Control, Motors and Efficiency

I was talking with some friends about control technology and made the observation that over the last decade the progress in the control field has been really amazing.  Particularly, the processor technology that is available for controlling electric motors is operating 1000 times faster than the control platforms of a decade ago.  We look at events in nanoseconds, not microseconds.

Increasing the control system’s frequency response is not signficant in itself.  But it does mean that software can be applied to problems that are more subtle in the operation of a particular system.  Observation of the phase relationship between the rotor and stator in an electric motor is now commonplace in 3 phase systems.  Algorithms for optimizing this relationship dynamically are also commonplace to adjust the power factor or reduce energy consumption in inertial loads like fans.

But this is not where the big energy gains will come from.  These improvements are smaller and more incremental.

Variable speed motors are systems that are made up of electric motors and power electronic systems.  Both are subject to losses in the form of heat.  In the motor bulk magnetizing of the stator, phase loss due to load, and copper losses due to the construction methods used are common.

Better metallurgy is needed to reduce losses associated with magnetizing the stator core.  The steel industry has attempted to address this issue, but the high cost of exotic alloy laminations prevents the advanced materials from becoming widely used.

Copper loss is improved in the segmented stator, but this manufacturing technique is most often found in more expensive servo motors, even though analysis suggests the cost is lower.  This may have to do with scale effect, since the servo motor world runs at much lower volumes than the AC motor world.

The other major dependency in the speed control is the power semiconductor.  The costs for power devices are falling and performance is improving.

So where are the big efficiency gains going to come from?

The control system strategy.  If the application is not well regulated you might be able to get a big increase in efficiency by measuring things more carefully.  In a cooling tower changing from a +/- 10 degree thermostat to a +/- 1 degree thermostat allowed me to implement a control system that reduced the energy consumption sufficiently to pay for the equipment in less than two years.

No new technology motor, nothing special about the variable frequency drive.  Just what was available at the time.  The big difference was the strategy.  Measuring what was important and organizing everything in the control system to achieve our objective.

Peak versus Continuous Power

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.

Progress In Europe

The SPS Drives show in Nuremberg Germany is one of the best attended shows in the world.  And one of the reasons why has to the with the outstanding technical effort and preparation that all the vendors put forward.  Demonstration systems are sophisticated and perform flawlessly.  Exhibitor personnel are all very professional and technically astute.  And multi-lingual which is great for me because my German is almost non-existent, so its great having so many people able to bridge the gap. Read more

Different Drive Concepts for Different Machine Tool Motions

Here’s a quick look at your options when selecting drives for use in machine tools.

In machine tools, typically the details of the installed drive technology are concealed. In principle, however, there are several possibilities for main, feed and auxiliary drives to carry out the needed machine movements.

Main Drives

Main drives are predominantly closed-loop controlled, electric synchronous, and asynchronous motors. Their applications include kit or housed motors for use in turning, milling and grinding machines as well as in machining centers. The traditional spindle drives with housed motors – mostly air-cooled – are also popular as main drives. In comparison with motor spindles they are less costly when considering the secondary costs of both systems. On the one hand, the interposition of gearboxes enables the rotational speed and torque to be tuned to the machining task. On the other hand the gear boxes cause unwanted radial forces, noise and increased wear. Read more