Symptoms of a Bad Capacitor on an Electric Motor [Why capacitors fail]

Capacitor failure is the most common problem encountered with AC single-phase motors. Every time I get a breakdown call from a client and they tell me their motor won’t start, I smile because I already know it’s going to be a simple fix. But before buying new parts you should be sure of the symptoms to verify it’s actually the capacitor and not something else at fault.

Symptoms of a bad capacitor on a motor include the motor laboring or failing to start completely, reduced motor efficiency, overheating, signs of physical damage to the body of the capacitor, an electrolytic burning smell, a loud humming noise when trying to start or nuisance tripping of breakers.

As you can see, the symptoms for bad motor capacitors can vary quite a bit depending on motor type and application, but after reading this article you will have the knowledge, confidence, and ability to diagnose and repair a bad capacitor yourself with minimal tools and without the expense of hiring an electrician.

blown capacitor

Bad AC Capacitor Symptoms

Electric motors fail to start

Capacitor start motors require the use of a start capacitor to give the motor the initial boost of current it needs to generate sufficient torque to begin rotation. When a capacitor is faulty, it becomes unable to hold the electrical charge needed to provide the necessary energy required to start the motor.

motor maintenance pdf

This can manifest as the motor laboring or coming under severe strain or pressure whilst trying to start or it may not start at all, especially if there is a load on the motor.

This lack of starting torque caused by a bad capacitor can cause all types of electrical problems in an AC induction motor, because if the capacitor is not doing its job by providing more energy to produce the torque needed, the motor will instead try to compensate by drawing more current from the supply to try to produce the extra torque.

The problem with this is that if the motor draws more current than the windings are rated for they will soon overheat and either burn out causing an open circuit, or the insulation will be damaged resulting in a short circuit, either way, the motor will need to be replaced or rewound.

Motor plate, rated for 4.42 Amps

A simple way of troubleshooting a bad start capacitor is by looking at the motor shaft while you energize the motor, if the start capacitor is faulty you will notice the shaft slightly rocking back and forth whilst making a droning sound like it is trying to start. If this sounds like your motor, it’s likely your start capacitor is bad and will need to be replaced.

Ruptured or leaking AC capacitor

A highly common and very noticeable symptom of a bad AC capacitor is physical signs of bulging or leaking dielectric material from the housing of the capacitor. Over time, the internal dielectric material can deteriorate or degrade, causing pressure to build up within the capacitor. This pressure can cause the capacitor casing to bulge or result in visible leakage of dielectric material, indicating internal damage.

ruptured capacitor
Leaking capacitor

Capacitors can leak for many reasons depending on the application of the motor and the working environment such as the ambient temperature etc… when capacitors are exposed to high temperatures for a prolonged period of time the housing becomes soft, allowing the internals to expand causing the capacitor to rupture and leak.

Another common reason AC capacitors can burst is due to overvoltage or transient voltages, when this happens the capacitor is subject to voltage spikes and overheating which exceeds the capacitors voltage rating causing internal stress and failure of the unit if exposed to these conditions for a prolonged duration.

Reduced AC electric motor performance

A failing capacitor can lead to reduced motor performance. Run capacitors are used to improve the motors power factor by reducing the inductive effect produced by the windings.

As the capacitor deteriorates, its capacitance decreases, which negatively affects the motor’s efficiency and power output. This will result in decreased torque output under load, slower rotation, or reduced overall motor performance. Another significant effect this will have is the cost of your utility bills.

If your running motors at a poor power factor, with the current lagging the voltage by much more than 0.78, you will be using far more electrical energy than is actually needed this will result in much higher utility bills, plus energy suppliers place a tariff on premises that have a poor power factor for the extra load it puts on the grid.

So if you suspect your motors aren’t running as efficiently as possible or you notice a sudden rise in utility bills it might be worth having your capacitors checked out.

Overheating AC motor

A bad capacitor can contribute to the motor overheating. This occurs when the capacitor’s internal components degrade or lose their ability to properly store and distribute electrical charge as needed.

When this happens, the motor will try to compensate for the loss in torque by drawing more current than necessary or experience power factor issues, leading to excessive heat generation and wasted power.

If this issue is not addressed quickly, prolonged overheating can cause damage to the motor windings or other components leading o the motor needing to be rewound or replaced which would cost a lot more than replacing a capacitor.

Excessive noise

When a capacitor malfunctions, fluctuating electrical currents can flow through the motor. These fluctuations can cause audible noise either while starting or during motor operation, the noise can sound like a droning, humming, or buzzing sound.

The motor may also vibrate more than normal if the run capacitor is bad, this is due to the motor largely running on momentum, especially if it’s a capacitor start capacitor run motor. If the motor continues to vibrate excessively for a prolonged period, eventually the frame the motor is mounted on can crack and break resulting in catastrophic failure of the machine.

So if you notice your motor is vibrating more than usual you should isolate the motor and test the capacitors using a multimeter set to μF or a capacitance meter, Fluke products are the most accurate meters on the market if you’re working on sensitive equipment or units such as HVAC or air conditioning to guarantee accurate readings.

Capacitor electrolyte smell

A pungent or burnt smell may be present in severe cases of capacitor failure. This odor can result from electrolyte substances inside the capacitor being released due to internal damage or component breakdown. The smell serves as a clear sign of a bad capacitor and significant capacitor malfunction, upon closer inspection, you may find a melted capacitor is the culprit.

Blown fuses, tripping of breakers, and overload relays

A malfunctioning capacitor can cause electrical issues, such as blown fuses, tripped breakers or constantly needing to reset the overload relay (O.L.R). The failure of the capacitor can create imbalances in the electrical system, leading to excessive current draw or other irregularities as mentioned above.

motor overload relay
Motor overload relay (O.L.R)

These electrical anomalies will trigger protective devices resulting in blown fuses or tripped circuit breakers to prevent further damage to the motor or equipment.

If this sounds like your motor you should isolate the motor and carry out an insulation resistance test between the windings and the windings and earth to verify there is no continuity between them when voltage is applied.

Check out this article to find out exactly how many ohms (Ω) of resistance you should have between phases and ground.

Three important factors to be considered when replacing a faulty capacitor

When replacing a faulty capacitor there are two important factors to take into consideration before buying a new one.

Capacitance rating

The capacitance rating of the replacement capacitor should match or closely match the capacitance value of the faulty capacitor. Capacitance is measured in microfarads (μF) and represents the ability of the capacitor to store and release electrical energy.

Selecting a replacement capacitor with a similar capacitance ensures that the motor operates within its designed specifications and maintains proper performance. If you find that your capacitor has melted and the label is unreadable you will need to first tell the difference between the start and run capacitor and then it’s simple to determine an approximate size capacitor to suit.

Capacitors voltage rating

The voltage rating of the replacement capacitor should be equal to or higher than the voltage rating of the faulty capacitor. The voltage rating indicates the maximum voltage the capacitor can safely withstand without risking insulation breakdown or failure.

Choosing a replacement capacitor with an adequate voltage rating ensures that it can handle the voltage levels present in the motor circuit without causing damage or compromising safety.

Two AC capacitors rated 50 volts, each wired in parallel, can maintain a supply voltage of 100 volts.

It is crucial to select a replacement capacitor that matches both the capacitance and voltage ratings to ensure compatibility and optimal motor performance.

Additionally, it is recommended to consult the motor’s documentation, and manufacturer guidelines to ensure you choose the correct size for your specific motor.

Polarity of the capacitor

AC capacitors come both polarised and non-polarised. This means the polarised capacitor must be connected with the phase (live) conductor connected to a specific terminal on the capacitor and likewise with the neutral having a specific terminal. It is crucial that these connections are not mixed up, if they are, the capacitor will short-circuit and fail instantly upon energization.

A non-polarised capacitor however can be connected either way and it will function correctly.

Luckily AC motor doesn’t mind if you use a polarised or non-polarised capacitor, so when you’re buying a new one just ask for a non-polarised to avoid any mistakes. You can tell by reading the label on the side of the capacitor which one it is, if it’s polarised it will have a + – symbol.

Conclusion

So as you can see, the list of bad AC capacitor symptoms is a long one, but luckily everything is easily accessible for investigation and diagnosis. By following the steps listed above you will be easily able to diagnose a bad motor capacitor.

Whether its an AC induction motor, air conditioning unit or HVAC system your working on, all motors work in the same way, and by first identifying the type of motor you have and then getting a good understanding of how motors work you will be able to diagnose any issue you may have now or in the future.

Gavin

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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