AC Drive: Power converter designed to vary the speed of an ordinary induction AC motor. The AC drive input AC voltage and frequency are fixed. The AC drive output voltage and frequency vary but the ratio of voltage to frequency is kept fixed allowing the drive to maintain, to a degree, constant output torque. The torque is constant only with motor speeds above 5HZ of the output voltage. Below 5Hz the output torque collapses and the motor stalls usually triggering an overload trip in the drive.

Vector Drive: it is an AC drive with the add on feature allowing precise calculation of motor torque. This enables the drive to ensure constant torque at zero motor speeds. The Vector Drive is a substantial improvement compared to an ordinary AC drive. However, its control system is more complex. A Vector drive can operate an ordinary AC motor but it wears it down quickly due to increased chopping frequencies within the drive. It is better to use a purpose design Vector motor with better design windings and separate cooling.

AC Servo Drive: It precisely monitors the motor speed and toque in order to allow exact control of the motor speed. It requires special motors with speed feedback devices. Far more expensive than a Vector drive.

DC Servo Drive: It works on similar principal to AC Servo Drive but delivers far higher speed accuracy and much higher torque.

DC Drive: Mains synchronised power converter used to control speed of shunt or series exited DC motors.  It is going out of fashion, replaced by the Vector drive but still very popular in legacy applications.