Certain types of single-phase AC motors can experience a problem of zero starting torque, due to the alternating magnetic field not producing a force in one direction for long enough to overcome the inertia. To overcome this, a secondary "starting coil" is sometimes included and is connected to the power supply through a starting capacitor, creating an LC circuit. As the AC current through a capacitor is 90 degrees ahead of the main phase, this initially provides power to the starting coil which gets the motor turning, before switching to the main coils once the motor is up to speed. Other motor designs use run capacitors to improve the power of the motor in operation.