Page 81 - Electric Drives and Electromechanical Systems

74 Electric Drives and Electromechanical Systems

discusses their impact on overall system performance, and concludes with the process
required to determine the detailed specifications of the motor and the drive.
The design parameters of the mechanical transmission system of the overall drive
must be identified at the earliest possible stage. However, it must be realised that the
system will, in all probability, be subjected to detailed design changes as development
proceeds. It should also be appreciated that the selection of a motor and its associated
drive, together with their integration into a mechanical system, is by necessity an iter-
ative process; any solution is a compromise. For this reason, this chapter can only give a
broad outline of the procedures to be followed; the detail is determined by the engineer’s
insight into the problem, particularly for constraints of a non-engineering nature, such
as a company’s or a customer’s policy, which may dictate that only a certain range of
components or suppliers can be used.
In general, once the overall application, and the speed and torque (or in the case of a
linear motor, speed and force) requirements of the system have been clearly identified,
various broad combinations of motors and drives can be reviewed. The principles
governing the sizing of a motor drive are largely independent of the type of motor being
considered. In brief, adequate sizing involves determining the motor’s speed range, and
determining the continuous and intermittent peak torque or force which are required to
allow the overall system to perform to its specification. Once these factors have been
determined, an iterative process using the manufacturer’s specifications and data sheets
will lead to as close an optimum solution as is possible.

3.1 Gearboxes

3.1.1 Conventional gears
In a gear train two or more gears are made to mesh with each other to transmit power
from one shaft to another. The design of the train used in a specific application
depends upon the gear ratio required and the relative position of the axes of shafts. A
number of approaches for configuring the gear train are possible, (i) simple gear train,
(ii) compound gear train, (iii) epicyclic gear train. In the first two types of gear trains,
the axes of the shafts over which the gears fixed relative to each other, in the case of
epicyclic gear trains, the axes of the shafts on which the gears are mounted may move
relative to a fixed axis, as discussed in Section 3.1.2.
As discussed in Section 2.1.3 in a simple gear train their will be a change in the
angular velocity and torque between an input and output shaft; the fundamental speed
relationship is given by,

u i N o
n ¼ ¼ (3.1)
u o N i
where N i and u i are the number of teeth on, and the angular velocity of, the input gear,
and N o and u o are the number of teeth on, and the angular velocity of, the output gear.