What does a electric motor capacitor do?


What does a electric motor capacitor do?

Jerry Walch

Electric motors fall into two broad categories, alternating current (AC), and Direct Current (DC) motors. There are two broad classes of alternating current motors, single-phase electric motors, and three-phase electric motors.
Induction start-induction run, capacitor start-induction run, and capacitor start-capacitor run are all types of single-phase AC electric motors. Before we explore what an electric motor capacitor does for a capacitor start-induction run and a capacitor start-capacitor run motor, we need to understand what a capacitor is and how it works.

A capacitor is an electrical device used to store electrical energy. In its simplest form, a capacitor is nothing more than two metal plates separated by a dielectric material, an insulator. In one of its earliest forms, the Leyden Jar, the dielectric between the metal plates was the glass itself. When a potential difference, a voltage, was applied across the terminal on a capacitor, one plate became depleted of electrons and became positively charged while the second plate built up an excess of electrons and became negatively charged. A good capacitor would hold this charge until the two plates were connected and the surplus of electrons flowed into the positively charged plate neutralizing the charge. Modern capacitors used with electric motors are of two types, electrolytic capacitors, and oil-filled capacitors. Electric motor start capacitors are of the electrolytic type while electric motor run capacitors are of the oil filled types.

Electric motor capacitors are rated by their operating voltage and by their capacitance in Microfarads. Both of these ratings are extremely important when selecting a replacement capacitor for an electric motor. Using a capacitor rated for use with 125-Volts on an electric motor connected to 240-volts will destroy the capacitor in an instant. The type of dielectric used and the thickness of the dielectric material determine the working voltage of a capacitor. Applying too high a voltage to a capacitor will cause the dielectric to puncture and the capacitor to short out. A capacitor with the wrong Microfarads rating will result in a wrong current phase-shift angle and the electric motor to run inefficiently.

Electric motors use motor capacitors for several reasons. Capacitors assist them in starting. Capacitors increase their starting torque. Capacitors increase their power factor and make them operate more efficiently.

Single-phase AC electric motors have two sets of stator coils, a “Run” winding and a “Start” winding. In order for an electric motor to start and run the magnetic fields produced by the start winding and the run winding has to be 90 degrees out of phase. As you may recall from high school physic a magnetic field is produced any time an electric current flows through a conductor. The start winding and the run windings are physically displaced on the stator but that displacement falls far short of the required 90 degrees. Adding a non-polarized electrolytic capacitor in series with an electric motor’s start winding creates a virtual displacement of the magnetic field. Without becoming bogged down in physics and applied mathematics, capacitors have a characteristic called Capacitive reactance that creates a phase shift between the current flowing through it and the voltage applied across it. The winding themselves have what’s known as Inductive Reactance which creates a phase shift between the current flowing through them and the voltage across them. The phase shift created by inductive reactance and capacitive reactance are in opposite directions. Current lags the voltage in one and leads the voltage in the other so a careful matching of the two produces the virtual displacement in the start and run windings needed the motor to start easily and produce maximum starting torque. It’s a common belief that you can use a capacitor with a higher capacity rating then the one that came with the motor but not one of a lower rating but it a wrong belief. The capacity of the capacitor affects capacitive reactance therefore it’s essential that you use one that the motor was designed for.

In a capacitor start electric motor the capacitor remains in the circuit for a very short period of time. These motors use a centrifugal start switch that disconnects the start winding and the starting capacitor when the motor reaches 2/3 of its operating speed.

In a split capacitor electric motor, a capacitor start-capacitor run motor, two capacitors are used. One is connected in series with the start winding through the centrifugal start switch just as in the capacitor-start motor. The run capacitor is connected in series with the Start and Run windings on the Start winding’s side of the centrifugal switch and remains in the circuit continuously. The run capacitor actually connects the start winding in parallel with the run winding once the start switch open producing a higher running torque and a more efficient electric motor. Split capacitor electric motors pull less current then a capacitor start-induction run motor of the same horsepower rating.

Start capacitor used with electric motors is non-polarized electrolytic capacitors easily recognized by the long, round, black plastic construction. Electrolytic capacitors use a paste electrolyte. Run capacitors, on the other hand are usually oil filled, and easily recognize by their metal can construction.

Thesis capacitors are one of the biggest causes for electric motors malfunctioning. A defective starting capacitor will keep a motor from starting under load. A defective run capacitor will keep a motor from producing full power.


The running capacitor in a single phase motor increases the torque for rootating the rotor.Check the capacitor for a possible replacement.