SEED Guides Unit Selection - Electric Motor < >3. Factors Affecting Motor Selection
|Mechanical Output Required|
The most likely starting point for motor selection is the output required from it to drive the system. This information must be found from a consideration of the other elements in the system. Most of the common elements are covered in other Guides in this series. The mechanical output can be sub-divided as follows:
Load is defined as the torque required to drive the system, including the driven machine.
This may be constant or may vary with speed. Thus a relationship between torque and speed, called a characteristic of the system, should be established. Some typical load characteristics are illustrated in Appendix 1. Some motor characteristics are shown in Appendix 2.
The nominal torque required by the system at normal running speed will be a function of the work done by the driven machine. It is usual to estimate the percentage overload which might be applied as a result of system malfunction before the motor reaches its stalling point. Figure 2 shows the relationship between the required data.
If the torque varies with time then the maximum range should be found. If it varies with speed then the relationship between the two is needed.
The Duty of the system involves the number of hours per day it is in use and the proportion of time the system is working at full load. Motors are rated according to the time they can maintain their full load without its temperature exceeding the maximum permitted by the insulation class. Continuous rating means it can work continuously and has a built in overload margin. Continuous maximum is similar but without the overload margin. Intermittent rating for a given time means that working is restricted to that time and a minimum time must be allowed for cooling between loaded periods. It follows that for a given motor the 'continuous' rating has the lowest value. In order to take account of the function of the driven machine and the duty cycle a service factor is used. Selected values of service factor are shown in Appendix 3.
AC motors starting a system having high inertia usually demand a high starting current. This may be inconvenient and may carry tariff penalties. Special starting devices may be appropriate. The inertia of the system and the frequency of starting should be determined. The starting characteristics of selected AC motors is shown in Appendix 4.
Acceleration of the system will depend on the difference between the torque required to drive the system at a given speed and the torque provided by the motor at that speed. It is necessary to ensure that this is sufficient and sometimes that it is not excessive.
The system mechanical efficiency accounts for all the 'losses' in the system. A guide to system efficiencies is given in Appendix 5.
The nominal power requirement of the system is given by:
Nominal Torque(Nm) x Nominal Speed(rad/s) x Service Factor / System Efficiency
The motor may drive a machine through a coupling directly or through some intermediate device such as a gearbox. In both cases the nature of the shaft coupling and the compatibility of the shaft geometry is important. Shaft couplings are the subject of another Guide in this series.
|Direction of Rotation|
Most systems are designed to rotate in a specific direction so it is necessary to determine which direction. Some systems require to rotate in both directions, i.e. to be reversible.
DC motors can be reversed easily by switching the polarity of the supply. AC motors running on a polyphase supply can be reversed by switching the phases. AC single phase motors are difficult to reverse and are usually designed for a specific direction of rotation.
If reversing of the system is required the frequency of reversals must be determined. The system inertia and acceleration characteristic will be important considerations.
For systems running at a nominally fixed speed it is necessary to determine that speed and the permitted variation or tolerance. For systems which are required to be driven at varying speed the speed range must be supplied as well. Many electric motors can be controlled easily over a wide range of speeds. However it is important to specify the minimum acceptable speed range since there are usually significant cost penalties involved.
DC motors may be controlled by varying the voltage of the supply with feedback if precision is required. AC motors run at a nominal speed related to the frequency of the supply and the number of field poles so changing either of these parameters can provide varying speed. In addition small variations of speed may be obtained by varying the supply voltage or the electrical resistance of the rotating part of the motor. Standard speed motors should always be specified if possible, on cost grounds.
The normal electrical supply to industrial installations in UK is 3-phase Alternating Current which implies 415 volts at 50 Hz frequency. Single phase AC machines can be supplied from one of the phases provided that the three phases are loaded as equally as possible. In some installations a Direct Current supply may be available or be derived from the AC supply with the use of rectifier circuits.
If DC supply is already available its voltage and maximum permitted current should be established. In some cases the maximum variation from the nominal voltage is important.
If the AC supply is to be used the number of phases available and frequency must be determined. Then the nominal voltage, the lowest allowable power factor and the maximum permitted current must be found. This last is particularly important for AC motors since during starting they tend to draw high currents.
The environment of the motor is taken to include its entire conditions of working. These include: the maximum ambient temperature of the cooling air: temperatures above 40š C reduce the nominal rating of the motor. As an approximate guide an increase of 5š C in the ambient air temperature reduces the power rating by 5%.
The altitude of installation: air density is reduced at significant altitudes above sea level and with it the capacity of the air to cool the motor. The following table may be used as a guide:
|Reduction in Power Rating (%)||8||1 5||2 5|
The combination of these two factors will affect the class of insulation to be specified for the motor since both affect the cooling rate. The following table gives a guide to the allowable temperature rise for the most common insulation classes.
|Max. Temp Rise (šC)||75||90||140|
|Insulation Class (BS 4999)||E||B||F|
The condition of the surrounding air: this may be contaminated with liquid and/or solid particles of varying sizes. This factor determines the type of enclosure to be specified. While there are theoretically many types available, the three most common are:
drip-proof (DP); totally-enclosed / fan -cooled (TEFC); Flameproof (FP)
These are defined in Appendix 6 and together with other types are specified by British Standards.
The installation requirements: the space available to manoeuvre, install and fix the motor must be determined and the means of handling it should be known especially if it is a large machine.
Motor casings may be mounted independently on flat surfaces or located on a driven machine using a flange. Standard casing dimensions cover a range of motor types and are specified by British Standards. Most manufacturers include dimensional information in their catalogues.
Motors must be firmly and accurately mounted to transmit the required torque and to avoid misalignment of the shafts. Special mountings may be used to adjust the motor's position or to reduce the effects of vibration. All mountings should be checked to ensure that they are suitable for the forces applied by motor torque, weight and inertia.
The cost of electric motors varies with power rating, type of motor, ancillary equipment required and, of course, manufacturer. Appendix 7 gives a guide to the comparative cost of motors on the basis of their 'type' and manufacturers should be asked for quotations for specific cases.
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