Page 109 - Electric Drives and Electromechanical Systems

102 Electric Drives and Electromechanical Systems

Select a motor-drive combination with a peak torque capability of at least 1.5e2
times the minimum motor-torque requirement to ensure a sufficient torque
capability.
Recalculate the acceleration torque required, this time including the inertia of the
motor which has been selected.
The peak torque of the motor-drive combination must exceed, by a safe margin of
at least fifteen per cent, the sum of the estimated friction torque and the accelera-
tion torque and any continuous torque loading which is present during accelera-
tion. If this is not achievable, a different motor or gear ratio will be required.
The motor’s root-mean-square (r.m.s.) torque requirement can then be calculated
as a weighted time average, using;
q ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
2
T rms T cm þ T þ dT 2 a (3.41)
f
where T cm is the continuous motor-torque requirement, T f is the friction torque at the
motor, T a is the acceleration torque, and d is the duty cycle.
The selected motor-drive combination is evaluated for maximum speed and
continuous torque capabilities as in Section 3.8.1.
If no motor of a given size can meet all the constraints, then a different, usually
larger, frame must be considered, and the procedure must be repeated.
In practice, it is usual for one or two iterations to be undertaken in order find an
acceptable motor-drive combination. The approximate r.m.s.-torque equation used
above not only simplifies computation, but it also allows an easy examination of the
effects of varying the acceleration/deceleration duty cycle. For example, the effects of
changes in the dwell time on the value of r.m.s. torque can be immediately identified.
Should no cost-effective motor-drive be identified, the effects of varying the speed-
reduction ratio and inertias can easily be studied by trying alternative values and
sizing the reconfigured system.
Sometimes, repeated selections of motors and drives will not yield a satisfactory
result; in particular, no combination is able to simultaneously deliver the speed and the
continuous torque which is required by the application, or to simultaneously deliver the
peak torque and the r.m.s. torque required. In certain cases, motor-drive combinations
can be identified, but the size or cost of the equipment may appear to be too high for the
application, and changes will again be required.

nnn
Example 3.6
Identify a suitable motor and drive for the application detailed below:
2
The load is a rotary system which has a moment of inertia of 1.5 kg m . Any external
load and the system’s friction has been assumed to be zero.