A Complete Guide To Electric Motor Capacitor

A Complete Guide To Electric Motor Capacitor

Do you know that different electric motors require capacitors to increase their efficiency and performance? Yes, if your motor does not provide the desired outcome, you may need a capacitor to increase its efficiency.

Electric motor capacitors boost the motor’s performance and help it start and run smoothly. They maximize the performance of electric motors which cannot generate high-output torque.


How do I choose a suitable capacitor without getting hoodwinked? 

Don’t worry. Let’s get over all the necessary information you need to acquire about the electric motor capacitor and it’s working. 

What is an electric motor capacitor?

A motor capacitor is an energy-storing device that stores energy in an electric field. The primary purpose of a capacitor in an electric motor is to generate a rotating magnetic field. 

Figure 1 Electric Motor Capacitor

Electric Motor with Capacitors

Let’s take battery as a potential example here. 

A battery stores energy that can be utilized when required to increase performance. A capacitor works the same way. 

However, a capacitor releases energy in a fraction of a second and is more economically viable than a battery.

A capacitor stores and releases electrical energy when the motor runs slower than the expected speed to boost the current and increase the output torque.

Does the capacitor release stored energy fast?

It releases energy at sixty times per second without causing any overheating or internal damage.

Wait a while!

How do I know if my electric motor needs a capacitor?

Many types of electric motors come with built-in capacitors following their sizing and design. 

However, your motor needs a suitable capacitor if you experience lower output torque, overheating, humming, or vibration. 

Related Guide: Single-Phase Motor With Capacitor

How does an electric motor capacitor work?

The working mechanism of electric motor capacitors is simple and well-structured. Here is how a capacitor works when connected in series with the auxiliary winding of an electric motor;

Firstly, the capacitors draw power from the source (battery) and store it on metallic conductors, mainly copper, silver, aluminum, and brass. 


Capacitors use a dielectric medium (insulator) between metal capacitors to increase their capacitance and preserve potential difference. Glass, ceramic, Mica, and paper are common dielectric materials used in capacitors.

A capacitor creates a rotating magnetic field and initial torque that the motor cannot produce to start or run. 

#Thing to Consider:

Always examine the capacitor’s sizing and tolerance before choosing it. We recommend consulting a technician to understand your motor requirements to find the correct sizing. 

Types of electric motor capacitors:

You must wonder if all electric motor capacitors are the same, right?

No, that’s completely wrong.

Electric motor capacitors are specified on five parameters such as; sizing, capacitance (μF), voltage rating (V), frequency (Hz), and ambient temperature (T).

Here are three common types of capacitors widely used in the industry for different applications. Let’s have a look.

Start Capacitor:

Figure 2 Starting Capacitors

Figure 2: Starting capacitors

A start capacitor is connected in series with auxiliary winding, which helps the motor to generate initial torque. The capacitance of start capacitors (Cs) ranges from 30μF to 370μF. Once the motor reaches the optimum speed required for starting a motor (75% speed), it is then drawn out with the help of a centrifugal switch.

A start capacitor is ideal for increasing the starting rotations of the non-self-starting motor because its voltage ratings are 125V, 250V, and 330V.

Run Capacitor:

Figure 3 Electric Motor Run Capacitors

Figure 3: Electric motor run capacitors

Run Capacitors are also called continuous-duty capacitors, which energize the secondary coil during the entire run time. The capacitance range of these capacitors is about 3 to 70 microfarad (uF). 

Almost all single-phase electric motors have a running capacitor installed to increase efficiency and operational performance. Like start capacitors, these motors also have dielectric to sustain the stored potential energy.

The voltage rating of these capacitors ranges from 370V and 440V.

What if I install the bad run capacitor?

No, that’s what we need to avoid. 

If you install the wrong capacitor, the motor will malfunction and become overheated. It will cause high noise and hesitation, which ultimately declines the performance. 

So, you must pay attention to sizing before choosing a capacitor.

If you are not sure where to get your next electric motor, read our latest guide: Finding The Best Chinese Electric Motor Service Provider In 2023


If your motor is not operating at high speed or generating enough torque, a capacitor can become a life savior. It not only increases efficiency but also reduces the energy consumption of an electric motor. 

Xinnuo Motor is an acclaimed electric motor manufacturer that delivers high-quality single-phase, three-phase, and OEM motors. The company provides free quotations and 100% customization services. 

For more information, feel free to contact us. We are available 24/7 to assist you.

Frequently asked questions:

How to test a capacitor?

You can test a capacitor with a voltmeter. Test the capacitor by charging on the maximum voltage a capacitor can tolerate and discharging to 0V. 

If the capacitor fails to retain that voltage range, it might have a defect.

What happens to the motor if the capacitor fails?

It can cause severe overheating or extreme noise. The motor fails to produce even a magnetic field if the capacitor fails. It can also start consuming high energy in case of failure. 

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