Motors and Drives
Fig. 4. Although ACS880 is not a servo drive, DTC enabled fast, accurate motor acceleration in both closed-loop and sensorless control modes. Comparing measured acceleration time to the motor’s mechanical time constant provides one measure of torque accuracy during extremely fast acceleration.
benefits.
Speed (rpm)
0 16
● Torque accuracy during ramping. ACS880 drive’s sensorless torque control accuracy was compared using a four-pole induction motor and a synchronous reluctance motor of 15kW size (at 50 per cent of rated speed). DTC held torque error to only a small percent of rated torque for both motor types (Fig. 3).
-3000 -6000
● Servo-class dynamic performance. Speed and angular position of a 1.5Nm, 6,000rpm PM synchronous motor were measured during speed reversal between ±6,000rpm in less than 25 milliseconds (ms). This is very close to the motor’s mechanical time constant of 24ms and a theoretical limit (Fig. 4).
DTC today, tomorrow
Updates over time have allowed DTC to reach beyond demanding, high-dynamic applications for which it was created. Software improvements and the commonality of high-power microprocessors make DTC drives economically justified for wider implementation. Ability to respond rapidly to changes in process variables, such as pressure, tension, or position using exceptional speed and torque control dynamics makes DTC attractive to
21 26 31 36 time (ms) Speed
6000
Angular position 3000
60 120 180 240 300 360
0
-360 -300 -240 -180 -120 -60
more industries. Moreover, AC drives offer dramatic energy savings
potential for large numbers of variable-speed pump/ fan applications. Because pump speed versus power has a cubic relationship, a process sequence that allows the pump to run at 50 per cent speed would use only 1
/8 of
full power. Direct torque control has shown a continuum of
hardware and software developments over its near 30-year lifespan. Based on firm theoretical foundations and DSP technology, DTC has overcome limitations of the early processors. Powerful processors now rapidly execute complex control algorithms, update motor model parameters, and switch the drive’s transistors for optimal performance. ●
For more information ✔ at
www.engineerlive.com/ede
Ere Jääskeläinen is with ABB Oy Drives, Helsinki, Finland.
www.abb.com/drives
from Pittman are available in the UK
Industrial grade servo motors T
he recently announced AB series industrial grade servo motors
from Mclennan Servo Supplies. These IP65 rated industrial grade
servo motors are well suited for use in harsh environments and supplied in NEMA 23, 34, and 56 frame sizes with various stack lengths to accommodate a wide range of medium performance automation applications where competitive price is important. The AB series
Angular position (degrees)
provide continuous stall torque output from 0.4Nm to 12.9Nm and peak torque from 1.3Nm to 38.3 Nm with rated speeds from 2800 to 5200rpm. The durable AB series feature
integral Hall sensor commutation and temperature overload protection with heavy-duty sealed output shafts and military-style multi-pin connectors. A wide choice of options includes
encoder feedback, external holding brakes, precision gearheads, and mating power and feedback cables. ●
For more information, visit:
www.mclennan.co.uk
www.engineerlive.com 47
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