DYNA JOURNAL ENGINEERING

There are many types of motor, from high cost high performance servos to models at the cheaper end of the range, including many specialist types of motors. Choosing the right one for any given application depends on many variables, but very often a standard industrial AC induction motor is the best answer. These are relatively low cost, reliable, efficient, and well understood by engineers across all industrial sectors. The main considerations for selection for this type of motor can be outlined as follows:

Power supply

Most mains power supplies are single phase or three phase, with the latter almost universal in industrial environments. Single or three phase motors must be used, depending on the supply. Three phase motors tend to be smaller and more powerful.

It is also imperative to consider the motor's voltage against the supply voltage; it is usual that the selected motor has a 'nameplate' voltage about 10 percent less than the supply voltage to allow for distribution losses. Mains supply is alternating current (AC, as opposed to direct current, DC), and its frequency, measured in Hertz, is fixed. The chosen motor is usually matched to the supply frequency, to within about 5 percent.

If the supply voltage and/or frequency fluctuate widely, motor performance will suffer and its working life may be shortened. However, it should be noted that motors can be used in conjunction with variable speed drives, which make the output speed adjustable by varying the infeed frequency.

System requirements

The driven equipment will require a certain performance from its motor, including rated speed (motor output speed), torque and power.

The rated speed depends on the supply frequency and the number of poles (magnets) for which the motor is wound. Speed increases with frequency but decreases with pole count. The theoretical speed of a motor is called synchronous speed, but the motor's actual speed is always lower due to electrical and mechanical losses. This is called the slip and is typically 5-20 percent.

Torque and power determine the size of motor required for each application. Torque is the turning force acting through a radius. Power increases with speed, and so is a measure of work done.

However, the torque characteristics of a motor must be understood. Starting torque is usually higher than full load torque; this typically drops slightly as the motor gains speed, then climbs again to a value in excess of starting torque before suddenly dropping to its full load torque, at which point the motor is producing its rated power (in kilowatts or horsepower). During this ramp up period, the motor will be drawing considerably more than its rated current from the mains, but settles down at full load speed.

The starting characteristics of a motor need to be selected for each application. In fact they can be used to advantage, for example by giving a gentle start to a loaded conveyor, or a rapid start to a ventilation fan.

Motor classification

While motor classification is a difficult area, some basic rules and common sense will certainly help. There are two main motor classification systems, IEC in Europe and NEMA in America and a good rule of thumb is that industrial applications will use one or other of these (there are specialist classifications for marine, aerospace and other fields). The details of the classifications are beyond the scope of this article, but can be looked up on line or explained by a reputable supplier.

However key points to consider include:

• The 'service factor', or the amount of overload a motor can take.
• The type of insulation, a critical issue for both safety and longevity
• Temperature: motors get hot when running and may need forced draught or even water jacket cooling
• Enclosures: if a motor has to work in a confined space, heat build-up can be very significant.

Carbon emissions

Energy efficiency ratings have been applied to electric motors for some years, but legislation in this field is tightening and low efficiency motors no longer comply with today's regulations.

Furthermore, the European Union has just introduced the Minimum Energy Performance Standards (MEPS). This measures not the efficiency of the motor but the efficiency of the overall drive system, including any mechanical drive train components, variable speed drive or softstart. Thus it is no longer acceptable to simply fit an energy efficient motor; the whole installation has to be certified.

At this point, it should be noted that the old practice of oversizing a motor so that it has extra power for coping with overloads or blockages for example is in effect illegal. One must include a variable speed drive to ensure these issues are properly handled.

Mechanicals

There are also some mechanical issues that have to be considered. The first is that the motor's internal bearings must be suitable for the duty they are likely to endure (also the seal that protects them requires consideration). This is generally covered by the Standards, but it is worth getting expert advice from your motor supplier if your application is in any way unusual.

The second mechanical issue is mounting. There are two common ways to mount a motor, flange- mounted (end on) or foot-mounted (side-on). In both cases you need to make sure the mounting is firm and will endure possibly years of use; with foot-mounting you also need to check that the motor shaft is at the right height to connect with the driven equipment. You may also need to think about noise and vibration, maintenance and demounting.

This article has covered just the basics of motor selection and installation. But by bearing the fundamentals in mind and using the expertise of reputable supplier, you should be able to make the correct choice.

It's also worth bearing in mind that the purchase price of a motor is likely to be about five percent of the lifetime cost of ownership, so deciding on the cheaper option is not really the best way to save money.