Start Capacitor Selection Guide
A start capacitor is used to briefly shift phase on a start winding in a single phase electric motor to create an increase in torque. Start capacitors possess a very large capacitance value for their size and voltage rating. As a result, they are only intended for intermittent duty. Because of this, start capacitors will fail after being left energized for too long due to a faulty starting circuit on a motor.
Start vs. Run Capacitors
Start capacitors give a large capacitance value necessary for motor starting for a very short (seconds long) period of time. 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.
Are start and run capacitors interchangeable?
Yes and no. 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 startcapacitor can not ever be used as a run capacitor, because it could not handle current continuously (only a couple of seconds).
Watch the video tutorial below to learn the differences between start and run capacitors.
What is a resistor and do I need one?
Most replacement start capacitors 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.
Learn how to install a bleed down resistor on your start cap.
How do I know if my start capacitor is bad?
Most electric start motor capacitor failures are one of two types:
"The Start Cap blew its guts out!" This is what we call catastrophic failure. It 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.
Ruptured Pressure Relief Blister Similarly but not quite as dramatic, 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?
The answer to this question is maybe. 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.
Watch the video below on how to replace a start capacitor.
Most start capacitor applications use a rating of 50-1200 uf capacitance and voltages of 110/125, 165, 220/250 and 330 VAC. They are also usually always 50 and 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.
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. The 440 volt unit will actually last longer. A capacitor will have a marked voltage indicating the accpetable peak voltage, not operational voltage.
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.
Select a capacitor with the Hz rating of the original. Nearly all replacement capacitors will be labeled 50/60.
Connection terminal style
Nearly every capacitor will use a ¼" flag style push-on connector. The next question is, "How many terminals per terminal post are needed for the application motor?" Most start capacitors have two terminals per post, and most run capacitors will have either 3 or 4 terminals per post. Verify that your selected capacitor has at least the number of connection terminals per connection post as the original motor capacitor.
Nearly all start capacitors have a round case. Round run capacitors are by far the most common, but many motors still use oval designs. Electrically speaking, there is no difference. Fit is the only question here. If space in the mounting box is not limited, the case style does not matter.
Just like case shape, overall size makes no difference electrically. Select a capacitor that will fit within the space provided.
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