Motors and Drives
150 Torque with ACS800 100 Torque with ACS880 50
Speed (rpm), Torque (%)
0 0 -50 Speed -100 1 2 3 4 5 6 7 8 9 10 11 12 13
Time (s) 14
Fig. 2. Both drives show remarkable sensorless control ability to operate for long periods in the near zero speed range, but the new ACS880 has less deviation from the torque reference and thus can provide better motor control performance.
15
-150
● High static and dynamic speed accuracy. ● No preset switching frequency. Optimal transistor
switching is determined for every control cycle, allowing the drive to more readily match driven load requirements.
DTC was originally developed for AC induction motors because of their popularity in myriad industrial and commercial applications. Indicative of DTC performance with induction motors is torque response time (to a 100 per cent torque reference step) that approaches the motor’s electrical time constant limit. Torque repeatability uncertainty under the same reference command is typically as low as 1 per cent of nominal torque across the drive’s speed range.
Beyond induction motors
Demand for higher power density and evolving international efficiency regulations have renewed interest in other motor topologies. The good news is that DTC has been extended to work with permanent magnet (PM) synchronous and new synchronous reluctance (SynRM) motors. The main difference for DTC occurs during motor starting, which ABB drives accommodate. Absence of rotor windings and slip- speed in PM synchronous and SynRM motors extends their efficiency gains over a wider torque-speed range compared to induction motors.
100 Torque reference
20 40 60 80
Actual and reference torques (%)
Fig. 3. Maximum torque error is slightly less for the synchronous reluctance motor than for the induction motor.
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-100 -80 -60 -40 -20 0
0 5
High torque/size ratio of these motors may allow a simpler drive train design. For example, a direct-driven low speed PM motor could eliminate a packaging machine gearbox. One drawback of PM synchronous motors is their reliance on so-called rare-earth magnets for best performance. Here is where SynRM motors provide an alternative - because they don’t use permanent magnets.
Recent performance validation
In mid-2012, ABB conducted a series of measurements to ensure that continuing improvements made to DTC technology are keeping its AC drives at peak performance. Here are some significant results:
● Torque stability near zero speed. ABB’s ACS800 and new ACS880 industrial drives were compared for torque control accuracy in sensorless (open-loop) operating mode. Each drive ran a 15kW, four-pole induction motor at its rated torque reference and with the load machine controlled to make slow speed reversals near zero speed. Fig. 2. shows customer
Torque error of the induction motor
Torque error of the synchronous reluctance motor
Time (s) 10 15 20 25 30 35
Induction motor torque
Synchronous reluctance motor torque
40
-20 -15 -10 -5 0 5 10 15 20
Torque error (%)
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