<P> Control can mean either manually adjustable - by means of a potentiometer or linear hall effect device, (which is more resistant to dust and grease) or it can also be automatically controlled for example by using a rotational detector such as a Gray code optical encoder . </P> <P> There are three general categories of electric drives: DC motor drives, eddy current drives and AC motor drives . Each of these general types can be further divided into numerous variations . Electric drives generally include both an electric motor and a speed control unit or system . The term drive is often applied to the controller without the motor . In the early days of electric drive technology, electromechanical control systems were used . Later, electronic controllers were designed using various types of vacuum tubes . As suitable solid state electronic components became available, new controller designs incorporated the latest electronic technology . </P> <P> DC drives are DC motor speed control systems . Since the speed of a DC motor is directly proportional to armature voltage and inversely proportional to motor flux (which is a function of field current), either armature voltage or field current can be used to control speed . Several types of DC motors are described in the electric motor article . The electric motor article also describes electronic speed controls used with various types of DC motors . </P> <P> An eddy current drive (sometimes called a Dynamatic drive, after one of the most common brand names) consists of a fixed speed motor (generally an induction motor) and an eddy current clutch . The clutch contains a fixed speed rotor and an adjustable speed rotor separated by a small air gap . A direct current in a field coil produces a magnetic field that determines the torque transmitted from the input rotor to the output rotor . The controller provides closed loop speed regulation by varying clutch current, only allowing the clutch to transmit enough torque to operate at the desired speed . Speed feedback is typically provided via an integral AC tachometer . </P>

In a dc drive which is used as a speed feedback device
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