Another aspect of applying electric motors to power mechanical systems is the relationship between peak power and continuous power. In mechanical systems the forces required to start a load may have no relationship to the power required to keep the system running. Further, the ideal demand for mechanical power may occur at a speed that has no relationship to the electric motor speed.
AC motors operate at fixed speeds unless they are controlled by a frequency inverter. So matching the electric motor to the demand for mechanical power requires some electrical sophistication. The most important factor in most energy conservation applications for inverters and AC motors is creating the right control strategy to match the demand for power to the to electric motor. (we’ve done some articles on this subject so I won’t repeat the comments here.
Interestingly, the same problem with continuous and intermittent ratings show up in a lot of situations. In the alternative energy arena, many systems are specified based on the peak power available from the equipment. Most of the photovoltaic systems being installed are flat panels which only reach maximum output for a couple of hours a day when the sun is perpendicular to the solar panels. During the rest of the daylight hours the photovoltaic panels put out considerably less power. So there’s a big “disconnect” between the cost of the technology and the value it produces.
Photovoltaic pricing is still very expensive. Residential installations that can produce enough power to take your home off the grid currently cost about $35,000 including installation. Most state programs and federal tax rebates will pay for about half the cost. But even at $15 to $20 thousand dollars, it costs more than most people can afford.
In the wind energy arena, the same rating problem exists. Wind power systems are rated at their maximum output. But that output can only be achieved a certain number of hours out of the year when the wind is blowing in the right speed range. Not too fast, because it’s hard for the power conversion systems to function, and not too slow or the wind won’t turn the generator.
So these million dollar machines must harvest the wind enough hours to make a profit. This means it’s all about “location, location, location”. The game is to find a location where there is enough wind for enough hours to generate electricity and a profit. And that’s not easy, and it’s not cheap. Locations that are suitable, like Altamont Pass in California, are remote and hard to get to. This make installation more expensive and losses from sending the electricity long distances, less efficient.
In general the difference in peak versus continuous rating wouldn’t bother me so much, but it’s systematic in the alternative energy community. It suggests a bit of misrepresentation as if to create a greater perception of value, when in fact, the systems being built take 8 years before they break even.
We can do better.