This is a switching circuit used for a switched reluctance motor. In this circuit there is a 47μF capacitor on the right side. What is the capacitors function in this circuit?
This capacitor is used to keep the 12V supply stable as possible.
According to the equation P=VI, when the motor draws a high current, the voltage across the motor drops, when the voltage drops, your motor speed reduces. Therefore if you have a capacitor in parallel to the motor, in times of voltage drops, your capacitor provides that necessary current to the motor so that your motor voltage remains stable, thus keeping a constant speed.