DRIVES, CONTROLS & MOTORS FEATURE


KEY CONSIDERATIONS FOR OEMS SELECTING ELECTRIC MOTORS


With numerous characteristics to consider, each impacting on the performance of a motor in its application, the motor selection process can seem daunting. Lammers Exico’s managing director Jerry Hodek offers some guidance


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electing electric motors can be critical for OEMs - If the motor is under


specified it can impact on systems performance and will potentially fail prematurely, and if it is over specified it may have high running costs. However, motor specification can bring many benefits to the OEM and end-user, including reduced size, lower running costs and overall competitive advantage. There are many parameters that need to be considered in any motor selection process, starting with speed, torque, load profile and duty cycle. Engineers may also need to consider the motor’s size and weight, while noise could also be important. Furthermore, how will the motor be controlled, and what are the energy efficiency requirements? What are the mounting arrangements and does the operating environment demand an IP rated unit, temperature resilience or washdown capabilities? However, there are likely to be other factors to consider too, and it may turn out that these cannot all be optimally addressed and that compromises are required. With each application having its own unique needs, engineers may need to prioritise operational parameters and look at the best compromise for others.


MOTOR LOAD CHARACTERISTICS Speed and torque will usually be the starting point for any motor selection process, determining the motor’s performance and size. Generally loads are either constant torque or variable torque, with the latter further divide into those with gradual torques changes, such as fans and blowers, and those with abrupt changes such as lifts and conveyors. When addressing sudden torque changes, engineers should look for a motor where the speed-torque curve exceeds the application’s load requirement. When torque load changes are gradual, it is more important to consider the highest continuous load point. This will typically be at the fastest speed. Starting torque is also a major consideration, particularly if the motor is


required to start under load. Considerations about the duty cycle


may be a bit counter intuitive. When a motor is to be run nearly constantly, there are firm conditions for sizing the motor and many of the performance parameters become stabilised. Intermittent duties may seem to offer the motor a less demanding working life, but high frequency inrush current at each start up may cause thermal stressing. Torque and speed calculations will lead to an overall motor power. A motor should not be underspecified in terms of power. This will lead to performance problems and potentially shortened life, both of which will impact on the OEM’s reputation. However, engineers should also avoid the temptation to over-specify a motor. Not only will this add to running costs for the end user, but it will also add to component costs and potentially increase the motor size to the point it can impact on the overall system footprint. An important issue, which is often


overlooked, is the consideration of a motor’s acceleration torque. For instance, a motor with good starting torque and pull-up torque will start much more quickly; so for applications where acceleration up to speed is important motors with this characteristic should be used. If the motor is to run in a field weakened mode (when VSD driven, for example), a motor with high ‘breakdown torque’ may be preferred, as it maintains torque output over a wide speed range. Finally, in terms of load characteristics,


overload possibilities must be reviewed. Motors are rated to run at a specified level of overload without damage. This may be stated as a percentage of rated power, or as a service factor, and must be considered for safety and to optimise motor life.


MOTOR EFFICIENCY RATINGS Much has been written in recent years about motor efficiency ratings and engineers should always look at motor power requirements to determine, at the


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very least, the minimum efficiency rating. The baseline is Correct motor


specification can bring many benefits to the OEM and end-user, including reduced size, lower running costs and an overall competitive advantag


now IE3, although if the motor is to be driven by an inverter, IE2 may still be applicable and may well be the best option. IE4 is also an option and IE5 motors are coming onto the market. There are cost implications when buying more efficient motors, but they will generally be smaller and may offer other advantages too.


ENVIRONMENTAL CONDITIONS Ambient environmental considerations will always be a key issue in motor selection. Most standard motors are designed for a clean, dry, environment at room temperature. A more highly protected motor will be needed if the eventual application is to be outdoors, or if there are high levels of dirt, dust, water splash or even washdowns. Marine and shoreside applications, for example, generally require special motors. High ambient temperatures, raised humidity, corrosive environments and even altitude, all will affect a motor’s long- term performance, so must be considered from the outset. For instance, the temperature may impact on the required insulation class, or may suggest the use of a cooling system. Similarly, the motor’s IP rating will give an indication of suitability for use in dusty or water- splash environments. Finally, within environmental considerations, engineers may need to assess whether motors will be operating in explosive atmospheres and look at ATEX ratings accordingly. With so many issues to consider, the best approach is to engage the expertise and experience of a reputable supplier, discussing applications requirements thoroughly and following advice.


Lammers Exico www.exico.co.uk


DESIGN SOLUTIONS | JUNE 2017 23


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