Electrical drives are electrical motor controls that control motor speed. For many years, most electrical drives used DC motors, but AC motor drives have now become the dominant type. The most common AC motor speed controls are variable frequency drives (VFDs) that control the speed of 3-phase induction motors. In the most common type of VFD, mains power is rectified to provide DC power. The DC power is then converted to 3-phase AC power using electronic switching circuits in an inverter configuration. The inverter controls both the frequency and the voltage supplied to the motor using pulse width modulation to control the voltage. The voltage supplied to the motor must be proportional to the frequency. To operate at half of normal speed, the motor must receive half of the mains frequency and half of the mains voltage. The motor can produce its full rated torque at reduced speed, but the power is reduced because power is proportional to torque multiplied by speed. Electrical drives are electronic power conversion devices. The electronic circuits operate at 3-phase mains voltage and carry the full motor current. Thus their power rating is the same as the motor power rating. Their efficiency is typically 95% to 98%, but that means they must dissipate 2% to 3% of the motor power as heat which can be quite a lot of heat.
To give you an accurate answer, I would need a bit more information where your coming from but I'll give it a shot. Some electrical motor applications have drives that vary the speed of the motor. To vary the speed of an AC motor, a drive board reduces the hz cycle of the current. In computer and robotics a driver runs motors called servos. The drivers in these settings not only vary the speed but also the position of the shaft in rotation. I know a little about them but not enough to explain them on here.
