitor calculator.

The formula in our calculator uses the standard energy equation for capacitors:

where:

• $E$ — Startup energy;
• $V$ — Voltage; and
• $C$ — Capacitance.

Rearranging the formula changes it to determine capacitance, which helps you determine the capacitor size based on voltage:

We suggest checking out our capacitor energy calculator. It helps you evaluate the amount of energy stored in a capacitor.

The following table shows some differences between start capacitors vs. run capacitors based on the purpose, operating time, and capacitance.

The ability to properly size motor capacitors has broad applications across residential, commercial, and industrial settings. This sizing is often performed by engineers and technicians when a system is being custom-designed, when a motor capacitor is being replaced, or when a motor is exhibiting performance issues.

The motor capacitors’ sizing can be applied to:

• Air conditioning and HVAC systems;
• Water pumps and irrigation systems;
• Air compressors;
• Fans;
• Refrigerators and other refrigerant systems;
• Conveyor systems;
• Industrial machinery and tools; and
• Commercial ventilation systems.

Sizing motor capacitors has many applications, so let’s look at some tips that can help provide the most useful results from a sizing calculator:

• Use the normal operating voltage of the motor, not the maximum voltage.
• Check to ensure the proper units are being used to avoid conversion errors.
• Most motor capacitors are rated in microfarads (μF), and if the value is calculated in farads, convert to microfarads.
• Check the sizing value against the motor manufacturer’s specifications when selecting a replacement motor capacitor.

We also have a capacitance calculator which you can try out. It is sure to come in handy.