Page 111 - Electric Drives and Electromechanical Systems

104 Electric Drives and Electromechanical Systems

This example illustrates a different approach to resolving the sizing problem encountered
earlier. The change in gear ratio can easily be achieved at the design state, and in all possibility
be cheaper that going to a larger motor and drive system.
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3.8.3 Inability to meet both the speed and the torque requirements

In the selection of motors, the limitations of both the motor and the drive forces a
trade-off between the speed and the torque capabilities. Thus, it is usually advanta-
geous to examine whether some alteration in the mechanical elements may improve
the overall cost effectiveness of the application. Usually the speed-reduction ratios
used in the application are the simplest mechanical parameter which can be investi-
gated. If the speed required of the motor is high, but the torque seems manageable, a
reduction in the gear ratio may solve the problem. If the torque required seems high
but additional speed is obtainable, then the gear ratio should be increased. The goal is
to use the smallest motor-drive combination that exceeds both the speed and torque
requirement by a minimum of ten to twenty per cent. Sometimes the simple changing
of a gear or pulley size may enable a suitable system to be selected.
A further problem may be the inability to select a drive that meets both the peak- and
the continuous-torque requirements. This is particularly common in intermittent-
motion applications. Often, the peak torque is achievable, but the drive is unable to
supply the continuous current required by the motor. As has been shown earlier, while
optimum power transfer occurs when the motor’s rotor and the reflected load inertia are
equal this may not give the optimum performance for an intermittent drive, hence the
gear ratios in the system need to be modified and the sizing process repeated.

3.8.4 Linear motor sizing

So far in this section we have considered the sizing of conventional rotary motors. We
will now consider the sizing of a linear motor. Due to the simplicity of a linear drive, the
process is straightforward compared to combining a leadscrew, ballscrew or belt drive
with a conventional motor. As with all other sizing exercises, the initial process is to
identify the key parameters, before undertaking the detailed sizing process. A suitable
algorithm is as follows:

Using the application requirements and the required speed profile determine the
required speed and acceleration.
Estimate the minimum motor force for the application as discussed in Section 2.2.
Select a motor-drive combination with a peak force capability of at least 1.5 to 2
times the minimum force requirement to ensure a sufficient capability.
Recalculate the acceleration force required, this time including the mass of the
moving part of the selected motor.
The peak force of the motor-drive combination must exceed, by a safe margin of at
least fifteen per cent, the sum of the estimated friction force and the acceleration