TechArchive

Original article date: February 1999

Though DC bus drives have been available for some time the first multiple drive system on a common DC bus incorporating its proprietary Direct Torque Control (DTC) inverters has now been launched by ABB Industrial Systems in the form of the ACS 600 Multidrive.

DC bus drives use common components to control and supply power to the inverter sections of AC drives thereby reducing overall capital costs. A single rectifier can supply several inverter sections from a common DC bus rather than using one rectifier per inverter as is the case with standalone units. Up to 25% less space is needed owing to the smaller component count particularly important where space is at a premium such as in marine applications.

This launch of the ACS 600 Multidrive follows in the wake of the launch in 1995 of ABB’s first standalone drives incorporating DTC technology. Here ABB claims that users are reporting capital cost reductions of up to 25% as encoders are no longer needed. The accurate torque and speed control provided by DTC (see panel) aims to eliminate the need for feedback devices on the applications served by many DC bus drive systems such as rollers and conveyers on the paper and metals industries.

On a DC bus drive because all the inverters are in one system they are much easier to control reducing the amount of external control and switchgear needed. Energy is used more efficiently as when an inverter is regenerating – for example on a crane application – energy is transferred to the bus system where it can be used by an inverter that is driving.

DTC inverter sections from 2.2kW to 2500kW can be included in the Multidrive giving a wide choice of inverter powers in one system. Users can specify as many inverter sections as they wish which ABB will then build to order. Inverter sections can be added to the DC bus easily after installation to accommodate plant expansions a less costly method than adding an entire drive.

The Multidrive has a wider power range than do previous systems – 5kVA to 3350kVA the higher power systems being designed to cover the growing machine sizes found in the pulp and paper and metals industries. The low power models are aimed at small machines for example in converting machinery such as paper coaters.

Control of the inverters can be by the ACS 600 standard software linked to a control panel or PC for simple applications such as pumping stations. For more complex applications such as paper machines an application controller is built in which has all the hardware and software facilities for the specific application. One controller can control up to 12 inverter sections. For large systems several application controllers can be linked by Advant Fieldbus 100.

For small customised system control such as a conveyor an alternative approach is used which includes a PLC as an overriding controller linked to the inverters via a fieldbus. All inverter sections now use IGBTs rather than the GTOs (Gate Turn-Off Thyristors) of previous systems. IGBT stands for Insulated Gate Bipolar Transistor and forms the basis for most modern AC Drives technology. IGBTs have many advantages such as allowing a smaller drive package and better motor control. Their faster switching enables the fast torque response times and speed accuracy of DTC to be exploited. The also contribute to lower noise and EMI levels.

Direct Torque Control – a reminder

Starting with a few basics in a variable speed drive (VSD) the basic function is to control the flow of energy from the mains to a process via the shaft of a motor. Two physical quantities describe the state of the shaft: torque and speed. Controlling the flow of energy depend on controlling these quantities.

In practice either one of them is controlled and we speak of “torque control” or “speed control”. When a VSD operates in torque control mode the speed is determined by the load. Torque is a function of the actual current and actual flux in the machine. Likewise when operated in speed control the torque is determined by the load.

Variable speed drives are used in all industries to control precisely the speed of electric motors driving loads ranging from pumps and fans to complex drives on paper machines rolling mills cranes and similar drives. So just what is behind a technology which is claimed to be so revolutionary that it is the ideal solution to every drive problem?

The publication of the theory of DTC actually goes back to 1971 when it was published by German engineer Blaschke. ABB’s interest dates back to 1988.

The idea is that motor flux and torque are used as primary control variables which is contrary to the way in which traditional AC drives control input frequency and voltage but is in principle similar to what is done with a DC drive where it is much more straightforward to achieve. In contrast traditional PWM and flux vector drives use output voltage and output frequency as the primary control variables but these need to be pulse width modulated before being applied to the motor. This modulator stage adds to the signal processing time and therefore limits the level of torque and speed response time possible from the PWM drive.

In contrast by controlling motor torque directly DTC provides dynamic speed accuracy equivalent to closed loop AC and DC systems and torque response times that are 10 times faster. The DTC principle is also claimed not to generate noise like that produced by conventional PWM AC drives. An the wider spectrum of noise means that amplitudes are lower which helps to control EMI and RFI emissions.

In DTC field orientation is achieved without feedback using advanced motor theory to calculate the motor torque directly and stator flux without using a modulator or a requirement for a tachogenerator or position encoder to feed back the speed or position of the motor shaft. Both parameters are obtained instead from the motor itself.

DTC’s configuration also relies on two key developments – the latest high speed signal processing technology and a highly advanced motor model precisely simulating the actual motor within the controller. A 40MHz digital signal processor is used together with ASIC hardware to determine the switching logic of the inverter.

The motor model is programmed with information about the motor which enables it to determine parameters including stator resistance mutual inductance saturation coefficients and motor inertia. The model also encompasses temperature compensation which is essential for good static speed accuracy without encoder.

Points to watch with IGBT Drives

Recently some application issues using IGBT drives have come to light. The fast turn-on time of the IGBT drives causes phenomena called “standing wave” to occur if there is enough distance between the output of the AC drive and the motor. The “standing wave” may produce enough voltage to damage the motor insulation leading to motor failure.

All AC drive output devise including GTOs (Gate Turn-off Thyristors) and BJTs (Bipolar Junction Transistors) can produce the standing wave effect at motor cable distances that are typically longer than most installations call for. IGBTs however can cause problems with motor insulation in as little as 13m of cable. The main reason that these high voltage spikes may be a problem is that motor construction can vary considerably. Motors are manufactured with 1000V 1200 and 1600V insulation ratings. Small frame motors (1hp and below ) are also typically randomly wound instead of form wound. Form winding reduces the voltage difference between adjacent windings. Smaller horsepower drives usually have faster rise times than the larger horsepower models.

If the installation is new purchase a 1600V rated motor. If the motor is existing than a line reactor either at the output of the drive or mounted at the motor can be used to slow down the rise time of the waveform. In larger HP sizes a line terminator or filter mounted at the motor will reduce the voltage to a safe level.

It is important to remember that standing waves only affect a small percentage of applications and the majority of motors do not fail due to this phenomenon.

• ABB
• Ian Rennie
• 01438 742366

February 1999