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Why Does My Motor Need A Capacitor?

Motor CapacitorIf you are using an AC pump to raise water from a sump to an overhead tank, chances are it uses a squirrel-cage type motor, which needs a capacitor to make it work. This is true for single-phase motors, where the capacitor creates an artificial second phase necessary to generate the rotating magnetic field and make the rotor start spinning. Once the rotor starts rotating, the interaction between the stator and rotor keeps the magnetic field spinning.

A single-phase motor has a primary winding and a secondary winding. If connected to the AC supply without the capacitor, both windings produce magnetic fields of the same phase resulting in zero torque. With a capacitor connected in series to the secondary winding, the magnetic field it produces lags behind the magnetic field generated by the primary winding. This difference in phases creates a starting torque and the motor starts to rotate.

Capacitors that allow a motor to start rotating are called start capacitors. Smaller motors usually have the start capacitor permanently connected in series to the secondary winding. Big motors require a larger capacitor to help them generate the starting torque, but they run more efficiently with a small capacitor in place, called run capacitor. Often both capacitors are housed in the same can, which then has three terminals in place of the customary two. Such motors have a centrifugal switch to disconnect the start capacitor when the motor has reached 70-75% of its full speed. Start capacitors are typically of high value of 100 or more microfarads, while run capacitors are smaller, of about 25-47 microfarads.

You will find motors with large start capacitors being used for several applications where it is necessary to generate considerable torque to begin moving the load. Such applications include mechanical conveyors, belted blowers and commercial garage door openers. These are mostly electrolytic capacitors, housed within a plastic or metal can. Inside the can are two metal foils rolled up with a flexible paper-like insulation separating the sheets. The paper, soaked with an electrolyte, forms the dielectric of the capacitor. The two metal foils are connected to two terminals. The assembly is sealed with epoxy and the two terminals are available for external electrical connection.

Large HVAC units sometimes need two run capacitors, because they have both a fan motor and a compressor motor. To save space, manufacturers combine the two physical capacitors into a single can. Such dual capacitors have three terminals and they are usually marked as Common, Fan and Compressor.

You will find a variety of combinations for dual capacitors, for example, 40 + 5uF, 370V or 100 + 25uF, 440V and others. Their shapes can be cylindrical with a round or oval cross-section. A capacitor’s ability to hold charge is measured in microfarads. As electrolytic capacitors age, their capacity reduces. That results in the motor failing to start or run at less than full speed.

Motors are not fastidious about the capacitance value of the capacitor used for starting. However, when replacing a faulty capacitor, you must never use a replacement that has a lower voltage rating. Always use a part with a voltage rating that is the same or higher than the rating of the capacitor you are replacing. Of course, it’s always preferred to replace a capacitor with another that has the exact electrical specifications for the best results – both in performance and safety.

The Ripple Rating of a Capacitor

Engineers do not prefer having ripples in their circuits and do their best to minimize its effects. For example, an AC source delivers power to an AC-DC converter that subsequently converts it to a steady DC output. It can be very inconvenient if the output were to have any source AC power appearing on top of the DC output in the form of small, frequency dependent variations. However, ripple may not be considered evil in all cases, as some digital signals could be useful to engineers as a necessary design function. Among these are signals that use changes in voltage levels to switch the state of a device and those generating clock timings.

As capacitors can store charge, they are useful for smoothening ripples in circuits. However, the designer must take care that the peak voltage does not exceed the voltage rating of the capacitor. It must also be noted that since there can be DC bias present in the circuit, the peak voltage will be the sum of the maximum ripple voltage and the DC bias. However, that is not enough for electrolytic capacitors.

Electrolytic capacitors are usually made with aluminum, tantalum and niobium oxide technologies and they have polarity. If the negative voltage of the ripple is allowed to drop below zero, this will cause a connected capacitor to operate under reverse bias conditions. Class II ceramic capacitors used in low frequency applications also suffer from this restriction.

A capacitor functions as a charge reservoir, charging with the rise of the incoming voltage and discharging into the load as it decreases – smoothening out the ripples in the process. Therefore, capacitors will see varying voltage. Additionally, depending on the power applied, the current through the capacitor will also vary, as will the intermittently pulsed and continuous power. This causes resultant changes in the electric field of the capacitor regardless of the incoming form and creates oscillating dipoles within the dielectric material, thereby self-heating the capacitor. Any parasitic inductance or ESL and resistance or ESR contributes to the energy dissipation.

That means a capacitor with low ESR, ESL and DF (dissipating factor), will heat up less than one with a dielectric characterized by high ESR and DF. However, as these parameters also depend on frequency, different dielectric materials offer optimum performance (lower heat generation) over different frequency ranges.

The dielectric in a capacitor is usually very thin constituting only a small amount of the overall mass of the capacitor. Other materials used in the construction also contribute to the heating when considering ripple – capacitor plates being one of the major contributors. Additionally, the conductive contacts also heat up to some degree when the capacitor carries an AC signal or current.

For example, at a certain frequency, if the capacitor with a 100mOhms ESR carries a 1A rms current, the power dissipated internally will be 100mW. If this power is supplied continuously, it will heat the capacitor internally until thermal balance is reached. Since this depends on ESR, the power dissipation is a function of frequency. However, the total thermal management will also depend on the capacitor’s environmental conditions, governing the heating up of the capacitor in an application.