Adjustable speed drives change the speed of a driven shaft to a speed selected by the operator. Additionally, these drives are used to maintain the speed of a machine to very close tolerances: ±0.1% is not uncommon. This is much more accurate than the speed control offered by conventional AC motors, which can vary 3% from no load to full. Important performance specifications to consider when searching for adjustable speed drives include output shaft speed, output torque, continuous output power, and speed control. The output shaft speed is the no load rotational speed of output shaft at rated terminal voltage. Output torque describes the torque capability of the drive under constant running conditions. The continuous output power is the mechanical power provided by the drive output. Choices for speed control include handwheel, lever, screw, electric, and solid-state.
Choices for drive types for adjustable speed drives include traction drives, belt and chain drives, gear drives or differentials, hydraulic drives, eddy current drives, and magnetic coupling. Traction drives depend upon friction between a speed adjusting mechanism and specially shaped input and output plates to achieve adjustable speed with relatively high efficiency. Belt and train drives operate with adjustable diameter sheaves or pulleys. Gear drives are the most durable, rugged, and efficient of all adjustable-speed drives, but they are capable of providing only a specific number of fixed gear ratios. In a hydraulic drive there are two main methods of hydraulically varying the speed of the driven load when the driving motor is operating at a constant speed, fluid coupling and hydraulic pump and motor. An eddy current drive is an adjustable speed drives in which a constant speed AC motor drives a drum assembly. In a magnetic coupling drive the magnetic coupling replaces the physical connection between motors and loads with a gap of air. The output shaft position on hydraulic drives can be horizontal, vertical up, or vertical down. Shaft orientation choices include in-line, offset parallel, and right angle. Mounting choices are base or flange.
Adjustable speed drives can have an integrated motor in a packaged configuration. The "packaged" drive consists of the adjustable speed section and an electric motor. The unit may also include a geared reducer. Choices for motor input include 115/120V 60 Hz, 208-230 / 240V 60 Hz, 460 / 480V 60 Hz, 575 / 600V 60 Hz, 50 Hz / International Power, DC motor, and hydraulic motor. The phase for adjustable speed drives can be single or three phase. Common features for adjustable speed drives include integrated gears, continuously variable, C-flange adapters, hollow shaft, speed feedback, and reversible. Motor enclosure options include open frame or frameless, drip-proof, dust-proof, and totally enclosed. Special or extreme environments that adjustable speed drives can be configured for include cleanroom, cryogenic, explosion-proof, radiation hardened, vacuum use, and washdown duty.
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Variable transmissions are used in industrial applications to alter the speed of an output shaft.