Motor Selection Considerations Ⅱ

•It should be emphasized that there is no infallible guide to selecting the best motor. There are always several workable configurations. Constraints (e.g., space, positioning resolution) can often eliminate several designs.

• The engineer must make the motor drive system work both electrically and mechanically. Look at the motor-to-load interface before looking at the electrical drive-to-motor interface.

• In a typical motion control application the requirement will be to overcome some load frictional force and move a mass through a certain distance in a specified time.

• The designer should weigh the following:
– Moment of Inertia
– Torque
– Power
– Cost

• Load Inertia
– For optimum system performance, the load moment of inertia should be similar to the motor inertia.
– When gear reducers intervene between motor and load, the reflected load inertia is JL/N2, where N is the gear ratio.
– If the motor inertia JM is equal to the reflected load inertia, the fastest load acceleration will be achieved, or conversely, the torque to obtain a given acceleration will be minimized.
– Therefore, matched inertias are best for fast positioning.
– Peak power requirements are minimized by selecting the motor inertia so that the reflected load inertia is 2.5 times
as large as the motor inertia. The torque will be increased but the maximum speed will be further reduced. A load
inertia greater than 2.5 times the motor inertia is less than ideal, but it should not present any problems if the ratio is
less than 5. A larger motor inertia implies that the same performance can be achieved at a lower cost by selecting a
smaller motor.
– There is a wide range of motor inertias on the market today. Overlap is extensive. An engineer can virtually consider any type of motor including brushless and stepper, at the first stage of the design-inertia match.