The use of capacitors.

Capacitors have many usage in electronic and electrical systems. They are so ubiquitous.

For example,to start the motor, a secondary winding is used in series with a non-polarized starting capacitor to introduce a lag in the sinusoidal current through the starting winding. When the secondary winding is placed at an angle with respect to the primary winding, a rotating electric field is created. The force of the rotational field is not constant, but is sufficient to start the rotor spinning. When the rotor comes close to operating speed, a centrifugal switch (or current-sensitive relay in series with the main winding) disconnects the capacitor. The start capacitor is typically mounted to the side of the motor housing. These are called capacitor-start motors, and have relatively high starting .

Energy storage capacitors are used for power supply, where they smooth out the full or half - wave rectifier. They can also be used in a charge pump circuit as an energy storage device for a voltage higher than the input voltage. For example, some motor capacitors are specially applied to water pumps and some motors.

Groups of large, specially constructed, low-inductance high-voltage capacitors (capacitor banks) are used to supply huge pulses of current for many pulsed power applications. 

Large capacitor banks (reservoirs) are used as energy sources for the exploding-bridgewire detonators or slapper detonators in nuclear weapons and other specialty weapons. Experimental work is under way using banks of capacitors as power sources for electromagnetic armour and electromagnetic railguns or coilguns.