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Run vs. Starting Capacitor

Run vs. Starting Capacitor


Single phase electric motors use a device called a starting capacitor to create a large phase shift to produce necessary torque during start up. Single phase motors will commonly have both a run capacitor and a starting capacitor. Starting capacitors differ from run capacitors by their relative capacitance value being much higher for a given physical size. The starting capacitor will usually be identified as having a black phenolic or plastic case and a recessed top where the connections are located.


As a byproduct of high capacity, they are only intermittent rated and can only be energized for a few seconds at a time. For this reason, a starting capacitor is usually one of the first components to fail on single phase electric motors.


Run Capacitors

A motor run capacitor is engaged in a circuit all the time for phase delay or power factor correction. They are common in HVAC units and larger single phase motors. They are oil cooled, in metal cases, and have fairly low capacitance ratings (such as 5 uf or 45 uf). Run capacitors are usually rated for 370V or 440V.


If you're sizing a capacitor to start with (and not merely replacing an old one), make sure that your capacitor voltage rating is 1.5x your line rated voltage. This is because the voltage ratings on AC capacitors are not RMS, but rather, peak to peak voltage.


On 240V system, run capacitors are generally 370-440V, and in 480V systems, you'll often see 600V capacitors used. This is to account for the peak to peak voltage levels in the system.


Starting Capacitor

Starting capacitors are usually physically bigger than run capacitors. They also tend to be larger in capacitance values (such as 430-516 uf). Their capacitance ratings come in a range instead of a set number, like in run capacitors. This is because the phase shift produced by running AC current through a start capacitor doesn't need to be as precise to get the motor turning during start up. Common voltage ratings for a starting capacitor are 110/125V, 165V, 220/250V, or 330V.


Duty Cycle

Run capacitors are 100% duty cycle. This means they can remain charged up in the circuit indefinitely.


A starting capacitor can only be charged up for a few seconds before they are damaged. The images below depict damaged start capacitors.


Blown Starting Capacitor Starting Capacitor Ruptured Blister

Can you use a run capacitor in place of a starting capacitor?

Yes, you can use run capacitors as start capacitors. The disadvantage of using run capacitors as start capacitors is that you would usually need to use more run capacitors than you have space for. For the start up of a motor to be able to develop torque, you need a very high capacitance value. To get a high enough capacitance value with run capacitors, you would generally need to have a whole array of run capacitors to compensate for the low capacitance values of each of the individual run capacitors. But if you have the mounting space available, this is a perfectly adequate solution.


However, you cannot use a starting capacitor in place of a run capacitor. When you engage a start capacitor in a current-carrying circuit, it will be good for a few seconds only. It cannot be charged up in the circuit continuously like run capacitors can.


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