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.

Electric Vehicles and Electric Motors

A friend of mine finally got delivery of a Tesla Roadster.  This prompted discussion of the drive train and the fact that Tesla has had to go from two speed transmissions which were failing to a transmissionless drive train.  The ultimate mechatronic challenge, the electric car, is also a challenger in terms of the precise  application of electric motor technology.

But it has to be said that the motor and drive solution for the electric car is not where the problem has to be solved.  Any motor can be made to run an electric car.  What is critical is how you apply it.  The starting conditions require high torque at low speed and the running conditions require low torque at high speed.  So, typically, what looks like a small 5 to 15 horsepower running requirement at full speed, becomes a 150 horsepower starting requirement depending on how quickly you would like to start.  If you want to keep up with a Corvette, it uses 450 HP to start.

And this produces a lot of confusion.  Why not use at 2 speed transmission to help the situation.  Fine, but the ones that are available can’t handle the dynamic response of the electric motor.

Can electronics help this situation?  Interestingly, yes.  There is a control algorithm generally called vector control which allows you to manage the rotor torque and stator torque separately.  By varying the phase angle between the two, like advancing and retarding the timing of a mechanical distributor cap on an internal combustion engine, you get different speed torque curves out of the motor.  COOL!  Is there any downside to this?

Yes.  You need more current to produce more torque.  That doesn’t change.  So you have to be able to supply the current, and you have to be able to manage the heat.  The heat is transitory since you only need the high current during starting, but it is best to have sophisticated software running to keep track of the RMS temperature of the motor.  Lower operating temperatures mean longer life and reduced risk of demagnetizing the motor.

So, yes, you can run an electric car with a garden variety AC motor, and with good electronics, you can make it run fairly efficiently.  With higher efficiency motors, the benefit is increased driving range from a given power source.  High efficiency motors are frequently smaller and lighter weigh, but a weight savings in the motor of 50 or even 100 pounds is not that big a factor in the driving range when the curb weight of the vehicle is 3000 pounds.

Basically, its F=ma.  If you can reduce the mass of the vehicle, you reduce the battery payload required to power the car.  Aluminum space frames, like on the Prowler, have been studied by the car industry and can reduce curb weight by 400 pounds and reduce cost by 10% at the same time.  We need to bring all the mechatronic leverage to the situation that we can, if we are going to make electric cars that make sense.  Before its too late for Detroit.

Mechatronic Top Ten – Hard Disk Drives

One of the mechatronic Top Ten applications has to be the hard disk drive.  Strangely, it is not an application that you hear much about.  That’s probably because unless you work on hard disk drive design, you pretty much take for granted that little black box that stores all your information. So the group that is actually pushing the design frontier of hard disk drive technology is a very finite group.  There are only a dozen companies actually making disk drives these days, after consolidation in the market has resulted from acquisitions and mergers over the last decade.

Worldwide consumption of hard disk drives is in the tens of millions per year, and like all things electronic and high volume, the industry produces ever more memory at ever lower prices.  The absolute value of hard disk technology is one of the most incredible bargains in the world.  The current state of the art is about 10 cents per gigabyte which is quite a bargain compared to the 1.5 Megabytes for the old 3.5″ mini floppy disk.  With seek times in the low milliseconds, memory is almost instantly available due to 7200 RPM platter speeds.  The 7200 RPM speed is the equivalent of 75 miles per hour at the edge of the platter.  Higher speeds have been delivered, but the thin aluminum platter is subject to “flutter” which can cause a head crash.

The spindle motor is a 3 phase dc brushless motor that is designed to accelerate the memory platter to the 7200 RPM running speed in just 2 or 3 milliseconds.  This is an incredible feat considering that the power available is limited to a small lithium battery.  Further, the spindle motor must coordinate it’s motion with a linear actuator to place the drive’s read head a few millionths of an inch above the platter surface at the exact target sector on the disk.  So, just getting the platter to spin, which is hard enough given the time constraints, is further complicated by the extreme challenge of coordinating the rotational motion with the linear motion of the read head.

What makes this all even more astounding is that the budget for the motor can only be a few dollars, given a retail selling price of $60 for the whole package including the memory.  I don’t know how these guys come up with the solutions, but they consistently do and they consistently do it at lower prices.  The last thing I remember reading about was the elimination of bearings in favor of fluidized bearings.  At 60 million units, saving money on bearings adds up to a lot of money.

One of the many ironies of the hard disk drive is that it is at the root of many improvements in industrial motion control.  The venerable 33035 controller chip from Motorola was developed specifically to run hard disk drives.  It later appeared in a number of industrial servo amplifier designs delivering precise control of higher current power to a variety of brushless dc servo motors.

You never know where the breakthroughs are going to come from, but we keep them coming.  Keep up the good work!

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.

Super Size my Motor?

There is an interesting problem with applying electric motors that is a constant source of difficulty, the nature of peak power versus continuous power.  The problem is that few systems operate at a statistical average power demand.  Frequently, this causes equipment designers to oversize the motor for the application.  At the same time, however, this can put the motor in a very low efficiency operating range.

So what’s the right solution?  Right sizing.  Yes, just like Goldilocks and the Three Bears, not too big, not too small, but just right.

There are some great DOE publications on motor sixing that can be very helpful on the AC motor side, so make sure to give those a look.  But the implications of how to deal with varying loads are complex, each requirement having its own unique conditions that need to be considered.  Is an underpowered application actually safer?  Sometimes, yes.  I recently noticed that a particular orbital sander had been designed so that if the unit became momentarily overloaded, it stalled.  Perfectly safe.  In fact, this design is to be preferred because it prevents accidentally damaging a work piece by burying the sander in the wood and removing too much material.  Who’d have thought of it?  Certainly not Tool Time Tim.  More Power!

In fact, most of us view more as better.  More power means more production.  Or does it.  In an increasingly energy conscious community, more power means more cost.  And that’s really what its all about.  The value of the motor is not just in the purchase price, but also in the operating cost.  Especially if the motor is expected to run for 8 years, 24/7.  (That’s what the life expectancy of large AC motors is)

There’s another trick to the power requirement problem.  How much time is spent at full load and how much time is spent at average power, or, what is the duty cycle?  If the system is starting and stopping frequently it puts different constraints on the motor.  If the system is typically starting only once an hour, then we can consider the thermal duty cycle of the motor.  The momentary peak power requirement is insignificant and the vendor can usually tell from their modeling and testing of their products how much impact the peak current will have on the motor’s average temperature.

After all, its Thermodynamics 101 in the final analysis.  Every energy transformation produces heat as a byproduct.  How much heat a given system can tolerate is the key to its operating life.  In electric motors, the key values are the insulation system’s temperature rating, usually in the range of 150 to 180 C and in the case of steppers, brushless dc and permanent magnet dc motors, the magnet’s ability to resist high temperature and high coercive magnetic fields that can be generated in the motor.  Both sets of limits are generally well considered by suppliers when electrically controller motors are shipped as motor/drive combinations.  This can get a little tricky when pairing motors from one vendor with controls from another vendor.

Materials and Motion

Most motor sizing programs deal with time and torque analysis.  The traditional tradeoff is more torque for less time.  As an aside, the increase in electric motor torque comes with increased motor inertia, so it’s not for free.  And the motor costs are always a factor.

But this assumes that inertia is fixed.  And that’s an OK assumption as long as the assumption is made consciously.  Because, it’s only an assumption for the convenience of doing a time/torque tradeoff analysis, not reality.

The fact is that there are lots of options on inertia, whether the mechanism is new or existing.  If the parts are existing, you know what your starting parameters are.  But the real issue is to NOT ignore the options.  There’s a lot of performance bandwidth to be had by exploring materials and inertia options.

Here are some typical material properties; steels are approximately 7.86gm per cubic centimeter.  Good old steel, cheap, strong, dense.  Typical strength is 50-60kpsi without getting into the exotic alloys.   This is where most mechanical designs start because of steel’s low cost.

Machinable grades of aluminum alloys are routinely available in the 30-40kpsi range.  And the density is only 2.7gm/cubic centimeter.  Roughly one third of steel.  That means only 1/3 the inertia.  And only 1/3 the amount of torque needed to achieve a given motion.  So even though aluminum costs three times as much as steel, the cost of the motor to drive the load is reduced significantly.  In addition, most machine shops can run aluminum parts twice as fast, so it costs less to machine.  A lot less.  So at the end of the day, even though we could start the comparison of steel versus aluminum at direct material cost, that comparison wouldn’t take into account all the benefits.

Then there are the engineering plastics which have gotten better over the years.  Polycarbonate, for example, has strength in the range of 10-13kpsi with density of 1.3gm per cubic centimeter.  Half the density of aluminum, good strength and very inexpensive.  So in some cases, you could use more polycarbonate volumetrically to replace aluminum and reach comparable strength requirements while reducing inertia at the same time.

This is all based on common off the shelf materials.  The  options get even more interesting when you start exploring more exotic materials.

Titanium is a great alternative to steel when high strength and light weight are required.  But it’s expensive material to buy and because it is so hard, the machine costs are typically much higher than steel.  But when you have to have it, you have to have it.  And I have had projects where we needed the inertia advantage, and the premium paid for the material made possible some applications that couldn’t have been done any other way.

A friend of mine developed a material called AlBeMet, a blend of aluminum and beryllium.  The beauty of this material is that it has the strength of titanium at the weight of aluminum, beryllium being much harder and lighter than aluminum.  Beryllium doesn’t alloy well, but that’s the part the folks at Brush Wellman were able to get done.  And the results are phenomenal.  Again, it is expensive material, but you don’t need much of it to get the job done.  And where strength and light weight is needed, this stuff is incredible.

But the real point is, keep your options open when working on mechatronic designs.

Common Terms are Not So Common

I have worked around electric motors for a long time. It still blows my mind that we can’t seem to settle on common terms and definitions of things we work with on a daily basis. When is a motor AC Synchronous or DC Brushless? They are identical. Same motor.

So you have to start from the beginning of the motor family tree and work your way through it. What makes a motor AC versus DC. The only definition I know of that makes any sense is this:

An AC motor has only one magnetic field, in the stator, and induces a second magnetic field through induction, into the rotor. The rotor becomes magnetized and follows the circulating magnetic field in the stator by passive attraction. This is why most AC motors are referred to as induction motors, because the magnetic polarity in the rotor is induces. AC motors as a result are constant speed machine based on the frequency of the electrical excitation. Since that’s mostly 60 hertz in the USA, its usually some multiple of 60, most commonly 1800 RPM (minus some rotor slip). AC machines require an inverter to change AC into DC and then DC into variable frequency AC in order to achieve variable speed performance. Due to decreasing costs for the AC motor and recent cost breakthroughs in inverter technology, AC variable speed is the dominant solution. Read more

Motor Industry and the New Year

Hope everyone had a great holiday and a successful year in 2007.

I was reviewing the Department of Commerce figures for electric motors. Hey, its just something I do.

A few years ago when I was hired to help a young high tech motor company in Denver understand the marketplace, we started with the Department of Commerce data for electric motors and generators. We studied the data and went into an interesting exercise in analyzing the market. At that time the US produced approximately $12B in electric motors. That includes all the industrial and high tech stuff, as well as starter motors on cars, alternators, generators and fractional horsepower fans you find around the house. Big numbers, lots of parts. Read more