It seems odd that after over 100 years of electric motor manufacturing, we have very few people in the industry that can really ‘boil it down’ to the basics of what is important in applying electric motors. An electric motor transforms electricity into mechanical work. That’s why most electric motor manufacturers employ mechanical engineers. And why controlling electric motors is almost exclusively done by electrical engineers. The two rarely meet.
Consistent with the Second Law of Thermodynamics, any time energy is converted from one form to another there are losses that are manifested as heat. Efficiency is one of the primary figures of merit that give us a reference point for how well the conversion process works. But more than just a relative measure, efficiency values across a range of technology options, leads to the ability to measure cost of operation for any given application.
So it’s important to understand the mechanical load requirement in some detail and then navigate the technology trade-offs. Purchase cost, installation cost, operating costs, maintenance costs and life expectancy all play a part in the life cycle evaluation. One of the great features of electric motors is that they are generally designed for low maintenance and life expectancies in excess of 50,000 hours. Most large equipment motors can be rebuilt and put back into service at relatively low cost. Which is why electric motors continue to be the ‘workhorse of industry’.
Navigating the trade off is not so easy. This is because over the years there have been so many variations produced that it is hard to make definite rules to guide the selection process. Rules that seem to work well for large motors don’t work so well for small motors because of scale effect and many more technology options. The decreased cost of motor control technology the use of electronics cost effective and especially important for long term operation of production equipment.
One basic distinction in the electric motor is variable speed construction direct current motors versus constant speed alternating current motors. Edison versus Tesla. Unfortunately even this basic distinction has broken down in presence of the variable frequency drive (VFD) that is so popular today for use with alternating current motors. In the past VFD’s had significant limitations so that it was possible to maintain the distinctions, particularly in areas requiring low speed operation of the AC motor. Modern VFD’s are able to provide torque at zero speed when position feedback devices are added to the motor and the rotor position can be fed back to the drive.
In the past the simplicity of the AC motor and the fact that it did not require expensive controls made it extremely popular and low cost to install. As the control cost has fallen, many features can be added to enhance the operation of the motor.
More next week