Page 107 - Electric Drives and Electromechanical Systems

100 Electric Drives and Electromechanical Systems

As discussed above, two operating regimes can be identified, with key features
summarised as.
In a continuous duty application, the acceleration and deceleration requirements
are not considered critical; the motor and the controller can be satisfactorily
selected by considering the maximum-speed and continuous-torque requirements.
An intermittent-duty application is defined as an application where the accelera-
tion and deceleration of the load form a significant part of the motor’s duty cycle
and need to be considered during the sizing process.
It should be noted that these approaches provide information for drive selection etc.
If a detailed analysis of closed loop performance is required, further analysis will be
required, normally using computer based modelling as discussed in Chapter 10,
Controllers for Automation.

3.8.1 Continuous duty

For continuous duty, where the acceleration performance is not of critical importance,
the following approach can be used:
Knowledge of the required speed range of the load, and an initial estimation of
the gear ratios required, will permit the peak motor speed to be estimated. In
order to prevent the motor from not reaching its required speed, due to fluctua-
tions of the supply voltage, the maximum required speed should be increased by
a factor of 1.2. It should be noted that this factor is satisfactory for most indus-
trial applications, but it may be refined for special applications, for example,
when the system has to operate from a restricted supply as is found in aircraft
and offshore-oil platforms.
Using the drive and the motor manufacturers’ data sheet, it will be possible to
locate a range of motors that meets the speed requirement when the drive
operates at the specified supply voltage. If the speed range is not achievable,
the gear ratio should be revised.
From themotor’s data, itwillnormally bepossibleto locate a motor-drivethat
meets the torque requirement; this will also allow the current rating of the drive
to be determined. A check should then be undertaken to ensure that the selected
system can accelerate the load to its required speed in an acceptable time.

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Example 3.5
Determine the motor’s speed and torque requirement for the system detailed below, and hence
identify a suitable motor and associated drive: