Today's Sensorless AC Drives Have More to Offer


A djustable-speed ac drives operating without an encoder or other external feedback device-so-called sensorless drives-represent a growing middleground between high-performance closed-loop control and simpler Volts/Hertz control for induction motors. As explained in the main article , a good deal of sensorless drives' popularity stems from improvements in speed regulation under changing loads, starting torque, and low-speed control. Most sensorless ac drives are based on one of various flux vector methods-loosely named sensorless vector (SV) control.

Control Techniques (Eden, Prairie, Minn.; Newtown, Powys, U.K.) sees a definite emergence of sensorless vector control because of the improved dynamic performance it offers over open-loop (V/Hz) control. However, open-loop drives still make up 50-60% of the business, according to Alan Baird, Control Techniques product manager for AC Drives, based in the U.K. 'At the other end of the drive spectrum, closed-loop flux vector control represents only 5-10% of CT's business, though several thousand units are installed overall,' adds Mr. Baird.

Product examples from Control Techniques that include SV control are its UniDrive with a size range of 1-1,000 hp (0.75-750 kW) and Commander SE drive (with a 0.25-15 kW power range), which has become the successor to the Dinverter product line. Importantly, UniDrive also incorporates three other motor control modes, see CE , March 1996, pp. 9, 89. Control Techniques is one of the 60-plus divisions of Emerson (St. Louis, Mo.).

Sensorless vector drives' present expansion comes from a general improvement and maturing of the underlying flux vector control technology. Wayne Kantarek, marketing manager, AC Drives, at Mitsubishi Electric Automation (Vernon Hills, Ill.), reflects, 'Competing with dc drives and motors, early flux vector control did not live up to its full promises, but now better algorithms, motor modeling, microprocessors, and other factors make it reality.'

Mr. Kantarek puts primary market applications of SV control drives in lift/hoist, textile, and printing industries. 'Future sensorless technology will improve speed control, torque control of motors, and extend the usable speed range,' he says.

Mitsubishi Electric currently offers two products featuring sensorless vector control. E500 Micro Inverter covers a range of 1/8-10 hp (0.09-7.7 kW) for 240 and 480 V ac applications, while A500 Inverter Series ranges from 1/2 to 1,250 hp (0.37-937 kW) for 240, 480, and 575 V ac supply. Larger models (above 75 kW), designated A500(L) Series, offer a 120:1 speed range while delivering over 200% torque at 0.5Hz.

Siemens Energy & Automation (Alpharetta, Ga.) concurs that SV control is growing due to lower cost and simpler approach without the need for a feedback device and cabling. Dave Kirkpatrick, AC Drives product manager at the Solutions Business Unit of Siemens E&A, believes that better torque response under dynamic loads and handling speed reversals from sub-cycle torque changes distinguishes SV drives from V/Hz-type drives.

MasterDrive Sensorless Vector Drive from Siemens uses a single logic platform for all variations of the drive from 1 to 5,000 hp. (V/Hz and flux vector control modes are also offered.) 'An autotuning feature allows users to select the dynamic performance of the application, which guides the autotuning to the degree of performance required,' says Mr. Kirkpatrick. The drive can work as a stand-alone unit or be connected on a common dc bus, using one of the front-end modules, such as a rectifier, regenerative rectifier, or active front-end.

The latest microprocessors push the capabilities of sensorless drives. 'Higher speed microprocessors with on-board peripherals that lend themselves to motor control are expanding the performance envelope to a broader speed range,' remarks Susan Bowler, drives marketing manager at Schneider Electric/Square D Co . (Raleigh, N.C.). Ms. Bowler also attributes sensorless ac drives' adaptability to wider applications and easier implementation to 'more powerful microprocessors available today.'

Schneider Electric's Altivar 58 drive incorporates third-generation sensorless vector control. Alitvar 58's features include 100:1 speed range, adaptive tuning of motor characteristics for ease of set up, improved dynamic response for faster adaptation to load changes, 'accelerating torque' for difficult to start loads, and continuous motor braking capacity up to 100% torque.

Testing some competitors

B aldor Electric Co. manufactures 'encoderless vector' (SV), closed-loop vector, V/Hz, and servo controls. A particular solution is dictated by the details of the application, as it should be. In Baldor's application and user experience open-loop control remains very effective.
To gain further insight about sensorless controls, Baldor tested SV drives from seven of its competitors. All SV drive brands were run under the same test procedures at Baldor's Fort Smith, Ark.-testing facility. Here are some performance results from the comparative evaluations provided by product manager Roddy Yates:

Low-speed torque-full rated torque capability was possible down to low speeds in the range of 10-150 rpm, depending on the manufacturer.

Speed regulation-an 'average' value of 1% base speed was obtained, with a variation of

Peak starting torque-was found to be within the range of 160-200% of rated.

These results are generally in line with, but somewhat lower than certain claims. It's possible that 'expert' operators from a given drive supplier could have tweaked more performance form a given unit. SV drives require a bit of fine-tuning compared to the simpler V/Hz units. Many customers prefer the simplicity.

'Sensorless vector drives usually don't work right out of the box without some motor information. Most inverters will at least rotate the motor out of the box and are still lower in cost,' says Mr. Yates.

Baldor Electric Co . (Fort Smith, Ark.) notes a 'healthy increase' in the growth of sensorless ac drives. At the same time, the open-loop market is not being diminished by the emergence of sensorless technology, according to Baldor product manager Roddy Yates. Rather, the growth is due to the more common availability of SV and V/Hz controls together in one drive. 'When sensorless technology is combined with tried and proven V/Hz control algorithms, a competitive package emerges to fit a wide variety of motor control applications,' he says.

Baldor makes a broad line of sensorless vector drives with input voltages of 230, 460, and 575 V ac. Called 'Encoderless Vector Controls,' they're available in single-axis or multi-axis designs and with NEMA 1 and 4X enclosures or in panel-mount styles. Output ratings cover the power range from 1 to 700 hp (0.75-525 kW). www,

Increasing the number of motor parameters measured will increase performance in SV drives. However, there is a tradeoff between the extra performance obtained (or needed in certain applications) and the cost/complexity of adding more sensor circuits. Particularly voltage feedback can raise the cost of SV drives. Russ Kerkman, engineering consultant at Rockwell Automation (Mequon, Wis.) mentions the so-called 'five-parameter' motor model used in standard SV drives. These current-based measurements are:

  • Stator resistance;

  • Rotor resistance;

  • Magnetizing inductance;

  • Stator leakage inductance; and

  • Rotor leakage inductance.

Higher performance drives expand the number of parameters measured by adding terminal voltage sensors. For other than high-end SV drives the question becomes 'How good can you get without motor voltage feedback?' according to Dr. Kerkman.

Rockwell Automation's latest ac drive, PowerFlex 700, features stator-flux-type control, but future models will add higher level SV control. The 'bookshelf-style' package has V/Hz and flux vector control as well. An LCD operator module shows grouped parameters/descriptions along with programming. troubleshooting, and startup information. Initial ratings of PowerFlex 700, released in March 2001 span 0.37-30 kW (0.5-40 hp) at three supply voltages-200-240, 400-480, and 600 V ac. High-end SV drives include Allen-Bradley 1336 Impulse and 1336 Force drives.

Reduced cost of ownership is viewed as the main market driver for sensorless ac drives at GE Toshiba Automation Systems (Salem, Va.). However, picking sensorless motor control isn't as simple as eliminating the cost of the encoder. Rodney A. Fickler, manager strategic marketing at GE Toshiba, thinks the initial cost difference is minimal between using a sufficient number of current and voltage sensors in an SV drive or using an encoder on the motor.

Real savings come from reducing the cost to install, commission, maintain, and replace the encoder over time. A related item is the smaller drive motor footprint that reduces cost and the task of mounting to the machine, important to OEMs.

`Massaging' the model for lower cost

To deliver higher performance sensorless drives need an enhanced motor model. '[The model has to be able] to estimate and adapt its estimation of primary and secondary leakage, magnetic saturation, skin effects, core and stray load losses, and resistance,' explains Mr. Fickler. Adaptation of the motor model is necessary to compensate for changes in the above parameters. Even more important is the model's ability to predict changes in rotor temperature due operating conditions.

Improved motor modeling and adaptation lead to lower initial capital cost, as well as cost of ownership of ac drives, according to GE Toshiba. Mr. Fickler also reports associated enhancement of auto-tuning drives during commissioning. 'Incorporating additional sensors in the drive, in conjunction with more accurate motor modeling, has allowed the auto-tuning feature to be more accurate-and in some cases, for rough setup, autotuning can be performed using only general motor parameters. This has saved weeks of commissioning time for projects involving 20 or 30 coordinated drives in a process,' he states.

GE Toshiba Automation Systems and its counterpart in Japan, Toshiba GE Automation Systems (Tokyo) are the two subsidiaries of a joint venture company called Toshiba GE Automation Systems International. Formed in October 2000, this joint venture company between GE (General Electric) and Toshiba has the mission of being the global integrator of products from both parent companies.

A separate joint-venture company- GE Fuji Drives USA (Salem, Va.)-also notes lower life-cycle costs for SV drives compared to full flux vector control drives. 'By eliminating speed or positional sensors, the user can reduce initial start-up costs and eliminate sensor maintenance and replacement costs during ownership,' says John A. Cline, manager of engineering at GE Fuji Drives.

AF-300 G11 Adjustable Frequency Drive from GE Fuji is a multi-control drive. 'Dynamic torque vector' control is the SV mode, with full flux vector and 'compensated' V/Hz control also user-selectable options. A multi-lingual keypad simplifies configuration, while built-in LAN communications and various options add up to meet demanding, high performance applications. Compared to traditional field-oriented drives, AF-300 G11's features are said to be easily set-up with a few simple keypad commands.

There is a distinction to note between the business sectors of the above joint venture companies. GE Fuji products fall into the standard drives arena, while GE Toshiba is active mainly in system drives and coordinated multi-section drives.

Different approach

As discussed in the main article, ABB takes a different approach to sensorless vector control with its Direct Torque Control (DTC) method.

'DTC relies on a sophisticated motor reference model for its success, which includes a separate model for torque control,' comments Michael Link, product manager, Standard drives at ABB Automation Products (Mannheim, Germany). Among DTC's notable features, Mr. Link mentions the torque and flux comparator, where differences between real (or observed) and ideal values are minimized. The comparator employs variable switching frequencies.

Users need only to supply motor nameplate data. The drive selects the necessary machine parameters using an 'identification' process. This simple identification procedure does not require spinning the motor and is adequate in most cases, according to Mr. Link. 'A more sophisticated identification procedure is also available. It involves spinning the motor at no load, but is rarely needed,' he concludes.

ABB's premium drives use the DTC technique, see below. Its general-purpose and HVAC drives (ACS 100, 140, and 400 Series) do not use DTC.

ACS 600 Series drives come in sizes up to 3.45 MVA at 480 V and 5.14 MVA at 690 V. Both water-cooled and air-cooled versions are offered. In the medium-voltage (MV) range several products are available. Among them is ACS 1000, which uses a three-level inverter with LC output filter to avoid high voltage spikes (dv/dt effects) detrimental to existing (older) induction motors. These drives are available up to 4,160 V and up to 6,700 hp.

A larger MV drive, ACS 6000sd also operates with a three-level inverter to control synchronous motors. It comes standard with an ARU (active rectifier unit) for very low line current harmonics. Availability is up 3.3 kV and up to 27 MW power.

The foregoing product information comes from Kalyan Gokhale, manager, AC Drives R&D at ABB Automation (New Berlin, Wis.).

Other products

Most drive manufacturers offer models with sensorless vector control. Here is a sampling of further SV drive products not mentioned elsewhere.

VLT 5000 Series drives from Danfoss Drives (Rockford, Ill.; Graasten, Denmark) incorporate VVCPlus, a new sensorless vector drive system for torque and speed control of induction motors. An Automatic Motor Adaptation function handles most of the setup adjustments needed. The software automatically optimizes drive to motor settings by reading/checking operating parameters. There is no need to spin the motor. VLT 5000 has a size range of 1-600 hp in normal overload mode, and supplies starting torque of 180%. The drive also features extended ac line voltages of 200-240 and 380-500 V ac

Eurotherm Drives (Charlotte, N.C.; Heppenheim, Germany) applies its MRAS (model reference adaptive system) technology for current and speed control to the sensorless vector control version of its 690+ AC Drive. Current control under MRAS is available in both open-loop and closed-loop modes. The drive is said to produce smooth running and robust, trip-free acceleration/deceleration even in demanding applications. The 690+ AC Drive has a power range of 0.37 to 90 kW. Drive models up to 355 kW are coming.

Drives with sensorless vector capability from Hitachi America Ltd. (Tarrytown, N.Y.) include SJ100, a variable-frequency microdrive with precise torque and speed regulation (0.1%), up to 200% starting torque, and autotuning that sets motor parameters. Available ratings are 1/4-10 hp at 230V (single phase and three phase) and 1/2-10 hp at 460 V (three phase).

SJ300, a larger, all three-phase series of drives comes in 230 V models from 1/2 to 30 hp and in 460 V models from 1 to 30 hp. Units with higher power rating are available on special order. SJ300 Series includes Advanced Sensorless Vector control, but can be configured for V/Hz or closed-loop vector control, as well. High starting torque and low-speed torque (200% at 0.5Hz, 150% at 0 Hz), online/offline autotuning, and built-in communications are other useful features.

These drives are sold by Hitachi Companies in North America and also through Inc .(Cumming, Ga.).

TB Wood's (Chambersburg, Pa.) offers WF2 Sensorless Vector Drive as the newest member of its E-trAC family. SV operation produces 150% torque down to 0.5 Hz. Drive configuration is said to be simple, via programming selections read in 'plain English' directly from an LCD screen built into the unit. WF2 comes with various enclosure options ranging from Chassis/IP21 to NEMA 12/IP55. Present size availability of WF2 drives is 1-5 hp at 230/460 V ac. Larger drives and more supply voltages will be added.

Yaskawa Electric America 's (Waukegan, Ill.) Varispeed-G5 is a general-purpose inverter with flux vector control. Varispeed-G5 combines four control methods of operation, one of which is Sensorless Flux Vector mode. The inverter offers a wide range of features such as, high torque at low speed, speed range of 100:1, and speed regulation of

Coming: more capable modeling, more parameters measured

Looking ahead, Dr.-Ing. Dieter Eckardt, manager of system technology, Standard Drives R&D, at Siemens Automation & Drives (Erlangen, Germany), notes several developments in the future of sensorless vector drives. More complete and sophisticated motor models will be developed. Also, added parameter measurements will give the controller deeper insight into the motor, allowing better performance from more information available. 'Sensorless vector drives will move into more mainstream applications. Performance will grow, but not reach a servo-like level,' says Dr. Eckardt.

Still higher performance SV drives are envisioned down the road at GE Fuji. 'These new drives will find wider application in small networked systems, with built-in LAN communications and Internet-ready configuration and monitoring software packages,' adds Mr. Cline.

Control Techniques' Mr. Baird sees evolutionary developments coming in the near term, such as newer power-switching devices (e.g., trench-gate IGBTs) and improved algorithms for sensorless control. 'Industry is waiting for the next quantum leap of technology,' he says.

GE Toshiba's Mr. Fickler sees a long-range goal to eliminate separate position- or speed-based sensors entirely. 'However, to do so will require more advanced control techniques to infer motor operating characteristics,' he adds. Also ahead are still more capable predictive/adaptive modeling methods for sensorless drives, coming from ever faster and more powerful computers

Overall, sensorless ac drives are making steady progress in capabilities and market share. However, the consensus is that for the highest demand, critical response applications closed-loop control will rule for the foreseeable future.

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