Manager, AC Drives R&D ABB Inc.
Edited by Kenneth Korane
To operate efficiently, a conventional flux-vector PWM drive needs to know the rotor flux angle inside an ac-induction motor. A pulse encoder typically supplies information about rotor speed and angular position relative to the stator field. The controller uses this data to regulate voltage, current, and frequency through a pulse-width modulator to the motor. Torque, therefore, is controlled indirectly.
This system provides good torque response, accurate speed control, and can produce full torque at zero speed, giving performance close to that of a dc drive. However, it requires rotor-position feedback. This adds cost and complexity to simple ac-induction motors. Also, using a modulator slows the motor's response to changes in the incoming voltage and frequency signals.
Direct torque control (DTC), on the other hand, achieves field orientation without feedback encoders or tachometers. Instead it relies on fast DSPs and an advanced mathematical model of the motor. In this system, motor current and voltage levels are inputs to the DTC controller. The embedded motor model uses this information to generate precise estimates of stator flux and torque every 25 µsec.
Thus, DTC controls torque and speed directly based on the electromagnetic state of the motor, similar to a dc motor but unlike traditional PWM drives that adjust motor frequency and voltage. Because torque and flux are directly controlled, there is no need for a modulator — as used in PWM drives — to control frequency and voltage. This dramatically speeds the drive's response to changes in required torque.
Another option for design engineers is sensorless-vector control (SVC), which evolved from field-oriented control. In other words, these are encoder-based full-field-oriented (vector) control systems modified for sensorless-torque control. In essence, algorithms eliminate the encoder. But most SVC algorithms use field-oriented control architecture as the starting point, and then try to estimate speed using motor current and voltage information instead of encoder feedback.
On the other hand, DTC was developed as a "native" sensorless-torque control, not an adaptation of the vector-control method. This technology was pioneered as a sensorless-control architecture and is one of the main reasons for its success.
DTC offers several benefits over traditional ac drive technology. One is better torque response. Traditional PWM drives use output voltage and output frequency as the primary control variables, but these need to be pulsewidth modulated before being applied to the motor. This modulator stage adds to the signal processing time and therefore limits the torque and speed response from a PWM drive.
The DTC's typical torque response is 1 to 2 msec, compared to >5 msec for both flux-vector and dc drives fitted with an encoder. Open-loop PWM drives (scalar drives) typically have a response time well over 100 msec. Even in newer sensorless drives, torque response is in the 10-msec range. Dynamic speed accuracy of DTC drives is about eight times better than open-loop ac drives and comparable to dc drives using feedback.
Another plus is accurate torque control at low frequencies, as well as full-load torque at zero speed. DTC, without an encoder, can provide 1 to 2% torque repeatability across the speed range. This is half that of other open-loop ac drives and on par with closed-loop ac and dc drives. Also, motor static and dynamic speed accuracy is far better than with PWM drives.
Setup and commissioning are also straightforward, thanks to the sophisticated motor model used in DTC. A user basically need only supply motor nameplate data. In this autotuning stage, the system determines data such as stator resistance, mutual inductance, and saturation coefficients, along with motor inertia. The simple identification procedure does not require spinning the motor and is generally adequate. A more-sophisticated procedure is also available that involves spinning the motor at no load, but is rarely needed.
From an operating point of view, entire machines and processes can be installed or retrofit with encoderless drives. It removes all the difficulties and downtime associated with trying to keep glass-disk encoders operable, particularly in extremely harsh environments such as paper mills and cement plants.
As just one example, operators of the Georgia Pacific Gypsum Paper mill in San Leandro, Calif., turned to DTC as part of an upgrade to increase capacity as well as improve control and reliability. The 400-foot-long mill converts corrugated and flyleaf wastepaper to eight-ply face and back-side paper for gypsum wallboard. At the heart of the upgrade were ABB variable-speed ac motors, drives, and encoderless operation.
Encoders, fragile glass disks wired to motors that provide critical feedback in traditional PWM drives, are the weakest link in the electrical chain, according to Fred Curcio, the plant manager. They must perform in a hostile, harsh environment and if they fail, the processing line grinds to a halt until maintenance personnel can identify and replace the faulty encoder.
ABB's DTC technology eliminated this consideration. "Encoderless operation of a paper mill is a quantum leap in simplification," said Mike Giraudo, vice president of Intec Solutions, Livermore, Calif, the system integrator for this project. "Mechanically, the encoders and wiring cease to be necessary; they're gone in one fell swoop."
The DTC drives — a total of 13 on the paper machine — have operated flawlessly for more than two years, according to Curcio.
The retrofit provided operators exceptional control of the machine. All drives and motors at San Leandro are wired to a master SAF controller platform, for which Intec created proprietary software. The programmable linear controller uses a processor with input cards and talks directly to the drives, via a fiber-optic connection. The fiber-optic cable is the I/O to the drives. SAF, in turn, reports to a SCADA program that provides historical trends of data for troubleshooting faults or identifying operating problems.
The drives also permit higher speeds with more precise draw control, giving the mill the ability to run tighter draws with fewer paper breaks — improving throughput and product quality. The papermaking machine's maximum capacity improved from 750 to 1,000 fpm, according to Curcio.
More-precise control also makes it easier to adjust the paper machine on the fly, he says, and the mill has begun producing heavier paper for G-P Gypsum's premium ToughRock wallboard. Capacity has improved from 175 to 225 tons/day.
Production is, in large part, a function of uptime, notes Curcio. And with the retrofit, the machine runs with very little delay. Downtime at the plant now averages about 2%, he says, well below the industry average of 7%.
Direct Torque Control (DTC) is an optimized ac-drive-control principle where inverter switching directly controls the motor variables: flux and torque.
The measured motor current and voltage are inputs to an adaptive motor model which produces actual values of flux and torque every 25 µsec.
Motor torque and flux two-level comparators compare actual and reference values produced by torque and flux-reference controllers. Depending on the outputs from the two-level controllers, the pulse selector directly determines the optimum inverter switch positions.
The inverter switch positions determine the motor voltage and current. These, in turn, influence the motor torque and flux and the control loop is closed.