Feature Articles

About the Author=20

Eugene Kocherov is a Controls = Engineer=20 at Princeton Power Systems, a company that = develops=20 advanced power conversion technologies, = including=20 AC-link=99, a patented control method that = provides a more=20 reliable and cost-effective means for converting = electric power cleanly and efficiently.=20 =

Princeton Power Installs an AC-Link Drive to Operate = Linear=20 Induction Motors

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Recently, Princeton Power Systems has installed a = Variable Speed=20 Drive (VSD), based on its innovative AC-link technology, at = Power=20 Superconductor Applications (PSA) Corporation=92s facility = in New=20 Castle, PA. The AC-link Clean Power VSD is used to = control=20 several linear induction motors used in ropeless elevators = that are=20 being developed for the U.S. Navy. This application = requires a=20 very precise control of frequency and current with very = clean output=20 waveforms. As other competing drives could not provide = an=20 adequate solution, PSA turned to Princeton Power. = Princeton=20 Power was able to deliver a drive that met all of the = customer=92s=20 criteria and offered outstanding performance and = reliability.

The Application-Ropeless = Elevators
Power=20 Superconductor Applications Corporation (PSA) is a leading=20 manufacturer of linear induction motors and levitation = apparatus for=20 guided ground transportation and roller coaster catapult=20 launchers. PSA conducts research in the fields of = cryogenics=20 and superconductivity and uses the acquired knowledge to = develop=20 magnetic levitation propulsion. PSA supplied linear = induction motors=20 for the nation=92s first passenger maglev system installed = at Old=20 Dominion University, Norfolk Virginia.

PSA was recently awarded a contract by the U.S. Navy to = develop=20 linear induction motors for ropeless elevators intended for = use on=20 aircraft carriers. The U.S. Navy is in the process of=20 developing all-electric ships, and is interested in = replacing=20 conventional, roped elevators with ropeless elevators = powered by=20 linear induction motors.

Ropeless elevators have significant advantages over their = hydraulic and roped, electric counterparts. Hydraulic=20 elevators are inefficient due to friction in fluid = transmission and=20 multiple energy conversion steps -- from electrical, to = mechanical=20 energy of the pump, to fluid energy, to mechanical energy of = the=20 piston, and so on. Also, conventional elevators are = limited to=20 vertical movement, the envisioned ropeless elevator system = can move=20 vertically, horizontally, or diagonally. This is = useful, for=20 example, in creating a =93highway=94 that can be shared by = multiple=20 elevator platforms.

The ropeless elevator experiments conducted by PSA are = powered by=20 linear induction motors, whose technology is very = similar to=20 rotary electrical motors. The only difference is geometric=20 shape. A linear motor is essentially a flattened = squirrel cage=20 motor where the =93stator=94 produces a linear force instead = of a torque=20 on the =93rotor=94. Therefore, the speed of a linear = motor can be=20 controlled in a similar way as a rotary motor using a = variable speed=20 drive.

So far, PSA has developed a subscale version of a = ropeless=20 elevator system. The subscale system can carry loads = up to 100=20 lb and consists of several different sections including = vertical,=20 horizontal, and inclined. To test these elevators, PSA = required a variable speed drive that could provide 100 amps = of=20 current at frequencies between 3 to 60 Hz and offer a low = output=20 harmonic distortion level.

Controlling Elevator Speed
Since = performance=20 characteristics of linear induction motors are different = than that=20 of rotary motors, certain aspects of the VSD had to be = tailored to=20 meet the criteria. As most commercial drives have = complex=20 control algorithms and hardware, it would not have been=20 cost-effective to modify any of these drives to control a = linear=20 induction motor. Also, to achieve low output harmonic=20 distortion, PWM-based drives require the use of very = expensive=20 filters, which would further add to the total cost. = After a=20 search for a drive that could meet the required criteria, = Dr.=20 Stephen Kuznetsov, president of PSA and chief scientist, = decided to=20 turn to Princeton Power, which offered to build a drive that = could=20 meet the customer=92s needs for a reasonable price.

The Solution: AC-Link
In a short = period of=20 time Princeton Power was able to develop a variable speed = drive that=20 met the customer=92s specifications, using its AC-link Clean = Power VSD=20 platform. This drive was able to deliver superior = quality=20 output waveforms without installing an expensive filtering = system,=20 the use of which would be required with existing PWM-based=20 drives. The drive performed flawlessly with several = linear=20 motors, which allowed PSA to successfully demonstrate the = operation=20 of the subscale version of ropeless elevators to the = Navy.

Behind the drive=92s excellent performance is AC-link = technology --=20 a revolutionary control method and circuit topology, which = utilizes=20 Silicon Controlled Rectifiers (SCRs) in a unique = configuration to=20 create clean sinusoidal power waveforms. Unlike other = drives,=20 clean power is inherent to AC-link technology, and with = flexible=20 software control it can be easily adjusted to meet the = requirements=20 of many applications without modifying the hardware.

Advantages of Clean Power
Clean power = of=20 AC-link drives is characterized by low input and output = total=20 harmonic distortion and voltage rate of change, also known = as=20 dV/dt. Low output harmonic distortion is very = important for=20 linear induction motors. Harmonic distortion creates a = backward component of thrust, which causes a motor to lose=20 efficiency. According to Dr. Kuznetsov, the loss in = efficiency=20 can be as high as 30%. In addition, it creates speed=20 fluctuations and dangerous overheating in a motor.

Input harmonic distortion is also a very important factor = to=20 consider when installing a variable speed drive. In = general,=20 harmonic distortion can overheat power system components, = trip=20 circuit breakers and damage a variety of equipment connected = to the=20 grid. AC-link drives have very low total input and = output=20 harmonic distortion of current and voltage, which is less = than 5%.=20 This level meets IEEE 519 standards for total harmonic = distortion=20 measured at the drive=92s input terminals.

Low rate of change in voltage is also very important = because it=20 prolongs the lifetime of a motor. High dV/dt produced = by=20 PWM-based drives causes wear of motor bearings and damages = winding=20 insulation that results in motor failures. Since the AC-link = drive=20 is a current source, unlike PWM-based drives that control = voltage,=20 it is able to limit dV/dt to 2 to 5 volts per = microsecond. =20 This is extremely low compared to 1500 to 3000 volts per = microsecond=20 produced by PWM drives.

These AC-link advantages resulted in excellent = performance of the=20 VSD with the linear induction motor application and led to = the=20 customer=92s satisfaction. As PSA continues to develop = linear=20 induction motors for various applications, including roller=20 coasters, ropeless elevators, aircraft launchers and many = others, it=20 plans to continue its relationship with Princeton = Power. =20 Currently, PSA is developing linear induction motors for = Maglev=20 trains. These motors will be able to carry 30-ton = trains at a=20 speed of 250 miles per hour. The motors will need 500 = kW =96 4MW=20 variable speed drives and AC-link technology will be an = ideal=20 solution to this new technological challenge.

Future Applications for = AC-Link
Currently,=20 Princeton Power is developing a commercial version of its=20 drives. In 2005, Princeton Power is introducing VSDs = with=20 features equivalent to drives on the market in addition to = providing=20 superior power quality. These features will include = process=20 PID control, speed jumps, various ramp options, jog, = programmable=20 pattern, and many others. AC-link Clean Power VSDs will also = have a=20 web-based user interface and will support the Modbus = communication=20 protocol. The drives will provide a customer with superior = power=20 quality as well as complete functionality, which will be = able to=20 solve many technical challenges for a variety of = applications.

AC-link is a revolutionary technology that can provide a = solution=20 to a variety of applications, including particularly = challenging=20 problems like the case of linear induction motors. The = AC-link=20 Clean Power drive=92s outstanding performance in this = application was=20 due to its simple hardware design, flexible software and a = superior=20 power quality =96 something that no competitors could = provide.=20

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