Page 106 - Electric Drives and Electromechanical Systems

Chapter 3 Power transmission and sizing 99

The maximum speed required by the application must be no greater than approxi-
mately eighty per cent of the maximum no-load speed of the motor drive combi-
nation; this allows for voltage fluctuations in the supply to the drive system.
The motor’s speed-torque characteristics must not be violated; in addition, with a
direct current (d.c.) brushed motor, the commutation characteristics of the motor
must not be exceeded.
It should be noted that if a linear motor is used in an application the same set of
constraints need to be considered, however force is considered to be the main driver as
opposed to torque in the sizing process.
The operating regimes of the motor and its associated controller must be considered;
two types of duty can be identified. The main determining factor is a comparison of the
time spent accelerating and decelerating the load, against the time spent at constant
speed. In a continuous duty application, the time spent accelerating and decelerating is
not critical to the overall application, hence the maximum required torque (the external-
load torque plus the drive-train’s friction torque) needs to be provided on a continuous
basis; the peak torque and the r.m.s. torque requirements are not significantly different
from that of the continuous torque. The motor and the controller are therefore selected
primarily by considering the maximum-speed and continuous-torque requirements.
An intermittent-duty application is defined as an application where the acceleration
and deceleration of the load form a significant part of the motor’s duty cycle. In this case
the total system inertia, including the motor inertia, must be considered when the
acceleration torque is being determined. Thus, the acceleration torque plus the friction
torque, and any continuous load torque present during acceleration, must be exceeded
by the peak-torque capability of the motor-drive package. Additionally, the drive’s
continuous torque capability must exceed the required r.m.s. torque resulting from the
worst-case positioning move.
The difference between these two application regimes can be illustrated by consid-
ering a lathe, as discussed in Section 1.2. The spindle drive of a lathe can be defined as a
continuous-duty application since it runs at a constant speed under a constant load for a
majority of the time, while the axis drives are intermittent-duty applications because the
acceleration and deceleration required to follow the tool path are critical selection
factors.
The confirmation of suitable motor-drive combinations can be undertaken by the
inspection of the supplier’s motor-drive performance data, which provides information
on the maximum no-load speed and on the continuous torque capability, together with
the torque sensitivity of various motor frame sizes and windings. In the sizing process it
is normal to initially consider only a small number of the key electrical and mechanical
parameters. If significant problems with motor and drive selection are experienced, a
detailed discussion with the suppliers will normally resolve the problem.