Power Transmission for Motion Control
- Wednesday, 01 December 2004
While technology has advanced and there are many new ways to accomplish useful motion within particular applications, electric motors and gearheads are still the preferred and most popular choices for power transmission throughout commercial, industrial, and automation applications. Motors are the components that convert electrical power to useful mechanical power. Gearheads transform the mechanical rotary power to the desired combination of speed and torque if the motor cannot do so directly. Benefits of this combination include low cost, simplicity, reliability, and versatility.
There are several styles and types of electrical motors and power transmission devices on the market today. Depending on the application, a designer might first choose between AC and DC motors, then between styles (e.g., induction, universal, and permanent magnet), and finally between the many types specific to each style. The designer is also challenged with the task of selecting a controlling device and amplifier to provide the proper input power for the electric motor. Within the layout for the power transmission, one can select between options such as direct drive, chain/belt drive, ball screws, direct gearing, and all of the types respective to those styles.
As designers put the pieces of the puzzle in place to resolve their applications, they usually hit a hurdle somewhere in the middle; the motor does not provide the proper output. For example, the motor may be capable of supplying the necessary power, but not match the speed or torque requirements. Most motors are rated to operate for peak output around a certain continuous load range of torque and speed. To operate the motor outside that range might either under utilize the motor and result in wasting money on an oversized motor, or over utilize the motor and potentially run the risk of damaging the motor.
Although each type and style of motor operates differently, smaller motors typically operate more efficiently at higher speeds and lower torques. Larger motors operate at higher torques and lower speeds. The dynamics of this is simple to follow once the motor is viewed as a large moment arm. The greater the radius of the rotor the larger the moment that can be created from the motor, resulting in greater torque.
High-energy permanent magnet servo motors have become extremely popular in the motion control industry of late. For these motors, a general rule of thumb is that if the length of the motor is doubled, then the output torque capacity of the motor is also doubled. However, the output torque capacity of the motor will be quadrupled if the length is kept the same and the diameter of the motor is doubled. For maximum efficiency, proper sizing, and a cost effective design, it is best to design the motor for operation at or slightly under its designed load point of speed and torque.