Motor Capacitor FAQ

Overview

Voltage

A capacitor will have a marked voltage indicating its acceptable peak voltage, not its operational voltage. Therefore, you can select a capacitor with a voltage rating at or above the original capacitor. If you're using a 370 volt capacitor, a 370 or 440 volt one will work, though the 440 volt unit will actually last longer. However, you cannot replace a 440 volt capacitor with a 370 volt one without dramatically reducing its lifespan.

Capacitance

Select a capacitor with a capacitance value (given in MFD, uf or microfarad) that is equal to the original capacitor. Do not deviate from the original value, as it sets the operational characteristics of the motor.

Frequency (Hz)

Select a capacitor with the Hz rating of the original. Nearly all capacitors will be labeled 50/60.

Connection Terminal Style

Nearly every capacitor will use a ¼" flag style push-on connector. When replacing a capacitor, you will need to know how many terminals per terminal post are needed for your motor. Most start capacitors have two terminals per post, and most run capacitors will have either 3 or 4 terminals per post. Verify that the replacement has at least the number of connection terminals per connection post as the original motor capacitor.

Case Shape (Round vs. Oval)

Nearly all start capacitors have a round case. Round cases are by far the most common, but many motors still use oval designs. Electrically speaking, there is no difference. If space in the mounting box is not limited, the case style does not matter.

Case Size

Just like case shape, overall size makes no difference electrically. Select a capacitor that will fit within the space provided.

Start vs. Run Capacitors

Start capacitors give a large capacitance value necessary for motor starting for a very short period of time (usually seconds long). They are only intermittent duty and will fail catastrophically if energized too long. Run capacitors are used for continuous voltage and current control to a motor's windings and are therefore continuous duty. They are generally of a much lower capacitance value.

In unusual circumstances, a run capacitor could be used as a start capacitor, but the values available are much lower than the values usually available for dedicated start capacitors. The capacitance and voltage ratings would have to match the original start capacitor specification. A start capacitor can never be used as a run capacitor, because it cannot not handle current continuously.

View our video tutorial below to learn more about the differences between start and run capacitors.

Start Capacitors

Applications

Start capacitors are used to briefly shift phase start windings in single phase electric motors to create an increase in torque. They possess very large capacitance values for their size and voltage rating. As a result, they are only intended for intermittent duty. For this reason, start capacitors will fail after being left energized for too long due to a faulty starting circuit on a motor.

Specifications

Most start capacitors are rated for 50-1200 uf and 110/125, 165, 220/250, or 330 VAC. They are usually 50/60 Hz rated. Case designs are typically round and cast in black phenolic or Bakelite materials. Terminations are usually ¼" push on terminals with two terminals per connection post.

How do I know if my start capacitor is bad?

Most start capacitor failures are one of two types. Catastrophic failure is usually caused by an electric motor's starting circuit being engaged too long for the intermittent duty rating of a start cap. The top of the start cap has literally been blown off, and the insides have been partially or fully ejected. Similarly, a start cap may just exhibit a ruptured pressure relief blister. In either case, it's easy to tell that the start cap is in need of replacement.

My motor is slow to start. Is my start capacitor bad?

Your start capacitor may have lost its capacitance rating due to wear and age, or you may have other non-capacitor related issues that have to do with other motor components. You will want to measure the capacitance of your start capacitor to find out.

My start capacitor has a resistor on it. Do I need a replacement capacitor with one?

Most replacement start caps will not include a resistor. You can check the condition of the old one by checking the resistance value, or just replace it with a new one. This should read somewhere around 10-20k Ohms and around 2 watts. The resistors are usually either soldered or crimped to the terminals. The purpose of the resistor is to bleed off residual voltage in the capacitor after it has been disconnected from the circuit after motor start up. Not all start capacitors will use one, as there are other ways to accomplish this. The important part is if your original capacitor had one, you'll need to replace it on the new capacitor.

Can I use a higher voltage rated capacitor than the original?

Yes. Click here for more detail.

Run Capacitors

Applications

Run capacitors are used to continuously adjust current or phase shift to a motor's windings in an effort to optimise the motor's torque and efficiency performance. They are designed for continuous duty, and as a result, have a much lower failure rate than start capacitors. They are commonly used in HVAC units.

Specifications

Most run caps are rated for 2.5-100 uf (microfarads) with voltage ratings of 370 or 440 VAC. They are usually 50/60 Hz rated. Case designs are round or oval, most commonly using either a steel or aluminum shell and cap. Terminations are usually ¼" push on terminals with 2-4 terminals per connection post.

When to Replace

As a general rule of thumb, a run capacitor will far outlast the same motor's start capacitor. A run cap will also fail or wear differently than a start cap, making troubleshooting slightly more involved.

When a run capacitor begins to perform outside the allowable range, it is most often indicated by a dropping of the rated capacitance value (the microfarad value has gone down). For most standard motors, a run capacitor will have a "tolerance" specified describing how close to the rated capacitance value that the actual value may be. This will be usually +/- 5 to 10%. For most motors, as long as the actual value is is within the 10% mark of the rated value, you're in good shape. If it drops outside of this range, you'll need to replace it.

In some cases, due to a defect in a capacitor's construction or sometimes caused by a non-capacitor related motor issue, a run capacitor will bulge from internal pressure. For most modern run capacitor designs, this will open the circuit, disconnecting the internal spiral membrane as a protective measure to prevent the capacitor from popping open.

If its bulging, time to replace. If you measure no continuity across the terminals, it is also time to replace.

Why did my run capacitor fail?

Below are some common reasons that run capacitors fail, but depending on how close the run capacitor is to its design life, it may be difficult to pinpoint the reason on a single factor.

Time - All capacitors have a design life. Several factors may be interchanged or combined to increase or reduce the life of a run capacitor, but once the design life is exceeded, the internals may begin to more rapidly decay and drop in performance. Simply put, a failure may be attributed to it being "just old."

Heat - Exceeding the design limit of operating temperature can have a big effect on run capacitor life expectancy. In general, motors that are operated in hot environments or with little ventilation will experience a dramatically reduced lifespan on their capacitors. The same can be caused by radiated heat from a generally hot running motor that causes the capacitor to run hot. In general, if you can keep your run capacitor cool, it will last a lot longer.

Current - When a motor is overloaded or has a failure in windings, it causes the current to climb, which can overload capacitors. This scenario is less commonly noticed, as it would usually be accompanied by a partial or complete failure of the motor.

Voltage - Voltage can have an exponential effect in shortening the design life of a capacitor. A run capacitor will have a marked voltage rating that should not be exceeded. For example, a capacitor is rated for 440 volts. At 450 volts, the life may be reduced by 20%. At 460 volts, the life may be reduced by 50%. At 470 volts, there is a 75% life reduction. The same can be applied in reverse to help increase the design life by using a capacitor with a voltage rating significantly higher than needed, though the effect will be less dramatic.

How long should my run capacitor last?

The life of a good quality aftermarket run capacitor (one that didn't come with your motor) would be 30,000 to 60,000 running hours. Factory-installed run capacitors sometimes have a much lower designed lifespan. In highly competitive industries where every part can have a significant impact on cost, or where a motor's intended use would likely be intermittent and infrequent, a lower grade of run capacitor may be selected with a design life of as little as 1000 hours. Additionally, all the factors from the section above ("Why did my run capacitor fail?") may dramatically modify the reasonable expected life of a run capacitor.

Dual Run Capacitors

Dual run capacitors are two run capacitors in one case. They have nothing else that makes them electrically special. They generally have connections marked "C" for "common", "H" or "Herm" for "Hermetic Compressor" and "F" for "Fan." They will also have two different capacitor ratings for the two different parts. You might see 40/5 MFD, meaning that one side is 40 microfarads (measurement of capacitance) and the other side is 5 microfarads. The smaller value will always be connected to the fan. The larger connection will always be connected to the compressor.

If I can't find a replacement for my dual run capacitor, can I use two separate run caps?

The dual run capacitor design's only advantage is that it comes in a small package with only 3 connections. There is no other difference. If there is enough space for mounting, using two separate run capacitors in place of your original dual run capacitor is an acceptable practice.