What's the Difference between a Start and a Run Capacitor?

Capacitors in General

A capacitor is an energy storing device. It is a medium by which energy is stored to either be released suddenly or over a period of time. The energy or capacitance of an electric capacitor is measured in the form of micro-farads. Essentially, two plates are separated by a material known as a dielectric or insulator. These insulators can be mica, ceramic, porcelain, Mylar, Teflon, glass, or rubber. Capacitors will also limit the current. They can be used to store voltage or build it up until the call for it to be released is present.

Start Capacitors

A start capacitor is found in the circuit of start windings when the motor is starting. This capacitor contains a higher capacitance than a run capacitor. It varies, but a start capacitor will measure between 70 and 120 micro Farads. The start capacitor provides an immediate electrical push to get the motor rotation started. Without a start capacitor when the voltage is applied, the motor would just hum. The start capacitor creates a current to voltage lag in the separate start windings of the motor. The current builds up slowly, and the armature has an opportunity to begin rotating with the field of current.

Run Capacitors

A run capacitor uses the charge in the dielectric to boost the current which provides power to the motor. It is used to maintain a charge. In AC units, there are dual run capacitors. One capacitor provides power to the fan motor. The other sends power to the compressor. Run capacitors measure in at approximately 7-9 micro-Farads. The value or rating of the run capacitor must be accurate. If the value is too high, the phase shift will be less than perfect and the winding current will be too high. If the capacitor value/rating is too low, the phase shift will be higher and the winding current will be too low. If run capacitors are not ideal, then the motor could overheat and the true torque will not be enough to drive current.

What is Equivalent Series Resistance?

The equivalent series resistance of a capacitor is the internal resistance that appears in series with the capacitance of the device. Almost all capacitors exhibit this property at varying degrees depending on the construction, dielectric materials, quality, and reliability of the capacitor. The equivalent series resistance (ESR) values range from a few milliohms to several ohms, and results into power losses, reduced efficiency, and instability of power supplies and regulators circuits.

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The aluminum electrolytic capacitors and tantalum ones, have higher ESRs than ceramic capacitors of the same capacitance and voltage rating. The polypropylene and polyester capacitors fall in between, but are not commonly used in the SMPSs due to their large physical sizes.

Main parts of an ESR

Metallic resistance

Electrolytic and paper resistance which is dependent on frequency and temperature

Dielectric which depends on frequency

Factors that increase the ESR value

Bad electrical connections; – The connection between the copper leads and the aluminum plates in the capacitor are usually welded or made using mechanical crimps. This type of connections introduces some series resistance, and is used because the aluminum cannot be soldered.

The drying of capacitor electrolyte solution. As the liquid component of the electrolyte dries out due to elevated temperatures, the electrical resistance increases.

ESR increases with increase in temperature and frequency. In power supplies with high currents, the power dissipation associated with the ESR may further increase the temperature and lead to capacitor failure.

Minimizing ESR in circuits

High performance applications use the low ESR capacitors such as the low ESR solid polymer capacitors, tantalum capacitors and the multilayer ceramic capacitors (MLCC).

Capacitors are connected in parallel in places such as the power supply smoothing circuits. Small value capacitors are connected in parallel as opposed to connecting a single large capacitor. This reduces the effective ESR in addition to reducing the ripple volatge, and allows the circuit to handle higher currents with less losses.