Will an Electric Motor Run With a Bad Capacitor [Motors explained]

Electric motors are incredible machines and are used for many many applications across the globe, but unfortunately when it comes to single phase motors, most of them share a common problem, failing capacitors. Recently a student asked me can an electric motor run with a bad run capacitor?

An electric motor will run with a bad capacitor. The run capacitor’s purpose is to maximize motor efficiency and improve power factor by reducing the inductive reactance XL caused by the run winding. In the case of a capacitor start run (CSR) motor, manual spinning the motor can initiate operation.

Before diagnosing a problem with a motor the first thing to do is figure out exactly which type of motor it is your working with. If your unsure of the type of motor you have, I would suggest taking a look at this article covering everything you need to know about the different types of motors and how each one works.

Single phase AC electric motors are generally relatively trouble free with the exception of one component, the capacitors. Unfortunately single phase motors are notorious for blowing capacitors and causing all types of problems, with the main problem being, the motor will struggle or fail to start all together.

motor maintenance pdf
single phase ac induction motor capacitor start CSR

Luckily if this happens their is another option to get the motor running again. To get a motor running you will need to manually spin the motor pulley by hand to initiate rotation of the rotor. Once the motor begins to run, it will continue to run, due to the run winding producing enough magnetic flux to interact with the rotor and create the torque needed to drive the motor.

Most single phase electric motors will come with either one or two capacitors depending on the design of the motor. One capacitor is called the start capacitor and the other is called the run capacitor.

In the case of capacitor start run (CSR) motor the same capacitor is used for both starting and running, if this capacitor fails it may cause starting problems or could shut off the motor while running. If this happens, the motor will still run but can experience spikes in voltage and will need to be started manually.

Start capacitors purpose in a motor

The purpose of a start capacitor in an electric motor is to provide an extra boost of power during motor startup. Start capacitors are used in certain types of single-phase motors, such as capacitor start induction motors. They are connected in series with the motor’s start winding and are responsible for creating an initial phase shift in the motor’s magnetic field.

When voltage is applied the current will then charge the capacitor, now the capacitor will store this electrical energy. During startup, the capacitor then releases this stored energy, which helps generate an additional rotating magnetic field. This phase shift and the resulting increased torque helps to assist the motor in overcoming the initial inertia and enable the motor to start smoothly and efficiently.

Once the motor reaches 75-80% of its full load speed, the centrifugal switch disconnects the start capacitor from the circuit. This ensures that the start capacitor is no longer in operation during normal motor running. Instead, the motor relies on its run capacitor (if equipped) or the inherent characteristics of the winding configuration for continued operation.

Function of a motors run capacitors

The run capacitor on the other hand is used to improve the overall performance, efficiency, and power factor of the motor during operation. Run capacitors are typically used in single-phase induction motors, such as capacitor start capacitor run (CSCR) motors.

The primary function of a run capacitor is to reduce the inductive reactance XL caused by the motor’s run winding. By adding increasing the capacitance in parallel with the winding, the run capacitor helps counterbalance the inductive effect, leading to a more balanced current flow and improved power factor. This results in increased motor efficiency and reduced energy consumption.

The run capacitor also assists in maintaining a steady and consistent torque output throughout the motor’s operation. It also helps stabilize the motor’s speed and prevents fluctuations or variations in performance, especially under varying load conditions. This does an excellent job at improving the motor’s reliability and performance, particularly in applications where constant speed and smooth operation are crucial.

Unlike the start capacitor, the run capacitor remains connected to the motor circuit throughout its operation. It continuously provides the necessary capacitance to optimize motor performance, power factor, and overall efficiency.

So if you have a motor that fails start up and run as normal, you first need to determine which exact motor you have. From there, you can carry out some basic fault finding by identifying which is the run capacitor and which is the start capacitor, then you can test each one with a multimeter to determine if either of the capacitors are blown or faulty.

Signs of a faulty run capacitor

Diagnosing a blown capacitor is a relatively easy job, and in most cases it will be blatantly obvious weather of not the capacitor is blown or not because in most cases the capacitor will rupture and leak dielectric material. Below is a checklist to help diagnose a bad capacitor.

bad ruptured blown capacitor
Ruptured capacitor
  • Motor Fails to Start: A faulty start capacitor can prevent the motor from starting or cause it to struggle during startup. If you hear a humming sound coming from the motor but it doesn’t start running, a bad start capacitor is likely the culprit.
  • Irregular Motor Operation: A failing run capacitor can lead to irregular motor operation. The motor may start and stop intermittently, exhibit inconsistent speed, or experience sudden changes in performance.
  • Overheating: A bad start capacitor can cause the motor to overheat. Excessive heat may be felt around the motor or even emitted as a burning smell. Overheating can damage the motor’s components and lead to premature failure if not addressed promptly.
  • Excessive Motor Noise: Capacitor issues can contribute to abnormal motor noises. Buzzing, humming, or grinding sounds may indicate a problem with the capacitor, causing motor vibrations and audible disturbances.
  • High Current Draw: A failing start capacitor can result in increased current draw by the motor. This higher electrical load can be detected by monitoring the motor’s current readings, which may exceed normal operating levels.
  • Bulging or Leaking Capacitor: In some cases, a visibly bulging or leaking capacitor is a clear sign of failure. Capacitors can become physically damaged or deteriorate over time, leading to noticeable deformities or fluid leakage.

If you notice any of these signs in your motor’s operation, it is advisable to have a professional technician or electrician inspect the capacitor and motor system. They can accurately diagnose the issue and recommend appropriate repairs or replacements to ensure the motor’s optimal performance and longevity.

How to test an electric capacitor

In order to diagnose if a capacitor is bad or not, you must first visually inspect the capacitor to check for signs of burning, overheating or damage. Usually when a capacitor fails it will rupture and leak, making it pretty obvious the capacitor is bad.

If you don’t notice any visible damage the next thing to do is remove the capacitor for the motor, paying careful attention to the terminals you are removing the wires from, it is crucial the capacitor gets wired up in the same way as it was taken apart.

Usually the only tools you will need to remove the capacitor is a small flat terminal screwdriver.

Once the capacitor is out, you then need to check the capacitance rating of the capacitor, this is usually printed on the side of the capacitor and should look like the one in the image below.

capacitor rating plate

As you can see from the picture, the capacitance rating of this capacitor is 8 micro farads + or – 10%. Now all you need to do is set your capacitance meter or multimeter to the correct setting, in this case it will be set in the range of 20 micros farads due to it being a 8 micro farad capacitor.

The meter should give you a reading of 8 micro farads plus or minus 10%, if you get a reading outside of these parameters you know that the capacitor is blown and needs to be replaced.

testing a bad capacitor

The capacitor being tested in the picture above is a 15 micro farad capacitor but as you can see, it is clearly ruptured and only giving a reading of 0.03 micro farads meaning the motor will no longer start with this start capacitor installed.

Will a bad capacitor trip a breaker

A bad capacitor in a motor has the potential to trip a breaker, although it may not always be the direct cause. When a motor starts up, it typically draws a higher starting current to overcome the initial inertia and start rotating. The start capacitor assists in providing the additional torque required during startup.

However, a faulty start capacitor will lead to irregular motor operation or excessive current draw. If the motor encounters difficulties starting, it may continue to draw high currents for an extended period. This increased current flow can overcome the capacity of the circuit breaker, causing it to trip and interrupt the electrical supply to the motor.

overload relay protective device
Overload relay

It’s important to note that a tripped breaker may also be triggered by other factors, such as a short circuit, overload, or faults in other components of the motor system. Therefore, while a bad capacitor can contribute to excessive current draw and potentially trip a breaker, it is not the sole cause for breaker tripping.

miniature circuit breaker MCB
Miniature circuit breaker (MCB)

If you experience frequent breaker trips in relation to a motor, it is advisable to have a qualified electrician or technician inspect the entire motor system, including the capacitor, for any faults or issues. Only then can you accurately diagnose the problem and recommend appropriate repairs or replacements to ensure safe and reliable motor operation.

Will a bad RUN capacitor trip a motors breaker

A bad run capacitor in a motor is less likely to directly trip a breaker. Unlike a start capacitor, which is primarily involved in the motor’s startup process, a run capacitor will stay continuously connected to the motor circuit during operation to improve performance and power factor.

However, a failing run capacitor can indirectly contribute to issues that may lead to a breaker trip. Here’s how:

  • Overheating: A deteriorating run capacitor can cause the motor to operate inefficiently, leading to excessive heat generation. If the motor overheats, the temperature rise can trip the motor’s internal thermal protection or the circuit breaker that protects the motor.
  • Increased Current Draw: A faulty run capacitor can disrupt the balance of the motor’s current flow, potentially leading to higher current draw from the power supply. This increased current can exceed the capacity of the circuit breaker and cause it to trip. However, it’s important to note that the direct cause of the increased current is usually associated with other motor issues, such as bearing wear or winding faults, rather than solely the run capacitor.

While a bad run capacitor may not be the primary reason for a breaker trip, it is possible for the run capacitor’s deteriorating condition to contribute indirectly to conditions that can cause a breaker to trip.

Therefore, if you suspect a problem with the run capacitor or experience frequent breaker trips in relation to the motor, it is advisable to have a qualified electrical technician inspect the entire motor system to identify and address any underlying issues that may be causing the breaker trips.

Will a single phase motor hum if the start capacitor is bad?

A motor may produce a humming sound if the start capacitor is bad or malfunctioning.

When the motor is energized, but the capacitor fails to function correctly, it can result in the motor being unable to initiate rotation. In such cases, the motor may emit a continuous humming noise, indicating that it is attempting to start but lacks the necessary torque to overcome the initial resistance, to get a better understanding of this you should check out this article on the basics of how a motor works.

The humming sound is a result of the motor experiencing high magnetic forces without the ability to achieve sufficient rotational movement. It is important to note that while a humming sound is commonly associated with a bad capacitor, it can also be caused by other factors such as mechanical issues, voltage problems, or motor winding faults.

If you hear a persistent humming noise coming from the motor, it is advisable to have a professional electrical technician inspect the motor and its components, including the capacitor. They can diagnose the cause of the humming sound and determine if the capacitor requires replacement or if there are other underlying issues affecting the motor’s performance.

Alternatively you can use the steps provided above to diagnose and test the capacitor yourself.


In conclusion, a motor may struggle to start or fail to run properly with a bad capacitor. The capacitor’s role in providing the necessary phase shift and additional torque during startup is essential. A faulty capacitor can hinder motor performance, leading to issues such as motor humming or failure to initiate rotation.

Prompt replacement is crucial for optimal motor operation, luckily the regular price of a new motor capacitor is only about $15 in most electrical stores and you can replace it yourself at home in ten minutes by just removing the wires from the terminals.


I'm Gavin and Iv been teaching electrical science to apprentice electricians in a local technological university since 2022. I hold an Electrical Level 6 QQI Qualification along with several NZEB Certifications.

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