Choosing Electric, Hydraulic or Pneumatic Drives

With electric drives, electrical energy is converted into mechanical kinetic energy. Inside the electric motor, magnetic fields in the stator and rotor interact; torque is created when they try to align themselves with each other, creating movement. The "precise hands" of electric drives and motors are ideal for offering high speed and precise accuracy.

In dc motors, a dc winding or a permanent magnet generates a fixed magnetic field in the stator. To deliver maximum torque, coils are wound onto the rotor and connected so that the rotor field is perpendicular to the stator field. Speed and power in dc motors can be controlled very effectively, but the carbon brushes that are required to switch the current in this type of motor are subject to wear and tear.

A synchronous motor features a three-phase winding configured in a circle. Three phase-shifted currents generate a rotating magnetic field. Since the rotor has a fixed magnetic field, it can only develop effective torque at a synchronous speed. With modern current converters, synchronous motors can be controlled as precisely as dc motors, but without any wearing parts. Synchronous motors have an excellent operating efficiency, above 90 percent, but require a complex electronic regulation system and costly permanent magnets.

Asynchronous motors also feature stators that generate a rotating field, but with a squirrel-cage winding. When the rotor is not following synchronously, a current is induced that counteracts the change in the magnetic field. Together with the magnetic field of the stator, this generates a torque that pulls the rotor along. The advantage is that asynchronous motors are less expensive, but because the current flowing through the rotor generates heat, the motor has a low efficiency level.

Hydraulic Drives

With hydraulic drives, fluid is pressurized to move a piston inside a cylinder. A pump supplies the volume flow. Depending on the force required to move a load, the corresponding pressure is developed in the fluid and the pump counteracts this pressure. In rotating drives, a hydraulic motor delivers torque instead of linear force.

Hydrostatic drives, which feature adjustable pumps that push pistons or turn hydraulic motors, are extremely efficient with years of development history. However, the speed of the system varies when the external forces change and there is no easy way to maintain a position once it has been attained. Consequently, the distance between the pump and the cylinder needs to be as short as possible.

In secondary control drives, the motor, not the pump, is regulated. The varied torque enables the hydraulic drive to adapt quickly to changes in force. This highly efficient technology allows for the regulation of speed, torque and position. However, the drives are expensive to build and the rapid adjustability is only required by users with special machine needs, like test bed manufacturers.

Drives with valve control are common in hydraulic systems for mobile equipment. Only a portion of the hydraulic energy generated is applied, via a valve. The unused portion is converted into heat. Valve control drives are extremely accurate with excellent control but lower efficiency. This can be improved with load-sensing technology, so that only the pressure that is needed is actually provided.

Pneumatic Drives

In pneumatics, air is compressed and the stored energy is converted to mechanical energy in cylinders, motors or other units. However, using compressed air in industrial applications is only cost-effective when low forces are required. The "nimble fingers" of pneumatics are used when small masses need to be moved at high speeds across short distances, like in clamping, transporting, screw-tightening and in other industrial, trade or medical tasks.

Pneumatic drive systems include three subsystems: air compression and processing; control (via valves); and output drive (a cylinder or motor). These components require little maintenance during operation and offer long service lives. Compressed air, which is readily available, poses no fire or explosion hazards. However, producing and preparing compressed air can be expensive and the noise of air exhaust may have to be muffled. An advantage is that compressed air is insensitive to temperature variations. If leaks occur, they have no effect on machine safety and do not contaminate the surroundings. The speed and force of the actuators can be controlled simply and continuously over a broad range, but it may be difficult to achieve constant and uniform piston speeds.

Amy DeFayette, is technical marketing manager for Bosch Rexroth Corp.