Electric motors are central to the industrial age. They are literally the replacement technology for the “work horse” of the 19th century. Which is probably where the term “work horse” came from in the first place. And when it comes to units of measure, we think about the electric motor in terms of horsepower. This not by coincidence, but rather as the result of James Watt’s invention of the term to describe the amount of work that a steam engine could perform in contrast to the work performed by a horse. It might seem silly in retrospect, but it may have been one of the most important inventions in mechanical engineering history. The unit of measure of work.
The electric motor is the transducer of electrical energy to mechanical work. This is probably the most important relationship to keep in mind at all times when working on a new design that requires mechanical power. What is sometimes missed is the opportunity that is afforded on existing equipment. The motor can be used in the opposite sense and will allow direct measurement of the load electrically. So by measuring current in an alternating current circuit, it is possible to directly examine the load behavior accurately. As the current waveform changes, the torque required by the load is measured directly.
This can be an invaluable tool to help diagnose machinery problems, or simply to reduce excess power usage. Often, electric motors are installed larger than needed because of concerns about the starting load or due to availability of the right size machine. If the machine in question is new and has never been built before, the oversizing is precautionary. Regardless of what the reason, a lot of energy is wasted in keeping the motor turning when it is too big, and several DOE studies over the years indicate that this is a very common situation.
Among the more common problems in applying electric motors is the speed mis-match. Most electric machines are high speed. Typical ac motor speeds are 1800 rpm and 3600 rpm. Most loads are low speed. Even fans for moving air are typically around 800 rpm. The common solutions are gear reduction, belt and pulley reduction and electronic speed controls. Depending on the motor type, size and load conditions, any of these solutions can be cost effective.
The underlying problem is that any conversion of energy results in losses, usually in the form of heat. And the list of losses extensive. In an era of rising electrical cost, energy efficiency is a growing concern to many industries and to all consumers. Electronics holds the answers to the inevitable declining cost of controls that impact electric motor operation. What is not clear is how, or if, this will change motor design in the near future. There are few examples of real innovation in the motor arena, Novatorque being an exception.
There is still plenty of room for innovation.