SEED Guides Unit Selection - Electric Motor < >

Before embarking on the selection procedure it is necessary to ensure that the need for an electric motor, as distinct from other forms of drive, has been carefully considered. The Guide System Selection -Rotary Power Transmission gives assistance in this process.

The successful selection of a suitable motor is the result of matching the requirements of the power transmission system with one of the range of motors offered by the manufacturers. Thus information about the system and information about available hardware is necessary.

The most important information is usually related to its purpose, constituent elements, life, performance, environment and the economic situation for which it is required. This information is needed in order to understand the total system so that the motor selected consistent with the rest of that system. The temptation to the mechanical engineer is to consider only purpose, performance and perhaps environment should be strongly resisted. Although this stage is sometimes difficult and time consuming it must be completed thoroughly if an appropriate unit is to be identified.

Factors which influence the choice of motor must now be identified. A listing of most of the important and common factors appears below. Not all of these factors are important on every occasion so careful study of the system is required to ensure that those considered are actually relevant. Refer to the Product Design Specification for the system.

Each factor should then be defined in terms which are as objective as possible. Thus, where appropriate, numerical information should be given, terms must be explained and vagueness avoided. Then the boundaries of satisfaction must be defined for each of the chosen factors. This helps the designer to decide which motors meet the requirements in each respect. The boundaries must be numerical whenever possible. When subjective judgements are necessary a means of comparison must be established.

Manufacturers' data should now be collected and arranged into a suitable format. There is a finite number of motors available from manufacturers and the selection process is constrained by the form and content of the information presented by them and the range of catalogues available. There is a good case for maintaining a 'rolling' catalogue library or data on reference cards (or microfilm/computer) since this stage can be very time consuming if a unique set of data is collected separately on each occasion. Data on, say, size, performance, cost can be noted in numerical form, giving a range where appropriate. In the case of less objective data a rating may be shown based on advice or opinion gathered.

Optimising the choice is now a process of finding the best compromise (in the opinion of the designer) between the priorities of the system and the availability of the hardware. As far as the factors involving numerical data are concerned, some yield a 'go/no-go' situation which will eliminate those motors which are e.g.: too costly, too heavy, too big etc.

Other requirements involving more subjective data should be compared on the basis of their ability to meet the criteria as laid down in the Product Design Specification. This may be an iterative process which converges on the best compromise.

The evaluation technique used here will be similar to that used elsewhere in the design activity.

Further advice on the detail of installation or specifying and ordering will be required from the manufacturer's information. Normally this would be available from the catalogue but sometimes it is necessary to communicate directly with a representative of the company.

Electric motors are classified by the electrical supply required, the environmental conditions prevailing and the mechanical drive characteristic produced. The designer must ensure that each of these is taken into account when specifying the chosen motor. If in doubt the literature or representative of the motor manufacturer should be consulted.